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9.0 Introduction ................................................................................................................... 6
Objectives ....................................................................................................................... 6
Experimental halls. ......................................................................................................... 6
Photon-induced damage. ................................................................................................. 9
Normal incidence ........................................................................................................ 9
Grazing incidence ..................................................................................................... 11
Diffraction ................................................................................................................. 12
Absorbers and attenuators ......................................................................................... 14
9.1 Commissioning and common usage diagnostics ........................................................ 15
9.1.1 Requirements ....................................................................................................... 15
9.1.2 Coherent radiation properties ............................................................................... 20
9.1.2.1 Spectrum ....................................................................................................... 20
9.1.2.1.1 low energy: grating spectrometers ......................................................... 20
9.1.2.1.2 high energy: crystal spectrometers......................................................... 20
9.1.2.2 Total energy (including profile) .................................................................... 20
9.1.2.2.1 Total energy, accurate, intrusive: calorimetery...................................... 20
9.1.2.2.2 Total energy, every pulse, non intrusive: ionization chamber ............... 20
9.1.2.2.3 Profile, intrusive: x-ray CCD camera .................................................... 20
9.1.2.3 Centroid location ........................................................................................... 22
9.1.2.3.1. Ionization chambers ............................................................................. 22
9.1.2.3.2 crossed wire arrays ................................................................................ 22
9.1.2.3.3 mirrors ................................................................................................... 22
9.1.2.4 Divergence .................................................................................................... 22
9.1.2.5 Temporal distribution.................................................................................... 22
9.1.2.6 Coherence: .................................................................................................... 22
9.1.2.6.1. Asymmetric Michelson interferometer ................................................ 22
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9.1.3 Spontaneous radiation properties ......................................................................... 28
9.1.3.1 Spectral structure of higher harmonics ......................................................... 28
9.1.3.2 Energy of higher harmonics .......................................................................... 28
9.1.3.3 Intensity of higher harmonics ....................................................................... 29
9.1.3.4 Angular distribution ...................................................................................... 29
9.1.3.5 Temporal distribution.................................................................................... 29
9.1.4 Other common usage diagnostics ........................................................................ 29
9.1.4.1 Synchronization between FEL and pump or probe laser .............................. 29
9.2 Optics and beamline components for the experiments ............................................... 30
9.2.1 Requirements ....................................................................................................... 30
9.2.2 Optical enclosures ................................................................................................ 32
9.2.2.1 Front end enclosure ....................................................................................... 32
9.2.2.1.1 Valve and seat ........................................................................................ 32
9.2.2.1.2 Vertical slit, and 9.2.2.1.3 horizontal slit ............................................... 32
9.2.2.1.3 Attenuator/absorber................................................................................ 34
9.2.2.2 First optical enclosure ................................................................................... 38
9.2.2.3 Inter-Hall transport........................................................................................ 38
9.2.2.4 Rear optical enclosure ................................................................................... 38
9.2.2.5 End station .................................................................................................... 38
9.2.3 Near Hall experiments ......................................................................................... 38
9.2.3.1 Atomic physics.............................................................................................. 38
9.2.3.1.1 Ellipsoidal focusing mirror .................................................................... 38
9.2.3.1.2 Apertures ................................................................................................ 38
9.2.3.1.3 Filters ..................................................................................................... 38
9.2.3.1.4 Other equipment..................................................................................... 38
9.2.3.1.5 Photon diagnostics ................................................................................. 38
9.2.3.2 Warm dense matter and plasma .................................................................... 38
9.2.3.2.1 Requirements ......................................................................................... 38
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9.2.3.2.2 Optical Design and Components ........................................................... 40
9.2.3.2.2.1 Focusing System ............................................................................. 40
9.2.3.2.2.1.1 Optical design .......................................................................... 40
9.2.3.2.2.1.2 lens fabrication ......................................................................... 43
9.2.3.2.2.1.3 Lens Survivability. ................................................................... 44
9.2.3.2.2.2 Apertures ......................................................................................... 45
9.2.3.2.2.3 Attenuator ....................................................................................... 46
9.2.3.2.2.4 Beam intensity monitors ................................................................. 48
9.2.3.2.2.5 Imaging detectors ............................................................................ 48
9.2.3.3 FEL commissioning and physics .................................................................. 49
9.2.3.3.1 Pulse slice, compression and transform-limiting systems ..................... 49
9.2.3.3.1.1 Pulse slice........................................................................................ 49
9.2.3.3.1.2 Pulse compression........................................................................... 53
9.2.3.3.1.3 Transform-limited pulse...................Error! Bookmark not defined.
9.2.3.3.2 Wavelength selection ............................................................................. 53
9.2.3.3.2.1 Monochrometers ............................................................................. 53
9.2.3.3.2.2 Grazing incidence mirrors............................................................... 56
9.2.3.3.3 Beam splitting ........................................................................................ 57
9.2.3.3.4 Pulse delay ............................................................................................. 58
9.2.3.3.5 Polarization control ................................................................................ 58
9.2.4 Far Hall experiments ............................................................................................ 58
9.2.4.1 Nano-scale dynamics .................................................................................... 58
9.2.4.2 Femto-second chemistry ............................................................................... 59
9.2.4.3 Biological structure ....................................................................................... 59
9.3 Facility requirements (access, personal protection, radiation shielding and beam
containment, toxic materials) ............................................................................................ 59
9.3.1 Requirements and calculations ............................................................................ 59
9.3.1.1 Photons.......................................................................................................... 60
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9.3.1.1.1 Bremsstrahlung from collimators .......................................................... 60
9.3.1.1.2 Bremsstrahlung from profile and intensity monitors ............................. 61
9.3.1.1.3 Gas bremsstrahlung ................................................................................ 61
9.3.1.2 Muons ........................................................................................................... 62
9.3.1.3 Neutrons ........................................................................................................ 62
9.3.1.4 Toxic materials.............................................................................................. 63
9.3.2 Personnel protection system (PPS) and access .................................................... 63
9.3.3 PPS beam stoppers ............................................................................................... 64
9.3.4 Burn through monitors ......................................................................................... 65
9.3.5 Shielding .............................................................................................................. 66
9.3.6 Beam containment system (BCS) ........................................................................ 66
9.3.7 Lead end stops...................................................................................................... 66
9.3.8 Muon stops ........................................................................................................... 66
9.4 Control and instrumentation ....................................................................................... 66
9.4.1 Control system objectives .................................................................................... 66
9.4.2 Control system layout .......................................................................................... 67
9.4.3 Motion controls .................................................................................................... 68
9.4.4 Photon beam stabilization at the sample .............................................................. 69
9.4.5 Timing system ...................................................................................................... 69
9.5 Other components ....................................................................................................... 71
9.5.1 Vacuum system and differential pumping ........................................................... 71
9.5.2 Debris catchers ..................................................................................................... 73
9.5.3 Solid target positioner .......................................................................................... 73
9.5.4 Gas target system ................................................................................................. 73
9.5.5 Displacement controllers ..................................................................................... 73
9.6 optical components research and development........................................................... 73
9.6.1 Experiment and simulation of x-ray photon-material interaction ........................ 74
9.6.1.1 Simulation ..................................................................................................... 74
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9.6.1.2 Experiment .................................................................................................... 77
9.6.2 Renewable and high flux-compatible optics ........................................................ 78
9.6.2.1 Liquid optics ................................................................................................. 78
9.6.2.2 Plasma lens.................................................................................................... 80
9.6.3 Advanced attenuators ........................................................................................... 81
9.6.3.1 Liquid metal attenuator ................................................................................. 81
9.6.3.2 Mixed gas/low Z metal attenuator ................................................................ 82
9.6.9 Application of generic optical techniques to the LCLS ....................................... 82
9.6.9.3 Multilayer optics ........................................................................................... 82
9.6.9.4 Replicated optics, e.g ellipsoidal mirrors ...................................................... 83
9.7
Summary ............................................................................................................... 85
9.7.1 Optical components required ............................................................................... 85
9.7.2 Diagnostics required ............................................................................................ 85
9.7.3 Access and personnel protection .......................................................................... 85
9.7.4 Other components ................................................................................................ 85
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Chapter 9
X-ray optics and diagnostics
9.0 Introduction
Objectives
This section concerns those elements between the undulator end and the User
experiments. It is written based on the User and FEL physics requirements for the x-ray
optics and x-ray diagnostics, and the facility requirements (i.e. the facility protocols and
guidelines). Some of the User and FEL physics requirements are found in the document
“LCLS First Experiments (September 2000)”; more details have been obtained by
working with individual User group representatives.
From the known requirements, the appropriate hardware can be determined. Both the
x-ray optical components, and the x-ray diagnostics, can be categorized as
a) those systems that are needed for FEL commissioning, or are required by
multiple User experiments
b) those that are specific to a limited number of the User experiments.
In the case of the diagnostics, those that are required for commissioning or are
required by multiple users are described first. Those specific to a particular experiment
are described in the section detailing the particular experimental layout and optical
requirements.
Experimental halls.
Two experimental halls are provided, one close to the undulator exit (Hall A, starting
~ 50 m from the undulator end) and one downstream of the undulator (Hall B, starting ~
400 m from the undulator end). The total experimental floor area is a requirement of the
first 6 experiments planned (including FEL physics); the locations are determined by
local access roads and topology. Optics in Hall B will experience a reduced power
density, by about a factor 15 (see below); this should allow many standard solutions and
materials to be applied to the optics. Hall A is necessary if maximum coherence is
important. Hall A is also necessary because a number of the optical elements and
instruments developed (at least conceptually) for the LCLS depend on close proximity to
the LCLS undulator for optimal operation or parameter values. These elements include
(and are discussed later):
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a) planar take-off mirrors (if these were 300 m farther downstream they would
need to be substantially longer, up to 1 meter/facet);
b) refractive lenses (whether solid state or gaseous, such lenses are efficiencylimited by their aperture diameter - even at distances of ~50 m from the
undulator their operation will be marginal);
c) chirped-multilayer compressor optics (these are also efficiency-limited by the
beam diameter);
d) multilayer-based transmission gratings (with present techniques it is doubtful
that the required high-quality and efficient gratings with apertures greater than
~100 m could be built).
An additional reason for a close hall (Hall A) involves the transmission of the
spontaneous synchrotron radiation (SR) to experiments. Close to the undulator, a ~1 cm
aperture through the vacuum system should transmit a usable fraction of this spectrum.
This aperture is at the limit at which the differential pumping systems we have specified
can operate effectively. If the same SR cone needed to be transmitted to the far hall, the
vacuum aperture would more than double, necessitating a complete redesign of the
vacuum components.
These re-specified components would significantly more
expensive.
The large flight distance to Hall B, and associated beam wander, is not expected to be
a problem (the specification of beam "wander" in the LCLS design study report is ~10%
of the beam diameter, i.e. independent of path length). The angular acceptance for SASE
saturation through the undulator is of the same order as the beam divergence, so any
beam angle excursions larger than this value would probably quench any coherent output
(a similar constraint applies to the position of the beam axis). In addition to this, an
active monitor and feedback system coupled to positional and attitude controls on the
mirrors (and possibly the undulator) are planned to help stabilize beam movement.
Figure 9.0.1 illustrates one possible configuration of the halls, in which various
optical elements are indicated (all described later in detail). An important flexibility for
various experimental applications is the possibility of separating, or suppressing, the
spontaneous radiation with respect to the coherent line spectrum. This spectral-angular
filtering function can be performed by:
a) the absorption cell
b) take-off mirrors or crystals, and
c) by the horizontally and vertically tunable x-ray slits.
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Front end
Front end
fast close
valve
valve seat
differential
vacuum
horizontal
slit/collimator
vertical
slit/collimator
differential
vacuum
calorimete
r
grey/black:
safety
yellow:
pumping
blue: beam
conditioning
orange:
diagnostic
differential
vacuum
collimators
differential
vacuum
monochrometer
spool
differential
vacuum
mirror
chamber 1
mirror
chamber 3
Hall A2
beam stopper
Hall A2
beam stopper
Hall A2
beam stopper
burn thru
monitor
burn thru
monitor
burn thru
monitor
Hall A3
beam
stopper
collimators
differential
vacuum
Hall B2
beam stopper
Hall B2
beam stopper
Front end
O ptical
O ptical
O ptical
Hall A1
beam stopper
Hall A1
beam stopper
Hall A1
beam stopper
burn thru
monitor
burn thru
monitor
burn thru
monitor
Hall A3
Hall A3
beam
stopper
differential
vacuum
mirror
chamber1
differential
vacuum
burn thru
monitor
Hall B3
beam
stopper
O ptical
O ptical
differential
vacuum
O ptical
Hall B1
beam stopper
Hall B1
beam stopper
Hall B1
burn thru
monitor
burn thru
monitor
burn thru
monitor
Hall B2
Figure 9.0.1 A schematic diagram of elements in the experimental halls.
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Hall B3
Hall B3
beam
stopper
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Will be modified to reflect new information. Note at present there are no requests
for mirrors.
Photon-induced damage.
In translating the User and facility requirements to hardware, the attributes of the FEL
x-ray output must be considered. Detailed properties of both the coherent and
spontaneous radiation are available. In particular, the photon energy, the energy per
bunch, and the beam cross section, imply damage will be an important issue in
component design; these parameters are shown in Table 9.0.1 for locations
approximating the front of Hall A, and in Hall B. Only the coherent light poses a
problem; the spontaneous emission is divergent and will usually be reduced by an upstream aperture. It is also mostly at larger energies than the fundamental, and the energy
absorbed in optical components is low.
Normal incidence
The exact criteria for absorption and damage during pulses of such high intensity as
those to be produced by the LCLS are not known, and will be part of the first atomic
physics experiments, but photo absorption and possible subsequent melt are used here.
Table 9.0.2 shows results for different materials used in Hall A1 (atomic physics
experiments), for the worst case of 827 e.V. where absorption is largest [1]. Dose rates
given here are for normal incidence, relevant for transmissive optics, calculated from
photo-ionization cross sections (  phot oi onizati on), with the photon beam areal density  phot on
calculated for a propagated Gaussian beam:
dose  E phot on phot on phot oionization
Ephot on is the photon energy. Comparing the dose predicted in eV/atom, and the dose
required to melt, one finds Be, B4C, C and Si can be used without melt (although Si is
already at > 0.5 the melt limit). Probably BeB, Li and LiH can also be used. Two other
criteria must be considered for transmissive optics; first the material should be such that a
 phase change occurs in a reasonably large (> 5 m) distance, to allow construction.
Second, the distance required for this phase change should be shorter than the penetration
depth, so that a reasonable fraction of the light is transmitted.
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Table 9.0.1. Parameters of the FEL x-ray photon output that are involved in estimates
of damage thresholds. Check divergence of SR
FEL photon
energy
0.828 keV (4.54 GeV electron
beam)
8.27 keV (14.35 GeV electron
beam)
Coherent
(fundamental)
spontaneous
Coherent
(fundamental)
spontaneous
Energy per pulse
(mJ)
3
1.4
2.5
22
Peak power (GW)
11
4.9
9
12
Photons/pulse
23x10
1.9x10
Divergence
(fwhm, rad)
9
Spot (fwhm) at 50
m (m) Hall A
610
130
Spot (fwhm) at
400 m (m) Hall
B
4400
570
Peak energy at 50
m (J. cm-2) Hall A
0.59
11.9
Peak energy at
400 m (J. cm-2)
Hall B
0.01
0.57
310
1
81
12
100
Table 9.0. 2. Parameters relating to material absorption, damage, and phase change
of 827 eV photons in Hall A1.
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w orst ca se FEL photon energy = 82 7 eV, Hall A1 front
do se
(EV/atom )
me lt
(eV/ato m)
ph ase depth
(le ngth for a
1*  p hase
chang e) ( m)
5.2
pe netra tion
de pth (um)
Li
Li H
B
BeB
B4C
C
Si
0.001
0.001
0.005
0.005
0.14
0.22
0.38
Al
0.27
0.2
1.8
Cu
po lystyrene
0.72
0.11
0.35
0.5
2.7
0.36
1.4
0.6
0.86
0.74
0.9
1.1
47
35
5.1
4.3
1.6
1
1.6
pe netra tion
de pth / ph ase
de pth (large is
go od)
9.04
3.64
1.11
1.45
Grazing incidence
Calculations for grazing incidence mirrors include the effect of energy density
dilution through the angle (the footprint area increases), reflectivity, and deposition
throughout an e-folding depth for the photons. The results demonstrate an interplay
between atomic number, incidence angle, and photon energy. The absorbed energy
density is [2]:
A (eV / atom) 
Ppeak 2    i 1  R 
 2 

q
 Dw   p # 
Here Ppeak is the peak coherent power,  is the standard deviation of the temporal
pulse length, q is the electronic charge, Dw [cm] is the beam diameter at the optic, p
[cm] is the 1/e penetration depth of the light into the material in a direction normal to the
surface, # [cm-3] is the atomic density of the material, and R the reflection coefficient.
Following conventional analysis [1,3], we show A vs. i in Fig. 9.0.2 for three candidate
reflecting materials: Au (high-Z), Ni (medium-Z), and Be (low-Z), for three
representative values of the LCLS's coherent fundamental and ~3rd harmonic lines (900
eV, 8500 eV, and 30000 eV). Selecting, e.g., A  0.01, a criterion suggested by earlier
experimental work at SSRL [3], and safe with respect to melt, we may, for example,
select an Au-coated mirror with I ~ 0.0001 for energies < 3 keV, and a Be-coated
reflector with I ~ 0.0005 for all energies >3 keV.
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100
Au 1 keV
Au 8.6 keV
Au 30 keV
Be 1keV
Be 8.6 keV
Ni 8.6 keV
Ni 30 keV
Ni 1 keV
Be 30 keV
A[eV/atom]
10
1
0.1
0.01
0.001
0.0001
10-5
-6
10
-5
10
0.0001
0.001
0.01
0.1
Grazing Incidence Angle
Figure 9.0.2. Peak power energy loading of candidate LCLS mirror materials vs.
(TE) grazing incidence angle and LCLS energy.
Diffraction
Assuming a crystal to be cut to thicknesses equal to the extinction depth, and operated
in a symmetric diffraction geometry, then a conservative estimate of power and energy
loading can be obtained. Assume: 1) the angles of incidence of the radiation (with
respect to the surface plane) are large enough to enforce negligible reflection; 2) on the
average, the diffracted beams travels through approximately the same distance in the
material as the 0th order beam; and 3) on average, the absorption depth, ta, for the
diffracted beams is the same as for the incoming beam. Then the energy loading is:
(eV / atom) 
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Ppeak 2 
2
qDw #t a
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Selecting Si, C*, and Be as the candidate materials, we tabulate A for selected
crystal plane orientations in Table 9.0.3. The lower absorptivities of the lower-Z
materials result in substantially lower energy loading, in fact down to levels comparable
to those that were presumed marginal for the mirror material. On the other hand, even at
1.5 Å, it is evident that carbon (i.e., diamond), ostensibly the material of choice, is
beginning to experience substantial loading. At wavelengths > 2-3 Å, the corresponding
numbers for all three materials would begin to warrant serious concern.
Table 9.0.3. Selected crystal and energy loading parameters under LCLS beam
conditions at 1.5 Å. Dw = 100 m. Assumed crystal thickness is ~te.
Material
Lattice Spacing dH [Å]
1st Order
Resolution [Dl/l]
A
Diffraction
(x10-6)
[eV/atom]
Angle [°]
Be (002)
1.7916
24.1.75
22.8
0.014
Be (110)
1.1428
41.02
7.1
0.009
C* (111)
2.0593
21.36
59.7
0.069
C*(220)
1.2611
36.49
19.3
0.042
Si (111)
3.1354
13.84
135
2.011
Si(220)
1.9200
22.99
57.7
1.232
Table 9.0.4. Summary of suitable materials for optical components, without any FEL
radiation attenuation
transmi ssion
grazing in cid ence
crystaldiff raction
multi layers
- 13 -
Front ha lls
0.8 keV
8 keV
Margin ally B e, C, Li,B e, B,C, Si,
Si
and lo w-z
compounds o f the
above
Au
Be
(ext reme
in cid ence)
none
Be, C*
Alllo w to
mod erate z
Alllo w to
mod erate z
0.8 keV
W M, Au OK
Rear hall s
8 keV
W,M, Au OK
W,M, Au OK
W,M, Au OK
Alllo w to
mod erate z
W,M, Au OK
Alllo w to
mod erate z
W,M, Au OK
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Table 9.0.4 summarizes the possible materials that can be used, if there is no FEL
radiation attenuation. In Hall B the increased spot size reduced the energy density by a
factor ~15, and more standard materials are possible at all photon energies. For example,
W, M and Au have doses < 0.3 eV/atom for both 8 and 0.8 keV photon energies.
Continued R&D into the topic of x-ray photon-material interaction and damage is
imperative to reduce the risk of component damage associated with many of the User
experiments. First, not all known physics has yet been included in the modeling
described above; see section 9.6.0. Second, and more importantly, the remarkable photon
areal densities available are expected to instigate new processes, which are the specific
topic of the atomic physics experiments (see "LCLS First Experiments (September
2000)". The effects of the intensity spikes within a single FEL ~ 250 fs pulse, the spikes
having characteristic times < 1 fs and intensities ~ 5 times the nominal value, are
unknown.
Absorbers and attenuators
Gas, liquid or metal attenuators (see section 9.5.2.3, and 9.6.2.5) will be constructed
to reduce the FEL beam intensity, both as an experimental control, and to allow the use of
standard optical component and diagnostic designs. It is emphasized that, while a design
for a gas attenuator has been provided, a basic question is how the actual LCLS pulses,
whose intensity and degeneracy parameters lie well outside the regime of weak-field
interactions, will interact with candidate absorbing media. We note the gas absorption
cell [4] can be used for initial studies of scattering of the LCLS pulses by absorbing
media. The chamber design, predicated on the initial use of xenon, can includes ports for
line of sight fluorescence detection, as well for the introduction of external magnetic and
electric fields. Due to its location inside the FFTB tunnel, provisions for a detector
shielding enclosure have been included.
References
[1] R. M. Bionta, “Controlling Dose to Low Z Solids at LCLS”, LCLS note LCLS-TN-00-3
[2] R. Tatchyn, "LCLS Optics: Technological Issues and Scientific Opportunities," in Proceedings of the
Workshop on Scientific Applications of Short Wavelength Coherent Light Sources, SLAC Report 414;
SLAC-PUB 6064, March 1993
[3] R. Tatchyn, P. Csonka, H. Kilic, H. Watanabe, A. Fuller, M. Beck, A. Toor, J. Underwood, and R.
Catura, "Focusing of undulator light at SPEAR with a lacquer-coated mirror to power densities of 109
watts/cm2," SPIE Proceedings No. 733, 368-376(1986)
[4] D. Ryotov and A. Toor, "x-ray attenuation cell", LCLS TN-00-10 (2000)
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9.1 Commissioning and common usage diagnostics
9.1.1 Requirements
The diagnostic requirements are derived from the User experimental requirements
(including FEL commissioning, and facility requirements (see section 9.3). Those
required for FEL commissioning, and those requested by multiple Users, are brought out
and described here., Other experiment-specific diagnostics are dealt with in the sections
describing each of the first experiments. Table 9.1.1.1 lists the diagnostics requested by
the Users (including FEL commissioning and physics), in addition to certain other
specific User requirements.
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Table 9.1.1.1A X-ray photon diagnostic (and other) requirements for Hall A
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Table 9.1.1.1 B X-ray photon diagnostic (and related) requirements for Hall B
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Based on the above table, the diagnostics are considered as being required for 827 eV
(1.5 nm) first harmonic operation and 8.28 keV first harmonic operation (0.15 nm), and
allowing for measurement of both the coherent and spontaneous parts of the spectrum.
9.1.2 Coherent radiation properties
9.1.2.1 Spectrum
9.1.2.1.1 low energy: grating spectrometers
Roman Tatchyn is providing this section
9.1.2.1.2 high energy: crystal spectrometers
Roman Tatchyn is providing this section
9.1.2.2 Total energy (including profile)
9.1.2.2.1 Total energy, accurate, intrusive: calorimetery
John Arthur is providing this section
9.1.2.2.2 Total energy, every pulse, non intrusive: ionization chamber
John Arthur is providing this section
9.1.2.2.3 Profile, intrusive and partially intrusive: x-ray CCD camera
The LCLS X-FEL needs x-ray imaging systems for measuring the spatial distribution
and divergence of the raw beam, for alignment and focusing of optical elements, and for
pulse-to-pulse monitoring of the beam shape, centroid, and intensity. Unfortunately a
single instrument cannot currently meet these requirements.
The most useful instrument, in the short term, will be a high resolution, CCD-like,
camera for measuring spatial distributions and for alignment and focusing of optical
elements. Traditional instruments have used phosphorus screens to convert x-rays to
visible light that can be recorded by a CCD. Even with a microscope objective to
magnify the screen, the spatial resolution is limited by the spatial resolution of the
phosphorus which is typically in the range of 10 to 50 microns. Such resolutions are of
marginal utility to the LCLS which has a beam diameter at 8 keV of 100 microns.
Recently workers at the ESRF synchrotron facility have discovered a crystal that when
used as a scintillator for x-rays achieves 0.8 micron resolution. The scintillator is a 5
micron thick Ce doped YAG crystal on a 100 micron YAG substrate.
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This camera will not work as a pulse-to-pulse beam monitoring device at LCLS
because of its slow readout speed and the susceptibility of the YAG crystal to damage in
the intense X-FEL beam. Nevertheless the YAG crystal technology could be utilized for
this purpose with slight modification as shown in figure 9.1.2.1.
FEL Beam
Objective
Be
Reflected beam
YAG
CCD
Figure 9.1.2.1 Concept for a pulse-to-pulse beam monitoring camera. A small amount
of the x-ray FEL beam is reflected by the Be foil onto a YAG scintillator whose emissions
are recorded by a fast framing CCD camera.
In this concept a thin foil of a low Z material such as Be acts as a beam splitter to
partially reflect a portion of the beam onto the YAG crystal. The YAG crystal imaging
system is the same as before except a fast framing CCD is substituted for the large format
CCD used above. The major foreseeable problem with this concept is the background xray radiation impinging on the crystal due to Compton scattering of the FEL beam by the
Be foil, and any fluorescence from an oxide layer on the foil surface. The existence of
fast framing CCDs with suitable format and frame rate for the LCLS is not an issue, as
suitable fast framing CCDs exist today.
An alternate proposal is to utilize Compton scattering of the beam off of a low Z gas
utilizing the crystal/fast framing camera.
Gas s cattering cell
FEL Beam
Slits
Compton
s cattered
x rays
YAG
Ob jective
CCD
Reflected b eam
Figu re 1 Pulse-to-pulse mon itor ing of beam pro file utiliz ing Comp ton
Figure 9.1.2.2 A schematic of a system to measure beam width using Compton
scattering from a gas cell
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9.1.2.3 Centroid location
9.1.2.3.1.
Ionization chambers
The centroid location can be measured by suitably instrumented ionization chambers,
as described in the section on the gas cell attenuator (9.2.2.1.3)
9.1.2.3.2 crossed wire arrays
9.1.2.3.3 mirrors
This measurement is performed by the less intrusive profile diagnostic, based on a
CCD camera, see section 9.1.2.2.2.
9.1.2.4 Divergence
This measurement is performed by multiple versions (at different locations) of the
less intrusive profile diagnostic, based on a CCD camera, see section 9.1.2.2.2.
9.1.2.5 Temporal distribution
Pulse length is measured by
Roman Tatchyn is providing this section
9.1.2.6 Coherence:
9.1.2.6.1. Asymmetric Michelson interferometer
Time-domain autocorrelation can be used to obtain information on the power spectral
density of a temporal signal. Specifically, it can provide information on the temporal
coherence length of a quasi-coherent source [1]. In the case of the LCLS, this is a
critically important parameter directly related to the FEL-induced microbunch structure
in the electron beam [2], and provisions for characterizing its statistics are presently
under study by the LCLS X-Ray Optics R&D group [3]. With regard to interferometer
design, a basic requirement is to minimize distortions in the temporal structure of the
pulse so that valid autocorrelation spectra can be generated. A second requirement,
arising from the extreme power density of the LCLS pulses is that the response of an
optical element interacting with an LCLS pulse should remain sufficiently uninfluenced
by the energy being absorbed during the interaction.
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A schematic of the proposed instrument is shown in Fig. 9.1.2.7.1. Although a
transmission grating splitter is explicitly shown, techniques based on
reflecting/transmitting foils [4] could also be considered. The split beams are reflected
off mirrors M1 and M2 and recombine at the detector plane. With the mirrors parallel the
path length difference between the two interferometer arms is 0. To induce a path length
difference M1 is rotated counterclockwise through an angle . At the same time, the
detector is rotated through the same angle and translated back a distance r2.
Figure 9.1.2.7.1 Schematic layout of an asymmetric Michelson autocorrelator based
on a grating splitter.
As  is tuned, the path difference r between the two arms is given by
sin( 1 )sec( 1  2 )  tan(1 ) 
r  (r0  r2 )  r1  2r0  

 tan( 1 )  tan(1  2  )

(1)
and the stroke distance r2 of the scanning/rotating detector by
tan(1 )  tan(1  2 ) 
r2  r0  

tan(1 )  tan(1  2 ) 
(2)
To illustrate parameter dependence, expand r about  = 0. This yields:
r  (2r0 1 )   (2r0 12 )   ...
2
and
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 

 2
 2 2  41
r2  r0  
1  1 
 ...


(




)
3
3
(




)
 1

1
To illustrate a typical parameter range, let r0 = 0.5m. In Fig. 9.1.2.7.2 the variables
r2 and r are plotted as functions of  for two values of 1.
r2 [m]
r [] (x100)
2
1.5
r2 (Theta 1 = 0.005r)
r2 (Theta1 = 0.01r)
Delta r (Theta 1 = 0.01r)
Delta r (Theta 1 = 0.005r)
1
0.5
0
0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045
 [rad]
Figure 9.1.2.7.2 Tuning curves for an asymmetric Michelson autocorrelator for two
1. r0=0.5m. The ordinate values for each curve scale
linearly with r0.
For a given starting value of 1 (viz., in the symmetric limit), the reflectivities of M1
and M2 are equal. As the device is tuned, the reflectivity of M1 will increase (assuming
it is a specular reflector), inducing an asymmetric scaling in the relative amplitudes of the
interferometer beams. In this regard, the maximal asymmetry, as well as the maximum
power loading condition, for specular interferometer facets will be determined by the
mirror material and 1. To illustrate the range of Z-dependent reflectivities accessible
with specular mirror materials curves spanning the 1 keV - 8.5 keV range of the LCLS
fundamental are plotted vs. 1 for gold and beryllium in Fig. 9.1.2.7.3. Corresponding to
these curves, the energy loading [eV/atom] induced in the mirror material - assuming
irradiation with the full unattenuated LCLS beam - is plotted in Fig. 9.1.2.7.4. It is
evident that in order to develop a value of r of the same order of length as the LCLS
pulse without excessive values of r0 or r2 the preferred value of 1 will lie in the > 0.005
rad range. In order to operate in this range, it consequently follows that substantial
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attenuation of the LCLS beam will be desirable prior to reflection in the interferometer.
In the present design this can be accomplished in part by the grating splitter, whose
diffraction efficiency can be controlled by adjusting the parameters (primarily the
thickness) of the transmission grating. Without upstream attenuation the grating itself is
likely to be damaged; however, a new grating, or grating area, could be inserted between
shots, which would also allow for spectral tuning. The basic requirements on the grating
parameters is that the dispersion be small enough to preserve the temporal structure of the
LCLS pulses and that the spectral bandwidth of the diffracted orders be substantially
larger than the bandwidth of the LCLS radiation.
1
Absolute reflectivity
Au 1 keV
0.8
Au 8.6 keV
0.6
Be 1 keV
0.4
Be 8.6 keV
0.2
10 -5
0.0001
0.001
0.01
0.1
Grazing Incidence Angle [rad]
Figure 9.1.2.7.3 Absolute reflectivities of Au and Be vs. grazing incidence angle at
photon energies of 1 and 8.6 keV.
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100
 A[eV/atom]
10
1
Au 1 keV
Au 8.6 keV
Be 1keV
0.1
Be 8.6 keV
0.01
0.001
0.0001
10 -5 -5
10
0.0001
0.001
0.01
0.1
Grazing Incidence Angle
Figure 9.1.2.7.4 Energy loading of Au and Be vs. grazing incidence angle at photon
energies of 1 and 8.6 keV. Irradiation with the unattenuated LCLS beam is assumed.
Issues include:
a) Path length and resolution. The practically attainable path length difference of
the instrument is of the order of 20 m, for interferometer lengths of 2 m or
less. However, since the resolution is given by dn/n ~ (l/r), this still
corresponds to a resolution of the order of 5x10-6 at 8.5 keV. For larger
values of 1 or substantially longer arm path lengths, correspondingly longer
path length differences (and resolutions) could be generated.
b) Recording. As the path length difference is tuned, the detector need not record
the detailed fringe pattern associated with a given pulse. A mask consisting of
apertures smaller than ~ l/(8(1-)), and with a varying period equal to the
varying fringe pattern period, followed by an intensity detector would be
adequate, provided the contrast ratio of the interference pattern remains
sufficiently high over the operating range of the instrument. A mask of this
type could be fabricated as a variable-period multilayer consisting of
alternating high-Z/low-Z materials and operated in transmission.
c) Photon flux. Estimates indicate that a detector that could record the fringe
pattern would also be feasible, even at 1.5 Å. This is based on two factors.
First, at small angles 1 of operation the wavelength of the interference
pattern will be dilated by the factor (2(1-))-1. In practical terms, periods
in the 50 -1500 Å range can be recorded. Second is that while recording
materials that operate down to this level of resolution (e.g., PMMA, or Ag- 26 -
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doped semiconductors) are known to require large amounts of energy per unit
area (O(1 J/cm2)), such (single-shot) exposure requirements could easily be
met by the LCLS, even far away from saturation.
d) Beam splitting. It is estimated that in order to attain a 1 in the >0.005 rad
range at 1.5 Å, grating periods of 300 Å or less will be required. Such
structures, similarly to the masks described above, could be fabricated as
multilayers and operated in transmission, a technique that has been developed
in recent years at LLNL (11,12). Blurring of the LCLS temporal structure due
to the splitter's dispersive effects could to a certain extent be mitigated by
pinhole aperturing of the incoming light. Splitting methods based on
homogeneous or perforated [4] foils operating in transmission/reflection could
also be investigated, particularly if the practical performance of the grating
splitter proves to be overly dispersion-limited.
In this context, the
development of broad-band, high-quality multilayers as alternatives to the
specular reflectors assumed here could be pursued as a means of scaling down
the length of the instrument, particularly for FTS or source-analysis
applications for which the bandwidth reduction would be acceptable.
e) Alignment. The most critical issue concerns the tolerances required on the
alignments, positions, and motions of the interferometer components.
Tolerance specifications will be determined to a large extent by the optical
components utilized in the interferometer, the type of detector, and mode of
operation of the instrument. For example, starting with the splitter, it is wellknown that the far-field diffraction pattern of a normal-incidence transmission
grating is invariant with respect to the transverse coordinates of the grating.
Moreover, while it is sensitive to the inclination of the grating away from
normal incidence, the dispersion angles vary with the cosine of the deviation
angle, making the tolerance on the deviation of this parameter for controlling
the lateral motion of the split beams to , say, the ~10-9 rad level fairly robust
(~10-100 mrad). However the following elements, the mirrors, will require
exceptionally stringent tolerances both on position and angle should, for
example, maintenance of the interference pattern's lateral position on the
detector plane to a fraction of the pattern's wavelength be required. This
requirement would be necessary if operating with a mask followed by an
intensity detector (as described above), and may well represent the limit on
the lowest attainable wavelength that a practical device could operate at.
However, these tolerances could be minimized if the detector was a resist that
recorded the interference pattern of each shot. In this case the autocorrelation
could be unfolded from the distribution and statistics of the interference
pattern, and these would be substantially less sensitive to its lateral position.
In the same context, it can be noted that the ability to control the interference
patterns' lateral positions could be considerably enhanced - even for
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dispersion lengths of 1-2 meters - by replacing the mirrors with transmission
grating splitters (the one corresponding to M1 being also rotatable), which
would result in the same tolerance reduction as for the incoming beam splitter.
Here again the copious flux of the LCLS would more than compensate for the
substantially lower efficiency of the grating deflectors. Needless to say, the
duration of the LCLS recording events will be so short that questions of
tolerance on any component's motion during recording can be completely
disregarded
References
[1] Fowles, G. R., Introduction to Modern Optics, New York: Holt, Rinehart and Winston, Inc., 1975, ch.
3, pp. 33-80.
[2] Murphy, J. B., Pellegrini, C. "Introduction to the Physics of the Free Electron Laser," in Frontiers of
Particle Beams, M. Month, S. Turner, eds., Lecture Notes in Physics No. 296, H. Araki et al, eds.,
Springer-Verlag, Berlin, 1988, pp. 163-212.
[3] Tatchyn, R., Arthur, J., Boyce, R., Cremer, T., Fasso, A., Montgomery, J., Vylet, V., Walz, D., Yotam,
R., Freund, A. K., Howells, M. R., "X-ray Optics Design Studies for the 1.5-15 Å Linac Coherent Light
Source (LCLS) at the Stanford Linear Accelerator Center (SLAC)," SPIE Proceedings 3154, 174-222
(1998).
[4] Moler, E. J., Duarte, R. M., Howells, M. R., Hussain, Z., Oh, C., Spring, J., "First measurements using
the ALS soft x-ray Fourier transform spectrometer," SPIE Proceedings 3154, 117-122 (1997).
9.1.3 Spontaneous radiation properties
Jerry Hastings is providing this section
9.1.3.1 Spectral structure of higher harmonics
Jerry Hastings is providing this section
9.1.3.2 Energy of higher harmonics
Jerry Hastings is providing this section
The reflectivity of perfect single crystals in the angstrom wavelength region is
understood quantitatively. Thus a single crystal x-ray spectrometer is the ideal tool to
study the intensity, wavelength and detailed spectral properties of the spontaneous
radiation. Such a device is both simple in concept, a precision rotation axis and a perfect
single crystal, preferably low Z to avoid damage and heating issues, and in practice. The
scattered intensity can be measured with high precision with any of several detectors that
are generically ion chambers measuring the photon intensity in a current mode.
The detailed structure of the spectrum is measured in an angular scan in the theta-two
theta mode with a wide open detector. In this way the integrated intensity as a function
of photon energy is measured. From this the incident spectrum can be quantitatively
recovered. By using the same spectrometer and measuring the angular positions of the
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LCLS CDR
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harmonics at plus and minus Bragg angles one can measure within the precision of the
rotary axis and the know ledge of the d-spacing of the crystal the wavelength of the
radiation without the need for determining the zero of the rotation axis, but rather just
needing the angular difference between the observed maxima at plus and minus angles.
All of these techniques are well documented in the literature and have been in use for
perhaps 50 years to characterize x-ray sources of various sorts.
9.1.3.3 Intensity of higher harmonics
Jerry Hastings is providing this secti
9.1.3.4 Angular distribution
Jerry Hastings is providing this section
The measurement of the angular distribution of the harmonics of the spontaneous
radiation requires an angular slit with microradian resolution. This is possible with two
perfect crystals set to diffract in the so-called plus-plus arrangement. In this arrangement
the crystal pair transmit radiation of a prescribed wavelength, determined of the crystal dspacing and the angle between the normals to the diffracting planes of the two crystals.
The angular acceptance of the pair is determined by the width of the diffraction profile of
the perfect crystals used. The choice of this width is made by selecting the appropriate
Bragg reflection. The mechanics required are straightforward. For the angular variation
of the crystal pair a precise tangent arm mechanism with resolution on the microradian
scale with perhaps a few milliradians of range is require. Furthermore the relative
settings of the two crystals must be established alas on the microradiatin scale and should
be stable over the time of the experiment. Often it is possible to choose a monolithic two
reflection system that eliminates the need for the precise relative alignment stage with the
restriction that it works at one wavelength.
9.1.3.5 Temporal distribution
Jerry Hastings is providing this section
9.1.4 Other common usage diagnostics
9.1.4.1 Synchronization between FEL and pump or probe laser
Dick Lee is providing this section
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9.2 Optics and beamline components for the experiments
9.2.1 Requirements
The User experimental requirements(including FEL commissioning and physics) are
summarized in table 9.2.1A and B.
Table 9.2.1A. A summary of experimental and optical requirements
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Table 9.2.1B. A summary of experimental and optical requirements, continued
Based on these requirements, the hardware associated with different parts of the
experimental area can be determined. These are now detailed below, for optical
elements. Safety, vacuum hardware and diagnostics are dealt with in separate sections.
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9.2.2 Optical enclosures
9.2.2.1 Front end enclosure
This includes a fast valve and seat, vertical and horizontal slits (2 of each), a gas
attenuator, a beam stopper, vacuum pumping, and a burn through monitor
9.2.2.1.1 Valve and seat
9.2.2.1.2 Vertical slit, and 9.2.2.1.3 horizontal slit
Two slit pairs (each consisting of a horizontal and vertical slit) will be used. At 15
GeV the X-ray beam entering the X-ray optics transport system following the beam dump
consists of an intense coherent FEL line with an FWHM angular divergence of ~1 mrad
(~9 mrad at 5 GeV) surrounded by a broad spontaneous distribution with an FWHM
angular width of ~100 mrad (~310 mrad at 5 GeV). For particular experimental
applications, the low energy spontaneous radiation can constitute a noise source and will
need to be removed. For other applications removal of the full off-axis spectrum may be
required. These considerations have led to the introduction of the two-slit-pair system
shown in Fig. 9.2.2.1.2.1. The first slit-pair is located just upstream of the absorption cell
so that low energy spontaneous radiation can be filtered out for scattering experiments
located at the cell. The second slit-pair, located ~15 m farther downstream, can also be
used as an independent aperture, or combined with the first slit-pair to provide an angular
collimator with an extremely small acceptance, providing a broad range of spectralangular filtering options, including the delivery of quasi-monochromatic beams. An
additional, equally important function of the slits (when operated in a collimator mode)
will be to protect the LCLS mirrors from excessive peak power damage stemming from
transient beam jitter.
The basic strategy is to control both the minimal aperture of each slit (ddv or ddh), as
well as the angles of incidence presented to the off-axis X-rays (tan-1((duv-ddv)/L) or
tan-1((duh-ddh)/L)). The basic structure of each slit segment (jaw) consists of a low-Z
substrate to minimize bremsstrahlung propagation, coated with a high-Z material to
maximize reflectivity at a given incidence angle (which in the present design is assumed
to be greater than the critical angle). For the actual structure we propose to use a
modified version of an existing SLC collimator design as presently employed in the
SLAC beam switchyard for collimator C-0 and momentum slit SL-2 [1]. The
modification will replace the existing jaws, which have curved surfaces, with the
geometry of Fig. 9.2.2.1.2.1. The most recent jaw design assumes an 0.5 m long block of
Al heat treated for exceptional dimensional stability. The collimating surface will be
lapped flat to a low surface roughness of 50-100 nm (RMS). The high-Z material will be
deposited onto this surface to a thickness of a few-to-several m and polished. Although
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average power deposition will be modest and cooling by thermal radiation appears
acceptable, the jaws will nevertheless be water-cooled for optimal dimensional stability
during operation. In the existing chamber design, the jaws are remotely adjustable by
means of stepper motors and can be differentially adjusted to control duv, ddv, duh, and
ddh, as well as the average vertical and horizontal midplanes of the slits. A maximal
incidence-angle range of ~0-1.5 mr is envisaged and the minimum aperture size will be
variable from 0 to >1cm.
Figure 9.2.2.1.2.1 The LCLS x-ray slit configuration. The detailed geometry of the
facets not shown. Gap dimensions duv, ddv, duh, ddh are independently adjustable.
Although the average X-ray beam power deposited into the jaws through the
reflecting surfaces is expected to be quite small, the total cross section of the spontaneous
radiation at both slit locations will be significantly larger than the upstream silt
apertures duv and duh. This means that not only can one or more of the slits absorb most
of the spontaneous X-ray power during operation, but that most of it will impact the jaws'
upstream facets at normal or near-normal incidence. Discussed earlier, the spontaneous
peak power density at normal incidence can attain off-axis values within three orders of
magnitude of that of the coherent line, which brings the peak power densities anticipated
for the jaws to levels at which little or no experimental data exists. Similar peak power
levels in the high-Z reflecting material (assuming ~99% reflectivity) can be expected for
scenarios where the LCLS coherent line impacts the jaw surface, whether due to jitter or
for other reasons.
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Calculations for the absorbed energy show if the coherent radiation is incident on the
high Z mirror surface (assumed 0.0015 radians incident angle, 99% reflection of the
fundamental, << 100% reflection for harmonics, beam waist = 80 m), then the W is
close to melting. Should the incidence angle attain substantially larger values (e.g., by
the coherent beam striking one of the slit jaws' upstream facets), the energy absorption
could approach catastrophic (e.g., vaporization) levels. For this reason, additional
protective apertures will be incorporated into the final slit assembly designs.
For the spontaneous harmonics, the total absorbed energy density per pulse is, even in
the worst-case limit, small enough to result in relatively safe temperature increases when
averaged over sufficiently long intervals of time. On the other hand, since for both the
fundamental and spontaneous radiation the absorbed energy is deposited in a time
interval too short for thermalization and diffusion processes to evolve, energy removal
may proceed via mechanisms that could induce alternative types of irreversible damage
in the jaw materials. Although there is some evidence of survival of mirrors exposed to
very high specific power densities from alternative sources, the processes that take place
in the temporal and spectral regimes of the LCLS [2] are still very poorly understood and
more experimental and theoretical studies will be needed.
References
[1] D. R. Walz, A. McFarlane, E. Lewandowsky, J. Zabdyr, "Momentum Slits, Collimators, and Masks
in the SLC," Proceedings IEEE 1989 Particle Accelerator Conference, IEEE Catalog # 89CH2669-0,
553(1989); SLAC-PUB 4965.
[2] R. Tatchyn, G. Materlik, A. Freund, J. Arthur, eds., Proceedings of the SLAC/DESY International
Workshop on the Interactions of Intense Sub-Picosecond X-Ray Pulses with Matter, SLAC, Stanford, CA,
Jan. 23-24, 1997, in preparation.
9.2.2.1.3 Attenuator/absorber
Attenuation of the coherent pulses of the LCLS can be accomplished by passage
through a gaseous, solid, or liquid medium. The nominal LCLS design is for a gas cell
operated with high pressure puff valves to introduce the absorbing gas into the path of the
coherent FEL photons (see Fig. 9.2.2.1.3.1) [1]. The axial dimensions of the chamber
and the number of valve nozzles must be adequate or numerous enough to allow a
sufficient thickness of the gas to provide four or more orders of magnitude of attenuation
over the full range of the LCLS fundamental (800 eV-8.3 keV). The combined axial and
transverse dimensions are determined by the requirement of maintaining an average
vessel pressure of <0.0075 Torr, corresponding to the Knudsen-through-molecular flow
regimes [2]. This pressure, which (for N2) is sufficiently low to be reduced to < 10-6
Torr by the differential pumping sections bracketing the chamber, will be determined
primarily by:
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1) the average volume of gas introduced into the chamber per puff,
2) its average pressure,
3) the axial conductance out of the gas cell,
4) the chamber volume,
5) the puff valve repetition rate, and
6) the capacity of the primary pump(s) connected directly to the chamber.
Figure 9.2.2.1.3.1 The conceptual design for the gas cell attenuator
The operation of the gas cell in the weak-field (linear) regime using xenon as an
absorber has been calculated for reference. In Fig. . 9.2.2.1.3.2 the absolute attenuation
of X-rays through xenon for four given pressure  tg [Torr-cm] products is plotted from
800 to 25000 eV. The curves indicate that a 7500 Torr Xe gas jet with tg=1 cm would
provide an attenuation factor of ~10-4 for an input intensity at which the absorption
mechanisms are essentially uni-molecular and linear. With suitable design and a
sufficiently low repetition (pulse) rate the loading of the vacuum system by the required
amount of gas should be maintainable at acceptable levels.
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Weak-Field Attenuation Curves for Xenon vs. (Pressure x Distance)
Absolute Attenuation
1
20000 Torr-cm
10000 Torr-cm
5000 Torr- cm
1000 Torr-cm
0.01
10 -4
10 -6
10 -8
10 -10
10 -12
10 4
1000
Photon Energy [eV]
Figure 9.2.2.1.3.2 Weak-field attenuation curves for xenon.
We note the absorption cell can be used for initial studies of scattering of the LCLS
pulses by absorbing media. The chamber design, predicated on the initial use of xenon,
can includes ports for line of sight fluorescence detection, as well for the introduction of
external magnetic and electric fields. Due to its location inside the FFTB tunnel,
provisions for a detector shielding enclosure have been included.
It must be emphasized that, while a design for a gas attenuator has been provided, a
basic question is how the actual LCLS pulses, whose intensity and degeneracy
parameters lie well outside the regime of weak-field interactions, will interact with
candidate absorbing media. A fundamental question to be resolved for scientific
experiments or pulse metrology utilizing the cell is the effect of its various scattering and
absorption processes on the temporal shape and the longitudinal and transverse coherence
of the pulses.
References
[1] D. Ryotov and A. Toor
[2] S. Dushman, Scientific Foundations of Vacuum Technique, John Wiley, New York, 1941.
The gas attenuator will be instrumented to allow detection of the total ionization
current (proportional to the incident x-ray photons) and the photon beam position. The
proportionality between ionization current and photon number breaks down at high
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LCLS CDR
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incident photon flux, because space charge is important. The limiting factor is the very
low ion mobility. The ions experience resonant charge exchange on the atoms, the crosssection is very large and, accordingly, the mobility is very low. So, in order to prevent a
build-up of the positively charged column, one has to apply very high voltages. This, in
turn, is limited by the Paschen breakdown constraint.
Helium is better than the heavier gases because of a favorable (1/M) dependence of
the mobility on the ion mass, a smaller charge exchange cross-section (which also leads
to an increase of the mobility) and a higher breakdown limit. However the gas-cell
proposal uses Xe because of its higher absorption efficiency and lower thermal velocity
(whence, a small throughput).
Therefore to allow measurements of the ionization current, the electrodes will be
situated at the rear of the chamber, where the photon flux is already two to four orders of
magnitude less than near the entrance. In this rear region, space charge and
recombination problems will be unimportant. The calibration volume will be separated
by a wall with a small hole (1 cm diameter, or less) from the rest of the volume, for
electrostatic insulation.
Position detection is accomplished using segmented electrodes (K. Sato, Proceedings
of the SPIE, detectors for cystallography and diffraction studies at synchrotron sources,
Vol 3774 p 114 (1999). Using 4 cm long electrodes, with the sawtooth base of 3 mm and
height 10 mm, and low noise current read-out, the horizontal and vertical centroid can
measured to < 10 m.
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Electro static iso lation
- HV
Main
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attenuator cell
2/17/16
area
X-ray
beam
Segmented electrod e
Segmented electrod e
Ic
Ib
Ia
z 
Ia  Ib
Ia  Ib
Id
Ic  Id
h 
Ic  Id
Figure. A sketch of the rear of the gas attenuator, showing segmented electrodes used
to measure the beam centroid
9.2.2.2 First optical enclosure
9.2.2.3 Inter-Hall transport
This must include pumping systems over the length.
9.2.2.4 Rear optical enclosure
9.2.2.5 End station
9.2.3 Near Hall experiments
9.2.3.1 Atomic physics
9.2.3.1.1 Ellipsoidal focusing mirror
Troy Barbee is providing this section
9.2.3.1.2 Apertures
9.2.3.1.3 Filters
9.2.3.1.4 Other equipment
9.2.3.1.5 Photon diagnostics
9.2.3.2 Warm dense matter and plasma
9.2.3.2.1 Requirements
The Warm Dense Matter (WDM) experimental program studies dense matter rapidly
heated to temperatures exceeding 10 eV/atom. Initial studies will use the LCLS X-FEL
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to heat thin samples, whose properties will then be measured by an array of diagnostics
placed around the sample. These diagnostics include an x-ray spectrometer, and a
Fourier Domain Interferometer (FDI) driven by a short-pulse optical laser. Interpretation
of these measurements requires knowledge of the x-ray photon flux incident on the
sample and the x-ray photon flux transmitted through the sample.
The experiment requires the use of the highest energy LCLS X-FEL photons, at 8.275
keV, in order to have enough penetrating power to uniformly heat the sample from front
to back. The required flux density at the sample is TBD photons/centimeter2. The heated
volume needs to have a radius of > 5 microns in order to create a large enough sample for
study.
Since the minimum required flux density is TBD x the peak flux density of the 8.275
keV beam in the near hall, the experiment requires x-ray focusing optics to achieve the
required flux densities. The use of focusing optics adds additional requirements for
alignment which include the ability to image the spot at the position of the sample and the
capability of running at high repetition rates at reduced intensity during the alignment
process.
250 mm
aperture
Incident Beam
Monitors
Back-scatter
x-ray
spectrometer100 mm
thick
sample
Imaging
detector
FDI
Laser
FEL
Beam
Focusing
Optic
Variable
beam
attenuator
FDI
Spectrometer
50-100 mm
aperture
Imaging
detector
(on sample
holder)
Outgoing
Beam
Monitor
Figure 9.2.3.2.1, A schematic of the warm dense matter experiment
Figure 9.2.3.2.1 shows a schematic of the WDM experiment including its auxiliary
hardware. The X FEL enters from the left and first passes through an aperture of slightly
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larger diameter which blocks most of the non FEL light, then through an adjustable
attenuator, an incident beam intensity monitor and on to the focusing optic. The focusing
optic is a variant of a blazed Fresnel zone-plate operating like a thin lens in visible optics.
After passing though the lens, the light passes through another incident beam monitor,
which serves to measure the intensity of the light directed to the sample. A smaller
aperture, closer to the sample, removes stray light scattered by the optic. The samples are
arranged in a grid on a movable stage at the position of the focal spot. The sample stage
also holds an imaging detector that can be moved into position for alignment and
characterization of the focal spot. The diagram shows the back-scatter x-ray
spectrometer that measures the x-ray spectra emitted by the sample and the path of the
optical FDI laser beam used to characterize the expansion of the sample. X rays passing
through the sample are measured by the downstream beam monitor and imaging detector.
9.2.3.2.2 Optical Design and Components
9.2.3.2.2.1 Focusing System
9.2.3.2.2.1.1 Optical design
The WDM experiment will be situated in the 4th shielded room in the near hall.
Table 1 shows the distances that are relevant for the optical design. The shielded rooms
are 13 meters long, but the PPS beam-stop limits the length available to experimenters to
10.6 meters. The sample is located approximately in the center of the room, 6.2 meters
from the upstream wall. It turns out that the flux requirements can be met with lenses
whose focal lengths are longer than the distance from the sample to the upstream wall,
but in the interests of minimizing interference between experiments, we have chosen a
position for the lens that is as far as possible from the sample, but still within the confines
of the WDM shielded room.
Table 9.2.3.2.1. Distances and specifications associated with the warm dense matter
optical components
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LCLS CDR
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Distances
From
To
units
Upstream wall
Downstream wall
13.00
m
Upstream wall
PPS Beam Stop
10.61
m
Upstream wall
Lens
1.20
m
Upstream wall
Sample
6.20
m
End of undulator
Upstream wall
72.11
m
Source at 8.275 Kev
End of undulator
19.50
m
Source at 8.275 Kev
Lens
92.80
m
Lens
Sample
5.00
m
Optic Specification
Lens Focal length
Beam size at lens position
4.74
m
131.12
microns FWHM
Diffraction limited spot
5.72
microns FWHM
Thin lens image size
Approximate focused spot size
2.74
6.34
microns FWHM
microns FWHM
The lens, placed 1.2 meters from the upstream wall, forms an image of the X-FEL
source at the sample. The source-to-lens and lens-to-image distances determine its focal
length. The X-FEL source point is at the position of the waist in the Gaussian model
which is located one Rayleigh length upstream of the exit of the undulator. As the table
shows, the resulting source-to-lens distance is 92.8 meters, and the lens-to-image distance
is 5 meters, requiring a lens focal length of 4.74 meters and giving a magnification of
0.05.
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Figure 9.2.3.2.2. Details of the lens design
Figure 9.2.3.2.2 shows details of the lens design. The lens is carved into the face of a
C disk and mounted over a hole drilled through a 25.4 mm diameter Cu mount. The C
disk is 650 microns thick except in the center where it thins down to 400 microns. The
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LCLS CDR
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active portion of the lens is 200 microns in diameter and consists of 6 concentric grooves
machined to a maximum depth of 18.8 microns. The plot of the beam profile at the lens,
figure 2g, shows that the 200 micron lens diameter nicely captures most of the beam.
The shape of the grooves was determined by calculating, at the position of the lens,
the phase change necessary to convert the diverging Gaussian X-FEL beam from the
undulator to a converging Gaussian waveform whose waist is at the sample position. The
radial phase profile was converted to a depth profile by multiplying by the optical
constant for C which, at 8.275 KeV, is 18.8 microns / 2 radians phase change ( with
respect to vacuum.)
The stage used to position the lens will have positioning precision of < 5 microns and
angular precision of < 1 mRad.
9.2.3.2.2.1.2 lens fabrication
Diamond tool
Diamond tool
carves lens pattern
here
Reference
flat
Al rod
Al rod
viewed in
cross section
Figure 9.2.3.2.3. Lens manufacturing technique
The C lens will be machined from a C rod by single-point diamond turning as
illustrated in figure 9.2.3.2.3. The tool first machines the end of the rod flat to provide a
reference surface. Then a disk of material is removed from the center providing a
recessed area for the protection of the lens. Then the lens profile is machined into the
base of the recess.
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LCLS CDR
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Glue
diamond turned
rod end
to glass
Glass slide
with surface
polished to
level of B 4 C
Mounting Block
Shim
B 4C
Polish back of lens
to level of B 4 C
Figure 9.2.3.2.4. Mounting the lens
After diamond turning, the end of the rod is carefully sliced off and its reference flat
is glued onto the polished glass surface of a specially prepared thinning fixture illustrated
in figure 4. The thinning fixture consists of a glass slide mounted on a mounting block
on top of a Cu shim whose thickness is equal to the desired final thickness between the
bottom of the lens, and the reference flat. Alongside of the blocks are a set of B4C (boron
carbide) sticks whose surfaces have been previously polished flat and parallel. Because
the B4C is much harder than the glass it is easy to polish the surface of the glass so that it
is level with the B4C. The reference surface of the lens is then glued to the glass surface
with a very thin glue. The shim is removed, lowering the mounted lens so that the
desired back surface of the lens is in the plane of the B4C sticks. The lens is then
polished to this level and removed for mounting over the hole in the Cu mount.
9.2.3.2.2.1.3 Lens Survivability.
C was chosen as the material for the lens based on its capability to withstand the
energy density of the full X FEL beam. Table 2 is a list of the lowest Z materials
amenable to single-point diamond-turning. The table gives the optical constants, the
calculated dose from the peak of the full beam at the lens position, and the dose needed to
bring the material up to its melting temperature. Cu and Al are ruled out because the
doses to these materials are enough to bring them to their melting temperature. C has
both a low dose and high melting point, and therefore will survive the full beam intensity.
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The dose to polystyrene is also very low, but it's large phase shift length results in too
large an aspect ratio for single-point diamond-turning.
Table 9.3.2.2. Doses to low-Z materials amenable to diamond turning
l = 0.15 nm
1 pi
phase shift atten
dose
dose
dose
length
length at lens to mp* through mp
micron
micron eV/atom eV/atom eV/atom
Cu
3.27
24.1
0.23
0.21
0.35
Al
9.34
85.7
0.09
0.09
0.2
C
9.41
997
0.004
0.86
0.86
9.2.3.2.2.2 Apertures
Survivability is also an issue in the design of the two apertures. The basic concept is
to utilize a laminate consisting of 4 mm of B4C, 150 microns of Al, and 200 microns of
Ta. As shown in table 3, this laminate has sufficient absorption to block x-rays up to the
3rd harmonic. Furthermore the B4C attenuates the direct X-FEL beam enough to prevent
damage to the Al which further attenuates the beam enough to prevent damage to the Ta.
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LCLS CDR
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Optional
hi-pass
Be
filter
4 mm
B4C
150 mm
Al
200 mm
Ta
1 mm
500 mm
250 mm
100 mm
Fixed
1 mm
apetrure
Selectable
Aper tures
onm oving
stage
Figure 9.2.3.2.5. Apertures for the warm dense matter experiment
A series of holes having diameters from 1 mm down to 100 microns will be drilled
through the laminate which will then be mounted on a movable stage that provides both
rotation and translation of the laminate. A second, fixed, laminate having a single 1 mm
diameter hole keeps light from passing through all but a single hole in the movable
laminate as shown in figure 9.2.3.2.5. The ability to rotate the laminate is necessary
because of the large aspect ratio of the holes. Using a downstream intensity monitor, and
starting with the largest diameter hole, the movable laminate will be rotated into a
position that maximizes the signal. The laminate will be shifted to the next smaller
diameter hole and rotated again to achieve highest intensity downstream. This process
will be repeated with successively smaller holes until the hole of the desired diameter is
positioned and aligned.
The stages used to position the apertures will have positioning precision < 10 microns
and angular precision of < 1 mRad.
9.2.3.2.2.3 Attenuator
The WDM experiment requires a local attenuator for calibration and to prevent
damage to sensitive components during alignment. Preventing damage at the focused
spot requires the highest attenuation. Table 9.2.3.2.3 shows an estimate of the dose to a
selection of materials at the focal spot in comparison to their tolerable doses (10% of
melting temperature.) To reduce the dose to Cu to a tolerable level requires an
attenuation of at least 10-4.
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LCLS CDR
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Table 9.2.3.2.3. Estimated dose to given materials at the focal spot
Material
Cu
Si
Al
C
B4C
polystyrine
Be
dose at
focused
spot
eV/atom
Tolerable
dose
eV/atom
Needed
attenuation
Needed
Thickness
B4C
mm
207.68
115.91
82.11
2.90
1.93
1.93
0.97
0.021
0.026
0.009
0.086
0.068
0.010
0.023
1.0E-04
2.2E-04
1.1E-04
3.0E-02
3.5E-02
5.2E-03
2.4E-02
16.3
14.9
16.1
6.2
5.9
9.3
6.6
The table also shows the thickness of B4C required to provide the needed attenuation.
Macroscopic thicknesses of B4C make good attenuators for 8.275 KeV radiation and, as
discussed in the section on apertures, can withstand the full (unfocused) beam intensity.
Table 9.2.3.2.4. Thickness of Boron carbide required for attenuation
desired
attenuation
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
B4C
thickness
mm
4.1
2.8
2.1
1.6
1.2
0.9
0.6
0.4
0.2
0.0
desired
attenuation
1.E-01
1.E-02
1.E-03
1.E-04
1.E-05
1.E-06
1.E-07
1.E-08
1.E-09
1.E-10
B4C
thickness
mm
4.1
8.1
12.2
16.3
20.4
24.4
28.5
32.6
36.7
40.7
Relative calibration of the downstream intensity monitors is best done with a
combination of variable attenuators that provide both linear and logarithmic variations in
intensity. Table 4 shows the sets of B4C thicknesses needed to vary the attenuation
linearly from 0.1 to 1 and logarithmically from 10-1 to 10-10.
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LCLS CDR
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Log attenuator
4 mm
Beam
B4C
1 cm
32.6 mm
4 mm
Linear attenuator
Figure 9.2.3.2.6. A linear attenuator
The attenuators will be fashioned from single plates of B4C milled in a staircase
pattern to the thicknesses specified in the table as shown in figure 9.2.3.2.6. The linear
and logarithmic attenuators will be mounted on separate translation stages allowing all
combinations of linear and logarithmic attenuation to be applied.
The attenuator translation stages will provide motion in the X and Y directions with a
precision of < 1 mm.
9.2.3.2.2.4 Beam intensity monitors
The beam intensity monitors are required to measure the absolute flux incident on the
samples and the amount of flux transmitted through the samples. These monitors will be
of the simple ion chamber type as described in the common diagnostics section (9.2...).
9.2.3.2.2.5 Imaging detectors
The WDM experiment requires imaging detectors to assist in the alignment and to
determine the size of the heated volume. Beam transport simulations described below
indicate that a spatial resolution of <5 microns is necessary for these purposes.
Furthermore it is desirable to obtain the entire image in a single shot. It would also be
desirable if the detector could take the full beam intensity but since the alignment could
be done in an attenuated beam this is not necessary.
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9.2.3.3 FEL commissioning and physics
9.2.3.3.1 Pulse slice, compression and transform-limiting systems
9.2.3.3.1.1 Pulse slice
Shorter than 200 fs pulses are of interest, e.g. to the biological experiments. A
chirped electron pulse creates a chirped photon pulse, so that slicing the pulse in energy
also slices the pulse in time. Three techniques have been proposed. First is to use an xray monochrometer with a bandpass << the width of the chirp (2%); the optimal choice is
for a bandpass that just corresponds to the zero bandwidth pulse duration, 6 fs. A
possible scheme uses a crystal (Si) monochrometer where the crystals are cut
asymmetrically to increase the energy acceptance. By adjusting the asymmetry, the Si
(111) reflection can be tuned to match that needed to slice the chirped x-ray pulse.
Second, reflection off multi-layers will allow a slice out of a chirped pulse [1]. The
efficiency, bandwidth and band shape can be controlled by the multilayer choice, and
should allow < 10 fs pulses with > 50% efficiency. Third is an off – axis zone plate in
combination with a slit [2]. This last is proposed for the first experiments on LCLS,
because of the similarity with of the required lens with what has already been achieved.
The experimental program outlined for the LCLS points toward scientific
opportunities with photon pulses that are shorter than the baseline design. The most
straightforward way to approach this problem is to introduce an photon energy-time
correlation (chirp) to the x-ray pulse and then select a small part of the energy width
yielding a small time width. Full 6D Simulations of chirped electron beams provide
photon beams with typically 1-2% energy bandwidths for the 230 fs baseline pulse
length. Thus an x-ray optic that can select a fraction of that bandwidth, say 0.1-0.2 %
would provide a pulse that is commensurately shorter, perhaps approaching the 10 fs
range. The needed bandwidths, of say 0.1% are perhaps 10 times wider than typical
bandwidths for conventional perfect crystal optics. However the need for broad
bandwidth crystal optics has been foreseen already for conventional synchrotron radiation
applications [ref Matsushita]. This approach of a pair of asymmetric reflections in the
traditional ‘channel cut’ arrangement using a Si monolith can provide the needed
bandwidth enhancement. It also provides a simple and robust optic that will preserve the
beam quality.
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l
> l
l
0
= l
l
0
< l
0
Z one plate axis
FEL Axis
Figure 1 - Image formation by zone plate struck off axis at different wavelengths.
During a wavelength chirpped FEL pulse the focused spot will move along the zone-plate
axis causing the beam to sweep across the plane of the central focus.
The concept, shown schematically in figure 1, is that a circular zone plate focuses
light to a point on its optical axis whose distance from the zone plate is a function of
wavelength. If the illumination is restricted to a small area near the outer edge of the
zone plate, it will still converge to the focal point on the zone plate axis, giving the
appearance that the small beam has been deflected sideways as if by a grating (but unlike
a grating, the light is also focused). In the case of a chirped FEL pulse, where the
wavelength decreases monotonically over the duration of the pulse, the focal spot will
move outward along the zone plate axis. An observer looking at a screen placed
perpendicular to the zone plate axis at the distance of the mid-pulse focus will see a small
spot of light moving across the screen during the pulse. Placing a small pinhole through
the screen in the path of the spot produces a brief pulse of light downstream whose
duration is roughly equal to the original pulse duration times the ratio of the width of the
pinhole to the total path length of the moving spot. Note that the entire zone plate does
not have to be manufactured, only the portion intercepted by the FEL beam. For large
offsets, the required off-axis zone plate portion closely resembles a grating having zones
of nearly equal thickness across its aperture.
We propose fabrication of a system for 8.271 keV, in the FEL physics hall, using
alternating layers of Be and B4C. At 8.271 keV, the optimal slice thickness for the Be
and B4C system is 33 m which gives a 1  phase shift between x rays emerging from
the Be and B4C layers. The attenuation through the 33 m thick optic is negligible, the
transmission through the Be is 99.5% while through the B4C the transmission is 98.2%.
The surface dose to the Be is 0.002 eV/atom and the surface dose to the B4C is 0.007
eV/atom at a position 15 meters downstream of the undulator exit. The design
parameters are shown in Figure 2, based on a 2% energy chirp and a 50 fs slice. An
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additional requirement is that the focused spot should be suitable for experiments, that
require 200 photons /A2.
Parameter
Value
Units
Object Distance
Image Distance
Focal length
Offset
Core radius
Inner zone width
53.99
2.67
2.587
277.5
177.5
1.08
meters
meters
meters
microns
microns
microns
Outer zone radius
Outer zone width
Number of fabricated zones
377.3
0.51
291
microns
microns
Figure 2: design parameters for the time-slice lens
The zone plate pattern is fabricated by sputtering alternating layers of Be and B4C
onto a wire core having a 177.5 m radius. The deposit is 200 m thick extending to an
outer radius of 377 m and contains 291 active zones. This zone plate is about 2 x larger
in diameter and active radius than previously fabricated sputtered-sliced circular zone
plates but it also has fewer and larger zones so its fabrication is probably within the
current state-of-the-art.
A numerical simulation of the system is based on the Gaussian properties of the FEL
beam and Kirchhoff’s diffraction theory. Figure 3 shows an overexposed gray level
intensity distribution of this modified electric field overlaid with a drawing outlining the
active area of the lens. The image was overexposed by altering the intensity to gray level
map to show the active boundaries of the lens.
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Outli ne o f
cyli ndrica l
zone plate
X,
m
Figure 3 An illustration of the xone plate and LCLS beam
Figure 4 shows the focused spots in the same area around the sample position for 3 of
the 52 calculated diffraction patterns. The top image is of the t = 0 fs step, the middle is
of the central t=125 fs step, and the bottom image is of the t = 255 fs step. The 3 images
clearly show the movements of the spot due to the wavelength change over the chirp and
the change in spot size as the z position of the focus moves from downstream to upstream
of the sample plane. The line shows the position of the samples fired from the injector
Figure 4 Focused spot at sample plane at beginning, middle, and end of chirp.
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Sections on slice by crystals to be provided by Jerry Hastings
9.2.3.3.1.2 Pulse compression
Pulse compression can be achieved by dispersing successive intervals of a chirped
photon pulse along trajectories of different lengths, and then recombining them within a
common time-space volume. Schemes [5,6] require a minimum of two dispersive
elementa (e.g. gratings, multilayers, crystals)
Details are to be provided by Jerry Hastings
References
[1] R. Tatchyn, in "Workshop on Methods and Instrumentation for X-FEL," DESY, Hamburg, June 26-27,
2000
[2] R. Bionta, “A transmissive optics approach to time-slicing the LCLS x-ray pules”, LCLS-TN-00-7, and
UCRL-ID-139011 (2000)
9.2.3.3.2 Wavelength selection
9.2.3.3.2.1 Monochrometers
Crystals and crystal-based instrumentation will undoubtedly prove to be critical to the
ultimate productivity of the LCLS [1,2]. Depending on the unit cell structure of the
material and its lattice spacing, a number of candidates exist that can be used to
efficiently diffract LCLS radiation in the <3-5 Å wavelength range, and for which a
broad depth of experience has been acquired at storage ring SR and other facilities over
the last twenty years, and for which photon absorption and consequent damage estimates
suggest they can survive at the shorter wavelengths (< 3 Å).
The most common materials used as single crystal monochromators for synchrotron
radiation are silicon, germanium, diamond and beryllium (in order of decreasing
perfection). Single crystals of silicon and germanium exist that have lattice planes
straight to better than 10-9 rad with their lattice plane distances defined to the same
accuracy: ∆d/d < 10-9. Also diamond single crystals of excellent perfection can be
manufactured. Thus these crystals are better with respect to "slope errors" than the best
man-made mirrors and the only problem is that they must be mounted with great care; the
same, of course, is true for mirrors. In contrast, beryllium is a mosaic crystal with a
reflection width of ~ 200 µrad and is chosen only for cases where more flux is needed at
the expense of resolution. In Table 9.6.9.2.1 below, selected structural and thermal
parameters, along with the absorption at 8 keV, are presented for all four materials. The
quantity k/ma can be considered as a figure of merit. It is seen that diamond has by far
the best performance both at room and cryogenic temperatures.
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Table 9.6.9.2.1 Comparison of monochromator materials42: beryllium, diamond
(C*), silicon and germanium
Material
Be
C*
Si
Ge
Atomic Number (Z)
4
6
14
32
Atomic Weight (A)
9
12
28
73
hcp
diamond
diamond
diamond
Lattice Constant a [Å]
2.286
3.567
5.431
5.658
Lattice Constant c [Å]
3.583
-
-
-
1188
1860
543
290
b)Absorption Coefficient m [cm-1]
1.8
7.5
1.41
402
a)Conductivity k [Wcm-1K-1]
1.93
23
1.5
0.64
a)Expansion Coefficient a [10-6K-1]
7.7
1.1
2.4
5.6
a)k/ma [MW]
0.14
2.78
4.4x10-3
2.8x10-4
c)k/ma [MW]
11
120
0.20
6.6x103
Crystal Structure
a)Debye Temperature T
D [K]
a)297 K; b)at 8 keV; c)77 K
A potential drawback of crystals compared to mirrors with respect to power loading is
the much bigger reflection angle. However, this problem can be mitigated by
asymmetrically cut crystals, where the lattice planes are not parallel to the crystal surface.
In this way the beam footprint can be increased in the vertical or in the horizontal
direction or both (the “rotated-inclined geometry”). The theoretical limit of asymmetry is
determined by total reflection, so that in principle the glancing angle of mirrors can be
approached. In that case a crystal diffracting hard X-rays is even able to specularly
reflect away the softer part of the spectrum and thus to protect itself from the power of
the soft radiation, which is much more heavily absorbed. Whereas meridional asymmetry
(in the diffraction plane) affects angular and spectral beam parameters, sagittal
asymmetry (inclined geometry) does not.
For incident beam divergences smaller than the Darwin width, increase of the beam
divergence after reflection must be taken into account. This is given by the sum of the
Darwin widths at the entrance and at the exit of the crystal. This means that in addition
to the increase of the positional beam width the beam will become progressively wider.
At the same time a chirp of the beam occurs, i.e. the isophases and the isochrones of the
reflected beam are no longer parallel and the isochrone is no longer normal to the beam
propagation direction. The beam widening and the chirp could be used for time structure
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experiments. On the other hand, if a second, antisymmetric crystal is inserted in the
reflected beam, the increase of the beam divergence, the angle-energy dispersion and the
chirp can be almost completely recovered.
The extreme meridional asymmetry outlined above is able to transmit almost fully the
coherent LCLS beam and will eliminate most of the unwanted photons and thus clean the
beam, provided that the absorbed part of the incident radiation does not destroy the
material. Detuning of the two crystals should be very efficient for eliminating harmonics.
It is, however, desirable to verify the predictions based on simple geometrical diagrams
in more detail with dynamical theory and to compare them experimentally, in addition to
what has already been done. If we want to narrow the bandwidth delivered by the LCLS,
we can no longer use meridional asymmetry to decrease the power density. In that case
the sagittal asymmetry can provide a considerable spread of the beam footprint, by about
the same amount as meridional asymmetry.
Another means of decreasing the absorbed power would be to make the crystals very
thin, approximately one extinction depth thick. Together with meriodanal asymmetry,
the softer radiation would be totally reflected and the harder radiation transmitted. It has,
however, turned out that it is very difficult to mount thin crystals without bending them.
In the double reflection scheme outlined above the two crystals must be aligned to much
better than the Darwin width at the exit of the first and the entrance of the second crystal
(i.e., to within 2.2 µrad), so that even a tiny mounting strain will severely hamper the
transmission.
Thus there are reasons for withholding an overly pessimistic judgment. First, we
have established that the best candidate materials for crystal optics are those of lowest Z,
most probably Be and diamond, of which the latter has a widespread infrastructure that
could support R&D critical to the LCLS. Second, as noted in the foregoing discussion, it
is possible to utilize asymmetrically cut crystal geometries that could further decrease the
absorbed power density, albeit at a probable cost in optical performance. Third, it is in
principle possible (although expensive) to adapt to overly rapid damage accrual by
employing dynamic optics; viz., by displacing the crystal to successively new exposure
points following a definite series of pulses. Ultimately, the actual longevity and
performance of candidate crystals will need to be experimentally determined.
References
[1] A. Freund, "Crystal Optics for the LCLS," presented at the SLAC/DESY International Workshop on
the Interactions of Intense Sub-Picosecond X-Ray Pulses with Matter, SLAC, Stanford, CA, Jan. 23-24,
1997.
[2] R. Tatchyn, "LCLS Optics: Technological Issues and Scientific Opportunities," in Proceedings of the
Workshop on Scientific Applications of Short Wavelength Coherent Light Sources, SLAC Report 414;
SLAC-PUB 6064, March 1993.
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9.2.3.3.2.2 Grazing incidence mirrors
Grazing incidence mirrors can be used for wavelength filtering, and beam steering.
An initial configuration developed for the mirrors and mirror chamber (tank) of the LCLS
is shown in Fig. 9.6.9.1.1. Multiple-mirror reflectors are stacked in a vertical bank to
allow the appropriate reflector to intercept the beam with a vertical motion. The mirror
width (shown as a nominal 4 cm) allows a large degree of mitigative redundancy against
surface damage by repeated vertical displacements. The length L of each facet is
determined by the value of i utilized for its particular reflector. Provisions for UHV
pumping and in situ cleaning [1], which may prove to be critical to the practical longevity
of the reflectors, are included. For illustrative purposes, a set of nominal mirror
dimensions and mirror bank parameters, is listed below in Table 9.6.9.1.1. Evidently,
facet length will be a sensitive parameter for higher-Z reflecting materials, and the initial
design parameters of the mirror enclosure may need to be revised, or additional
enclosures added, to accommodate multiple reflectors.
Figure 9.6.9.1.1 LCLS mirror chamber geometry.
Table 9.6.9.1.1 LCLS mirror bank parameters. Dw ~100 m.
Energy Range
m
L [m]
i[rad]
T[rad]
Beryllium
3-30 keV
1
0.25
0.0004
0.0004
A (x10-4)
[eV/atom]
6-0.12
Gold
< 3 keV
2
1
0.0001
0.0004
200
Mirror
Material
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In view of the ubiquitous importance of specular reflection, which, for angles of
incidence spanning 0 < I < , generally accounts well for those fractions of sufficiently
weak radiation reflected and absorbed by sufficiently smooth optical surfaces, the
extension of the work reported herein into the parameter regime of the LCLS is expected
to be of critical relevance to the LCLS project and its scientific programs. Fundamental
areas of investigation include the theory and modeling of the interaction of ultra-intense,
ultra-short X-ray pulses with matter [2]. Such studies, coupled with selected
experimental investigations, are needed to shed light on a number of physical effects
with the potential of inducing damage. These include radiation field / surface interactions
vs. surface roughness and surface contaminants; radiation/surface interactions as a
function of polarization; photoemissive surface stresses induced by radiation pulses; and
bulk and surface ionization-related damage effects. Regarding the mirror tank and the
mirrors, directed engineering R&D and design will be required for the ongoing
identification and study of alternative mirror materials with optimal properties, the
development of adequate in situ cleaning processes for those that are selected, and the
fabrication, polishing, and testing of prototypes.
References
[1] T. Koide, T. Shidara, K. Tanaka, A. Yagishita, S. Sato, "In situ dc oxygen-discharge cleaning system
for optical elements," Rev. Sci. Instrum. 60(7), 2034(1989).
[2] R. More, "Interaction Experiments with Advanced Photon Sources," presented at the
SLAC/DESY International Workshop on the Interactions of Intense Sub-Picosecond XRay Pulses with Matter, SLAC, Stanford, CA, Jan. 23-24, 1997.
9.2.3.3.3 Beam splitting
To completed by Brian Stephenson
Beam splitters can be made as multilayers and operated in transmission, a technique
that has been developed in recent years at LLNL [1,2]. Blurring of the temporal structure
due to a splitter's dispersive effects [3] could to a certain extent be mitigated by pinhole
aperturing of the incoming light. Splitting methods based on homogeneous or perforated
(6) foils operating in transmission/reflection could also be investigated, particularly if the
practical performance of the grating splitter proves to be overly dispersion-limited. In
this context, the development of broad-band, high-quality multilayers as alternatives to
the specular reflectors assumed here could be pursued as a means of scaling down the
length of the instrument, particularly for FTS or source-analysis applications for which
the bandwidth reduction would be acceptable
References
[1] Bionta, R. M., Jankowski, A. F., Makowiecki, D. M., "Fabrication and Evaluation of Transmissive
Multilayer Optics for 8 keV X Rays," Mat. Res. Soc. Symp. Proc. Vol. 103, 1993, pp. 257-263.
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[2] Bionta, R. M., "Transmission gratings that diffract 8 keV x rays," Appl. Phys. Lett. 51(10), 725-727
(1987).
[3] Tatchyn, R., Materlik, G., Freund, A., Arthur, J., eds., Proceedings of the SLAC/DESY International
Workshop on the Interactions of Intense Sub-Picosecond X-Ray Pulses with Matter, SLAC, Stanford, CA,
Jan. 23-24, 1997, SLAC WP-12.
9.2.3.3.4 Pulse delay
To be provided as part of beam splitting by Brian Stephenson
9.2.3.3.5 Polarization control
To be provided by John Arthur
9.2.4 Far Hall experiments
9.2.4.1 Nano-scale dynamics
Apertures
For the LCLS, we expect to use beam-defining apertures similar to those currently
used
on
beamline
8-ID
at
APS
(at
the
web
site
http://www.imm.aps.anl.gov/UserInfo/Layout/). Here are two aperture sets, each of
which consists of two sets of two edges at right angles with horizontal and vertical
translations. The edges are in vacuum to keep them clean, and the motions are taken up
by bellows in the beam pipe. One set takes up about 250mm of space along the beam.
The beam-defining apertures should be mounted on the same table as the sample chamber
(see below). The estimated of cost of apertures is $40K per 2 aperture sets.
Sample chamber
The chamber should sit on a support table with horizontal and vertical motion, step
size and stability about 1 micron, probably about 0.75 by 1.25 m in size. A medium-size
vertical axis goniometer will be required e.g. Huber 420 or 430) with two tilts and two
translations (e.g. Huber 5203/5102), supporting a medium-size chamber (e.g. 150 mm
diam, 250 mm long) with a 200-degree Be window. The vacuum/atmosphere of the
chamber will be separate from the beamline at LCLS. Estimate of costs are: table,
goniometer, and chamber, $250K
The the biggest challenge with the layout is that as much space as possible needs to
be left between the sample and detector. typically be 5 to 10 meters is required to resolve
the speckle. Only the monochrometer and splitter will require significant space (maybe
1.5 m each). The apertures, monochrometer, and splitter all will be in vacuum or in clean
He atmosphere, separated by Be windows from the beamline upstream, and the sample
chamber downstream. Using He would allow simple intensity monitors looking at scatter
from the gas, and may help with minor heating and thermal drift problems.
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9.2.4.2 Femto-second chemistry
9.2.4.3 Biological structure
9.3 Facility requirements (access, personal protection, radiation
shielding and beam containment, toxic materials)
9.3.1 Requirements and calculations
The radiation concerns related to the LCLS project fall into three distinct areas:
• radiation safety
• radiation background in experiments
• machine protection
In each of these areas one or more of the following radiation sources needs to be
considered:
1) bremsstrahlung from beam/halo interactions with beamline components
2) gas bremsstrahlung
3) synchrotron radiation
4) neutrons
5) muons
6) induced activity
7) secondary electrons and positrons
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9.3.1.1 Photons
We are concerned with radiation effects in the areas downstream of the undulator,
taking into account any relevant radiation source, located upstream, inside, or
downstream of the undulator. Potentially all sources, 1 through 7 above, can contribute
to the radiation background in experiments. Machine protection mainly concerns
possible radiation damage to mirrors and collimators which will intercept the
spontaneous synchrotron radiation from source number 3, accompanying the laser beam
proper. The radiation safety items include shielding, beam containment system (BCS)
and personnel protection system (PPS). Since the LCLS electron beam power will be
comparable to that of the FFTB, the existing enclosure and dump shielding as well as
PPS and BCS should be adequate without major modifications. New designs of the three
safety items are only required for the optics enclosure and experimental hutch
downstream of the electron beam dump, taking into account radiation sources 1 through
6. The ultimate goal is to have sufficiently accurate estimates of radiation sources 1
through 7 and their effects. Initial modeling of some of the dominant sources has been
completed and the status of the calculations is summarized in the text below. More
comprehensive calculations, as described below, will be continued in the future.
9.3.1.1.1 Bremsstrahlung from collimators
Two copper collimators, each 10 cm long and with an internal diameter of 0.2 cm,
will be placed between the dog-leg and the undulator. The purpose of the first collimator
is to reduce the electron beam halo, while the second should intercept any mis-steered
beam which could hit and damage the undulator. The first collimator, continuously
intercepting about 1% of the beam, will be a constant source of forward-directed
bremsstrahlung and muon radiation. The second one should interact with the beam only
in exceptional cases and is not expected to contribute substantially to the radiation field
under normal operating conditions.
Bremsstrahlung radiation produced in the first collimator will hit the PPS photon
stoppers which must be inserted into any beamlines when access is allowed in any
downstream enclosure. Two calculations have been made with the FLUKA code [1] for
the first of such photon stoppers, assumed located 20 m downstream of the undulator:
electrons in the beam halo were assumed to hit the collimator on its front edge in one
case, and inside the aperture in the second case. The first case was found to generate the
higher energy deposition. For a 1% loss of a 14.35 GeV electron beam of 1.722 kW (1
nC/pulse at 120 pps) the calculated energy deposition rate in the stopper was less than
0.02 mW. The shielding requirements for Giant Resonance photo-neutrons generated in
the stoppers located in the on-line hutches of the Near-Field Hall are being evaluated by
means of the SHIELD11 code considering an equivalent electron beam depositing the
same power in the stoppers. However, bremsstrahlung from collimators is neither the
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only nor the main source of radiation to be considered for shielding design: other
radiation components (bremsstrahlung from profile monitors, muons, X-rays) must also
be taken into account.
9.3.1.1.2 Bremsstrahlung from profile and intensity monitors
A Profile Monitor will be inserted into the electron beam upstream of the undulator
for diagnostic purposes. It consists of a wire scanner with tungsten wires of 20 m
diameter. During a scan, 20% of the electron beam will be intercepted on average. A
FLUKA simulation was made assuming electrons hitting an equivalent wire of square
cross section 17.72 m m thick 3 m upstream of the undulator. For a 14.35 GeV beam of
1.722 kW at 120 Hz and 20% interception, the absorbed power in a photon stopper is
0.46 mW. This would cause unacceptable radiation levels in hutches downstream of the
stopper when access is allowed. Therefore, it has been decided to allow use of the profile
monitor only with a 10 Hz beam, thus reducing the power absorption to 0.04 mW. This
condition will be part of the PPS interlock logic for access. The calculated radiation level,
together with that due to collimator-halo interaction, will be used to evaluate shielding
requirements for on-line hutches.
X-ray Intensity Monitors will be used at several locations along the undulator as
diagnostic devices for the photon beam. However, the electron beam will also be
intercepted. There will be 10 or 12 of them, but calculations have been made for the one
located in the last 10 m section of the undulator. The material is diamond, 0.5 mm thick:
but because the beam strikes it at an angle of 45˚, the effective thickness traversed is
0.707 mm. The 5 W deposited in the photon stopper result in radiation levels which are
too high to allow access downstream of these devices. Therefore, they will be
interlocked with the PPS system. Scattered bremsstrahlung and photo-neutrons produced
in the stoppers must be taken into account in the stopper shielding design.
9.3.1.1.3 Gas bremsstrahlung
Interaction of the electron beam with residual low-pressure gas molecules in the
vacuum pipe will give rise to forward-directed gas bremsstrahlung. This type of radiation
has been thoroughly investigated at circular storage rings, where the beam current is
much more intense. However, at LCLS the straight length over which bremsstrahlung is
produced will be much longer (120 m between the dog-leg and the first bending magnet
before the electron dump). The residual gas pressure and the electron energy will also be
higher.
A preliminary estimation of gas bremsstrahlung dose rate, made by an empirical
formula reported by Ferrari et al. [2], gives 60 mrem/h at a 20 m distance from the end of
the undulator, in absence of any shielding. The dose rate behind the photon stopper
shielding would be much smaller and would represent a negligible contribution compared
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with that of bremsstrahlung due to collimators and to beam diagnostic devices, but since
the formula was derived for very different conditions (lower energies and much smaller
straight lengths), it is planned to get a more accurate estimate by carrying out more a
detailed Monte Carlo calculation.
Bremsstrahlung, will co-propagate with any insertion device radiation until it hits a
medium with an adequately short extinction depth. This will be either a mirror or a
crystal. The impact will, in general, impulsively excite an intense thermal neutron field
which will decay with a time constant on the order of milliseconds. To suppress any
possible interference with LCLS experiments operating over similar time intervals, the
layout includes a lead/polyethylene shield wall between the crystal and specular take-off
optics tanks and the experimental end stations. Since both beam lines pass through this
wall at off-axis angles and via small apertures, the suppression of this source of
interference by the wall is expected to be very effective.
9.3.1.2 Muons
Muons produced by electron interactions in the Beam Switchyard and upstream of it
are already ranged out at present by 55 feet of iron and cannot constitute a concern.
Muons can be created in the diagnostic area (by losses upstream of and inside the dogleg, in the collimators and in the profile monitors); there will be other possible muon
sources inside the undulator (X-ray intensity monitors) and the electron dump. These
muons will be either bent away by magnets downstream of the undulator or shielded by
iron shielding located on the top of the electron dump.
However, persons accessing the on-line hutches and the research yard downstream of
the Near-Field Hall and possibly the Far-Field Hall could be exposed to several other
muon sources, which are produced when high-energy bremsstrahlung hits the photon
stoppers. Muons can also constitute an important radiation background for experiments.
The presently available calculation tools (programs MUCARLO [3] and MUON89 [4]
can only address muon production in thick targets and have been applied to estimate the
contribution of the electron dump. The FLUKA code can simulate with great accuracy
muon transport, but not yet muon production by photons. Work is in progress to include
this effect and it is planned to use this code for a detailed simulation as soon as the
upgrade will take place. It is expected that not more than 3 m of iron will be needed to
shield any of the known muon sources.
9.3.1.3 Neutrons
Photo-neutrons can be generated on the zero-degree line in any object hit by electrons
and by bremsstrahlung (and, to a much lesser degree, by muons). Such objects include
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the electron dump, the transport line to the dump, photon stoppers outside and inside the
experimental Halls, and any optical device in the X-ray line. Neutrons generated outside
the Near-Field Hall can penetrate to the Hall through the concrete shielding or streaming
through the X-ray beam pipe. Neutron shielding design must consider both personnel
protection and reduction of experimental background. For the latter, time-dependent
calculations may be necessary in order to take advantage of the slowing down time which
could allow to gate out thermal neutron contribution. Preliminary estimations can be
easily made for the Giant Resonance neutron component, which is practically isotropic
and whose yield is known to be proportional to deposited power. Higher energy
components require a more detailed study which can be carried out by the FLUKA code.
9.3.1.4 Toxic materials
Both Li and Be are materials under consideration for optical components, Add
section on how to deal with Be
9.3.2 Personnel protection system (PPS) and access
In light of the above sections, the access and personnel protection system (PPS) is
designed to prevent access to experimental areas where beams are present and to prevent
beams from entering an area during personnel access.
The experimental hall shielding will consist of fixed and moveable parts. The
experimental hall perimeter walls and central beamline walls are planned to be fixed
shielding consisting of appropriate material for the energy spectra of expected radiation.
The experimenter hutches may have movable walls to adjust for experimental
requirements. The moveable wall configuration will activate the current radiological
configuration control system when changing the hutch shielding. The experimental walls
will have the capability of adjusting to the different angles of any hutch branch lines.
The access control system will be capable of retaining integrity and reliability, while
compensating for wall placement.
The PPS functions as an access control system to the central beamline and each
experimenter hutch. The PPS access control components consist of entry modules (from
the experimental control area into the hutch, and entry points from the experimenter
hutch to the central beamline), interlocks for photon stopper and electrical hazards, logic
for access, and status and control both locally and remotely.
The hutch will be designed to contain all radiation background so that the dose rates
outside the hutch are acceptable when photons from the FEL are inside the hutch. Access
to the hutch will be controlled by a Hutch Protection System (HPS) modeled after
existing SSRL hutches. The key parts of the HPS are a keyed access door, photon
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stopper interlocks, and area security system. The HPS allows either permission for
personnel access or for beam to enter the hutch. It contains the logic interlock circuits
that govern the sequence of access operations centered on the status of the stoppers. It
also captures or releases the hutch door keys, acknowledges completion of a personnel
security search, and keys the experiment enclosure on-line or off-line. Access to the
hutch is permitted only if all photon stoppers are closed.
Access to the front end enclosure requires the FFTB stoppers inside the FFTB tunnel
to be in. The LCLS HPS will control the operation of photon stoppers per other areas or
hutches, which are required to be in. Each stopper is protected by two ion chambers and
a burn-through monitor. A list of possible PPS violations and system responses is
contained in Table 9.3.21.
Table 9.3.2.1 PPS violation and response.
1. FFTB tunnel
security fault
Any FFTB tunnel security fault, while dump D2 and stoppers ST60 and ST61 are not closed,
will shut-off the LCLS beam, close FFTB stoppers, turn off the electrical hazards.
2. Inside tunnel
BSOICs trip
These BSOICs continuously monitor the radiation inside the tunnel. If the detected radiation
level exceeds the preset limit, the PPS will shut-off the LCLS and any other beams in the
Linac. The BSOIC interlocks are by-passed when the FFTB tunnel is in the No Access state.
3. Outside tunnel
BSOICs trip
These BSOICs continuously monitor the radiation outside the tunnel. If the detected radiation
level exceeds the preset limit, the PPS will shut-off the LCLS beam and close the beam
stoppers.
4. HPS security
fault
Any HPS security fault while the hutch photon stoppers and the FFTB beam stoppers are not
closed will shut-off the LCLS beam and close the hutch and FFTB stoppers.
References
[1] A. Fassò, A. Ferrari, J. Ranft, P. R. Sala, “New developments in FLUKA modeling hadronic and EM
interactions”, SARE3 Workshop, KEK 7-9 May 1997. Ed. H. Hirayama, KEK Proceedings 97-5 (1997), p.
32-43
[2] A. Ferrari, M. Pelliccioni, P. R. Sala, “Estimation of fluence rate and absorbed dose rate due to gas
bremsstrahlung from electron storage rings”, Nucl. Instr. Meth. B83, 518 (1993)
[3] L. P. Keller, “Muon background in a 1.0 TeV Linear Colider”, SLAC-PUB 6385, 1993
[4] W. R. Nelson and Y. Namito, Computer Code MUON89, SLAC Radiation Physics Dept., 1989
9.3.3 PPS beam stoppers
Two Personnel Protection System (PPS) beam stoppers will be required to allow
entry into the experimental hutches while the e- beam is being delivered to the undulator
and deflected into the dump. The function of these stoppers is to block and absorb any
coherent or incoherent  or X-radiation from the undulator, as well as bremsstrahlung
from anywhere in the beam transport system. These stoppers are patterned after an SLC
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design used in Sector 10 of the SLAC linac and in the PEP-II extraction lines [1]. The
design energy is 12-15 GeV and the assumed power for continuous exposure is Pav ~5
kW. The absorbing element in each stopper is a Cu/W block assembly with an overall
length of 20.32 cm [~22 radiation lengths (Xo)]. The beam first encounters the 3.7 Xo
long Cu section. The remaining 12.3 Xo are provided by a block of free-machining
tungsten (W~90%,  ~17 g/cm3) which is brazed to the Cu. The shower maximum of the
electromagnetic cascade for 12-15 GeV occurs at a depth of 5.5-5.7 Xo. A built-in burnthrough monitor is located at that z-depth. It consists of a pair of cavities separated by a
Cu diaphragm. The first cavity is pressurized with dry N2. Its return line contains a
pressure switch with the trip level set to 15 psig. Should excessive beam power be
deposited in the stopper block, the diaphragm will perforate, allowing the N2 to escape
into the second cavity, which is open to atmospheric conditions on the outside. The
pressure switch will interrupt beam delivery within 2-3 linac pulses. The transverse
dimensions of the stopper block are 3.16 cm wide x 2.9 cm high. The minimum radial
attenuation distances are then, respectively, 3.5 Xo and 3.1 Xo in the Cu section and 3.5
Xo and 3.1 Xo in the free-machining W. This is more than adequate to attenuate the
radial shower.
The absorber block can be vertically inserted or removed from the beam channel by
means of a remotely controlled air cylinder. The block has a water-cooled heat sink
attached to the top surface which allows continuous and safe dissipation of up to 5 kW of
beam power.
A third stopper will be installed which is essentially an ordinary pneumatic valve with
no special heat-removal provisions. It is intended for protective insertion when the linac
is running in an ultra-low peak or average current mode, for which the average radiation
power is on the order of mW or less.
References
[1] D. Walz, "Beam Stopper for PEP II Injection," SLAC Memorandum, Sept. 28, 1994.
9.3.4 Burn through monitors
A built-in burn-through monitor is located in the PPS stoppers. It consists of a pair of
cavities separated by a Cu diaphragm. The first cavity is pressurized with dry N2. Its
return line contains a pressure switch with the trip level set to 15 psig. Should excessive
beam power be deposited in the stopper block, the diaphragm will perforate, allowing the
N2 to escape into the second cavity, which is open to atmospheric conditions on the
outside. The pressure switch will interrupt beam delivery within 2-3 linac pulses.
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9.3.5 Shielding
9.3.6 Beam containment system (BCS)
9.3.7 Lead end stops
9.3.8 Muon stops
9.4 Control and instrumentation
For instrumentation and control purposes, the LCLS X-ray optics system can be
divided into three parts: 1) the X-ray transport line, 2) the X-ray beam lines, and 3) the
experimental stations.
1) The X-ray transport line carries the X-ray beam from the undulator into the Xray beam line. The X-ray transport line will be installed inside the FFTB
tunnel and will include the following diagnostic equipment: differential
pumps to isolate the high vacuum systems, horizontal and vertical adjustable
collimators(slits), a gas attenuation cell, and calorimeters.
2) The X-ray beam line carries the X-ray beam from the transport line and directs
it to the experimental stations.
3) The experimental stations will also be installed in hutches. A shielded
partition will separate the beam line optics and the experimental stations. The
experimental stations will include e.g. adjustable slits, sample positioners,
detectors, and vacuum equipment to allow the continuous vacuum transport
line to connect to the UHV experimental chamber. Additional equipment will
be required for external pulse and time-resolved detection experiments.
9.4.1 Control system objectives
The X-ray optics and experiment controls have two major objectives. The first is to
provide local control of the X-ray optical elements. The second objective is to provide
local control of experimental data collection equipment. The equipment includes the
detectors and associated high speed electronics.
The LCLS X-ray optics and experimental control hardware is designed to be
compatible with the SSRL beam lines optics and experimental control systems, including
database and protocols. The software and applications programs that have been written
for SSRL's beam lines will be available for use on the LCLS.
The X-ray optics and experiment control system will control the operation of the
various motion controls and actuators in the X-ray transport line, X-ray beam line, and
experimental stations. A functional layout of typical X-ray optics motion & actuator
controls is shown in Fig. 9.4.1.1.
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Figure 9.4.1.1 Functional layout for the X-ray optics motion and actuator controls.
9.4.2 Control system layout
The optics and experiment control system will be installed in the LCLS Experimental
Halls. A workstation will control and monitor the operation of the X-ray optics and the
acquisition of data from the detectors. It will control the operation of any mirror and
crystal monochromator movers, optical tables, adjustable slits, attenuation cells sample
positioners, calorimeters, and detectors. Control of the hardware, and acquisition of data
from the sensors and diagnostics instrumentation will be done by a CAMAC and/or VXI
based system(s). The workstation will include a PCI-based Interconnect Highway Driver
that will be linked through a fiber-optics cable to CAMAC or VXI interconnect crate
controllers (see Fig. 2.5.3.1).
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Figure 9.4.2.1
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Schematic layout for the X-ray optics and experimental stations
The Interconnect is specifically designed to facilitate synchronous data acquisition
over a distributed area while maintaining the time coherency of the data. It operates over
a wide range of sampling rates and throughputs as well as collects data from a number of
modular standard I/O chassis including VXI, CAMAC, and VME. The implementation
of the Interconnect is based on 125 MHz bit-serial technology and a protocol that
provides throughputs of 10 Mbytes/second. For short distances (up to 5 m) a coaxial
cable can be used. For longer distances, or where good isolation is required, a fiber optic
link is supported. Fiber optic links up to 2 km between nodes are available. The
interconnect is designed as a single master (host) with up to 126 addressable slave nodes
or I/O chassis. Similar configurations using workstations with Alpha CPUs and Grand
Interconnect are used by all the SSRL beam lines.
9.4.3 Motion controls
To operate these systems requires position control and position readback. The design
goals, such as positioning accuracy, position encoder linearity and resolution, and the
processing electronics resolution differ from mover to mover. They strongly depend on
the mechanical design parameters, e.g., the motor gear ratio and the position encoder
range of travel.
The mechanical position will be measured directly with linear variable differential
transformers (LVDTs). These were chosen for their resolution (essentially determined by
the number of bits in the read-out ADC and the LVDT range of travel), their linearity
tolerance (<0.15%), and their ease of use.
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The number of steps and directions that each motor needs to take will be calculated
by the X-ray optics and experiment control computer. Stepper Motor Control (SMC)
commands will be sent out to the drivers in a form of the number of steps and direction to
move. Currently the mover controls design is based on a CAMAC system and SLAC
SLC-type motor drivers. The final design may use a VXI-based system and commercially
available stepper motor drivers.
9.4.4 Photon beam stabilization at the sample
Due to the small waist of the LCLS X-ray beam, experiments that require irradiation
of a fixed sample point (e.g., diffraction from an individual microstructure) may require
the stable positioning of the beam to within a fraction of its diameter. Factors
contributing to positional beam jitter or drift at the sample plane might include: 1) power
supply and control system errors in the RF gun-to-linac system, 2) vibration or positional
drift in the linac and undulator structures, and 3) vibration or positional drift of the X-ray
optics system components. For factors contributing to beam motion that have sufficiently
long time constants, detection of jitter or drift and their stabilization may be
accomplished with suitable detectors providing feedback to any of the upstream LCLS
system elements that govern beam position and attitude.
Detection of positional and attitudinal jitter will be accomplished with nondestructive photon beam position monitors (see section 9.5.1.3) The detector signal
could be fed back to positional/angular controllers in a mirror or crystal tank, or, in case
the direct LCLS beam needs to be utilized, to electrical current correctors installed in the
last few gain lengths of the undulator.
9.4.5 Timing system
Most LCLS x-ray experiments require synchronization of the experimental stations'
equipment with the electron beam, which is phased to the 476 MHz of the SLC master
clock. Temporal jitter between the RF and the beam is specified to less than 0.5 ps rms.
The jitter allowed for in the triggers that activate the detectors is less that 1 ns. The jitter
tolerance for the timing pulses that synchronize the user laser for the External Pulse and
the Time-Resolved Detection experiments is less than 1 ps rms. To minimize the jitter of
the 476 MHz timing pulses at the LCLS experiment stations, the RF signal needs to be
derived from the SLC RF system at a location as close as possible to the LCLS
Experimental Hall. The timing pulses used currently at the FFTB facility are derived
from the linac RF at sector 30 (end of the linac), optically modulated, and transferred by
an optical fiber link to the FFTB building 407B (a distance of ~600 m). There they are
demodulated to electrical pulses. This 476 MHz RF signal is divided in frequency as
necessary to generate the various triggers. A custom-made optical fiber cable is used
because it is less sensitive to environmental changes (e.g., temperature changes) than
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coaxial cables. For the short link between building 407B and the LCLS Experimental
Hall (~50 m) a temperature-stabilized heliax coaxial cable should be sufficient. The
following are the timing requirements and techniques for three types of LCLS X-ray
experiments: 1) split pulse, 2) external pulse, and 3) time-resolved detection.
1) split pulse (X-ray probe/probe) With this technique there are no timing
stabilization problems. Nevertheless, the activating window for the detectors
has to be synchronized with the FEL 120 Hz source and delayed accordingly.
2) external pulse (X-ray or laser pump/X-ray probe) The required triggering
range for the external laser is from -10 ms to +10 ms. This class of
experiments requires that the synchronization between the user laser and the
FEL X-ray beam have a timing jitter <1 ps.
3) time-resolved detection experiments. This requires that the synchronization
between the user laser and the FEL X-ray beam have a jitter of approximately
1 ps or less.
The operation of the external-pulse (laser pump/X-ray probe) timing system is now
described below (see Fig. 9.4.4.1). The wide triggering range is achieved by using a
pulse selection stage. The main components of the pulse selection stage are two
polarizers, a Pockels cell, an HV pulser, and a pulse delay unit. The pulse delay unit
(SRS) 120 Hz input trigger is synchronized with the laser oscillator (119 MHz).
Therefore, the selection of any laser pulse can be done by setting the SRS digital delay.
Figure 9.4.4.1 Timing techniques for LCLS experiments.
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Figure 9.4.4.2
experiment.
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Schematic layout of the timing system for an external-pulse
laser oscillator produces very narrow light pulses (~10 fs) every ~8 ns. To select
a given laser pulse requires that it pass the Pockels cell while the cell is pulsed by the
high voltage pulser. A pulse width of 6 ns assures that only one laser pulse is selected at
a time. The fine synchronization between the X-rays and the user laser is achieved by the
delay stage. The delay stage consists of a prism that is installed on a stepper motor
controlled translation stage and two mirrors. The position of the prism determines the
light travel time through the delay stage. A translation stage with a linear motion of ~ 33
cm and resolution of 1 mm provides a total delay of 2 ns.
The
9.5 Other components
9.5.1 Vacuum system and differential pumping
The basic spectral requirement of the LCLS R&D facility is to generate coherent
radiation in the fundamental over the 800 eV - 8.3 keV range. This necessitates the
unobstructed transport of the radiation (viz., with no normal-incidence barriers or
windows) from the undulator exit out to the experimental instrumentation and samples.
Furthermore, since the net power absorbed by a number of the components that intersect
the LCLS X-rays at extreme grazing incidence is very sensitive to surface contamination,
all such vacuum-sensitive components must be bracketed by differential pumping
sections featuring large isolation ratios. The goal of the design is to maintain, on the
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average, Ultra High Vacuum (UHV) environments (viz., <10-9 Torr) in designated
regions.
The provision for windowless (line-of-sight) radiation transport through the LCLS Xray optics system will be accomplished with conventionally implemented vacuum
pumping at all locations generating non-negligible gas loads and by special Differential
Pumping Sections (DPSs) bracketing all the chambers in which Ultra High Vacuum
(UHV) environments need to be maintained. The DPS chosen for this task [1] develops a
large isolation ratio across its length by effectively suppressing the "straight-through"
molecular flux propagating along the axis of its line-of-sight aperture. This flux
component, which is usually the limiting factor in the effective conductance of straightthrough ducts, is blocked by generating a high-density electron cloud along a section of
the duct volume. The electrons, generated, trapped, and accumulated by a structure
similar to that of a conventional ion pump, ionize the propagating molecules, allowing
them to be deflected and trapped on getters or duct walls by various well-known
mechanisms [2]. A drawing of a typical pump structure (omitting the external magnet) is
shown in Fig. 9.5.1.1 The performance of the pump in isolating N2 ambient pressures is
shown in Fig. 9.5.1.2. In continuing LCLS X-ray optics engineering studies, the DPS
design will be optimized for adequate pumping of the noble (and other) gases that may be
employed in the LCLS X-ray transport and optics systems.
Figure 9.5.1.1 Differential pump chamber and connecting flange geometry.
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Figure 9.5.1.2 Projected differential pump performance curves for N2 vs. aperture
diameter. Aperture length 15 cm.
The pipeline between Hall A and Hall B will be evacuated to .. by…
References
[1] Differential Pump DP-01, XHA X-Ray Instrumentation Associates, Mountain View, CA 94043.
[2] J. F. O'Hanlon, A User's Guide to Vacuum Technology, John Wiley & Sons, New York, 1980.
9.5.2 Debris catchers
9.5.3 Solid target positioner
9.5.4 Gas target system
9.5.5 Displacement controllers
9.6 optical components research and development
Here we discuss optical components that are called for, but which require significant
R&D before they are applied to LCLS User experiments, to reduce the risk of failure.
They are an intrinsic part of the FEL physics experiments. The section is initiated by a
discussion of damage issues.
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9.6.1 Experiment and simulation of x-ray photon-material interaction
9.6.1.1 Simulation
Of primary concern, and a topic of the first experiments to be undertaken under the
heading of atomic physics, is the damage resulting from photon-material interaction. We
are concerned mainly with mechanisms associated with the short-lived high temperature
and high pressure caused by the absorption of x rays. The primary processes which we
anticipate will cause damage, or a change of optical properties during a pulse, are melting
and spallation. Also possible are coulomb effects, phonons effects, and peeling from
stresses induced by local heating.
A number of physical processes, parameters, and effects must be theoretically
analyzed, understood, and calculated. Taking as an example the gas cell attenuator, the
primary parameters are the cross sections of the various scattering and energy conversion
processes encountered by the radiation (see Fig. 9.6.1.1.1). At the field strengths of the
LCLS, absorption mechanisms that are ordinarily negligible for weak fields can become
activated [7] and grow strong enough to significantly influence the total absorption cross
section. Potentially prominent ones include:
1) multi-photon absorption or ionization, and
2) field (tunneling) ionization.
A second set of critical parameters includes the time constants associated with the
various absorption and scattering processes. Here a distinction is usually drawn between
processes taking place at the individual molecular level and those involving collective
interactions. Again, the high field strength and degeneracy parameter of the LCLS can
complicate a quantitative assessment. For example, if the spatial density of coherent
photons is of the order of the absorber's atomic density, it may be necessary to treat some
or all of the ordinarily-independent absorption mechanisms as collective phenomen.
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Figure 9.6.1.1.1 Energy-conversion and scattering channels for radiation passing
through an absorbing medium.
In addition to the above-mentioned cross sections, the dynamics and characteristic
time constants of the dominant energy-conversion channels in the absorbing medium
must be known. In a broad discussion of the physical interactions representative of a gas
or liquid absorber, it is useful to refer to six fundamental time constants:
LCLS, the temporal interval between LCLS pulses (for 120 Hz operation,
LCLS = 8.33 ms);
pulse, the FWHM temporal pulse length (~280 fs);
spike, the FWHM temporal length of the average random spike contained in
the full LCLS pulse (~100 as);
EL, the time constant associated with the fastest (electronic) scattering,
absorption, or ionization mechanisms (e.g., resonant scattering, single and
multiple photon ionization, tunneling ionization, multiple-photon scattering,
etc.);
F, the characteristic duration of fluorescence processes; and
A, the time constant of the longest (collective) energy-conversion and
thermodynamic processes in the medium (e.g., thermalization, charge
neutralization, relaxation back to equilibrium, etc.).
In general, the relations spike <pulse < LCLS and EL < F < A will apply
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Figure 9.6.1.1.2. Response of (static) gaseous (or liquid) medium to LCLS pulses. The
FWHM widths EL, F, and A (not drawn to scale) represent the medium's
characteristic electronic, fluorescence, and equilibration time constants. A here is
assumed to be < LCLS.
For an assumed static medium, the propagation of the LCLS can be represented as in
Fig. 9.6.1.1.2. A fundamental question to be resolved for scientific experiments or pulse
metrology utilizing the cell is the effect of its various scattering and absorption processes
on the temporal shape and the longitudinal and transverse coherence of the pulses.
Although to first order it can be assumed that there will be an unscattered (as well as
elastically scattered) fraction of photons [1] that will retain their incoming coherence
properties, an accurate quantitative estimate of this fraction may, for various regimes of
LCLS parameter space, be difficult to obtain [2]. Additionally, non-linear absorption
mechanisms may substantially modulate the photons' temporal distribution. For example,
apart from the non-linearities associated with high fields, those that collectively saturate,
or "use up" available electronic transitions can subject the downstream part of the pulse
to stronger absorptivities than its upstream end [3]. Finally, if the interval LCLS
becomes too short, the overall model for propagation will acquire additional complexity
due to the fact that the medium's state will bear the imprint of preceding pulses. For
sufficiently short intervals (< ~O(1 ns)), this will probably become an important effect for
gas or liquid jets with even the highest attainable flow velocities. For the LCLS
operating at the present 120 Hz repetition rate, we note that this is not expected to be a
concern.
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We plan a program of modeling of these processes, both to aid the experiments under
consideration (see below), to assist in optical component design, and in preparation for
the atomic physics experiments.
9.6.1.2 Experiment
Our knowledge of the damage mechanisms and damage thresholds for LCLS optical
elements relies heavily on theoretical calculations. Experimental confirmation of these
calculations is highly desired. However, no x-ray source currently exists with the
appropriate properties (high fluence, short pulse, monochromatic) to study damage
processes. Instead, we propose to utilize a high power, ultrashort pulse optical laser to
experimentally simulate the interaction of LCLS x rays with a target.
There exist both similarities and differences in irradiating a target with an optical
laser compared to an x-ray laser. In both cases the energy is initially imparted to
electrons in the material. However, the initial electron energies are much higher for x
rays than for optical (keV versus eV). Since the keV electrons stop in about 20 fs, (much
less than the LCLS pulse length of ~300 fs) this difference should not be critical. We can
straightforwardly match the pulse length and average dose in the two scenarios. This will
lead to the same energy densities in the two scenarios, and to the extent that the energy
thermalizes, the same temperatures. Another important parameter is the depth of energy
penetration. The energy penetration for the x-ray case depends on both the x-ray
penetration depth and the range of the electrons. For the optical case we must match the
x-ray energy penetration depth. This can be done to some extent by choosing the target
material, the laser wavelength (although only a few wavelengths are available), and the
angle of incidence of the optical beam.
Experiments are proposed on the Jan-USP laser at LLNL. This laser operates at a
wavelength of 800 nm and can produce pulses up to 10 J in energy at pulse lengths down
to 300 fs. It is readily available to the LCLS optics group. We propose initial optical
experiments using two materials of similar atomic number but different optical
properties, aluminum and silicon. By having similar atomic number the materials will
have similar x-ray absorption properties. By having different optical properties, we can
simulate different x ray ranges due to different x ray photon energies. In both cases we
will simulate small-angle x ray irradiation with energy penetration depths less than 1 µm.
To simulate hard LCLS x rays, with longer ranges, we propose to use a silicon target. An
experiment is proposed to simulate an LCLS photon energy of 8.3 keV, incident at angles
of order the critical angle. In this case the LCLS energy penetration depth is set by the
electron collisional range and is about 0.5 µm (see Figure 1a, the ranges will be nearly
the same in Si and Al). For Si, the melting temperature is 1687K, at a dose of 0.36
eV/atom. We estimate an optical fluence of 0.12 J/cm2 will produce this dose.
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To simulate lower-energy LCLS x rays we will use an aluminum target. For 2 keV x
rays the expected energy penetration depth is 10-2 µm in Al, which matches well with the
optical range. The melting temperature for Al is 934K, expected at an optical fluence of
6x10-4 J/cm2.
In both cases we will study the damage threshold by a variety of optical and x-ray
methods, including post-shot optical microscopy and x-ray reflection measurements, and
by during-the-shot x-ray reflection. We will determine the threshold optical fluence to
cause either visible damage or a reduction in x-ray reflectivity. By relating the optical
experiments to the expected LCLS environment through the dose, we can then project
damage thresholds for LCLS optics. A typical parameter expected to characterize the
damage threshold is the melting point of the material. We expect that heating the
material to the melting temperature will certainly damage the optic, and therefore the
dose to achieve melting will be a hard upper limit to the allowable dose. Other damage
mechanisms, such as spallation and ion dislocations, may set in a somewhat lower doses.
The optical damage experiments will test the hypothesis that the damage threshold will
be close to the melting point of the materials.
References
[1] R. P. Feynman, Quantum Mechanics and Path Integrals , McGraw-Hill, New York, [1965])
[2] R. More, "Interaction Experiments with Advanced Photon Sources," presented at the SLAC/DESY
International Workshop on the Interactions of Intense Sub-Picosecond X-Ray Pulses with Matter, SLAC,
Stanford, CA, Jan. 23-24, 1997.
[3] R. H. Pantell and H. E. Puthoff, Fundamentals of Quantum Electronics, Wiley, New York, 1961.
9.6.2 Renewable and high flux-compatible optics
9.6.2.1 Liquid optics
A new approach to high power-loads on optics and adaptive optics based on the use
of liquid optical elements (planar and shaped mirrors, liquid diffraction gratings, and
zone plates) has been proposed [1]. This new class of centimeter-size optical elements
consists in most cases of thin liquid films over porous substrates. The principal features
associated with these optics are: 1) the film thickness is in the range from a few microns
to a hundred of microns, 2) the film’s behavior is strongly affected by capillary forces,
and 3) electrostatic and jB forces are used to control the shape of the surface of the film.
The optical elements, in most cases, can be arbitrarily oriented with respect to the
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rate. A significant advantage of optics for rep-rate applications is that the parameters of
the optical system can be monitored and changed between two successive pulses at a
frequency of ~ 10 Hz.
The shape of the free surface of the mirrors is controlled by capillary forces and
magneto-electrostatic forces (whence the acronym “CAMEL”=”Capillary-MagnetoElectrostatic”). In the rep-rate mode, one can replace the film material in every pulse.
The possibility of using planar liquid-film mirrors in the “upside-down” orientation, with
viscous inhibition of the gravitational instability, has been discussed. The film material
will be renewed within the time short compared to the instability e-folding time.
Reflecting diffraction gratings can be created by perturbing the film surface by the JB
force generated by an array of embedded fine wires. Gratings on the surface of dielectric
liquids can be produced by electrostatic forces created by submerged conductors.
Parabolic mirrors and reflecting zone plates can be created both electrostatically and
electromagnetically. The focal length and the direction of the optical axis of the
parabolic mirror can be changed by controlling voltages on a segmented electrodes,
without any need for introducing mechanical actuators. Figure 1 illustrates an
electrostatically controlled mirror.
For rep-rate applications, an important element of the CAMEL optics is a porous
substrate (perhaps, made of fused capillaries): a thin liquid layer is created over the
substrate and then removed by pressing the fluid in and out through the capillaries. This
has been evaluated and deemed possible. A set of equations which could serve as a
starting point for a more detailed analysis of the proposed system has been presented.
Fig. 1. Creating a focusing mirror on the surface of a conducting fluid by driving a
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current between the immersed electrodes
A key issue in the experimental tests of the concept is the production of a sufficiently
good quality liquid surface, produced by pressing the working fluid through the porous
substrate There is no doubt that thick-enough films with a flat surface can be produced.
What has to be determined experimentally, is the minimum attainable film thickness (at a
good surface quality). The answer may depend on the cleanness of the substrate and its
macroscopic uniformity. Direct experiments are required to address this issue.
References
[1] D. Ryotov and A. Toor,
9.6.2.2 Plasma lens
In [1-3] it was experimentally shown that a small ~300-400 m diameter evacuated
micro-capillaries can form concave electron density profile constituting a plasma
waveguide for laser radiation. The results demonstrated guiding of optical laser radiation
with intensities 108 -1017 W/cm2 over a distance of ~10 Rayleigh lengths. In [2,3] soft xray laser shadowgraphy, plasma emission spectroscopy and theoretical modeling were
used to study the plasma. Model computations show that the plasma is azimuthally
symmetry with a minimum density on axis, suitable for focusing.
Micro-capillary plasmas are generated by exciting a small diameter, 8-10 mm long
evacuated channel in polyacethal (CH2O)n, LiH or other isolating materials with a current
pulse of 0.5-1.5 kA peak amplitude and 150 –1000 ns half-period duration [1-3]. The
plasma is generated from material ablated from the capillary walls; see figure 9.6.2.2.1.
Prior to excitation the pressure in the vacuum chamber containing the capillary discharge
is < 110-5 Torr.
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Fig. 9.6.2.2.1. An experimental setup used to drive capillary discharge [1].
Modelling results show that the micro-capillary plasma is initially non-uniform, but
that is rapidly evolves into a highly symmetric column with minimum electron density on
axis needed for plasma waveguides or lenses for radiation. Ray-tracing calculations
inside a 30 cm long capillary find focal distances between 30 cm and 160 cm for 1.5 to 4
keV (3-8A) radiation, with the possibility of 1-2 m spot size diameter.
References
[1] Y. Ehrlich, C. Cohen, A. Zigler, J. Krall, P. Sprange, and E. Erasay, Phys. Rev. Lett. 77, 4186,
(1996). Also D. Kaganovich, P. V. Sasorov, Y. Ehrlich, C. Cohen, A. Zigler, Appl. Phys. Lett. 71(20),
2925, (1997).
[2] J. J. Rocca, F. G. Tomasel, M. C. Marconi, J. L. A. Chilla, C. H. Moreno, B. R. Benware, V. N.
Shlyaptsev, J. J. Gonzalez and C. D. Macchietto, SPIE vol. 3156, pp. 164, (1997).[3] M. C. Marconi,1, C.
H. Moreno, J. J. Rocca, V. N. Shlyaptsev, and A. L. Osterheld, Phys. Rev. E, Vol.62(5), 7209, (2000)
9.6.3 Advanced attenuators
9.6.3.1 Liquid metal attenuator
An alternative system to that of a gas cell attenuator, with a substantially smaller
vacuum loading effect, could be based on a liquid metal, such as gallium [1,2]. The
surface area of a liquid stream exposed to the vacuum could, with proper reservoir and
baffle design, be kept small even with the use of continuous pumping, while the timeaveraged vacuum loading could be further reduced with suitably designed rotating beam
shutters. In Fig. 9.6.3.1.1 a set of attenuation curves is plotted over the 800-25000 eV
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range for three values of thickness tl. A jet with a thickness controllable over a range of
0.1 - 1 mm, could evidently provide effective attenuation over the entire fundamentalthrough-3rd-harmonic range of the LCLS.
Weak-Field Attenuation Curves for Liquid Gallium vs. Distance
Absolute Attenuation
1
0.5 mm
0.1 mm
0.01 mm
0.01
10 -4
10 -6
10 -8
10 -10
10 -12
10 4
1000
Photon Energy [eV]
Figure 9.6.3.1.1 Weak-field attenuation curves for gallium.
References
[1] E. H. Kawamoto et al, "X-ray reflectivity study of the surface of liquid gallium," Phys. Rev. B 47(11),
6847(1993).
[2] D. H. Bilderback, C. Henderson, J. White, R. K. Smither, G. A. Forster, "Undulator heat loading studies
on X-ray monochromators cooled with liquid gallium," Rev. Sci. Instrum. 60(7), 1973(1989).
9.6.3.2 Mixed gas/low Z metal attenuator
9.6.9 Application of generic optical techniques to the LCLS
9.6.9.3 Multilayer optics
Multilayers are depth-periodic synthetic layered structures of high enough quality to
be considered equivalent to crystals. Layers of materials A and B having significant
differences in their electron densities (scattering powers for x-rays) and of uniform
thicknesses ta and tb are combined using vapor deposition technology to form samples
of uniform in-depth period d0 (= ta + tb). Synthesis processes applied to x-ray
multilayers are based primarily on multiple source physical vapor deposition using
thermal evaporation or sputter sources and moderate vacuum. They are probably inferior
- in terms of damage sustainability - to the elemental crystals. First, at the angles of
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incidence required for efficient operation, the typical depth presented to the radiation will
be similar to or greater than the values of te listed for crystals. In view of the fact that
one of the materials will typically have a moderate-to-high Z, the corresponding energy
loading will be substantially more severe than for the homogeneous low-Z crystals.
Second, the bonding between the layer materials, as well as their internal homogeneities
are on the average substantially poorer than in perfect crystals, leading to the acceleration
of damage mechanisms [1]. Finally, the disparity between the bulk and surface
properties of the substrate (which would almost certainly need to be a low-Z material
such as Be or C) and the multilayer constituents could also be expected to contribute to
the activation or exacerbation of irreversible damage processes.
Nonetheless, the exposure of multilayers to the LCLS pulses is of inherent interest,
and the operation and investigation of multilayers at the LCLS can be expected to
become a continuing R&D activity. Specifically, WSi2-S and MSi2SI laters have been
successfully constructed and tested.
References
[1] E. J. Gray, L. V. Knight, B. J. Peterson, J. M. Thorne, T. W. Barbee, Jr., A. Toor, "Laser plasma
damage to multilayer mirrors," SPIE Proceedings 831, 136 (1988).
9.6.9.4 Replicated optics, e.g ellipsoidal mirrors
We describe a new class of x-ray optical devices that have a tubular shape that is open
at both ends. The interior surface of the optic is highly reflective to x-rays within a
wavelength band of interest. X-rays enter one end, undergo a single reflection at the
interior surface, and exit from the other end with a different direction of travel. The
shapes of the optics are truncated paraboloidal shells or ellipsoidal shells of revolution.
The shape can also be polynomial. Paraboliodal shells including linear cones are often
used as collimators and ellipsoidal shells are used as concentrators.
These optical structures are made by sputter depositing layers of materials onto supersmooth mandrel thus forming the optic from the inside outwards. When the mandrel is
separated from the sputtered foil, the innermost layer replicates the mandrel smoothness
and serves as the reflecting surface. Both grazing incidence reflectors, and multi-layers,
can be used. Typical optics of that have been fabricated have a length of approximately
10 cm, a large end diameter of 3-4 cm and a small end diameter of 2-3 cm. Usually the
small end of the optic will be placed 3-5 cm from an x-ray point source. A beam block is
can be used to stop the direct unreflected x-rays from exiting the optic as shown in Fig.
9.6.9.4.1
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Figure 9.6.9.4.1 Artists drawing of an LLNL collimator.
For example, an x-ray reflective coating is made by dc magnetron sputtering, a
technique which has been proven in the past to be suitable for deposition of ultrasmooth,
uniform coatings. The mandrel with the sputtered coating is then electroplated with a
thick multilayer coating (1 mm or greater) in order to improve the sturdiness of the part.
The finished part is then easily removed from the mandrel.
The first LLNL x-ray collimators fabricated were near-conical polynomial shaped
structures having highly reflective interior surfaces. The shape of the reflecting surface is
designed to transform a portion of the spherical radiation pattern produced by a point
source into a quasi-parallel beam of x-rays capable of illuminating a full print field at
near normal incidence angles. A beam block stops direct irradiation of the mask by
unreflected rays. Figure 9.6.9.4.2. displays two photographs of the gold-reflector
collimator used in initial tests; the photograph on the right shows the collimator in a
mounting fixture that provides pointing and positioning capability.
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LCLS CDR
2/17/16
Figure 9.6.9.4.2 Photographs of the collimator optic used in tests.
It is expected that such optics can be used on the LCLS for focusing.
9.2.
Summary
As a summary, we provide a list of the optical components, diagnostics and access and
personnel protection hardware systems required .
9.7.1 Optical components required
Table of optical components required
9.7.2 Diagnostics required
Table of diagnostics required
9.7.3 Access and personnel protection
Table of safety components required
9.7.4 Other components
Table of other components required
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