X-Ray Diagnostics for the
LCLS
Jan. 19-20, 2004 UCLA
General Assumptions (See CDR)
• Undulators Fixed in Position
– Exception: Small motion for alignment
• Fixed Gap Undulators
• Permanent Magnet Quads
• Cavity BPMs following each undulator and
prior to the 1st
• Full suite of diagnostics every third
undulator
Current Thinking
• Use NdFeB
– Radiation Damage Issue
•
•
•
•
Real benefit of Sm2Co17 still no known
Must determine acceptable losses
APS is doing this for operational reasons
7 Additional undulators planned for use in regular
maintenance schedule
– Temperature Compensation
• Sm2Co17 slightly more than a factor of 2 better. Not
enough.
Current Thinking
• K Adjustment/Control Strategy
Cant
(mrad)
Horizontal
alignment
accuracy (mm)
Range of hort. motion
DBeff/Beff
over 10 mm (mm) for DBeff/Beff =
±1.5x10-3 (±5.5x104 )b
2
0.50
3.0x10-3
±5.0 (±1.8)
0.010
3
0.33
4.5x10-3
±3.3 (±1.2)
0.015
4
0.25
6.0x10-3
±2.5 (±0.9)
0.020
a All
Incremental
thickness (mm)
of mech. shims
calculations were made according to a tolerance of 1.5x10-4 and a total range of
1.5x10-3 (20 Gauss) for DBeff/Beff. Spacer thickness steps were chosen to allow full
compensation at half travel of the total range of motion.
b The range of horizontal motion listed in the parentheses corresponds to ±1°C
temperature compensation. To allow for temperature compensation of ±1°C the
additional range of motion listed in parentheses should be provided.
Effect of K Errors
Undulator segment tolerances
Parameter
Specified Value
Trajectory excursion (both planes)
2 mm
Radiation amplitude deviation
2%
Phase slippage between undulators
10° a
Vertical positioning error
50 mm
a Total allowed phase slippage including all errors. The error in the effective
magnetic field Beff, totally dominates the contributions.
Phase Error Correction Applied
Calculated gain length and increase in saturation length using
a random uniform distribution of K values from one undulator
to the next with end-phase corrections applied for the LCLS
beam parameters.a
a
DK/K
Gain length (m) b
Increase in sat. length (m) c
±3.5x10-4
±7.0x10-4
6.46 *
6.50
~ 1.7 m
±10.0x10-4
6.60
~ 3.4 m
Normalized beam emittance was 1.5 mm-mrad, beam energy spread 2.1x10-4, FODO lattice strength
0.112 m-1, and peak current 3.5 kA. Other parameters same as in Table 1.
b The gain length was derived over the next-to-last super period of six undulators.
c The increase in saturation length was estimated near 100 m by determining how much additional
distance was needed to reach the same level of ln|J|2.
* Same value as for DK/K = 0.
Current Thinking
• Undulator System Fully Installed on day 1
• Start at 800 eV
– Why?
Cell structure of the LCLS
Undulator Line
3420
852
627
UNDULATOR
11528 mm
Horizontal Steering Coil
Vertical Steering Coil
Beam Position Monitor
X-Ray Diagnostics
33 Undulators
~ 130-m
Overall Length
Quadrupoles
Pioneering
Science and
Technology
Office of Science
U.S. Department
of Energy
PRIOR WORK
FOR INTRA-UNDULATOR DIAGNOSTICS (CDR April 2002)
Electron Beam Diagnostics (Section 8.11, Glenn Decker and Alex Lumpkin)
Table 8.9 Undulator electron beam diagnostics
Quantity
Location
BXY Notes
Type
*Beam position monitor
48
All stations
Cavity / Button combo
OTR imaging diagnostic
13
Every third station
Thin aluminum foil screen
Wire scanners
4
Upstream of Undulator segments
*Cherenkov Detectors
33
All stations
Fused silica / PMT combo
2
Upstream/downstream of
undulator
Toroid transformer
*Current Monitors
* Non-intercepting
X-ray Diagnostics (Section 8.13, Efim Gluskin and Petre Ilinski)
On-axis diagnostics
Diamond (111), 4 – 9 keV, cooled silicon PIN diode.
Off-axis diagnostics
Hole crystal, 2q = 90, CCD detector.
PRIOR WORK (OBSERVATIONS)
 Good starting point for non-interceptive diagnostics (BPM,
Cherenkov detectors, current monitors, etc.)
 Laid good foundation for low-power interceptive diagnostics
(OTR, on-axis x-ray detectors, etc.)
 Low photon energy (827 eV or 1.5 angstrom) x-ray
diagnostics was not addressed.
 High-power interceptive diagnostics was not addressed.
IMPLICATION OF HIGH PEAK POWER
(Extrapolation from CDR Section 9.1)
Table 9.1 Characteristics of the FEL x-ray beam (Adapted)
0.828 keV (4.54 GeV)
8.27 keV (14.35 GeV)
FEL
Spontaneous
FEL
Spontaneous
Energy per pulse (mJ)
Peak power (GW)
Divergence (mrad FWHM)
Spot size at 50 m (mm FWHM)
Spot size in undulator (mm FWHM)
Average power (W)
Average power density (W/mm2)
3
11
9
610
75
0.4
52
1.4
4.9
780
Aperture limited
0.2
2.5
9
1
130
66
0.3
42
22
81
250
Aperture limited
2.6
Table 9.2 Normal-incidence peak energy dose and damage to materials (Adapted)
Melt
Evaporation*
Dose at 50 m (eV/atom)
Dose in undulator (eV/atom)
(eV/Atom)
(eV/atom)
827 eV
8.27 keV
827 eV
8.27 keV
0.1
1.8
0.02
0.0005
1.3
0.002
Li
0.3
3.8
0.08
0.001
5
0.004
Be
0.5
4.8
0.2
0.003
13
0.01
B
0.9
0.4
0.007
26
0.03
C (graphite)
0.2
3.9
0.4
0.2
26
0.8
Al
0.4
5.2
0.6
0.2
40
0.8
Si
0.3
1.1
0.4
73
1.6
Cu
* Data from thermal measurements: fusion and evaporation latent heat, plus heating up with specific heat (25C).
Material
Screen material is lost similar to laser ablation!
MAIN OBJECTIVES
(in orders of priority)
(0)
Measure the e-beam centroid position at all breaks
To characterize electron trajectory (RF BPM)
(1A) Measure the x-ray beam profile and its overlap with e-beam at all
long breaks
To characterize the amplifier in more details
(1B) Measure monochromatic x-ray beam intensity at all long breaks
To characterize the FEL amplifier at chosen photon energy
(2)
Measure the electron beam profile at all long breaks
To characterize the electron beam transport
(3)
Analyze the electron and x-ray beam in 6-dimensions (x, y, x’, y’, t)
with adequate resolution
To study FEL physics
Three Groups of LCLS Intra-Undulator Photon Diagnostics
At saturation, the LCLS x-ray beam is capable of severe surface damages at 40 m
downstream of the undulator exit (CDR Ch. 9). Inside the undulator, where the FEL
beam is at or near saturation, the power intensity is 30 to 60 times higher than that on the
user optics. No solid surfaces can avoid damage.
(1) Lower
Power Diagnostics (LPD)
Before amplified beam power exceeds spontaneous radiation power, conventional
technology can be used with judicious choice of materials. Installation may cover
the entire undulator during commissioning period. Reduce to about 50% (six to
seven stations) at normal (full power) operations.
(2) High
Power Diagnostics (HPD)
Depending on where the amplified beam reaches saturation, in as much as fifty
per cent of the undulator the beam power can be very damaging, especially at low
photon energy end. A maximum 6 stations are needed to provide diagnostics
without solid screens in the beam.
(3) End
of Undulator Diagnostics (EOU)
(Overlaps with LLNL and SLAC work)
End of Undulator Diagnostics
At the end of the undulator, the electron beam separates from the x-ray beam, and the
spatial limit in the breaks no longer exists. We have a unique opportunity to perform
more beam measurements.
(1) Gamma Ray diagnostics
 Gamma ray detector array: (use with wire scanner, beam loss monitor).
(2) Electron beam diagnostics
 Current monitor at the end of undulator and at the dump
 Bunch length (streak camera using OTR source, or electro-optic technique)
 Energy spread and emittance (characterize the extraction of beam energy)
(3) X-ray beam diagnostics: Intermediate power handling capabilities to
complement user beamline diagnostics. Provide micro-bunching
diagnostics for:
 During commissioning or machine study period, using the main x-ray beam,
before saturation is achieved.
 During full power x-ray operation, using one of the following sources in nonintercepting mode:
o Coherent dipole radiation,
o Use a dipole to deflect the electron beam before the last undulator, and
o Off-axis undulator radiation from the main beam?
Major Subsystems of the Low Power Diagnostics (LPD)
o Scanning wire assembly
o X-ray intensity monitor
o Optical transition radiation (OTR) imaging station
o X-ray monochromator / imaging detector
Low Power Diagnostics (LPD) Summary
Physics issues:
(1) Determine what screen is safe in the beam, for the undulator and itself
(Scattering calculations and power load calculations).
(2) Select suitable monochromator elements covering 800 eV to 8 keV (Search for
materials, techniques and vendors).
(3) Find linear detectors covering 800 eV to 8 keV (Explore detection techniques
and vendors).
Expect to use quantitative analysis to put the proposed approaches on solid footing.
Prototypes are to be used to experimentally verify the models.
Engineering issues:
(1) It is a challenge to accommodate all mechanisms under the stringent spatial
constraint.
(2) All LPD instruments in the same chamber need to be designed “together” to
minimize difficulties and design time.
Mechanical / vacuum design can start when an engineer is available and ready.
Major Subsystems of the High Power Diagnostics (HPD)
o X-ray intensity monitor (gas phase or disposable solid surfaces)
o Laser wire assembly
o Optical diffraction radiation (ODR) or off-axis undulator radiation monitor
The high power region is defined as where no solid surface is safe from damage
(or total evaporation) by the x-ray beam. Hence the interceptive diagnostics use
gas phase media or expendable solid surfaces. Some of the diagnostics in this
region are totally non-interceptive.
To date, we do not have a reasonably viable approach for characterizing the
overlap of the electron and photon beam. Approaches considered were:
o Focused ion beam (in magnetic field free region EOU)
o Molecular beam (spatial resolution)
o Gas phase fluorescence imaging
High Power Diagnostics (HPD) Summary
Physics issues:
(1) The approach to x-ray intensity measurements needed to be studied (gas cell / jet
proposed) with numeric modeling.
(2) The
approach to optical e-beam divergence measurements needed to be identified
(ODR imaging proposed) with numeric modeling.
(3) Laser
wire represents significant investment in cost and maintenance commitment,
although its resolution should be adequate. Alternative needs to be studied.
Major analysis work has to be performed to justify the proposed approaches. We
have not proposed a technique to measure x-ray beam size with confidence.
Engineering issues:
(0) Gas cell / jets put heavy load on the vacuum / pumping system.
(1) Laser wire requires line-of-sight transport, which should be built in the concrete
support base being designed.
Mechanical / vacuum design cannot start earnestly until major physics issues are
resolved. Integration of final physics approaches could prove challenging.
High Power Diagnostics Strategy
It can be seen that the main development effort is in the high-power segments. The
following strategies have been proposed to manage (delay or avoid) the tasks.
(1) Start with all lower power diagnostics (written in WBS)
Initial LPD installation covers the entire undulator. Replace stations at highpower end as the commissioning progresses.  Gain time for development for
high-power stations.
(2) Tune beam / undulator at short x-ray wavelengths (discussed with
Galayda, no conclusion)
Since absorption is roughly proportional to the third power of photon energy, the
absorbed power is dramatically reduced at the high photon energy end.  Do not
need high-power diagnostics.
(3) Tune beam / undulator at low charge (SLAC Pat Krejcik)
Use interceptive diagnostics only at low charge.  Do not need high-power
diagnostics.
Commissioning Workshop will formalize these strategies!
ENTRANCE SECTION
Gamma Ray Detector
Issues
• Quad Fixed to the Undulator
– Has implications on using the cant for tuning
• Requires separate motion
• Tunnel Temperature Control
– Would like better than +/- 0.2 Degrees C.
• Supports will probably need this or better
• 800 eV
– Makes life very difficult for the x-ray diagnostics
– Would prefer to start a > 2 KeV
Issues
• Motion Capability
– We need some for K tuning but…
– Do we need the ability to completely remove
the undulators?
• Aside: 0.1 G deflects the 14 GeV beam by ~1.5 um
in 3.7 m.
• High Power Damage
– Can we even make a diagnostic that is both
useful and can handle the power densities of
the FEL?
Issues
• Radiation Damage
– We will use NdFeB
– We must protect the undulator from radiation
• This has implications on intraundulator interceptive
diagnostics
• Canted Poles
– BBA
• There will be some arbitrary offset and anle
through the undulator
• How does this impact the K tuning?