LCLS Beam-Based Undulator K
Measurement Workshop
Summary Discussion
John Arthur
SLAC
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Workshop Objective
Define a strategy for using spontaneous undulator radiation to
measure the K value of every individual LCLS Undulator Segment
after installation in the Undulator Hall.
To reach the objective, the physics and technologies necessary need
to be identified. Workshop discussions will include
Usable spectral features of spontaneous radiation
Strategies for beam-based K measurements
Specifications for suitable instruments
Scheduling issues
Three Work Packages have been defined and assigned to three
different groups. Work described by these Work Packages has been
carried out in preparation of the workshop and will be presented and
discussed at the workshop.
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Work Package 1: Angle Integrated Measurement
Group: B. Yang, R. Dejus
Task: Examine robustness of angle-integrated measurements of undulator
spectrum. Consider effects of errors in beam alignment, undulator magnet
structure, straightness of vacuum pipe, alignment of spectrometer, etc.
Consider effects of location of undulator segment being tested. Determine
what are realistic values for the precision with which the value of K can be
determined for an undulator segment at the beginning, middle, and end of
the undulator.
This task explores the use of the high-energy edge of the fundamental
spectral peak (the third harmonic may also be considered) of a single
undulator to measure its K parameter. The measuring spectrometer will be
located in the LCLS FEE, roughly 100 m downstream from the final
undulator segment. Realistic values for the angular acceptance of the
measurement (limited by beam-pipe apertures, or apertures at the
measuring point) should be considered.
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
6
FLUX (10 PHOTONS/nC/0.01%BW)
Marking the FEATURES
locationOFofLCLS
a spectral
UNDULATORedge
SPECTRUM (n = 1)
1.6
1.4
1.2
Peak Flux
 = 3.5000
 = 13.64 GeV
1 = 8265.7 eV
1.0
0.8
0.6
0.4
HALF PEAK ENERGY
(8267.2 eV)
0.2
Peak Energy
We will watch
0.0
8000
8100
8200
8300
8400
how the following
PHOTON ENERGY (eV)
property changes:
HALF PEAK PHOTON ENERGY
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
8500
Effects of Aperture Change (Size and Center)
UNDULATOR SPECTRA THRU SQUARE WINDOW
C
1.6
1.4
A
B
K = 3.5000
E = 13.64 GeV
APERTURE = 140 mrad
(5mm@35m)
1.2
1.0
0.8
30 mrad
(5mm@167m)
6
PHOTONS/nC/0.01%BW)
FLUX (10
0.6
160 mrad
25 mrad
0.4
X-RAY SPECTRAL FEATURE OBSERVED
(OBSERVED THROUGH A SQUARE APERTURE)
8272
HALF-PEAK ENERGY (eV)
Plot the half-peak photon
energy vs. aperture size
Edge position stable for 25
– 140 mrad  100 mrad
best operation point
Independent of aperture
size  Independent of
aperture center position
K/K = 2.4 x 10-4
8270
K/K = 2.4 x 10-5
8268
8266
8264
20 mrad
15 mrad
0.2
10 mrad
0
8000
8050
8100
8150
8250
8300
PHOTON ENERGY (eV)
November 14, 2005
Summary
8200
8350
8400
50
100
150
APERTURE (mrad)
John Arthur
jarthur@slac.stanford.edu
200
Effects of Undulator Field Errors
Electron beam
parameters
E = 13.640 GeV
sx = 37 mm
sx’ = 1.2 mrad
sg/g = 0.03%
Detector
Aperture
80 mrad (H)
48 mrad (V)
Monte Carlo integration for 10 K particle histories.
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Comparison of Perfect and Real Undulator Spectra
Filename: LCL02272.ver; scaled by 0.968441 to make Keff = 3.4996
First harmonic spectrum changes little at the edge.
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Measure fluctuating variables
Charge monitor: bunch charge
OTR screen / BPM at dispersive point: energy centroid
Hard x-ray imaging detector: electron trajectory angle
(new proposal)
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Summary of 1-undulator simulations
(charge normalized and energy-corrected)
Applying correction with electron charge, energy and trajectory
angle data shot-by-shot greatly improves the quality of data
analysis at the spectral edge.
Full spectrum measurement for one undulator segment
(reference)
The minimum integration time to resolve effective-K changes is
10 – 100 shots with other undulator segment (data processing
required)
As a bonus, the dispersion at the flag / BPM can be measured
fairly accurately.
Not fully satisfied:
Rely heavily on correction calibration of the instrument
No buffer for “unknown-unknowns”
Non-Gaussian beam energy distribution ???
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Differential Measurements of Two Undulators
Insert only two segments in for the entire
undulator.
Steer the e-beam to separate the x-rays
Use one mono to pick the
same x-ray energy
Use two detectors to detect
the x-ray flux separately
Use differential electronics
to get the difference in flux
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Differential Measurement Recap
Use one reference undulator to test another undulator
simulataneously
Set monochromator energy at the spectral edge
Measure the difference of the two undulator intensity
Simulation gives approximately:
HISTOGRAM OF DIFFERENCE COUNTS
PHOTON ENERGY = 8265.7 eV
6
TOAL COUNTS = 0.644  10
N_avg = 64 (bunches)
1500
K = 3.49999
FREQUENCY
K = 3.50001
1000
• To get RMS error K/K <
we need
only a single shot (0.2 nC)!
• We can use it to periodically to log minor
magnetic field changes, for radiation damage.
• Any other uses?
0.710-4,
November 14, 2005
Summary
500
0
-4
-2
0
2
3
4
DIFFERENCE COUNTS (10 PER BUNCH)
John Arthur
jarthur@slac.stanford.edu
Yang Summary (The Main Idea)
We propose to use angle-integrated spectra (through a
large aperture, but radius < 1/g) for high-resolution
measurements of undulator field.
Expected to be robust against undulator field errors and electron
beam jitters.
Simulation shows that we have sufficient resolution to obtain
K/K <  10-4 using charge normalization. Correlation of
undulator spectra and electron beam energy data further
improves measurement quality.
A Differential technique with very high resolution was
proposed: It is based on comparison of flux intensities from
a test undulator with that from a reference undulator.
Within a perfect undulator approximation, the resolution is extremely
high, K/K =  3  10-6 or better. It is sufficient for XFEL applications.
It can also be used for routinely logging magnet degradation.
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Yang Summary (Continued)
Either beamline option can be used for searching for the
effective neutral magnetic plane and for positioning
undulator vertically. The simulation results are
encouraging (resolution ~1 mm in theory for now, hope
to get ~ 10 mm in reality).
What’s next
Sources of error need to be further studied. Experimental
tests need to be done.
More calculation and understanding of realistic field
Longitudinal wake field effect,
Experimental test in the APS 35ID
More?
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Work Package 2: Pinhole Measurement
Group: J. Welch, R. Bionta, S. Reiche
Task: Examine robustness of pinhole measurements of undulator spectrum.
Consider effects of errors in beam alignment, undulator magnet structure,
straightness of vacuum pipe, alignment of pinhole and spectrometer, etc.
Consider effects of location of undulator segment being tested. Determine
what are realistic values for the precision with which the value of K can be
determined for an undulator segment at the beginning, middle, and end of
the undulator.
This task explores the use of the fundamental spectral peak (the third
harmonic may also be considered) of a single undulator, as seen through a
small angular aperture, to measure its K parameter. The measuring
spectrometer will be located in the LCLS FEE, roughly 100 m downstream
from the final undulator segment. Realistic values for the angular
acceptance of the measurement should be determined, and the effects of
misalignment of the aperture or undulator axis should be carefully
considered.
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Basic Layout
Basic
Scheme
Slit width must be
small to get clean
signal. 2 mm
shown.
Useg #1 is worst
case
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Scan range
+ / - 1 mm
X and Y
Aligning the Pinhole
Actual beam
Axis 0.5, 0.5
Simple 2D scan, one shot
per data point, 0.1 mm
steps, no multi-shot
averaging
Error is added to geometry
term.
November 14, 2005
Summary
“Measured”
Beam axis
0.33, 0.34
John Arthur
jarthur@slac.stanford.edu
Simulated K Measurement
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
8.26 keV Transmission Grating
Sputter-sliced SiC / B4C multilayer
P = 200 nm
N = 500
D = 100 mm

P
 D  3750  mm
Interference Function
D  5 m
33 mm thick
Single Slit Diffraction
Pattern
Observed Intensity
100 mm
Beam
angle
E 1
  2 10 3
E
N
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
200 nm period x 33 microns works
33 mm
200 nm
period
diffraction peaks in far-field
Waveguide coupling limits us to periods > 200 nm
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Thick Slit
5 cm Ta
capped with
1 cm B4C
FEL Transmission Grating1st Order
Diffraction
Spectrometer
Peaks
Transmission
Grating
200 nm Period
100 micron Aperture
4 mm
1 mm
50-100 micron
5m
Very high energy photons go
through everything
Thin Adjustable Slit
1 mm Ta
November 14, 2005
Summary
YAG Scintillator
50 microns thick
6m
John Arthur
jarthur@slac.stanford.edu
Monte Carlo Generation of Photons
from Near-Field Calculations
Photons are aimed at
Sven’s near field
distributions…
… but allowed to
reflect off of the
vacuum pipe or get
absorbed in the
breaks
November 14, 2005
Summary
Slits, gratings
and scintillator
placed in beam
John Arthur
jarthur@slac.stanford.edu
Bionta Summary
Investigated 100 micron aperture FEL Transmission
Grating for use in measuring K
Sensitivities are roughly at the limit of what is needed
Signal level is too low by at least a factor of 200.
More aperture, say 1.4 x 1.4 mm would help. Larger
focal distance would allow larger periods
Signal:Backgrounds with thin scintillator are at least
1:1
Beam stability and pointing (relative to the 100
micron aperture) will be an issue that is not
investigated here
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Work Package 3: Single-Shot Spectral
Measurement
Group: J. Hastings, S.Hulbert, P.Heimann
Task: Assume that a single shot spectral measurement is needed
for an LCLS spontaneous undulator pulse. What are the best
options for doing the measurement? What spectral resolution can
be obtained using these methods? What are the effects of beam
jitter, spectrometer misalignment, etc?
This task explores the design and performance of x-ray
spectrometers capable of providing centroid or edge position with
high resolution, on a single-shot of radiation from a single LCLS
undulator. The spectrometer will most likely be located in the LCLS
FEE, about 100 m downstream from the final undulator segment.
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Possible spectrometers
Bent Bragg (after P. Siddons-NSLS)
Mosaic crystal
Bent Laue
Zhong Zhong-NSLS
X-ray Grating
P. Heimann-ALS, S. Hulbert-NSLS
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Bent Bragg
Spectrometer
surface
normal
Strip detector
(200 strips)
76 mm
Si (422)
2 mm
Cu foil
November 14, 2005
Summary
R=3.9 m
John Arthur
jarthur@slac.stanford.edu
Und-pinhole distance 200 m Pinhole 2.0 x 0.02 mm2
6. 00E +12
5. 00E +12
4. 00E +12
Ser i es1
Ser i es2
3. 00E +12
Ser i es3
2. 00E +12
1. 00E +12
On axis
+0.5 mm
0. 00E +00
7. 75
7. 8
7. 85
7. 9
7. 95
8
8. 05
8. 15
8. 1
8. 2
Photon energy (keV)
November 14, 2005
Summary
+1.0 mm
John Arthur
jarthur@slac.stanford.edu
Bent Bragg to do list
Simulation considering position dependent
spectrum
Role of jitter
Test K sensitivity with simulated data
(including noise)
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Mosaic crystal spectrometer
180-2Q
2 x Mosaic spread
2Q
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Andreas Freund, Anneli Munkholm, Sean Brennan,
SPIE, 2856,68 (1996)
24 keV
10 keV
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Mosaic crystal to do list
Crystal uniformity ?
Ultimate resolution ?
Experimental geometry (20 m crystal to
detector distance)
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Design criteria
Goals
Photon energy range: 800 – 8000 eV.
Spectral resolution: / < 1 x 10-3 set by the FEL radiation
bandwidth
Spectral window / > 1 x 10-2 set by the single undulator
harmonic energy width
Single shot sensitivity for single undulator spectra.
Consider damage for FEL radiation
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
LCLS grating spectrometer layout
Type
M11,
M12
G1
Plane
elliptical
mirrors
VLS plane
grating
Detector
Coating and Dimensions
blank
(mm)
material
Pt-coated
300 x 50,
silicon
300 x 50
Pt-coated
silicon
50 x 20
Radius
(m)
98,
294

Incidence Grating period Distance
angle(°)
order
from
source (m)
88.5
100
89.5
87.991189.3473
1/300
-1
110.3
111.3
One VLS grating in -1 order
Length of spectrometer 1.3 m
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Raytracing of the grating
spectrometer: 8000 eV
7992 eV
8000 eV
Source
90 mm diameter (fwhm)
7992, 8000, 8008 eV
or 7600, 8000, 8400 eV
At the detector
1.1 mm (h) x 2 mm (v) (fwhm)
E = 14 eV (6x102 RP , limited
by detector pixel size 13 mm,
in FEL case could use inclined
detector)
November 14, 2005
Summary
8008 eV
7600 eV
800 mm
8000 eV
8400 eV
John Arthur
jarthur@slac.stanford.edu
Is there single shot sensitivity
for spontaneous radiation?
Undulator (1)
Flux F = 1.4 x 1014 N Qn I = 3 x 106 1/(pulse 0.1% bw)
Bandwidth E/E = 1/N = 8.8 x10-3
Divergence sr‘ = /2L = 15 mrad (800 eV) and 4.8 mrad (8 keV)
Spectrometer
Vertical angular acceptance 60 mrad (800 eV) and 20 mrad (8 keV)
Efficiency e = RM1.eG = 0.13 (800 eV) and 0.08 (8000 eV)
Flux at detector 2 - 4 x 105, N noise ~ 0.2 %
Yes
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Summary: the Grating Spectrograph for
the LCLS
Photon energy range: 800 – 8000 eV.
Resolving power: E/E = 2000 at 800 eV and 300 at 8 keV.
For FEL radiation the resolution could be improved with an
inclined detector.
Spectral window: E/E = 10%.
Single shot sensitivity for single undulator spectra.
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Workshop Charge
Characterize the spectral features of spontaneous synchrotron
radiation that are usable for beam-based K-measurements.
Identify the most appropriate strategy for beam-based Kmeasurements.
Specify suitable instruments for the identified beam-based Kmeasurement strategy.
List expected performance parameters such as resolution of K
measurement as function of beam charge, and segment location as
well as expected tolerances to trajectory and energy jitter.
List any open questions regarding the feasibility of the most
appropriate strategy.
List the R&D activities, if any, needed before the design of a
measurement system can be completed and
manufacturing/procurement can start.
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu
Response to Charge
Are the spectral features robust?
Yes.
Angle-integrated or pinhole?
What’s the difference? For LCLS they are very similar.
Need detailed design.
Scanning spectrometer or single-shot?
Single-shot and scanning.
What kind of spectrometer?
Crystal or grating? What R&D is needed?
Create a PRD giving required specs
November 14, 2005
Summary
John Arthur
jarthur@slac.stanford.edu