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John Arthur
Summary [email protected]
November 14, 2005
Summary Discussion
John ArthurSLAC
LCLS Beam-Based Undulator K Measurement Workshop
John Arthur
Summary [email protected]
November 14, 2005
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.
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.
Workshop Objective
John Arthur
Summary [email protected]
November 14, 2005
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.
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.
Work Package 1: Angle Integrated Measurement
John Arthur
Summary [email protected]
November 14, 2005
Marking the location of a spectral edge
We will watch how the following property changes:
HALF PEAK PHOTON ENERGY
FEATURES OF LCLS UNDULATOR SPECTRUM (n = 1)
PHOTON ENERGY (eV)
8000 8100 8200 8300 8400 85000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Peak Flux
HALF PEAK ENERGY
Peak Energy
(8267.2 eV)
Κ = 3.5000Ε = 13.64 GeVω1 = 8265.7 eV
John Arthur
Summary [email protected]
November 14, 2005
Effects of Aperture Change (Size and Center)Plot the half-peak photon energy vs. aperture sizeEdge position stable for 25 – 140 rad 100 rad best operation pointIndependent of aperture size Independent of aperture center position
X-RAY SPECTRAL FEATURE OBSERVED (OBSERVED THROUGH A SQUARE APERTURE)
APERTURE ( )rad0 50 100 150 200
- ( )HALF PEAK ENERGY eV
8264
8266
8268
8270
8272
Δ /K K = 2.4 10x -4
Δ /K K = 2.4 10x -5
UNDULATOR SPECTRA THRU SQUARE WINDOW
PHOTON ENERGY (eV)8000 8050 8100 8150 8200 8250 8300 8350 8400
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6APERTURE = 140 rad
= 3.5000K = 13.64 E GeV
30 rad
25 rad
20 rad
15 rad 10 rad
C A B
160 rad
(5 167 )mm@ m
(5 35 )mm@ m
John Arthur
Summary [email protected]
November 14, 2005
Effects of Undulator Field Errors
Electron beam parameters
E = 13.640 GeV
x = 37 m
x’ = 1.2 rad
= 0.03%
Detector
Aperture
80 rad (H)
48 rad (V)
Monte Carlo integration for 10 K particle histories.
John Arthur
Summary [email protected]
November 14, 2005
Comparison of Perfect and Real Undulator SpectraFilename: LCL02272.ver; scaled by 0.968441 to make Keff = 3.4996
First harmonic spectrum changes little at the edge.
John Arthur
Summary [email protected]
November 14, 2005
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)
John Arthur
Summary [email protected]
November 14, 2005
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 instrumentNo buffer for “unknown-unknowns”Non-Gaussian beam energy distribution ???
John Arthur
Summary [email protected]
November 14, 2005
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 separatelyUse differential electronics to get the difference in flux
John Arthur
Summary [email protected]
November 14, 2005
Differential Measurement Recap Use one reference undulator to test another undulator simulataneouslySet monochromator energy at the spectral edgeMeasure the difference of the two undulator intensity
HISTOGRAM OF DIFFERENCE COUNTS
DIFFERENCE COUNTS (103 PER BUNCH)
-4 -2 0 2 4
FREQUENCY
0
500
1000
1500K = 3.49999
PHOTON ENERGY = 8265.7 eVTOAL COUNTS = 0.644 × 106
_ = 64 ( )N avg bunches
= 3.50001K
Simulation gives approximately:
• To get RMS error ΔK/K < 0.710-4, 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?
John Arthur
Summary [email protected]
November 14, 2005
Yang Summary (The Main Idea)We propose to use angle-integrated spectra (through a large aperture, but radius < 1/) 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 ΔK/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, ΔK/K = 3 10-6 or better. It is sufficient for XFEL applications.It can also be used for routinely logging magnet degradation.
John Arthur
Summary [email protected]
November 14, 2005
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 m in theory for now, hope to get ~ 10 m 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 fieldLongitudinal wake field effect,Experimental test in the APS 35IDMore?
John Arthur
Summary [email protected]
November 14, 2005
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.
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.
Work Package 2: Pinhole Measurement
John Arthur
Summary [email protected]
November 14, 2005
Basic Layout
BasicScheme
Slit width must be small to get clean signal. 2 mm shown.
Useg #1 is worst case
John Arthur
Summary [email protected]
November 14, 2005
Aligning the Pinhole
Simple 2D scan, one shot per data point, 0.1 mm steps, no multi-shot averaging
Error is added to geometry term.
Actual beamAxis 0.5, 0.5
Scan range+ / - 1 mmX and Y
“Measured”Beam axis0.33, 0.34
John Arthur
Summary [email protected]
November 14, 2005
8.26 keV Transmission Grating
31021 −⋅==
ΔNE
E
P = 200 nmN = 500
D = 100 m
Sputter-sliced SiC / B4C multilayer
33 m thick
Beam100 m
Interference Function
Single Slit Diffraction Pattern
Observed Intensity
angle
mDP
λ⋅=⋅ 3750
mD ⋅=5
John Arthur
Summary [email protected]
November 14, 2005
200 nm period x 33 microns works
diffraction peaks in far-field
200 nm period
33 m
Waveguide coupling limits us to periods > 200 nm
John Arthur
Summary [email protected]
November 14, 2005
1st Order DiffractionPeaks
4 mm
FEL Transmission Grating Spectrometer
Thick Slit5 cm Ta
capped with 1 cm B4C
YAG Scintillator50 microns thick
Thin Adjustable Slit1 mm Ta
Very high energy photons go through everything
50-100 micron1 mm
6 m
TransmissionGrating200 nm Period100 micron Aperture
5 m
John Arthur
Summary [email protected]
November 14, 2005
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
Slits, gratings and scintillator placed in beam
John Arthur
Summary [email protected]
November 14, 2005
Bionta Summary
Investigated 100 micron aperture FEL Transmission Grating for use in measuring KSensitivities are roughly at the limit of what is neededSignal 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 periodsSignal:Backgrounds with thin scintillator are at least 1:1Beam stability and pointing (relative to the 100 micron aperture) will be an issue that is not investigated here
John Arthur
Summary [email protected]
November 14, 2005
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.
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.
Work Package 3: Single-Shot Spectral Measurement
John Arthur
Summary [email protected]
November 14, 2005
Possible spectrometers
Bent Bragg (after P. Siddons-NSLS)
Mosaic crystal
Bent LaueZhong Zhong-NSLS
X-ray GratingP. Heimann-ALS, S. Hulbert-NSLS
John Arthur
Summary [email protected]
November 14, 2005
2 mm
Si (422)
surface normal
R=3.9 m
76 mm
Strip detector(200 strips)
Bent Bragg Spectrometer
Cu foil
John Arthur
Summary [email protected]
November 14, 2005
0.00E+00
1.00E+12
2.00E+12
3.00E+12
4.00E+12
5.00E+12
6.00E+12
7.75 7.8 7.85 7.9 7.95 8 8.05 8.1 8.15 8.2
Series1
Series2
Series3
On axis
+0.5 mm
+1.0 mmPhoton energy (keV)
Und-pinhole distance 200 m Pinhole 2.0 x 0.02 mm2
John Arthur
Summary [email protected]
November 14, 2005
Bent Bragg to do list
Simulation considering position dependent spectrum
Role of jitter
Test K sensitivity with simulated data (including noise)
John Arthur
Summary [email protected]
November 14, 2005
2 x Mosaic spread2
180-2
Mosaic crystal spectrometer
John Arthur
Summary [email protected]
November 14, 2005
Andreas Freund, Anneli Munkholm, Sean Brennan,SPIE, 2856,68 (1996)
10 keV
24 keV
John Arthur
Summary [email protected]
November 14, 2005
Mosaic crystal to do list
Crystal uniformity ?
Ultimate resolution ?
Experimental geometry (20 m crystal to detector distance)
John Arthur
Summary [email protected]
November 14, 2005
Design criteria
Goals
Photon energy range: 800 – 8000 eV.
Spectral resolution: Δλλ < 1 x 10-3 set by the FEL radiation bandwidth
Spectral window Δλλ > 1 x 10-2 set by the single undulator harmonic energy width
Single shot sensitivity for single undulator spectra.
Consider damage for FEL radiation
John Arthur
Summary [email protected]
November 14, 2005
LCLS grating spectrometer layout
One VLS grating in -1 order
Length of spectrometer 1.3 m
John Arthur
Summary [email protected]
November 14, 2005
Raytracing of the grating spectrometer: 8000 eV
Source 90 m diameter (fwhm)
7992, 8000, 8008 eV
or 7600, 8000, 8400 eV
At the detector1.1 mm (h) x 2 m (v) (fwhm)ΔE = 14 eV (6x102 RP , limited by detector pixel size 13 m, in FEL case could use inclined detector)
7992 eV
8000 eV
8008 eV
7600 eV
8000 eV
8400 eV
800 m
John Arthur
Summary [email protected]
November 14, 2005
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 ΔE/E = 1/N = 8.8 x10-3
Divergence r‘ = λ/2L = 15 rad (800 eV) and 4.8 rad (8 keV)
Spectrometer
Vertical angular acceptance 60 rad (800 eV) and 20 rad (8 keV)
Efficiency = RM1.G = 0.13 (800 eV) and 0.08 (8000 eV)
Flux at detector 2 - 4 x 105, N noise ~ 0.2 %
Yes
John Arthur
Summary [email protected]
November 14, 2005
Summary: the Grating Spectrograph for the LCLS
Photon energy range: 800 – 8000 eV.
Resolving power: EΔE = 2000 at 800 eV and 300 at 8 keV.
For FEL radiation the resolution could be improved with an inclined detector.
Spectral window: ΔEE = 10%.
Single shot sensitivity for single undulator spectra.
John Arthur
Summary [email protected]
November 14, 2005
Characterize the spectral features of spontaneous synchrotron radiation that are usable for beam-based K-measurements.
Identify the most appropriate strategy for beam-based K-measurements.
Specify suitable instruments for the identified beam-based K-measurement 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.
Characterize the spectral features of spontaneous synchrotron radiation that are usable for beam-based K-measurements.
Identify the most appropriate strategy for beam-based K-measurements.
Specify suitable instruments for the identified beam-based K-measurement 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.
Workshop Charge
John Arthur
Summary [email protected]
November 14, 2005
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
