Title: Summary Discussion
1Summary Discussion
LCLS Beam-Based Undulator K Measurement Workshop
John Arthur SLAC
2Workshop 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.
3Work 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.
4Marking the location of a spectral edge
- We will watch
- how the following
- property changes
- HALF PEAK PHOTON ENERGY
5Effects of Aperture Change (Size and Center)
- 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
6Effects 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.
7Comparison 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.
8Measure fluctuating variables
- Charge monitor bunch charge
- OTR screen / BPM at dispersive point energy
centroid - Hard x-ray imaging detector electron trajectory
angle (new proposal)
9Summary 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 ???
10Differential 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
11Differential 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
- To get RMS error DK/K lt 0.7?10-4, 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?
12Yang Summary (The Main Idea)
- We propose to use angle-integrated spectra
(through a large aperture, but radius lt 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 DK/K lt ? 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, DK/K ? 3 ? 10-6
or better. It is sufficient for XFEL
applications. - It can also be used for routinely logging magnet
degradation.
13Yang 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).
Whats 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?
14Work 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.
15Basic Scheme
Basic Layout
Slit width must be small to get clean signal. 2
mm shown.
Useg 1 is worst case
16Aligning the Pinhole
Scan range / - 1 mm X and Y
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.
Measured Beam axis 0.33, 0.34
17Simulated K Measurement
188.26 keV Transmission Grating
Sputter-sliced SiC / B4C multilayer
P 200 nm N 500 D 100 mm
Interference Function
33 mm thick
Single Slit Diffraction Pattern
Observed Intensity
100 mm
Beam
angle
19200 nm period x 33 microns works
33 mm
200 nm period
diffraction peaks in far-field
Waveguide coupling limits us to periods gt 200 nm
20Thick Slit 5 cm Ta capped with 1 cm B4C
FEL Transmission Grating Spectrometer
1 mm
50-100 micron
YAG Scintillator 50 microns thick
Thin Adjustable Slit 1 mm Ta
6 m
21Monte Carlo Generation of Photons from Near-Field
Calculations
Photons are aimed at Svens 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
22Bionta 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 - SignalBackgrounds with thin scintillator are at
least 11 - Beam stability and pointing (relative to the 100
micron aperture) will be an issue that is not
investigated here
23Work 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.
24Possible spectrometers
- Bent Bragg (after P. Siddons-NSLS)
- Mosaic crystal
- Bent Laue
- Zhong Zhong-NSLS
- X-ray Grating
- P. Heimann-ALS, S. Hulbert-NSLS
25Bent Bragg Spectrometer
Strip detector (200 strips)
76 mm
surface normal
Si (422)
2 mm
Cu foil
R3.9 m
26Und-pinhole distance 200 m Pinhole 2.0 x 0.02
mm2
On axis
0.5 mm
Photon energy (keV)
1.0 mm
27Bent Bragg to do list
- Simulation considering position dependent
spectrum - Role of jitter
- Test K sensitivity with simulated data (including
noise)
28Mosaic crystal spectrometer
180-2Q
2 x Mosaic spread 2Q
29Andreas Freund, Anneli Munkholm, Sean
Brennan, SPIE, 2856,68 (1996)
24 keV
10 keV
30Mosaic crystal to do list
- Crystal uniformity ?
- Ultimate resolution ?
- Experimental geometry (20 m crystal to detector
distance)
31Design criteria
- Goals
- Photon energy range 800 8000 eV.
- Spectral resolution Dl/l lt 1 x 10-3 set by the
FEL radiation bandwidth - Spectral window Dl/l gt 1 x 10-2 set by the
single undulator harmonic energy width - Single shot sensitivity for single undulator
spectra. - Consider damage for FEL radiation
32LCLS grating spectrometer layout
- One VLS grating in -1 order
- Length of spectrometer 1.3 m
33Raytracing of the grating spectrometer 8000 eV
7992 eV 8000 eV 8008 eV
- Source
- 90 mm diameter (fwhm)
- 7992, 8000, 8008 eV
- or 7600, 8000, 8400 eV
7600 eV 8000 eV 8400 eV
- At the detector
- 1.1 mm (h) x 2 mm (v) (fwhm)
- DE 14 eV (6x102 RP , limited by detector pixel
size 13 mm, in FEL case could use inclined
detector)
800 mm
34Is 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 DE/E 1/N 8.8 x10-3
- Divergence sr l/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
35Summary the Grating Spectrograph for the LCLS
- Photon energy range 800 8000 eV.
- Resolving power E/DE 2000 at 800 eV and 300
at 8 keV. - For FEL radiation the resolution could be
improved with an inclined detector. - Spectral window DE/E 10.
- Single shot sensitivity for single undulator
spectra.
36Workshop 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 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 RD activities, if any, needed before
the design of a measurement system can be
completed and manufacturing/procurement can start.
37Response to Charge
- Are the spectral features robust?
- Yes.
- Angle-integrated or pinhole?
- Whats 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 RD is needed?
- Create a PRD giving required specs