Summary Discussion

1
Summary Discussion
LCLS Beam-Based Undulator K Measurement Workshop
John Arthur SLAC
2
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.

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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.

4
Marking the location of a spectral edge

• We will watch
• how the following
• property changes
• HALF PEAK PHOTON ENERGY

5
Effects 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

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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.
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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.

8
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)

9
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 ???

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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

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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

• 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?

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Yang 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.

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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).

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?

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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.

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Basic Scheme
Basic Layout
Slit width must be small to get clean signal. 2
mm shown.
Useg 1 is worst case
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Aligning 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
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Simulated K Measurement
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8.26 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
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200 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
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Thick 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
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Monte 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
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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
• 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

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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.

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Possible spectrometers

• Bent Bragg (after P. Siddons-NSLS)
• Mosaic crystal
• Bent Laue
• Zhong Zhong-NSLS
• X-ray Grating
• P. Heimann-ALS, S. Hulbert-NSLS

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Bent Bragg Spectrometer
Strip detector (200 strips)
76 mm
surface normal
Si (422)
2 mm
Cu foil
R3.9 m
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Und-pinhole distance 200 m Pinhole 2.0 x 0.02
mm2
On axis
0.5 mm
Photon energy (keV)
1.0 mm
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Bent Bragg to do list

• Simulation considering position dependent
  spectrum
• Role of jitter
• Test K sensitivity with simulated data (including
  noise)

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Mosaic crystal spectrometer
180-2Q
2 x Mosaic spread 2Q
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Andreas Freund, Anneli Munkholm, Sean
Brennan, SPIE, 2856,68 (1996)
24 keV
10 keV
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Mosaic crystal to do list

• Crystal uniformity ?
• Ultimate resolution ?
• Experimental geometry (20 m crystal to detector
  distance)

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Design 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

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LCLS grating spectrometer layout

• One VLS grating in -1 order
• Length of spectrometer 1.3 m

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Raytracing 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
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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 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
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Summary 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.

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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 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.

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Response 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