Summary Discussion - PowerPoint PPT Presentation

About This Presentation
Title:

Summary Discussion

Description:

Title: Slide 1 Author: heleno Last modified by: John Arthur Created Date: 4/22/2004 6:04:17 PM Document presentation format: On-screen Show Company – PowerPoint PPT presentation

Number of Views:57
Avg rating:3.0/5.0
Slides: 38
Provided by: hele2201
Category:

less

Transcript and Presenter's Notes

Title: 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.

3
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

6
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.
7
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 ???

10
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

11
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?

12
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.

13
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?

14
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.

15
Basic Scheme
Basic Layout
Slit width must be small to get clean signal. 2
mm shown.
Useg 1 is worst case
16
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
17
Simulated K Measurement
18
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
19
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
20
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
21
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
22
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

23
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.

24
Possible spectrometers
  • Bent Bragg (after P. Siddons-NSLS)
  • Mosaic crystal
  • Bent Laue
  • Zhong Zhong-NSLS
  • X-ray Grating
  • P. Heimann-ALS, S. Hulbert-NSLS

25
Bent Bragg Spectrometer
Strip detector (200 strips)
76 mm
surface normal
Si (422)
2 mm
Cu foil
R3.9 m
26
Und-pinhole distance 200 m Pinhole 2.0 x 0.02
mm2
On axis
0.5 mm
Photon energy (keV)
1.0 mm
27
Bent Bragg to do list
  • Simulation considering position dependent
    spectrum
  • Role of jitter
  • Test K sensitivity with simulated data (including
    noise)

28
Mosaic crystal spectrometer
180-2Q
2 x Mosaic spread 2Q
29
Andreas Freund, Anneli Munkholm, Sean
Brennan, SPIE, 2856,68 (1996)
24 keV
10 keV
30
Mosaic crystal to do list
  • Crystal uniformity ?
  • Ultimate resolution ?
  • Experimental geometry (20 m crystal to detector
    distance)

31
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

32
LCLS grating spectrometer layout
  • One VLS grating in -1 order
  • Length of spectrometer 1.3 m

33
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
34
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
35
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.

36
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.

37
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
Write a Comment
User Comments (0)
About PowerShow.com