Summary Discussion

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

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.

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.

Marking the location of a spectral edge We will watch how the following property changes: • HALF PEAK PHOTON ENERGY

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

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.

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.

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)

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 ???

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

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 < 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?

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 DK/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, DK/K =  3  10-6 or better. It is sufficient for XFEL applications. • It can also be used for routinely logging magnet degradation.

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?

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.

Basic Scheme Basic Layout Slit width must be small to get clean signal. 2 mm shown. Useg #1 is worst case

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

Simulated K Measurement

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

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

1st Order Diffraction Peaks Transmission Grating 200 nm Period 100 micron Aperture 4 mm 5 m Very high energy photons go through everything 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

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

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

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.

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

Bent Bragg Spectrometer Strip detector (200 strips) 76 mm surface normal Si (422) 2 mm Cu foil R=3.9 m

Und-pinhole distance 200 m Pinhole 2.0 x 0.02 mm2 On axis +0.5 mm Photon energy (keV) +1.0 mm

Bent Bragg to do list Simulation considering position dependent spectrum Role of jitter Test K sensitivity with simulated data (including noise)

Mosaic crystal spectrometer 180-2Q 2 x Mosaic spread 2Q

Andreas Freund, Anneli Munkholm, Sean Brennan, SPIE, 2856,68 (1996) 24 keV 10 keV

Mosaic crystal to do list Crystal uniformity ? Ultimate resolution ? Experimental geometry (20 m crystal to detector distance)

Design criteria • Goals • Photon energy range: 800 – 8000 eV. • Spectral resolution: Dl/l < 1 x 10-3 set by the FEL radiation bandwidth • Spectral window Dl/l > 1 x 10-2 set by the single undulator harmonic energy width • Single shot sensitivity for single undulator spectra. • Consider damage for FEL radiation

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

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

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

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.

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 R&D activities, if any, needed before the design of a measurement system can be completed and manufacturing/procurement can start.

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

Summary Discussion