CSR Workshop - Zeuthen January, 17 2002

1. Experimental Results and Computational Modeling of Pulse Compression and High Gain at the VISA SASE FEL (and related topics) James Rosenzweig UCLA Department of Physics and Astronomy CSR Workshop - Zeuthen January, 17 2002

2. Acknowledgments • VISA is a large collaboration (BNL, LLNL, SLAC, UCLA). C. Pellegrini (UCLA) is spokesman • UCLA was lead on experimental data-taking and analysis • Aaron Tremaine (post-doc, ex-UCLA student). Experiment. • Alex Murokh (student). Experiment. • Ron Agusstson (student). ELEGANT simulation (originally for ATF compressor expt.) • Sven Reiche (post-doc). GENESIS • JBR (Expt. diagnosis; simulations), CP (theory) • Stealth collaborators in expt/data analysis: P. Emma, H-D. Nuhn • Extremely difficult experiment to perform and understand.

3. VISA Beamline • Gun and Linac Section (1.6 cell photo-emission gun and 2 SLAC type linac structures operating at S-Band, generate 71 MeV beam) • 20° double-bend dispersive transport section • Beamline III, with VISA matching optics and 4-m strong focusing undulator (K=1.26)

4. Measurements on Electron Beam at Linac Exit • Emittance was measured with the quad scan after the linac. For a typical charge of 200-500 pC emittance was optimized at • The beam current in the linac was measured by applying a linear chirp to the beam and measuring its profile after 20° bend. With wake-field correction the current value is found:

5. Beam Profile Monitors 8 diagnostic ports Undulator Diagnostics Beam Profile Monitors FEL Optical Diagnostics

6. new tune old tune Tune “Optimization” • Initially an FEL radiation pulse energy was measured ~ 1-10 nJ,in accord with the measured beam brightness. • In the attempt to compensate for the dispersion, a new tune was developed: • With the new tune the FEL radiation intensity went up to ~ 10 µJ. Why?

7. M. Xie numerical model: 18.7 cm gain length at 140 pC charge and 50 Amp peak current corresponds to the sliced emittance of <0.35 mm-mrad Saturation and Physical Model • With the high gain an FEL saturation in 3.6 m was observed: Lg = 18.7 cm • How does the gain length measurement agree with the high gain SASE-FEL theory? Not that well if we believe beam parameters at linac exit…

8. very unstable, 100% fluctuations stable condition low gain high gain G ~ 103 G ~ 107 More inconsistencies in the data • Highest gain observed after changing rf phase of linac • Change of the tune significantly altered all SASE radiation properties, indicating changes of basic electron beam properties: many spikes spike width ~ 0.1% centered at 830 nm single spike spike width ~ 1% centered at 845 nm (old tune) (new tune)

9. 4/30/01: Bunch compression hypothesis • High gain observed for running ~4-5 degrees forward of crest; horizontal beam size expands inside of undulator • Strong bunch compression in the dispersive section was suggested, due to mistuning of linac energy from the nominal value. Effective R56 can change sign, order of magnitude due to T566, off energy operation. • Increase in peak current reduces FEL gain length, explains the observed spectral behavior (watch for egrowth due to dispersion mismatch…) • Longitudinal transformation highly nonlinear • Measure compression in final VISA runs!

10. e-beam 1 0.9 90° polarizer 0.8 0.7 FEL light 0.6 Transmission 0.5 Golay cell 0.4 parabolic mirrors 0.3 0.2 0.1 0 removable low-pass filter 0 20 40 60 Wavenumber [cm-1] Single Golay Cell Experimental Set-up System allows following measurements: • Scanning linac RF phase and observe CTR signal (test for a possible bunch compression). • Inserting a remote controlled low-pass filter for a quantitative measure of a bunch length when compared to PARMELA/ELEGANT model.

11. sx [mm] Using Collimator to Map Linac RF Phase • To understand the nature of the compression, one has to keep a track of the linac RF phase jitter. 1.5 cm aperture • It was found that the bending dipole to ATF Beamline 1 acts as a scraper with the 1.5 cm aperture. • Charge loss at the scraper depends on the beam energy and is very sensitive to changes in the RF phase. Measuring the charge loss at the collimator allows to • Calibrate the linac RF phase shot-by-shot. • Use the same system operating point for FEL measurements.

12. Results of the Measurements • Initial test indicated strong CTR signal dependence on linac RF phase. Peaked SASE Signal • Filter in/out comparison (R=0.68) indicated short (sub-40 µm) bunch length. • Ratio measurement at the operating point established a benchmark for the PARMELA/ELEGANT numerical model of the system.

13. Low energy tail of the beam lost at the collimator PARMELA/ELEGANT Analysis • PARMELA reproduced the beam properties measured after the linac, and ELEGANT simulated bunch compression in the double-bend line. • ELEGANT is input off-design energy, with appropriate chirp for high gain case PARMELA output after linac ELEGANT output after dispersive section (no collimation). Note width!

14. Comparison with CTR Measurements • Manipulating the beam energy and chirp (equivalent to linac RF phase detuning) allowed to reproduce the bunch compression measured experimentally. Simulated CTR from the ELEGANT beam current output; good agreement with measurement.

15. Emittance Growth in Dispersive Section CSR effect on emittance is insignificant : DeCSR> ~ 0.3 mm-mrad Residual dispersion, nonlinearities dominate: eDp/p> ~ 7 mm-mrad Slice emittance of the lasing beam core stays below eslice> < 4 mm-mrad

16. SASE Diagnostics Golay cell Complete Set of Data for Simulations GENESIS ELEGANT PARMELA at peak lasing: LG ~ 18.5 cm LSAT ~ 3.6-3.8 m ESAT ~ 20 µJ D ~ 1.2 % (single spike) (at FEL operating point) Dp/p ~ 0.14 - 0.20 % transmission ~ 70 % compression ~ x 5 (CTR) Q ~ 200 pC IP ~ 55 Amp Dp/p ~ 0.05 % (uncorrelated) e(projected) ~ 1 - 2 mm-mrad

17. Constraints of start-to-end model • PARMELA must reproduce conditions at end of linac • Measured emittance, charge, energy, energy spread • ELEGANT fed PARMELA output, exact quad settings • ELEGANT output benchmarked by measurements • CTR bunch length • Beam size (dispersive emittance growth) • RF phase • GENESIS input from ELEGANT output • GENESIS must reproduce FEL results • Gain length, saturation • Angular and wavelength spectra • Higher harmonic gain and bunching • RF phase dependence • This effort took six months…

18. Measured angular profile GENESIS simulations GENESIS simulations: main results • GENESIS output is in excellent agreement with FEL gain, angular profile • Statistics of saturation also benchmarked with start-to-end model

19. SASE statistics and saturation • In exponential gain, statistics are consistent with single spike model • In saturation, picture changes radically in data and model

20. Extended work for model: SASE harmonics • Fundamental saturation allows deep beam modulation - harmonics • “Nonlinear gain” observed on 2nd and 3rd harmonics • Gain profiles consistent with scaling Lg,n=L g,1/n (Z. Huang, K-J Kim theory)

21. Microscopic view: CTR microbunching v. SASE Another detailed benchmark with UCTR

22. Effect of CSR on compressed beam • Beam bunch length is T516/T526/emittance limited (emittance must be ~2 mm-mrad) • CSR provides energy loss mechanism during bends • This can interact with the T516/T526 terms to produce longer beam • No CSR case has 300 A, not 250 A - GENESIS gain is far too large. No CSR CSR Correlated cut due to collimator, T516/T526 Width set by T516/T526

23. Future CSR experiments: expected signatures • UCLA fabricating compressor for BNL ATF • Very short beams possible • CSR power measured with Golay cell and filters • Momentum spectrum • Transverse phase space tomography. Why? ELEGANT simulation through chicane and beamline 1

24. Previous experience: bunch compression at Neptune • Neptune = UCLA advanced accelerator laboratory (photoinjector/laser) • Short beams needed for wakefield (source), beatwave (probe) experiments • Relatively low energy system • 12 MeV maximum • Concentrates on velocity fields • Components of compression system • Hardware • Linac + chicane (lens + drift) • Pulse Length Diagnostic • CTR measurement of subpicosecond bunches • Emittance Diagnostic • Current increase at what cost? • Beam physics in the compressor: phase space monitoring

25. The Neptune Compressor • Horizontally focusing edge angles fore and aft • Mitigate vertical focusing, no cross-over in chicane Edge provides horizontal focusing (and steering) 22.5º bend angles

26. CTR interferometry for pulse length • Data gives filtered autocorrelation of the temporal beam profile. Need to take into account “missing” long wavelengths • Short beam, some ancillary structures • Near resolution limit Interferogram for shortest pulse length

27. Emittance Growth in the Compressor • The compressor, pulse length, and emittance diagnostics allow us to examine the issue of emittance growth in bends. • In particular, the slit based measurement permits us to view the the evolution of the transverse phase space as the emittance increases. • Experimental Procedure: • Set bend angle to design value of 22.5°, keep R56 constant • Measure linac phase and pulse length, map compression • Vary phase and measure emittance

28. Emittance Versus Linac Phase Maximum compression Sharp increase is a consistent feature in data

29. Phase space reconstruction shows bifurcation

30. Simulation of experiment • Different codes model different processes (acceleration fields versus velocity fields.) • Codes employed: • TREDI: Full story, but noisy.. • PARMELA: Provides input distributions for TREDI. Point-to-point space charge for comparison, no acceleration fields. Noisy. • ELEGANT: only acceleration fields, approximate. • Heuristic calculation of space-charge between longitudinal slices. • Initial simulations indicate that for this experiment, acceleration fields do not contribute much emittance growth, the space charge fields are the dominant effect.

31. Simulation results • Simulation is difficult. Number of macro-particles is low because of time-intensive space-charge calculations. • Sharp emittance increase when “fold over” begins is missing in simulations. • Improve existing tools, use heuristic model

32. Heuristic analysis • To analyze the effect of space-charge in the compressor, we model the beam as a series of longitudinal slices. • Since the beam energy spread is heavily correlated to slice position, we assume that there is no energy spread within a single slice • Space-charge forces push a slice based on the fields at its centroid due to the other slices. • Use standard envelope equations to evolve the sizes of single slices.

33. Evolution Without Space-charge Beam “folds over” in configuration space. Configuration Space Long. Phase Space

34. Effect of space-charge in the model • Slices repel strongly in (and after) the last magnet • This destroys the dispersion cancellation at the compressor exit ( & ’  0) • Space-charge + dispersion grows emittance after the compressor as well

35. Slice Model Simulation • With space-charge beam “fold over” is not perfect as seen in configuration space. • In phase space, this shows up as a bifurcation • We see evidence for a two-peak initial longitudinal profile. Presently adding to this to all simulations, expect enhanced bifurcation

36. Summary and conclusions • Proper understanding of compression and beam performance requires large effort in diagnosis and simulation — in tandem • At 70 MeV, ELEGANT/GENESIS combination very robust • “Pathological” running conditions at VISA explained • Some verification of CSR importance at VISA • Computational tools are developing to meet experimental demands • The more details of beam 6D phase space revealed, the better • FEL is excellent “phase space diagnostic” • Phase space tomography is high energy analogue of slits • CSR spectrum should also be very useful • High brightness beams have a wealth of applications, equal wealth of problems to solve…