Experimental Results and Computational Modeling of Pulse Compression and High Gain at the VISA SASE FEL (and related topics)

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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

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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.

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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 (K1.26)

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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

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Undulator Diagnostics
Beam Profile Monitors
FEL Optical Diagnostics
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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?

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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

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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)
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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!

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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.

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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.

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Results of the Measurements

• Initial test indicated strong CTR signal
  dependence on linac RF phase.

Peaked SASE Signal

• Filter in/out comparison (R0.68) 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.

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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!
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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.
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Emittance Growth in Dispersive Section

• CSR effect on emittance is insignificant
• De?CSRgt 0.3 mm-mrad

Residual dispersion, nonlinearities dominate
e?Dp/pgt 7 mm-mrad
Slice emittance of the lasing beam core stays
below e?slicegt lt 4 mm-mrad
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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)
Q 200 pC IP 55 Amp Dp/p 0.05
(uncorrelated) e?(projected) 1 - 2 mm-mrad
(at FEL operating point) Dp/p 0.14 - 0.20
transmission 70 compression x 5 (CTR)
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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

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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

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SASE statistics and saturation

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

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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,nL
  g,1/n (Z. Huang, K-J Kim theory)

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

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The Neptune Compressor
Edge provides horizontal focusing (and steering)
22.5º bend angles

• Horizontally focusing edge angles fore and aft
• Mitigate vertical focusing, no cross-over in
  chicane

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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
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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

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Emittance Versus Linac Phase
Maximum compression
Sharp increase is a consistent feature in data
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Phase space reconstruction shows bifurcation
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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.

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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

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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.

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

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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

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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

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