Future UCLA Work in High Brightness Beams and FEL at BNL ATF: Compression and VISA II

1
Future UCLA Work in High Brightness Beams and FEL
at BNL ATF Compression and VISA II
James Rosenzweig UCLA Department of Physics and
Astronomy

• ICFA Sardinia
• July 4, 2002

2
VISA I

• VISA completed last year, measuring a large range
  of phenomena
• Exponential gain and saturation
• Statistics
• Angular, wavelength spectrum
• Microbunching with CTR
• Operating point was pathological
• Strong compression due to nonlinear properties of
  beamline 3
• Extremely sensitive system to rf phase jitter
• Information on initial longitudinal phase space
  lost
• Needed brave effort on experiment and
  start-to-end simulation to unfold
• Can we do a more linear experiment, with
  desired longitudinal phase space?
• Can we compress the beam in a more standard way?

3
UCLA Compressor _at_ ATF

• Compressor project begun 2.5 years ago to provide
  ATF with chicane
• ONR funding (C. Roberson)
• Extrapolation of Neptune/FTF experience to higher
  energies
• Brought need to model systems with CSR (VISA
  start-to-end!)
• CSR, longitudinal and transverse phase effects
• Status
• Magnets shipped 5/02
• Vacuum vessel ships now
• New funding!

4
Physics Goals of Compressor/VISA II

• Compress beam to 20 micron bunch length level
• Study CSR production directly in chicane
• Longitudinal phase space after chicane
• Transverse phase space diagnosis (a la Neptune?)
• Mitigate severe nonlinearity of bend beamline
• Diagnose beam completely at end of transport
• FEL studies with compressed beams
• Use FEL as diagnostic of compressor
• FEL studies with chirped, but uncompressed beams
• Impact LCLS schemes using chirping
• Study gain v. chirp
• Direct diagnosis of FEL longitudinal pulse
  characteristics

5
Compression experiments

• New funding from Roberson
• Very interested in CSR problems
• Personnel involved (JBR)
• Ron Agusstson
• Alex Murokh
• Sven Reiche
• Compression effects
• Tokyo polychromator
• Remove SDL ambiguities
• Energy spectrum measurements
• CSR instability
• What kind of beam do we expect?
• Can we take send this beam to the VISA undulator
  in useful form?

6
VISA I Review Beamline
(Beamline 3)
Insert compressor here

• 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)

7
Transport to Beamline 3

• Initially an FEL radiation pulse energy was
  measured 1-10 nJ, in agreement 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?

8
Saturation Measurements 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 believed beam parameters at linac exit

9
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)
10
Bunch compression in beamline 3

• High gain observed for running 4 degrees forward
  of crest horizontal beam size expands inside of
  undulator (dispersion error)
• 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.

-10

• 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
  horizontal/long. coupling
• Measure compression in final VISA runs!

11
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, mainly due to T512
12
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
Now we have a detailed, benchmarked model of the
beam sent into the undulator
13
Single Golay Cell Measurements

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

Peaked SASE Signal

• Low pass 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.

14
Comparison with CTR Measurements

• Manipulating the beam energy and chirp
  (equivalent to linac RF phase detuning) allowed
  reproduction of the bunch compression measured
  experimentally.

Simulated current
Simulated CTR from the ELEGANT beam current
output good agreement with measurement.
15
Start-to-end GENESIS simulations

• GENESIS output is in excellent agreement with FEL
  gain, angular profile

• Statistics of saturation also benchmarked with
  start-to-end mode
• Details count!!! Must know as much about phase
  space as possible.

16
Example 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
17
Compression experiments

• New funding from Roberson
• Very interested in CSR problems
• Personnel involved (JBR)
• Ron Agusstson
• Alex Murokh
• Sven Reiche
• F. Chou (Cline group)
• INFN-SPARC invited to join experiments
• What kind of beam do we expect?
• What are the interesting measurements?
• What can we do with this beam?

18
Note compressor and BL3 bends are orthogonal
ELEGANT study of compression/transport
19
Coherent radiation experiments

• Study CSR and CTR
• Coherent synchrotron radiation from bend 3/4
  edges
• Dedicated port
• Beam is shortest there!
• Enhanced radiation
• CTR after final dipole
• Wavelengths too short for interferometry? (SDL
  meas.)
• Spectrum of direct interest
• Tokyo polychromator
• Poor-mans version w/filters

CSR port, aimed at Dipole 4 entrance
20
Phase space measurements

• Momentum spectrum from horizontal bend
• Transverse phase space tomography
• Does the phase space bifurcate (S. Schreiber says
  yes)?
• Momentum (not time) resolved tomography

21
VISA-II prospects

• LCLS demands path to shorter (few fs!) pulse
  lengths
• Chirped beam/slicing
• Chirped beam/compression
• Proposed experiments using
• Chirped, compressed beam
• Chirped, uncompressed beam
• Compress FEL output. Use FROG to diagnose?
• ONR funding on the way for compressor/VISA-b this
  summer
• First job for FEL fix transport in BL3 to allow
  chirping
• Offset linear negative R56 (compressor setting)
• Correct T566

Saturation in 3 m with compressed beam (GENESIS)
22
Mitigation of Nonlinearity Originally Studied for
S-Bahn at Neptune
Simulation results after S-Bahn from Elegant
Without Sextupoles (T566 -1.9 m)
RAMPED BEAM
WITH Sextupoles (T566 0)
23
Sextupole correction of BL3 dynamics
Initial Phase Space
Final Phase Space (NO Sextupoles)
T566 -10 m
Final Phase Space (WITH Sextupoles)
T566 1.1 m
24
Sextupoles for Neptune and ATF

• Length 5cm Gap 4cm
• B0-0.000074 T
• K10.836m-2(Nept), 0.138m-2 (ATF)
• B0.034 T/m
• K2914 m-3(Nept), 152 m-3 (ATF)
• B 36.9 T/m2
• Op Current 100 Amps

25
Issues for VISA II

• What kind of energy spreads can be transported
• Better transport in BL3
• Need to develop methods to confirm linearity of
  longitudinal transport
• CTR
• Uncompressed beam can use streak camera
• Photon measurement really demands ways of
  measuring time structure
• Nonlinear harmonic crystal (crude measurement)
• SS autocorrelation with crystal
• FROG

26
People and time

• UCLA people
• Pellegrini, Rosenzweig
• Murokh, Reiche
• Agusstson, Andonian
• Collaborators
• BNL
• SLAC
• INFN-SPARC
• A. Tremaine (LLNL)
• Schedule
• Compressor begins now
• VISA inspection soon
• Sextupoles in queue