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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. VISA I. VISA completed last year, measuring a large range of phenomena Exponential gain and saturation Statistics

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future ucla work in high brightness beams and fel at bnl atf compression and visa ii

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

visa i
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?
ucla compressor @ atf
UCLA Compressor @ 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!
physics goals of compressor visa ii
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
compression experiments
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?
visa i review beamline
VISA I Review: Beamline

(Beamline 3)

  • 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)

Insert compressor here

transport to beamline 3

new tune

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?
saturation measurements and physical model

M. Xie numerical model:

18.7 cm gain length at 140 pC charge and ~50 Amp peak current corresponds to slice emittance of <0.35 mm-mrad

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…
more inconsistencies in the data

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)

bunch compression in beamline 3
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!
parmela elegant analysis

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, mainly due to T512

emittance growth in dispersive section
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

Now we have a detailed, benchmarked model of the

beam sent into the undulator

single golay cell measurements
Single Golay Cell Measurements
  • Initial test indicated strong CTR signal dependence on linac RF phase.

Peaked

SASE Signal

  • Low pass 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.
comparison with ctr measurements
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.

start to end genesis simulations

Measured angular profile

GENESIS simulations

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
  • Detailscount!!!Must know as much about phase space as possible.
example effect of csr on compressed beam
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

compression experiments17
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?
slide18

Note: compressor and BL3 bends are orthogonal

ELEGANT study of compression/transport

coherent radiation experiments
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-man’s” version w/filters

CSR port, aimed at

Dipole 4 entrance

phase space measurements
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
visa ii prospects
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)

slide22

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)

slide23

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

sextupoles for neptune and atf
Length = 5cm; Gap = 4cm

B0=-0.000074 T ;

K1=0.836m-2(Nept), 0.138m-2 (ATF)

B’=0.034 T/m

K2=914 m-3(Nept), 152 m-3 (ATF)

B’’ = 36.9 T/m2

Op Current = 100 Amps

Sextupoles for Neptune and ATF
issues for visa ii
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
people and time
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