csr workshop - zeuthen january, 17 2002

January 17, 2002 J. Rosenzweig

Experimental Results and Computational Modeling of Pulse Compression and High Gain

at the VISA SASE FEL (and related topics)

CSR Workshop - Zeuthen January, 17 2002

James RosenzweigUCLA Department of Physics and Astronomy


AcknowledgmentsAcknowledgments

• 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.


VISA BeamlineVISA 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)

Linac Sections

Gun

Transport

20° Dispersive SectionMatching Line

Undulator


Measurements on Electron Beam at Linac ExitMeasurements 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

n 1.3 2.7 mn 1.3 2.7 m

• 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:

Ip 55 AmpIp 55 Amp


Beam Profile Monitors

8 diagnostic ports

e-beam

vacuum chamber

actuator

OTR to CCD

FEL light out

Undulator DiagnosticsUndulator Diagnostics

FEL Optical Diagnostics

Beam Profile Monitors


Tune “Optimization”Tune “Optimization”

• Initially an FEL radiation pulse energy was measured ~ 1-10 nJ, in accord with the measured beam brightness.

-300.0000

-200.0000

-100.0000

0.0000

100.0000

200.0000

300.0000

400.0000

0 5 10 15 20 25

x-beta [m]

dispersion [cm]

y-beta [m]

old tune-300.0000

-200.0000

-100.0000

0.0000

100.0000

200.0000

300.0000

400.0000

0 5 10 15 20 25

x-beta [m]

dispersion [cm]

y-beta [m]

new tune

• With the new tune the FEL radiation intensity went up to ~ 10 µJ. Why?

• In the attempt to compensate for the dispersion, a new tune was developed:


Saturation and Physical ModelSaturation and Physical Model

• With the high gain an FEL saturation in 3.6 m was observed:

• 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…

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

Lg = 18.7 cm


(old tune) (new tune)

More inconsistencies in the data 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:

800 820 840 860 880 900

G ~ 103

many spikesspike width ~ 0.1% centered at 830 nm

800 820 840 860 880 900

G ~ 107

single spikespike width ~ 1% centered at 845 nm

low gain stable condition high gainvery unstable, 100% fluctuations


4/30/01: Bunch compression hypothesis4/30/01: 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 growth due to dispersion mismatch…)

• Longitudinal transformation highly nonlinear• Measure compression in final VISA runs!

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

-0.02 -0.01 0 0.01 0.02

R56

dp/p

z f zi R56p

p

1

2T566

p

p

2

z f zi R56p

p

1

2T566

p

p

2

T566 R56

p p 7 m rad2T566 R56

p p 7 m rad2

R56 p0 0.15 m


Single Golay Cell Experimental Set-upSingle Golay Cell Experimental Set-up

System allows following measurements:1. Scanning linac RF phase and observe CTR signal (test for a possible

bunch compression).2. Inserting a remote controlled low-pass filter for a quantitative

measure of a bunch length when compared to PARMELA/ELEGANT model.

e-beam

Golay cell

90° polarizer

parabolic mirrors

FEL light

Wavenumber [cm-1]

Tra

nsm

issi

on

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

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0.9

1

0 20 40 60

40 µm

detector window cut-off

100 µm

400 µm

removable low-pass filter

filter transmission


Using Collimator to Map Linac RF PhaseUsing 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.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 5 10 15 20 25

x [mm]

• 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 to1. Calibrate the linac RF phase shot-by-shot.2. Use the same system operating point for FEL measurements.

1.5 cm aperture


Results of the Measurements Results of the Measurements

0

0.005

0.01

0.015

0.02

0.05 0.1 0.15 0.2 0.25 0.3

Linac RF phase span of 2°

Charge [nC]

Goa

ly S

igna

l [m

V]

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

0

0.02

0.04

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0.08

0.1

0

0.2

0.4

0.6

0.8

1

0.1 0.12 0.14 0.16 0.18 0.2

without/filter

with/filter

filter/fit

no filter/fit

ratio

Charge [nC]

• Filter in/out comparison (R=0.68) indicated short (sub-40 µm) bunch length.

Peaked SASE Signal

• Ratio measurement at the operating point established a benchmark for the PARMELA/ELEGANT numerical model of the system.


PARMELA/ELEGANT AnalysisPARMELA/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!

Low energy tail of the beam lost at the

collimator


Comparison with CTR MeasurementsComparison 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.

0

50

100

150

200

250

-800 -600 -400 -200 0 200 400 600 800

I (A

)

z (m)

After injector

After beamline 3


Emittance Growth in Dispersive SectionEmittance Growth in Dispersive Section

CSR effect on emittance is insignificant :

CSR> ~ 0.3 mm-mrad

Residual dispersion, nonlinearities dominate: p/p> ~ 7 mm-mrad

Slice emittance of the lasing beam core stays below

slice> < 4 mm-mrad


Complete Set of Data for SimulationsComplete Set of Data for Simulations

Q ~ 200 pCIP ~ 55 Ampp/p ~ 0.05 % (uncorrelated)(projected) ~ 1 - 2 mm-mrad

at peak lasing:LG ~ 18.5 cmLSAT ~ 3.6-3.8 mESAT ~ 20 µJ ~ 1.2 % (single spike)

Linac Sections

Gun

Transport

20° Dispersive SectionMatching Line

Undulator

Golay cell

SASE Diagnostics

(at FEL operating point)p/p ~ 0.14 - 0.20 % transmission ~ 70 %

compression ~ x 5 (CTR)


Constraints of start-to-end modelConstraints 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…


GENESIS simulations: main resultsGENESIS simulations: main results

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

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

simulationsmeasurements

SA

SE

Int

ensi

ty [

µJ]

z [m]

• Statistics of saturation also benchmarked with start-to-end model

Measured angular profile

GENESIS simulations


SASE statistics and saturationSASE statistics and saturation

• In exponential gain, statistics are consistent with single spike model

• In saturation, picture changes radically in data and model


Extended work for model: SASE harmonicsExtended 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)

0

50

100

150

200

250

300

240 320 400 480 560 640 720 800 880

Cou

nt (

Am

plitu

de)

Wavelength (nm) 10-2

10-1

100

101

102

103

104

105

2 2.5 3 3.5 4

Harmonic Energy vs. Distance

Ene

rgy

(nJ)

z(m)

3rd harmonic

2nd harmonic

Fundamental


Microscopic view: CTR microbunching v. SASEMicroscopic view: CTR microbunching v. SASE

0.2

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1

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2nd Harmonic Bunchingvs. SASE

CT

R (

pJ)

SASE (J)

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30 35

Fundamental Microbunching vs. SASE

CT

R (

pJ)

SASE (J)

b22 exp z /Lg,2 exp 2z /Lg,1 b1

4

Another detailed benchmark with UCTR

Un N 2e2bn

2

8 x y z

nkr

41 x

2 1 y

2


Effect of CSR on compressed beamEffect 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.

CSR No CSR

Width set byT516/T526

Correlated cutdue to collimator, T516/T526


Future CSR experiments: expected signaturesFuture 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


Previous experience: bunch compression at NeptunePrevious 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 monitoringBeam physics in the compressor: phase space monitoring


The Neptune CompressorThe Neptune Compressor

2'-4"

Edge provideshorizontal focusing

(and steering)22.5º bend angles

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


CTR interferometry for pulse lengthCTR interferometry for pulse length

• Data gives filtered filtered autocorrelation of the temporal beam profile. Need to take into account “missing” long wavelengths

• Short beam, some ancillary structures

• Near resolution limit0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

10 12 14 16 18 20 22

Autcorrelation Data

Nor

mal

ized

Sig

nal

Delay [psec]

t = 0.63 ps

Interferogram for shortest pulse length


Emittance Growth in the CompressorEmittance 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


Emittance Versus Linac PhaseEmittance Versus Linac Phase

5

10

15

20

25

55 60 65 70 75 80 85 90

Norm

aliz

ed

Em

itta

nce [

mm

mra

d]

Linac Phase [deg]

Sharp increase is a consistent feature in

data

Maximum compression


Phase space reconstruction shows bifurcationPhase space reconstruction shows bifurcation


Simulation of experimentSimulation 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.


Simulation resultsSimulation results

0

5

10

15

20

25

55 60 65 70 75 80 85 90

DataPARMELATREDI

Em

itta

nce

[mm

mra

d]

Linac Phase [deg.]

• 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


Heuristic analysisHeuristic 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.


Evolution Without Space-chargeEvolution Without Space-charge

Configuration Space Long. Phase Space

Beam “folds over” in configuration space.


Effect of space-charge in the modelEffect of space-charge in the model

0

5

10

15

20

25

260 280 300 320 340 360 380 400 420

Parmela Simulation

No

rma

liz

ed

Em

itta

nc

e [

mm

mra

d]

Z [cm]

0

5 10 -6

1 10 -5

1.5 10 -5

2 10 -5

2.5 10 -5

3 10 -5

-0.2 0 0.2 0.4 0.6 0.8 1 1.2

Slice Model

No

rma

liz

ed

Em

itta

nc

e [

m r

ad

]

Z [m]

• 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


Slice Model SimulationSlice 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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-4

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0

1

2

3

-1 -0.5 0 0.5

Trace Space

X' [

mra

d]

X [mm]

-1

-0.5

0

0.5

-1.5 -1 -0.5 0 0.5

Configuration Space

X [m

m]

Z [mm]


Summary and conclusionsSummary 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…

csr workshop - zeuthen january, 17 2002

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