Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Patrick KrejcikR. Akre, P. Emma, M. Hogan, (SLAC),

H. Schlarb, R. Ischebeck (DESY), P. Muggli (USC Los Angeles),

A. Cavalieri (University of Michigan)

Sub-Picosecond Electron Bunch Length Measurements at SLAC

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Motivation behind ultra-short electron bunches

Compressing electron bunches from a linac to reach very high peak currents (kAmps)

Enables them to lase in a long undulator

4th generation light sources: LCLS, TESLA …

Ultra-short pulse for probing experiments down to femtosecond level

Short pulse x-rays SubPicosecond Pulsed Souce, SPPS

Advanced accelerator studiesPlasma wakefield acceleration

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Diagnostics Challenge for Measuring Sub-Picosecond Bunch Length

The SPPS e- bunch is 80 fwhm (12 m rms)Conventional streak camera technology ~1/2 ps

Ideally look for resolution <100 fs

Single pulse measurement important in linacs

Reconstruction of bunch length charge profile

Fast, relative measurements for feedback control

timing measurement relative to fs laser

Diagnose new instabilities – microbunching instability

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Bunch length diagnostic comparisonDevice Type Invasive

measurementSingle shot measurement

Abs. or rel. measurement

Timing measurement

Detect bunchin

g

RF Transverse Deflecting Cavity

Yes: Steal 3 pulses

No: 3 pulses Absolute No No

Coherent Radiation Spectral power

No for CSR Yes for CTR

Yes Relative No Yes

Coherent RadiationAutocorrelation

No for CSR Yes for CTR

No Absolute(2nd moment

only)

No No

Electro Optic Sampling

No Yes Absolute Yes No

Energy Wake-loss

Yes No Relative No No

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Relative bunch length measurement at SPPSbased on wakefield energy loss scan

Relative bunch length measurement at SPPSbased on wakefield energy loss scan

Energy change measured at the end of the linac

as a function of the linac phase (chirp) upstream of the compressor chicane

Shortest bunch has greatest energy loss

Predicted wakeloss___(P. Emma)

For bunch length z __

Predicted shape due to wakeloss plus RF curvature

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Bunch Length Measurements with the RF Transverse Deflecting CavityBunch Length Measurements with the RF Transverse Deflecting Cavity

yy

Asymmetric parabola indicates incoming tilt to beam

Y = A * (X - B)**2 + CA = 1.6696E-02 STD DEV = 1.3536E-03B = 28.23 STD DEV = 3.084C = 1328. STD DEV = 8.235RMS FIT ERROR = 23.63

-80 -40 0 40 80

SBST LI29 1 PDES (S-29-1)

1.7

1.6

1.5

1.4

1.3

X103

****

***

**

**

*

****

********

-80 -40 0 40 80

SBST LI29 1 PDES (S-29-1)

1.7

1.6

1.5

1.4

1.3

X103

E

-80 -40 0 40 80

SBST LI29 1 PDES (S-29-1)

1.7

1.6

1.5

1.4

1.3

X103

-80 -40 0 40 80

SBST LI29 1 PDES (S-29-1)

1.7

1.6

1.5

1.4

1.3

X103

-80 -40 0 40 80

SBST LI29 1 PDES (S-29-1)

1.7

1.6

1.5

1.4

1.3

X103

-80 -40 0 40 80

SBST LI29 1 PDES (S-29-1)

1.7

1.6

1.5

1.4

1.3

X103

E

MANUAL STEPPING. STEPS = 30

1-APR-03 20:21:16

Cavity on

Cavity off

Cavity on- 180°

Bunch length reconstructionMeasure streak at 3 different phases

z = 90 m

(Str

eak

size

)2

0 180

2.4 m 30 MW

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Calibration scan for RF transverse deflecting cavity

Beam centroid[pixels]

Cavity phase [deg. S-Band]

• Bunch lenght calibrated in units of the wavelength of the S-band RF

Further requirements for LCLS:

•High resolution OTR screen•Wide angle, linear view optics

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

OTR Profile Monitor in combination withRF Transverse Deflecting Cavity

Simulated digitized video image

Injector DL1 beam line is shown

Best resolution for slice energy spread measurement would be in adjacent spectrometer beam line.

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

4 THz

LCLS BC2 Bunch length monitor spectrum

BC2 bunch length feedback requires THz CSR detector

Demonstrated with CTR at SPPS

Bunch profile

200 fs

Bunch spectrum

>>z

THz spectromete

r

THz power detector

B4 Bend

Bunch Compressor Chicane

CSR

Vacuum port with reflecting foil

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Far-Infrared Detection of Wakefields from Ultra-Short Bunches

Wakefield diffraction radiation wavelength comparable

to bunch length

Pyroelectric detector

foilLINAC

FFTB

Comparison of bunch length minimized according to

wakefield loss and THz power

GADC

-26 -24 -22 -20 -18 -16 -14 -120

100

200

300

400

500

600P

yrom

eter

sig

nal [

arb.

uni

ts]

linac phase offset from crest [deg. S-Band]

FFTB Pyrometer Signal

-26 -24 -22 -20 -18 -16 -14 -12200

250

300

350

400

450

500

ener

gy lo

ss [

MeV

]

Linac Wake Loss

Linac phase

Wake energy loss

THz power

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

400 GHz1.2 mm

BC1 Bunch Length Monitor

CSR Power spectral density signal for bunch length feedback

CSR Power spectral density signal for bunch length feedback

Spectral lines accompanying micro-bunching instability

– Z. Huang.

Spectral lines accompanying micro-bunching instability

– Z. Huang.

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Dither feedback control of bunch length minimization - L. Hendrickson

Dither time steps of 10 seconds

Bunch length monitor response Feedback correction

signal

Linac phase

“ping”

optimum

Jitter in bunch length signal over 10 seconds ~10% rms

Jitter in bunch length signal over 10 seconds ~10% rms

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Transition radiation is coherent at wavelengths longer than the bunch length,>(2)1/2 z

P. Muggli, M. Hogan

Limited by long wavelength cutoff

Interferometer for autocorrelation of CTR

12 m rms

e-

VariablePositionMirror

Interferometer Pyro Detector

12.5 µm MylarBeam Splitters

RT≈0.17

12.5 µm MylarVacuum Window

(3/4” dia)

Ref. Pyro Detector

Alignment Laser

1 µm Titanium Foil

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

0

10

20

30

40

0

0.2

0.4

0.6

0.8

1

1.2

-300 -200 -100 0 100 200 300

CTRautocSigmaz12.7_3

Au

toco

rre

latio

n w

/o F

ilte

r (a

.u.)

Au

toco

rrela

tion

with

Filte

r (a.u

.)

Delay (µm)

z=14 µm

z=9 µm10-17

10-15

10-13

10-11

10-9

10-7

10-5

0.001

0.1

10

10-4

10-3

10-2

10-1

100

10 100 1000CTRFSpecSigmaz20Mylar12.5_3

Po

we

r S

pe

ctru

m (

a.u

.) F

ilter A

mp

litue

(a.u

.) Wavelength (µm)

Mylarresonances

Gaussian, z=20 µm, d=12.7 µm, n=3 Mylar window+splitter

Effect of Mylar Window and Beam Splitter

• Smaller measured width:

Autocorrelation < bunch !

• Modulation/dips in the interferogram

Simple model:

• Fabry-Perot resonance: =2d/m, m=1,2,…

• Signal attenuated by Mylar: (RT)2 per sheet

• Fabry-Perot resonance: =2d/m, m=1,2,…

• Signal attenuated by Mylar: (RT)2 per sheet

P. Muggli, M. Hogan

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Electro Optic Bunch Length Measurement

Probe laser

Defining aperture

Beam axis

M1 M2EO xtal

Geometry chosen to measure direct

electric field from bunch, not wakefieldModelled by H. Schlarb

electrons

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Resolution limit in temporal-to-spectral translation

0res CT T TBW limited pulse Short chirp

Long chirp

Temporal profile

Spectral profiles

However, recent work shows this limit can be overcome with noncollinear cross correlation of the light before and after the EO crystal

S.P. Jamison, Optics Letters, 28, 1710, 2003

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Er

ErP

Elevation view End view

Plan view

electrons

EO Xtal

Temporal to spatial geometry under test at SPPS

Er

Principal oftemporal-spatial

correlation

Line image camera

polarizer

analyzer

xtal

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Jitter determination from Electro Optic sampling

Er

Principal oftemporal-spatial correlation

Line image camera

polarizer

analyzer

EO xtal

seconds, 300 pulses: z = 530 fs ± 56 fs rms t = 300 fs rmsseconds, 300 pulses: z = 530 fs ± 56 fs rms t = 300 fs rms

single pulse

A. Cavalieri

centroidwidth

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

EO resolution limit due to wakefields – H. Schlarb

r

e-

•Apparent change in z when measured at

increasing radii relative to the aperture from the edge of the laser mirror

• negligible perturbation if EO crystal is closer to beam than mirror edge.

•Apparent change in z when measured at

increasing radii relative to the aperture from the edge of the laser mirror

• negligible perturbation if EO crystal is closer to beam than mirror edge.

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

thanks!

Patrick Krejcik

BIW04 pkr@slac.stanford.edu

May 3-6, 2004

Timing system requirements

Synchronization of fiducials in low-level RF with distribution of triggers in the control system

1/360 sLinac 476 MHzMain Drive Line Sector feed

Fiducialdetector

MasterPattern

Generator

SLCControlSystem Event

Generator360 Hz Triggers8.4 ns±10 ps

128-bit wordbeam codes

119 MHz

360 Hz fiducials phase locked to low level RF