EOS at SPPSAdrian L Cavalieri, David M Fritz, SooHeyong Lee,

Philip H Bucksbaum, David A Reis

(FOCUS Center, University of Michigan)

Holger Schlarb (DESY)

Patrick Krejcik, Jerome Hastings (SLAC/SSRL)

Timing for Ultrafast X-ray Science

• SPPS is an R&D facility for the Linac Coherent Light Source (LCLS)

and is a test bed for future beam diagnostics

SLAC LinacSLAC Linac

1 GeV1 GeV 20-50 GeV20-50 GeV

SPPSSPPSRTLRTL

9 ps9 ps 0.4 ps0.4 ps<100 fs<100 fs

50 ps50 ps

3km

• Electron bunches at SPPS are as short as 80fs FWHM, comparable

to the bunches that will drive future XFELs

– EOS delivers shot-to-shot bunch length to machine operators

• 2m undulator after bunch compression delivers 80fs FWHM hard

x-ray pulses

– EOS delivers shot-to-shot arrival time to users

EOS and “Pump-Probe”

system response

0S2S3S4S 6S7S 5S8S9S1S time

impulse

• Electro-Optic Sampling (EOS) delivers arrival time to users

– Pump-Probe experiments now possible at XFELs

– Machine jitter exploited to sample time-dependent phenomena

• Typical time resolved experiment utilizes intrinsic synchronization

between pump excitation and probe

Geography

EOS timing applicable

IF optical path lengths

remain constant

osc amp

EOS

X

undulator

t

t

Electro-Optic Sampling

x

• Crystal is affected by applied DC electric field

– Principle axes of crystal system are modified

– Index of refraction along these axes changes

• Probe laser field is decomposed in primed coordinate system

• Phase shift between components can be detected

•

y

DCE

x

y

x

ylaserE

DCE

Electric Field of 30GeV Electron Bunch

• Approximate field assuming:

–

– Steady-state

•

•

• Accurate calculation requires

numerical analysis

• Crystal is in a 6” 6-way vacuum

cross so approximation is good

12

80

z m RMS

fs FWHM

rE

zr

5 400 /peakE mm MV m

3.4nC

02r

z

qE

r

k

k

k

k

k

28.5GeV 28.5GeV 28.5GeV 28.5GeV 28.5GeV

EO Crystal

Spatially Resolved Electro-Optic Sampling (EOS)

k

k

k

k

k

k

k

k

k

k

Laser probe later relative to electron bunchLaser probe earlier relative to electron bunch• Spatially resolved EOS can deliver measurements with high enough

resolution to capture electron bunches at SPPS

– technique pioneered using table-top systems by Heinz et. al. in 2000

– spectrally resolved EOS cannot be used due to fundamental bandwidth

limitation

• Resolution limit of technique dominated by EO crystal thickness

input chirped

EO Crystal

Spatially Resolved EOS

v

v

k

k

k

v

(long bunch)

polarizing beamsplitter

laserk

s polarizedk

p polarizedk

time

; space

Arrival time and duration of bunch is encoded on profile of laser beam

Spatially Resolved EOS

time

time

integrated intensity

integrated intensity

SPPS Facility

AcceleratorTunnel Laser Room

&X-ray Hutch

LINAC

Effect of Long Pulse Probe Laser

• Probe pulse longer than e-bunch

– EO signal will be broadened

– If probe pulse shape is very well

known, we should be able to

deconvolve e-bunch shape

– Signal to background problems

introduced

• Probe pulse uncompressed

(~10’s of picoseconds or longer)

– Measurement will yield no

spatially dependent signal

v

k

Ultrafast Laser Transport

2

2

2

Pulse Shaper

• 640-pixel SLM can introduce arbitrary dispersion

• Genetic Algorithm used to find configuration that minimizes total

transport dispersion

• frequency doubled throughput provides feedback

• nearly transform limited pulse delivered when combined with grating

pair to compensate for GVD

SLM640-pixel

f f f f

Ultrafast Laser Transport: 3rd Order Correction

• Data taken with a 110m spool of test fiber at UofM with 128-pixel SLM

• Spectral width of transmitted pulse is ~10nm

Without Pulse Shaper With Pulse Shaper

FWHM 160fsFWHM 600fs

Transport Results

• Autocorrelation used for

measurement of laser pulse

arriving at EO chamber

• Find pulse is not transform limited

• Improvement can be made in

chromatic aberration in pulse

shaper

• Fixed phase mask can be used to

increase capacity of pulse shaper

135fs (FWHM)

PMT

BBO

2

Schematic

polarizing beamsplitter

laserk

s polarizedk

p polarizedk

time

; space

Spatially Resolved EOS Data

time

time

integrated intensity

integrated intensity

Single-Shot Data acquired with ZnTe

~ 300 fs

200 m

Single-Shotw/ high frequency filtering

iCC

D counts

time (ps)

color representation

Timing Jitter Data(20 Successive Shots)

time (ps)

shot

Effect of accelerator parameters on EO signal:Observation of resolution limit

• Changing Linac Phase detunes electron bunch compressor

Estimate Frequency Response Cut-Off

EO crystal sign of effect accumulated effect

fs

offwalkoffcut

176 pulse THzGaussian for

crystal) m200(for THz5.2-

1-

EO crystal imaging

2 f 2 f

fibe

r cou

ple

• EO signal/feature small ~50um extent

• Vacuum ports and other optics reduce angular resolution

• Object does not lie in a plane perpendicular to optical axis

• High resolution is required over a large depth of field (for adequate single-shot window)

Effect of Poor Imaging on Single-Shot Data

• Broadening also due to

insufficient resolution in

imaging system

• 100fs feature corresponds to a

30um feature in the beam

• Current chamber limits

achievable resolution

• Broadening is still due

predominantly to crystal

thickness and cut-off frequency

poor imaging

reference

Imaging Solution

45 3015

• Depth of field fixed by single-shot window

• Rate of time-sweep fixed by incident angle

• Resolution requirement of imaging system fixed by incident angle

New EO Chamber

• Accepts 15, 30, and 45 degree

angle of incidence

• Thick and thin crystal on

actuator, user chooses which

crystal is used for the EOS

• No internal optics

• Shorter path length to exit port

(use shorter focal length

imaging optics)

E-beam/X-rays

Pump Laser(Experiment)

EOS

The Flowchart

RF

Optical Path Length Jitter

• Long term thermal drift caused by:

– Optical fiber transport

– RF reference cable

• Short term drift caused by vibration

• Feedback used to stabilize fiber

length and keep signal in single

shot window

• Tracking changes made in optical

path to keep timing information

valid

EO/Streak Camera Correlation(no fiber stabilization)

A. MacPhee, LBL

• 5000 shots recorded at 10Hz rate

• EO arrival time accuracy: <30fs

• Streak Camera arrival time accuracy: ~150fs

• Correlation between measurements is .707

EO/Melting Correlation(no fiber stabilization)

• 30 shots recorded at 1Hz rate

• EO arrival time accuracy: <30fs

• Melting arrival time accuracy: ~50fs

• Agreement between two measurements is 60fs RMS

1010

Is the measurement plausible?

Red line: bunch charge distribution

– sum of two gaussians, one with

100% amplitude, 80fs width; the

other 20% amplitude, 1ps width

• Instrument function estimated to

be gaussian - 200fs width

– combine laser pusle duration with

imaging imperfections

– returns narrow feature in data

• Blue line is the convolution of the

two – corresponds to expected

EO signal

K. Gaffney

time (fs)

Expected EO signal

Is this measurement plausible?

• Good agreement between test

function and the experimental data

• Instrument function – reasonable

estimate that returns the sharp

central feature

• Choice for charge distribution

convolved with instrument function

matches data, but does not match

simulation

Single-shot data

w/ convolution function

Red line: real single-shot data

Blue line: convolution function

Synchronization Using RF Reference

170 fs rms170 fs rms