Singleshot, Subpicosecond, EO bunch measurements at FELIX

1
Single-shot, Sub-picosecond, EO bunch
measurements at FELIX

• Steven Jamison, Giel Berden, Allan MacLeod
• Allan Gillespie, Jingling Shen, Dino Jaroszynski,
  
• Britta Redlich, Lex van der Meer
• School of Computing and Advanced Technologies,
  University of Abertay, Dundee, UK
• Department of Physics, University of Strathclyde,
  Glasgow, UK
• FOM Institute for Plasma Physics Rijnhuizen,
  Nieuwegein, The Netherlands

2

• Single-shot electro-optic detection of
  ultra-short bunches - spectral-decoding -
  temporal decoding
• FELIX electron bunch measurements real time
  bunch profile adjustment bunch timing
  jitter charge dependence

3
Electro-optic detection of Coulomb field
Spectral decoding method
1.7ps
Temporal decoding method
450fs
4
Time-resolution of spectral decoding.
Time domain
Fouriertransform
Frequency domain
Mixing of optical spectrum with neighbouring
frequencies w ? W
approximate temporal limitation based on
bandwidth
5
Time-resolution function NOT a smoothing
function. Can cause artifacts!! Should be
considered as temporal limitation
e.g balanced detection measure change in probe
spectrum
6
Lab-bench test-bed for EO techniques
able to produce mimic of Coulomb field with
unipolar THz pulse from GaAs emitter
Multi-shot scanning optical delay
Single-shot spectral decoding
7
Observe oscillating features on shorter time
scale than usual time resolution
Jamison et al. Opt. Lett. 18 1710 (2003)
8
Temporal decoding

• Same chirped pulse encoding of Coulomb field
  information.
• Decode information through time-domain
  measurement of probe intensity

Cross-correlation of EO modulated pulse,
IOpt(t) EOpt2(t) with transform limited gate
pulse, Igate(t)
(balanced detection example)
Time resolution set by gate pulse duration (lt50fs)
9
Temporal decoding at FELIX
100mJ 30 fs
1mJ 21 ps
Chirped probe focussed on ZnTe in beamline
(sampling field at single point)
Probe expanded and collimated forcross-correlator
(for temporal-spatial mapping)
10
30-50MeV electron beam measurements FELIX FEL
facility, Rijnhuizen, Netherlands
11
Second-harmonic generation (single shot)
cross-correlation
Time window, t

• Beam diameter, D
• Angle of incidence F

t 9ps for D5mm, F15o
Symmetric crystal geometry 400nm walk-off
orthogonal to time-axis
Phase matching in BBO achievable for incident
angles F lt 33o
Able to choose time window from fs ? 30ps for
1cm beam (outside beamline!)
12
Single-shot temporal decoding of optical probe
Temporal profile of probe pulse ? Spatial image
of SHG
Symmetrical optical arrangement ? No temporal
blurring with crystal thickness
13
FELIX FEL facility, Rijnhuizen, Netherlands
CCD camera
short pulse
chirped probe
14
(No Transcript)
15
EO without any upstream mirror
ZnTe Reflection geometry.
High frequency signal components

• cancel during counter-propagating path
• sampled during co-propagating path

Low frequency components

• averaged during counter-propagating path
• sampled during co-propagating path

doubled retardation
Have applied this technique to single-shot
laser-plasma EO bunch measurements (May
August 2003)
16
Early temporal decoding measurements at FELIX
(July 2003)
30MeV?110fs resolution (at 1mm from beam)
800fs
Berden et al. Proceedings of FEL2003
600fs
28 July, 2003
17
Temporal decoding measurements at FELIX December
2003
450fs FWHM
G. Berden, S.P. Jamison et al. Accepted for Phys.
Rev. Lett.
18
Temporal decoding measurements at FELIX December
2003
Bunch timing jitter
19
Bunch profile versus EO signal

• Zero background signal
• Signal E2

Crossed-polarisers

• Small modulation on large background
• Signal E

Balanced detection
20
Determining Coulomb field from EO signal
Using crossed-polariser configuration ? EO
signal ? Coulomb Field2
Ecoulomb EO signal1/2
Quadratic dependence of EO signal on field
confirmed by varying bunch charge ( EO signal
? Q2 )
21
Confirmation of time-resolution and quadratic EO
dependence with monochromatic FEL radiation
22
Temporal decoding, spectral decoding comparison
temporal decoding measurement NOT
time-resolution limited
(crossed polarisers configuration)
23
Real time beam adjustment (beam-steering towards
crystal)
24
Real time monitoring and bunch profile
modification
. ? 450 fs FWHM Coulomb field on sub peak
Bunch profile modified by changing the buncher
and accelerator phase.
25
Need even better time-resolution. (measuring
sub-200fs bunches) .
Spectrum-envelope interdependence
(spectral-decoding)
gt300fs bunch duration limitation Solved by
temporal decoding 30-50fs possible
gt200fs limitation for undistorted
measurement Rigorous retrieval of true field
possible
EO phase matching
Alternative EO materials. (GaP, GaSe,
DAST) Approximate interpolation 30fs possible
(?)
EO crystal absorption
Time resolution approx. optical pulse duration.
30 fs. Improvements through X-FROG, .?
Optical bandwidth
26

• CONCLUSIONS FOR TEMPORAL DECODING..

• Demonstrated non-destructive longitudinal
  profile measurement with 650fs FWHM bunches
  
• 450fs FWHM substructure observed
• Estimated time resolution 300fs (2.8THz),
  due to ZnTe response and low g of bunch
• Real-time bunch shape monitoring and adjustment

27
Supported by EPSRC Basic Technology Programme
(alpha-X project on wavefield accelerator driven
FEL)
Allan MacLeod Allan Gillespie Steve Jamison
Jingling Shen Dino Jaroszynski
(Capital Normal University, Beijing)
Giel Berden Britta Redlich Lex van der Meer