Electro-optic Longitudinal Profile Diagnostics
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Electro-optic Longitudinal Profile Diagnostics
S P Jamison, Accelerator Science and Technology Centre,
STFC Daresbury Laboratory
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Electro-optic effect for bunch diagnosticsCoulomb field of relativistic bunch
probe laser
encodingof bunch information into
laser
decodingof information
from laser pulse
Measure electric fields of bunch : Coulomb field, CSR, CTR, wakefields, ...
Spectrum of field important
for capability &
technique choice
E(t) Coulomb Field E(w)
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Coulomb spectrum shifted to optical region
Coulomb pulse replicatedin optical pulse
envelope optical field
Electro-optic longitudinal diagnosticsPhysics : Frequency mixing between Coulomb field (or CSR, CTR,FEL …) pulse and probe laser
c (2)(w;wthz,wopt)
wopt + wthzwthz
wopt
wopt - wthz
woptEO cr
ysta
lCoulomb field
probe laser
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Electro-Optic Techniques...
Spectral Decoding
Spatial Encoding
Temporal Decoding
Spectral upconversion**
Variations in read-out of optical temporal signalo Chirped optical input o Spectral readouto Use time-wavelength relationship
o Ultrashort optical inputo Spatial readout (EO crystal)o Use time-space relationship
o Long pulse + ultrashort pulse gateo Spatial readout (cross-correlator crystal)o Use time-space relationship
o monochomatic optical input (long pulse)o Spectral readouto **Implicit time domain information only
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
In general spectral decoding resolution limited by chirp
Spectral DecodingAttractive simplicity for low time resolution measurements e.g. injector diagnostics
Rely on t-l relationship of input pulse for interpreting output optical spectrum Resolution limits come from fact that EO-generated optical field doesn't
have same t-l relationship
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
ALICE Electro-optic experimentso Energy recovery test-accelerator
intratrain diagnostics must be non-invasive
o low charge, high repition rate operation typically 40pC, 81MHz trains for 100us
Spectral decoding results for 40pC buncho confirming compression for FEL commissioningo examine compression and arrival timing along traino demonstrated significant reduction in charge requirements
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Measured EOSDSignal (40pC)
Electro-optic spectral decoding on ALICE
Model bunch-profile
EOSD response fnc.
expected EOSD signal
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Direct Temporal techniques...
Temporal decoding
• Encoding of signal exactly as before..• measure temporal profile of probe pulse directly
using spatial-temporal cross-correlation
envelope optical field
Spatial encoding
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Temporal decoding
Temporal profile of probe pulse Spatial image of 2nd harmonic
o Limited by gate pulse duration … … “frequency resolved optical gating” (FROG) solutions?
o Complex of laser & optical transport systems
o EO interaction produces optical replica of Coulomb fieldo Readout via 2nd Harmonic Generation & optical cross-correlation
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Electro-optic Temporal decodingFLASH, 400 MeV, ~500pC)
65mm thick GaP
Benchmarked against RF deflecting cavity• provides a unique “calibrated” THz source...• confirms understanding of material properties
Berden et al. Phys Rev Lett. 99 (2007)
ALICE, 30 MeV, 60pC
monitoring compression & arrive time (Lattice and beam properties)
Signal-nose issues at low charge
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Time Calibration....
probe laserbunchgate laser
measure the same electron bunch twicewith known measurement time delay
CDR feedback on CDR feedback offConfirmation of feedback systems
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Spectral upconversion diagnosticmeasure the bunch Fourier spectrum...
... accepting loss of phase information & explicit temporal information
... gaining potential for determining information on even shorter structure
... gaining measurement simplicity
Long pulse, narrow bandwidth, probe laser
d-function
NOTE: the long probe is still converted to optical replica
same physics as “standard” EO
different observationaloutcome
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
difference frequency mixing
sum frequency mixing
Spectral upconversion diagnosticFirst demonstration experiments at FELIX
Jamison et al. Applied Physics Letters, 96 231114 (2010)
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
FELIX temporal profile THz spectrum pr
edict
ion
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Current status, future improvementsLow time resolution (>1ps structure)
• spectral decoding offers explicit temporal characterisation• robust laser systems available• diagnostic rep rate only limited by optical cameras
High time resolution (>60 fs rms structure)• proven capability• significant issues with laser complexity / robustness
Very higher time resolution (<60 fs rms structure)• EO material properties (phase matching, GVD, crystal reflection)• Laser pulse duration (TD gate, SE probe)Limited by
Accelerator wish list - Missing capabilities o Higher time resolution (20fs rms for CLIC)o Higher reliability, lower cost (high resolution systems)o solution for feedback.
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Can we achieve even better time resolution ...?
Detector Material:– GaP – Move to new material? ( phase matching, c(2) considerations )– Could use GaSe, DAST, MBANP .....?– use multiple crystals, and reconstruction process
Gate pulse width ~ 50 fs– Introduce shorter pulse– Use (linear) spectral interferometry– Use FROG Measurement (initially attempted at FELIX, 2004)
Encoding
Decoding
or Alternative techniques: spectral upconversion
If drop requirement for explicit time information at high frequencies, other options also become available
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
c (2)(w;wthz,wopt)wopt + wthz
convolution over all combinations of optical
and Coulomb frequencies
Electro-optic detection as sum- and difference-frequency mixing
wthz
wopt
wopt - wthz
wopt
frequency domain description of EO detection...
THz spectrum(complex)
propagation& nonlinear
efficiencygeometrydependent
(repeat for each principle axis)
optical probe
spectrum(complex)
EO c
ryst
al
Refractive index formalism comes out as subset of solutions(restriction on laser parameters)
This is “Small signal” solution. High field effects c.f. Jamison Appl Phys B 91 241 (2008)
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Time resolution & bandwidth
Many variants of EO…
… all involve conversion of Coulomb field “pulse” to optical pulse
• Manageable relative bandwidth• Exploit ultrafast laser diagnostic techniques
CLIC requirements: 20 fs time resolution
Coulomb field …. 0.1 – 20 THz (octave spanning bandwidth) Converted to optical field …. 300 THz +/- 20THz (10% bandwidth
• Implies 20-30 THz detection bandwidth• Uniform (or known) response function
over full bandwidth
Time profile
Spectra
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
200fs
GaP
Effect of Material response...
100fs
100fs 50fs
ZnTe ZnTe
GaP
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Solution in multiple crystals and crystal orientations…
From Shi et al. Appl. Phys. Lett 2004
Tuneable phase matching of laser and THz pulse…
Coulomb spectral component to be
measured…
… crystal angle to achieve phase matching
Questions on how to “splice” data.
• Response amplitude can be measured from detection of tuneable THz source
• Spectral complex response can be measured from THz-TDS from linear THz-TDS … if we have known ultrashort source
GaSe Many candidate crystals
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
time
Cross-correlation method
• Optical probe with electron bunch info• ultrafast “gate” for time->space readout
• Resolution is limited by gate duration (+phase matching)
Practical implementation limits gate to >40fs fwhm ( laser transport, cross-correlator phase matching/signal levels )
• Weak probe due to EO material damage limits…• Compensated by intense gate
Signal/noise issues from this mismatch in intensities
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
frequ
ency
time
• Obtain both time and spectral information• Sub-pulse time resolution retrievable from additional information
FROG measurements of DL fibre laser (Trina Ng)
Higher resolution through “X-FROG “ cross-correlation, frequency resolved optical gating
standard FROG ultrafast laser diagnostics
Auto-correlation, not cross correlationSingle shot requires more intensity than reasonable from EO material limitation
R&D goals • Develop XFROG with realistic EO intensities - signal/noise issues; non-degenerate wavelengths (?)• Develop & demonstrate retrieval algorithms
- including “spliced data”
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Pushing the time resolution of electro-optic diagnosticsElectro-optic Materials - Bandwidth of Coulomb to Optical conversion - EO efficiency
Single-shot optical characterisation - bandwidth of single-shot optical readout - single to noise - single-shot X-FROG development Practical diagnostic system issues Minimising laser requirements - Reliability, robustness“Non- invasiveness” - signal-noise, time resolutionFeedback or tune-up -repetition rate, absolute vs relative temporal info
Current R&D focusIn collaboration with CERN (CLIC project) & University of Dundee
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
Overall Summary
o Electro-optic techniques available for different parameter regimes
o Proven capability for explicit temporal characterisation up to ~100 fs rms electron bunch structure
o Highest time resolution time-explicit techniques limited by - material properties
- optical pulse duration
- laser system robustness
o Multiple-crystal detectors & novel materials to be investigated
o “FROG-TD” will solve laser pulse duration limitation - amplified laser essential
- data-splicing procedure to be determined
o Spectral-upconversion offers solution for feedback - with multiple-crystal arrangement
S.P. Jamison, Daresbury Injector Workshop, June 30, 2011
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