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 diagnostics Coulomb field of relativistic bunch probe laser encoding of bunch information into laser decoding of 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(  ) S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Coulomb spectrum shifted to optical region Coulomb pulse replicated in optical pulse envelopeoptical field Electro-optic longitudinal diagnostics Physics : Frequency mixing between Coulomb field (or CSR, CTR,FEL …) pulse and probe laser  (2) (  thz,  opt )  opt +  thz  thz  opt  opt -  thz  opt EO crystal Coulomb 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 signal o Chirped optical input o Spectral readout o Use time-wavelength relationship o Ultrashort optical input o Spatial readout (EO crystal) o Use time-space relationship o Long pulse + ultrashort pulse gate o Spatial readout (cross-correlator crystal) o Use time-space relationship o monochomatic optical input (long pulse) o Spectral readout o **Implicit time domain information only S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

In general spectral decoding resolution limited by chirp Spectral Decoding Attractive simplicity for low time resolution measurements e.g. injector diagnostics Rely on t- relationship of input pulse for interpreting output optical spectrum Resolution limits come from fact that EO-generated optical field doesn't have same t- relationship S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

ALICE Electro-optic experiments o 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 bunch o confirming compression for FEL commissioning o examine compression and arrival timing along train o demonstrated significant reduction in charge requirements S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Measured EOSD Signal (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 envelopeoptical field Spatial encoding S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Temporal decoding Temporal profile of probe pulse  Spatial image of 2 nd 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 field o Readout via 2 nd Harmonic Generation & optical cross-correlation S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Electro-optic Temporal decoding FLASH, 400 MeV, ~500pC) 65  m 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 laser bunch gate laser measure the same electron bunch twice with known measurement time delay CDR feedback onCDR feedback off Confirmation of feedback systems S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Spectral upconversion diagnostic measure 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  -function NOTE: the long probe is still converted to optical replica same physics as “standard” EO different observational outcome S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

difference frequency mixing sum frequency mixing Spectral upconversion diagnostic First demonstration experiments at FELIX Jamison et al. Applied Physics Letters, (2010) S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

FELIX temporal profile THz spectrum prediction S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

Current status, future improvements Low 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,  (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

 (2) (  thz,  opt )  opt +  thz convolution over all combinations of optical and Coulomb frequencies Electro-optic detection as sum- and difference-frequency mixing  thz  opt  opt -  thz  opt frequency domain description of EO detection... THz spectrum (complex) propagation & nonlinear efficiency geometry dependent (repeat for each principle axis) optical probe spectrum (complex) EO crystal 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 (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 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 fs 50fs 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

frequency 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 correlation Single 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 diagnostics Electro-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 resolution Feedback or tune-up -repetition rate, absolute vs relative temporal info Current R&D focus In 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