Electrooptic Longitudinal Profile Diagnostics S P Jamison Accelerator

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 decoding of information from laser pulse probe laser encoding of bunch information into laser Measure electric fields of bunch : Coulomb field, CSR, CTR, wakefields, . . . Spectrum of field important for capability & technique choice E(t) S. P. Jamison, Daresbury Injector Workshop, June 30, 2011 Coulomb Field E(w)

Electro-optic longitudinal diagnostics Physics : Frequency mixing between Coulomb field (or CSR, CTR, FEL …) pulse and probe laser Coulomb field wthz probe laser wopt EO crystal c (2)(w; wthz, wopt) wopt + wthz wopt - wthz wopt Coulomb spectrum shifted to optical region Coulomb pulse replicated in optical pulse envelope optical field S. P. Jamison, Daresbury Injector Workshop, June 30, 2011

Electro-Optic Techniques. . . Variations in read-out of optical temporal signal Spectral Decoding o Chirped optical input o Spectral readout o Use time-wavelength relationship Spatial Encoding o Ultrashort optical input o Spatial readout (EO crystal) o Use time-space relationship Temporal Decoding o Long pulse + ultrashort pulse gate o Spatial readout (cross-correlator crystal) o Use time-space relationship Spectral upconversion** o monochomatic optical input (long pulse) o Spectral readout o **Implicit time domain information only S. P. Jamison, Daresbury Injector Workshop, June 30, 2011

Spectral Decoding Attractive 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 In general spectral decoding resolution limited by chirp 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 40 p. C, 81 MHz trains for 100 us Spectral decoding results for 40 p. C 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

Electro-optic spectral decoding on ALICE Measured EOSD Signal (40 p. C) Model bunch-profile EOSD response fnc. expected EOSD signal S. P. Jamison, Daresbury Injector Workshop, June 30, 2011

Direct Temporal techniques. . . Temporal decoding Spatial encoding • Encoding of signal exactly as before. . • measure temporal profile of probe pulse directly using spatial-temporal cross-correlation envelope optical field S. P. Jamison, Daresbury Injector Workshop, June 30, 2011

Temporal decoding o EO interaction produces optical replica of Coulomb field o Readout via 2 nd Harmonic Generation & optical cross-correlation 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 S. P. Jamison, Daresbury Injector Workshop, June 30, 2011

Electro-optic Temporal decoding FLASH, 400 Me. V, ~500 p. C) ALICE, 30 Me. V, 60 p. C 65 mm thick Ga. P Benchmarked against RF deflecting cavity • provides a unique “calibrated” THz source. . . • confirms understanding of material properties Berden et al. Phys Rev Lett. 99 (2007) 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. . measure the same electron bunch twice with known measurement time delay pro r se be l a el t ga bu as nc er h Confirmation of feedback systems CDR feedback on CDR feedback off 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 same physics as “standard” EO d-function different observational outcome NOTE: the long probe is still converted to optical replica S. P. Jamison, Daresbury Injector Workshop, June 30, 2011

Spectral upconversion diagnostic First demonstration experiments at FELIX sum frequency mixing difference frequency mixing Jamison et al. Applied Physics Letters, 96 231114 (2010) S. P. Jamison, Daresbury Injector Workshop, June 30, 2011

n tio di c THz spectrum S. P. Jamison, Daresbury Injector Workshop, June 30, 2011 pr e FELIX temporal profile

Current status, future improvements Low time resolution (>1 ps 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) Limited by • Laser pulse duration (TD gate, SE probe) Accelerator wish list - Missing capabilities o Higher time resolution (20 fs 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. . . ? Encoding Detector Material: – – Ga. P Move to new material? ( phase matching, (2) considerations ) Could use Ga. Se, DAST, MBANP. . . ? use multiple crystals, and reconstruction process Decoding Gate pulse width ~ 50 fs – Introduce shorter pulse – Use (linear) spectral interferometry – Use FROG Measurement (initially attempted at FELIX, 2004) 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

Electro-optic detection as sum- and difference-frequency mixing frequency domain description of EO detection. . . wthz wopt geometry dependent (repeat for each principle axis) EO crystal c (2)(w; wthz, wopt) convolution over all combinations of optical and Coulomb frequencies wopt + wthz wopt - wthz wopt propagation & nonlinear efficiency THz spectrum (complex) optical probe spectrum (complex) 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 CLIC requirements: 20 fs time resolution • Implies 20 -30 THz detection bandwidth • Uniform (or known) response function over full bandwidth Spectra Time profile Many variants of EO… … all involve conversion of Coulomb field “pulse” to optical pulse Coulomb field …. 0. 1 – 20 THz (octave spanning bandwidth) Converted to optical field …. 300 THz +/- 20 THz (10% bandwidth • Manageable relative bandwidth • Exploit ultrafast laser diagnostic techniques S. P. Jamison, Daresbury Injector Workshop, June 30, 2011

Effect of Material response. . . Zn. Te 200 fs 100 fs Ga. P 100 fs S. P. Jamison, Daresbury Injector Workshop, June 30, 2011 50 fs

Solution in multiple crystals and crystal orientations… Tuneable phase matching of laser and THz pulse… Ga. Se Many candidate crystals Coulomb spectral component to be measured… … crystal angle to achieve phase matching From Shi et al. Appl. Phys. Lett 2004 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 S. P. Jamison, Daresbury Injector Workshop, June 30, 2011

Cross-correlation method • Optical probe with electron bunch info • ultrafast “gate” for time->space readout time • Resolution is limited by gate duration (+phase matching) Practical implementation limits gate to >40 fs 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

Higher resolution through “X-FROG “ cross-correlation, frequency resolved optical gating • Obtain both time and spectral information • Sub-pulse time resolution retrievable from additional information frequency standard FROG ultrafast laser diagnostics time FROG measurements of DL fibre laser (Trina Ng) 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

Current R&D focus In collaboration with CERN (CLIC project) & University of Dundee 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 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