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Electro-optic Longitudinal Profile Diagnostics

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

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Electro-optic Longitudinal Profile Diagnostics

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  1. 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

  2. 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, ... Coulomb Field E(w) E(t) Spectrum of field important for capability & technique choice S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

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

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

  5. 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

  6. ALICE Electro-optic experiments • Energy recovery test-acceleratorintratrain diagnostics must be non-invasive • low charge, high repition rate operation typically 40pC, 81MHz trains for 100us Spectral decoding results for 40pC bunch • confirming compression for FEL commissioning • examine compression and arrival timing along train • demonstrated significant reduction in charge requirements S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

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

  8. envelope optical field 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 S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

  9. Temporal decoding • EO interaction produces optical replica of Coulomb field • Readout via 2nd Harmonic Generation & optical cross-correlation Temporal profile of probe pulse Spatial image of 2nd harmonic • Limited by gate pulse duration … … “frequency resolved optical gating” (FROG) solutions? • Complex of laser & optical transport systems S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

  10. Electro-optic Temporal decoding FLASH, 400 MeV, ~500pC) ALICE, 30 MeV, 60pC 65mm thick GaP Benchmarked against RF deflecting cavity monitoring compression & arrive time (Lattice and beam properties) Signal-nose issues at low charge • provides a unique “calibrated” THz source... • confirms understanding of material properties Berden et al. Phys Rev Lett. 99 (2007) S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

  11. probe laser gate laser bunch Confirmation of feedback systems CDR feedback on CDR feedback off Time Calibration.... measure the same electron bunch twice with known measurement time delay S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

  12. 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 observationaloutcome NOTE: the long probe is still converted to optical replica S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

  13. 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

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

  15. 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 fsrms 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 • Higher time resolution (20fs rms for CLIC) • Higher reliability, lower cost (high resolution systems) • solution for feedback. S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

  16. 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

  17. Electro-optic detection as sum- and difference-frequency mixing frequency domain description of EO detection... c (2)(w;wthz,wopt) wopt + wthz wthz wopt - wthz EO crystal wopt wopt propagation & nonlinear efficiency geometry dependent (repeat for each principle axis) THz spectrum (complex) optical probe spectrum (complex) convolution over all combinations of optical and Coulomb frequencies 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

  18. Time resolution & bandwidth CLIC requirements: 20 fstime resolution • Implies 20-30 THz detection bandwidth • Uniform (or known) response function over full bandwidth Time profile Spectra 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 +/- 20THz (10% bandwidth • Manageable relative bandwidth • Exploit ultrafast laser diagnostic techniques S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

  19. Effect of Material response... ZnTe ZnTe 200fs 100fs GaP GaP 100fs 50fs S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

  20. Solution in multiple crystals and crystal orientations… Tuneable phase matching of laser and THz pulse… GaSe 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

  21. 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 >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

  22. 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 standard FROG ultrafast laser diagnostics frequency FROG measurements of DL fibre laser (Trina Ng) time Auto-correlation, not cross correlation Single shot requires more intensity than reasonable from EO material limitation • Develop XFROG with realistic EO intensities • - signal/noise issues; non-degenerate wavelengths (?) • Develop & demonstrate retrieval algorithms - including “spliced data” R&D goals S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

  23. 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

  24. Overall Summary • Electro-optic techniques available for different parameter regimes • Proven capability for explicit temporal characterisation up to ~100 fsrms electron bunch structure • Highest time resolution time-explicit techniques limited by - material properties - optical pulse duration - laser system robustness • Multiple-crystal detectors & novel materials to be investigated • “FROG-TD” will solve laser pulse duration limitation - amplified laser essential - data-splicing procedure to be determined • Spectral-upconversion offers solution for feedback - with multiple-crystal arrangement S.P. Jamison, Daresbury Injector Workshop, June 30, 2011

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