Electro-Optic Longitudinal Profile Diagnostics for CLIC Current Status & Future Development

1. Steven Jamison, CTC, CERN September 21, 2010 Electro-Optic Longitudinal Profile Diagnostics for CLIC Current Status & Future Development S.P. Jamison,Accelerator Science and Technology Centre, STFC Daresbury Laboratory W.A. Gillespie, Carnegie Laboratory of Physics, University of DundeeA.M. MacLeod , University of Abertay Dundee G. Berden, FELIX, ‘Rijnhuizen’ FOM, Netherlands

2. Spectral Decoding FELIX DESY LBNL ... Spatial Encoding SLAC DESY LBNL, ... Temporal Decoding FELIX DESY RAL(CLF)MPQ Jena, ... spectral upconversion FELIX Steven Jamison, CTC, CERN September 21, 2010 Electro-Optic Techniques... several different techniques • same underlying physics of electron-beam interaction • differing optical implementation • differing capability.... Choices available, based on time resolution & simplicity /robustness our CLIC proposal • Temporal decoding for resolution down to ~60fs • Spectral up-conversion to be further developed & demonstrated for shorter time scale system

3. Spectral Decoding Steven Jamison, CTC, CERN September 21, 2010 Low time resolution... Attractive simplicity 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 Resolution related to laser parameters... BUT “ideal” laser systems still limited to > 1.0 ps resolution (for all reasonable observation windows) • Laser requirement can be modest • Suitable turn-key commercial systems widely available (fibre laser)

4. Steven Jamison, CTC, CERN September 21, 2010 Low time resolution... Spectral Decoding • Laser requirement can be modest • Suitable turnkey commercial systems widely available (fibre laser) • - majority of expts with complicated Ti:S systems • can be done with freq. doubled Erbium fibre lasers Could be developed to directly link to the optical timing distribution (if “pulsed” clock distribution employed) • Robust stand-alone Yb-fibre laser systems being developed by PSI & DESY low energy injector, >> 1ps diagnostics low charge, low energy measurement on ALICE (Daresbury) Compact fiberised assembly at PSI (from Steffen et al. DIPAC09) 40pC, single shot, Spectral decoding

5. Spatial Encoding Temporal Decoding Steven Jamison, CTC, CERN September 21, 2010 Explicit high time resolution approaches... Two distinct techniques demonstrated • Profile retrieval not yet adequately demonstrated • - complications of specific test • Potential for moderately complex laser systems (fibre) • Potential for all-optical integration with timing distribution system • Demonstrated 120fs FWHM, 60fs rms resolution • Benchmarked against RF deflecting cavity • Requires high pulse energy laser system (>50 uJ per pulse) capable of explicit temporal profiles with features down to 120fs FWHM ...will return to definition of time resolution Further improvements face significant issues laser: - generation & transport of <30fs pulses (physics & engineering) - ultrafast optical characterisation (physics) non-linear materials: bandwidth limitations (physics)

6. Steven Jamison, CTC, CERN September 21, 2010 Electro-optic experiments on FLASH... • Remote adjustment & alignment control required • Accelerator location required (ultrafast laser transport) • Vacuum transport of Ti:S to tunnel • Hands-on operation of amplified Ti:S CLIC context... • reliable remote diagnostic alignment achievable(ALICE@DL goal) • true turn-key laser systems not available now.…CLIC time scale..? • solution for high-power ultra-short laser transport..?

7. high time resolution (+ wakefields ?) 65mm thick GaP Steven Jamison, CTC, CERN September 21, 2010 Electro-optic experiments on FLASH realtime control-room profiles observing stabilising effect of slow feedback CDR feedback on CDR feedback off Control-room diagnostic… BUT only with laser room preparation and support

8. Steven Jamison, CTC, CERN September 21, 2010 Temporal decoding time resolution comparison with RF deflecting cavity Electro-optic shot-shot variations Transverse Deflecting Cavity Phys Rev Lett 99, 164801 (2007) Phys. Rev. ST, 12, 032802 (2009) not observable in(existing) EO... Resolution limited by both material properties & laser pulse duration

9. Coulomb field of relativistic bunch Decoding of information from laser pulse probe laser non-linear crystal(electro-optic effect) Steven Jamison, CTC, CERN September 21, 2010 Temporal resolution limits What are current limits? How to overcome them. • choices for complexity • limiting factor for “spectral decoding” Encoding EOTD & EOSD • Temporal resolution limits are primarily in material properties • Laser and “decoding” limits follow not far behind Encoding process physics: non-linear frequency mixing between Coulomb field and laser phase-velocity matching of full Coulomb spectrum with laser (refractive index variations near phonon resonances ) Constraints: Decoding physics: ultrafast optical pulse characterisation Constraints: ultrafast laser generation & transport

10. Coulomb pulse replicatedin optical pulse envelope optical field Steven Jamison, CTC, CERN September 21, 2010 Physics of EO encoding... time varying refractive index is a restricted approximation to physics Coulomb spectrum shifted to optical region New concepts & understanding of very high time res techniques come from generality of frequency mixing physics description) Jamison et al. Opt. Lett 31 1753 (2006) Jamison., Appl.Phys B. Laser & Opt. 91 241 (2008)

11. Steven Jamison, CTC, CERN September 21, 2010 Encoding Time Resolution...material response R(w) • Velocity mismatch of Coulomb field and probe laser • Frequency mixing efficiency, c(2)(w) GaP ZnTe • Phonon dips...missing data • Origin of ringing artefacts

12. Steven Jamison, CTC, CERN September 21, 2010 Time resolution from frequency response • Encoding “time resolution” not a RMS measure... • Better described as “temporal limitation” • structure on longer time scale recorded faithfully • short time scale structure not observed, PLUS ringing artefacts

13. Steven Jamison, CTC, CERN September 21, 2010 ...Time resolution from frequency response 50 fs FWHM bunches, separated by 110fs 100 um thick GaP 50 um thick GaP 10 um thick GaP • practical problems with 10um thickness • ringing not solved by thinner crystals • Laser pulse duration NOT included in calculations • At sub-50fs resolution, short pulse preservation needs to be addressed

14. Steven Jamison, CTC, CERN September 21, 2010 achieving even better resolution ...? Encoding Detector Material: • Move to new material? ( phase matching, (2) considerations ) • Could use GaSe, DAST, MBANP ..... or poled organic polymers? • use multiple crystals, and reconstruction process • Introduce shorter pulse lasers (<30 fs) • Use (linear) spectral interferometry • Use FROG Measurement (initially attempted at FELIX, 2004) Decoding or Alternative techniques: spectral upconversion If drop requirement for explicit time information at high frequencies, other options also become available

15. GaSe varying crystal angles polymer (MA9 in PMMA) Steven Jamison, CTC, CERN September 21, 2010 alternative EO materials.. • alternative materials exist with higher frequency response • all materials have resonances & bandwidth limitations from Cao et al Opt Lett (2002) from Ding et. al, J. non-linear Opt. materials (2003) Leads to consideration of multi-crystal EO detectors Explicit temporal reconstruction of combined crystal measurements…? removal of “ringing” from individual crystals ??

16. Spectral upconversion diagnostic Steven Jamison, CTC, CERN September 21, 2010 Two approaches.. • Explicit temporal information Temporal decoding for ~60 fs resolution Higher resolution with alternative materials & reconstruction • Implicit spectral information from spectral-upconversion Full spectral information, [mm wavelength -> optical ] Robust capability [feedback..?] use long pulse, narrow band, probe laser ... accepting loss of phase information & explicit temporal information ... gaining potential for determining information on even shorter structure ... gaining measurement simplicity same physics as “standard” EO.... different observational outcome. • laser complexity reduced, reliability increased • laser transport becomes trivial (fibre) • problematic artefacts of spectral decoding become solution

17. What’s different...? Steven Jamison, CTC, CERN September 21, 2010 Spectral decoding &Temporal decoding laser bandwidth bunch spectral extent >> • issues of • laser transport • laser complexity/expense/reliability • material effects (e.g. group velocity dispersion) Spectral upconversion laser bandwidth bunch spectral extent << • low power, ‘simple’ lasers OK • fibre transport now an option • simple – linear – spectral detection without artefacts of spectral decoding Important: can measure non-propagating long wavelength components not accessible to radiative techniques (CSR/CTR/SP)

18. Spectral upconversion diagnostic Steven Jamison, CTC, CERN September 21, 2010 experiments at FELIX For these experiments… • Free-space optical transport • Synchronised 5-50ps Ti:S and YAG lasers

19. Spectral upconversion diagnostic sum frequency mixing difference frequency mixing experiments at FELIX Appl. Phys. Lett. 96 231114 (2010) FELIX temporal profile inferredFELIXspectrum

20. Spectral upconversion with FEL radiation... Explicit confirmation of frequency upconversion process with tunable, monochromatic, Far-IR and mid-IR optical side bands from l=150mm FEL radiation Appl. Phys. Lett. 96 231114 (2010) High fields effects !!! x10 increase in field (…charge) Standard “EO” expectation

21. Steven Jamison, CTC, CERN September 21, 2010 Proposed spectral upconversion system fibre optic transport to optical spectrometer turn-key commercial fibre lasers commercially available link to telecoms technology (robustness) Compact crystal assembly in beam line fibre optic distribution to diagnostic location in accelerator • multiple crystals azimuthally offset • laser split and independently sent to each crystal Required development: • Short bunch testing of spectral upconversion • Testing of alternative materials, including in accelerator environment • Simultaneous testing of multiple-crystals

22. Steven Jamison, CTC, CERN September 21, 2010 Pathway to meeting CLIC requirements Materials investigation • Testing with laser generated THz (material properties) • FEL experiments for non-linearity and mid-IR capability EO technique development and demonstration: • Reducing laser requirements of EOTD • FROG extension of EOTD…allows sub-laser duration measurements • Multi-crystal reconstruction methods…theory -> expt test • Spectral upconversion • High field effects… Tests at short-bunch test facilities (PSI, FACET, …?)

23. Steven Jamison, CTC, CERN September 21, 2010 Summary Low time resolution (>1ps structure) • spectral decoding offers explicit temporal characterisation • relatively robust laser systems available High time resolution (>60 fs rms structure) • proven capability • significant issues with laser complexity / robustness Very higher time resolution (<60 fs rms structure) • Limited by EO material properties (& laser) • Spectral up conversion + alternative (multiple) EO materials gives implicit ultra-short bunch structure • Development & short-bunch demonstrations required (including alternative materials, GaSe, ...)

24. additional information...

26. Steven Jamison, CTC, CERN September 21, 2010 Electro-Optic Longitudinal Profile Diagnostics for CLIC • Current capabilities & options • Readiness for implementation • Outstanding issues • Development & Prototyping Pathways • time resolution vs simplicity • multi-bunch operation • integration in optical timing distribution systems Steve Jamison, CTC, Sept 2010

28. Spatial Encoding Rely on t-x relationship between laser and Coulomb field In principle: expect same/similar capabilities as TD less widely demonstrated: SLAC and DESY expts had significant additional complications of long transport in fibre... SPPS (SLAC) measurements FLASH (DESY) measurements from Cavaleria et al. PRL 2005 from A. Azima et. al EPAC06 + FERMI@elettra system under test (SPARC...) possible solution for simplifying laser requirements BUT.... cannot improve on temporal decoding not upgradable to FROG-like measurement without adding TD complexity (limited to laser pulse duration  issues of laser transport)

29. Frequency domain description of EO detection... Electro-optic encoding is a consequence of sum- and difference-frequency mixing c (2)(w;wthz,wopt) wthz Coulomb field wopt + wthz EO crystal wopt - wthz wopt probe laser wopt for arbitrary probe and Coulomb pulses... • convolve over all combinations of optical and Coulomb frequencies. • includes field phase (chirp), general phase matching, optical GVD etc Refractive index formalism comes out as subset of solutions (restriction on laser parameters)

30. alternative ways forward... Current limitations are from material properties Phonon-resonance at 3-15 THz (material dependent) All materials will have some phonon resonance effects Can we use a set of crystals to cover larger range? requires (uncertain) reconstruction to find temporal profile (relative phase shifts, phase matching, efficiency between crystals) complication of system would multiply If reconstruction needed anyway, reconsider spectral techniques ... BUT traditional spectral techniques have difficulties : long wavelength / DC component transport extreme (“100%”) spectral bandwidths for detection A solution : Electro-optic spectral upconversion

32. Steve Jamison, CTC, Sept 2010

33. Laser transport... in FLASH experiments EO Temporal decoding setup More complicated setup Less access to setup in accelerator • Vacuum laser transport from laser “hut” to tunnel. • Some relay lenses in transport • Remote adjustments on many mirrors

34. Spectral upconversion diagnostic new slide 20 Aim to 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 use long pulse, narrow band, probe laser same physics as “standard” EO  d-function different observationaloutcome • laser complexity reduced, reliability increased • laser transport becomes trivial (fibre) • problematic artefacts of spectral decoding become solution NOTE: the long probe is still converted to optical replica

35. In Summary ... • Proven capability for explicit temporal characterisation up to ~100 fs rms electron bunch structure • high time resolution techniques have problems with reliability & necessary infrastructure... • ... but there exist avenues available for improving time resolution & robustness (depending on beam diagnostics requirements) • For high time resolution, both alternative materials & spectral upconversion are under investigation

36. Selected References (Daresbury-Dundee Group): Free-electron laser pulse shape measurements with 100fs temporal resolution using a 10fs Ti:sapphire laser and differential optical gatingX.Yan, A.M. MacLeod, W.A.Gillespie et al. Nucl. Instr. Meths. Phys.Res. Vol A429 (1999) 7- 9Sub-picosecond electro-optic measurement of relativistic electron pulsesX. Yan, A.M. MacLeod, W.A. Gillespie, G.M.H. Knippels, D. Oepts, A.F.G. van der Meer. Physical Review Letters 85 (2000) 3404-7 Single-shot electron bunch length measurementsI. Wilke, A.M. MacLeod, W.A. Gillespie, G. Berden, G.M.H. Knippels, A.F.G. van der Meer Phys. Rev. Lett. 88 No 12 (2002) 124801/1-4 Real-time, non-destructive, single-shot electron bunch-length measurements G. Berden, S.P. Jamison, A.M .MacLeod, W.A. Gillespie, B. Redlich and A.F.G. van der Meer Physical Review Letters 93 (2004) 114802 Temporally resolved electro-optic effect S.P.Jamison, A.M. Macleod, G. Berden, D.A. Jaroszynski and W.A. Gillespie Optics Letters 31, 11 (2006) 1753-55 Benchmarking of Electro-Optic monitors for Femtosecond electron bunches G. Berden, W.A.Gillespie, S.P. Jamison, B. Steffen, V. Arsov, A.M. MacLeod, A.F.G. van der Meer, P.J. Phillips, H. Schlarb, B. Schmitt, and P. Schmüser Phys. Rev. Lett. 99 043901 (2007) Electro-optic time profile monitors for femtosecond electron bunches at the soft X-ray free-electron laser FLASH B. Steffen, V. Arsov, G. Berden, W.A. Gillespie, S.P. Jamison, A.M. MacLeod, A.F.G. van der Meer, P.J. Phillips, H. Schlarb, B. Schmitt, and P. Schmüser Physical Review Special Topics – Accelerators and Beams 12 032802 (2009)

37. Temporal decoding Rely on EO crystal producing a optical temporal replica of Coulomb field measure optical replica with t-x mapping in 2nd Harmonic Generation Temporal profile of probe pulseSpatial image of 2nd harmonic • limited by • gate pulse duration (although FROG etc could improve) • EO encoding efficiency, phase matching Practical limitations: complexity of laser systems transporting short pulse laser (gate pulse only)