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Lasers and Electro-optic diagnostic for characterising ultrafast electron bunches P.J. Phillips W.A. Gillespie University of Dundee S P Jamison ASTeC, STFC Daresbury A.M. Macleod University of Abertay Dundee Introduction Electro-optic (EO) technique Measurements at FLASH Comparison of Temporal decoding with Transverse Deflecting Cavity ALICE Setup for EO detection Migration to fibre of EO technique High field effects Fibre lasers in an accelerator enviroment Summary Electro-optic longitudinal bunch profile measurements Convert bunch Coulomb field into optical intensity variation. Coulomb field encoded into optical probe Decoding: temporal intensity Propagating variations in single laser pulse e-bunch electric field F ~ ETHz Effective polarisation rotation proportional to Coulomb field Temporal Decoding - the chirped laser pulse behind the EO crystal is measured by a short laser pulse with a single shot cross correlation technique - approx. 1mJ laser pulse energy necessary Encoding Time Resolution... material response, R(w) • velocity mismatch of Coulomb field and probe laser • frequency mixing efficiency [c (2)(w)] ZnTe Theoretical (but based on some experimental data) GaP Experimental setup at the VUV-FEL Resulting e-bunches at 450 MeV with 1000 pC in a < 100 fs spike during FEL operation at 32 nm. Laser hutch and optical setup at FLASH Temporal Decoding EO signal 80 fs 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 time [ps] Benchmarking EO by Transverse Deflecting Cavity 450 MeV, 1nC ~20% charge in main peak Transverse Deflecting Cavity (TDC) Comparison of EO and TDC signals LolaS 2006 03 01 03 08 0719 3.5 4 4.5 5 5.5 time [ps] SASE conditions 6 6.5 EOS@VUV-FEL by FELIX, DESY, Dundee, Daresbury 01-May-2006 squared Lola signal EO signal EO signal/sqared LOLA signal EO signal/sqared LOLA signal EOS@VUV-FEL by FELIX, DESY, Dundee, Daresbury 01-May-2006 LolaB 2006 03 01 01 43 3380 7 7.5 8 8.5 9 time [ps] 9.5 10 ACC1 phase 3° overcompression EO at first bunch, LOLA at second bunch in the same bunch train Electro-optic capability • Proven capability at ~80fs rms at ~450MeV • capability is maintained (or improved) as energy increased… Limited by non-linear optical effects Deflecting cavity capability… • ~15fs rms* at DESY, at 450 MeV • ~30-40fs rms*, at LCLS at 5 GeV Limited by electron beam optics/rigidity *Treat numbers with caution…these are my inferences (may be slightly better or worse) EO capability already comparable to deflecting structures. At higher energies expect demonstrated EO to overtake deflecting cavity capabilities…. (but no time slice information) ALICE....(formerly ERLP) Accelerator and Lasers in Combined Experiments Beam position monitor EO beamline section Beam profile monitor Synchrotron radiation diagnostics EO diagnostics table Energy ~ 35 MeV Temporal bunch length ~ 0.4ps Bunch charge 80pC Migration of EO capability to fibre laser technology… • Robustness & reliability • Potential Integration into accelerator timing system Erbium doped systems: 1.55mm; likely timing distribution system frequency doubled to 770nm => similar to Ti:S Ytterbium doped systems: 1030 nm phasematched with Galium Phosphide (GaP) E.O. material Ti:S (free-space) systems: Yb system being built at Darsebury... Commissioning ALICE EO testbed with Ti:S systems • temporal decoding... signal-noise; pulse energy, ... • spectral decoding... modified concepts for feedback • peak current arrival-time monitoring Tests of fibre EO system on ALICE... • • • • phase-matching signal-noise detector capabilities integration with fibre-laser timing distribution system Fibre laser EO demonstration at FELIX beam-time in February 2009 • e-bunch characterisation with ultrafast Yb system High field effects… “Standard” theory of EO measurement does not apply for very strong fields Standard description… Coulomb field induces refractive index change phase retardation Our “standard” description Frequency mixing of coulomb field new optical field Both descriptions include assumption of small-signal limit… (does the “new optical field” itself undergo an EO interaction..?) Development of high-field/large-signal EO theory starting point... wave equation with co-propagating non-linear source optical “sidebands” in small signal theory “sidebands” on sidebands ...on sidebands ...... describes probe depletion ...high-field/large-signal EO theory can associate EO interaction with matrix operation on input probe spectrum ..... where End result.... • “simple” matrix exponential solution to probe spectrum • Fourier transform to get time domain solution S.P. Jamison. Appl. Phys. B 91, 241-247, (2008) Experimental confirmation of theory... broadband THz, or Coulomb field, experiments not suitable... source not suitably characterised with monochromatic THz can get unambiguous observation of sideband cascading FELIX FEL user beamtime January/May 2008 monochromatic THz from FEL, synchronised laser probes. probe laser NOT ultrafast... require probe to also be quasi-monochromatic lasers in an accelerator... • machine diagnostics • electron generation • phase space manipulation • experimental photon sources • master RF clock • RF timing distribution • laser wakefield acceleration Schematic layout of a optical synchronisation system Fibre link stabilisation schematic setup Summary... • EO capabilities near expected requirements (potentially surpassing TDCav at higher energy) • Now addressing issues of robustness/reliability • Developed and experimentally confirmed high-field EO theory Experimental plans • ALICE test bed to be commissioned December • Fibre/e-beam test scheduled at FELIX February 2010