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EO@BC2 – A robust bunch length monitor

EO@BC2 – A robust bunch length monitor. Comissioning and first results of a possible standard diagnostic tool. Laurens Wissmann Bernd Steffen, Jonas Breunlin EO@BC2 – A robust bunch length monitor FLASH seminar , 2011-12-20. Outline. Basics

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EO@BC2 – A robust bunch length monitor

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  1. EO@BC2 – A robust bunchlengthmonitor Comissioningandfirstresultsof a possiblestandarddiagnostictool Laurens Wissmann Bernd Steffen, Jonas Breunlin EO@BC2 – A robust bunchlengthmonitor FLASH seminar, 2011-12-20

  2. Outline • Basics • The electro-opticeffect, measurementsetups • Electro-opticspectraldecoding • Setup • Schematic • Laser systemandlasersynchronisation • Electro-opticfrontend, electronics • Results • EstablishingOverlap • Data acquisition, time calibration • Long rangescan, lowchargecapability • Beam shapemeasurement vs. LOLA, resolutionlimitation • Upgrade • Exchange of EO crystal, newresults, summary Laurens Wissmann – EO@BC2 – Page 2/18

  3. Electro-OpticEffect • Electricfieldofrelativisticelectronbunch: THz-pulse in laboratoryframe • THz pulse changesrefractiveindex in the EO crystal • Polarisation of a copropagatinglaser pulse accordinglychanges • For example: crossed polariser setting (CP) picturedhere • Analyserchangespolarisationmodulation in amplitudemodulation Laurens Wissmann – EO@BC2 – Page 3/18

  4. Measurement Setups • EOS – electro-opticsampling • Least complex • multishottechnique • lowlaser power necessary • EOTD – electro-optic temporal decoding • Most complex • singleshot • requires ~100 µJ laserpulses • Other – EOSpD, Frequencymixing, etc. • General temporal resolutionlimitations: • EO crystalresonances • Laser pulse length Laurens Wissmann – EO@BC2 – Page 4/18

  5. EOSD – Electro-OpticSpectralDecoding • Short laser pulse has a broadbandwidth (0.1 ps at 1030 nm => 10 nm) • Chirped pulse: not transform limited, frequencycomponentssorted • Chirpedlaser pulse copropagateswiththeTHzfield in thecrystal • Different spectralcomponentsacquire different polarisationmodulation • Translation intoamplitudemodulation, readout via spectrometer • Medium complex, singleshot, requires large bandwidthlaserpulses • Resolution limited toΔ by frequency mixing Laurens Wissmann – EO@BC2 – Page 5/18

  6. EO@BC2 Setup - Schematic • All components in tunnel • Lead shielded box for • Laser • Electronics • Readoutcamera (10 Hz) • Remote control on • Crystal-to-beam-position • Analyserwaveplatesetting • Laser status • Laser synchronisation • Laser-to-bunchtiming Laurens Wissmann – EO@BC2 – Page 6/18

  7. EO@BC2 Setup – Ytterbium DopedFibreLaser (YDFL) • Commercial system (Menlo) • Ytterbium-dopedfibrelaser • Very robust design • Virtuallynomaintainance • Pulse lengthchirpedto7 ps Laurens Wissmann – EO@BC2 – Page 7/18

  8. EO@BC2 Setup– YDFL Synchronisation • Temperaturestabilisedlaser • Cavitylengthadjustment • Rough: Motor actuator • Fine: Piezo fibrestretcher • VME based digital controlloop • Goodlongtermperformance (days, weeks,…) Laurens Wissmann – EO@BC2 – Page 8/18

  9. EO@BC2 Setup – The Electro-OpticFrontend • Designedat PSI • Installedduring 2010 shutdown • Equippedwith all necessarybulkoptics • Requires 20 cm beam pipe • Fibrecoupled, motorised • Different dive-in depthswithoutadjustingoptics • Wave platesmotorised • EO crystal: 0.5 mm GaP Laurens Wissmann – EO@BC2 – Page 9/18

  10. EO@BC2 Setup – Electronics, Trigger, andReadout Box • Lead shielded box with • YDFL, spectrometerandInGaAs Cam • RF electronics, AOM • Power supplyunitwith piezo driver • VME cratewih RF lock controlrunning on a DSP, delaycards, ADC`s, triggerenhancementboard, AOM driverboard • Laser power supplyunit • 95/5 Coupler, Photodiode, RF amplifiers, otherstuff • SRS DG535 for Gate generation • Fibrelengthtooptical front end: 2 m Laurens Wissmann – EO@BC2 – Page 10/18

  11. Measurements– EstablishingOverlap • Fine timing: scanlaser in stepsof 1 ps w.r.t. bunch, lookatcameraand PD • Oncefound, timingdoes not changemuch • Roughtiming: comparepick-upantennasignaltolaser pulse arrival time • Set correcttimervalue for AOM and Cam Laurens Wissmann – EO@BC2 – Page 11/18

  12. Measurements – Data Acquisitionand Time Calibration • Reference spectrumtaken • Modulatedspectrumtaken • Phase retardationiscalculatedfromtheirrelation • Phase retardationis proportional totheTHzfieldstrength • Time calibrationbyshiftingthelaserwithrespecttothee-bunch • -28 channel/ps (bunchhead on theright) • 6.4 ps detectorrange Laurens Wissmann – EO@BC2 – Page 12/18

  13. Measurements – Long Time Scan, Low Charge Ability • Subsequent setsofdata, concatenated after requiry • Clearlyvisibleartifactat 11 ps due toreflection in EO crystal • Ringing for severalhundredps • Signals for bunchchargesaslowas 50 pChavebeenmeasured Laurens Wissmann – EO@BC2 – Page 13/18

  14. Measurements – Bunch Shapes: EO@BC2 vs. LOLA • Straight through BC3, measure same bunch • Goodagreement in shapemeasurementofordinarybunches • Oscillationsoccurwhen a steepedgeisproduced Laurens Wissmann – EO@BC2 – Page 14/18

  15. Measurements – Resolution Limit • Steepedges -> Oscillationsoccur • Frequencymixing • Simulationshavebeendone, herewithgaussianbunches Laurens Wissmann – EO@BC2 – Page 15/18

  16. EO@BC2 – Upgrade (2011 EasterShutdown) • Crystal exchange • 0.5 mm GaP -> 5 mm GaP • strongerphaseretardation (larger signal) • Shiftofthereflectionartifactfrom 11 ps to 110 ps • Longeropticalfibre • Strongerchirpedpulses (7 ps -> 10 ps) • Enhamncementofthedetectorrange • Trigger enhancementboardintegrated • Lesstimingjitter for theopticalgating • Decreaseofamplitudejitter • Laser hadtobereturned a second time Laurens Wissmann – EO@BC2 – Page 16/18

  17. EO@BC2 – CurrentStatus andOutlook • Hardware • Monitor provedtoworkasplanned • Resolution sufficient for longbunches after BC2 • Software • All measurementsweretakenwith MATLAB scripts -> not userfriendly • A Matlab GUI isavailable -> moreuserfriendly • A dedicated DOOCS serverisbeingdevelopped -> operatortool • Future perspective • Usefultuningtool, for example, for tailoredbunches • Minor changesmightbeinteresting (integrating a pulse compressor in thefrontend) • The frontend will be a partofthe XFEL diagnostics, with a different lasersystem Laurens Wissmann – EO@BC2 – Page 17/18

  18. The End Thankyou for yourattention. …anyquestions?

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