length monitor Comissioning and first results of a possible standard diagnostic tool Laurens Wissmann Bernd Steffen Jonas Breunlin EOBC2 A robust bunch ID: 1003499
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1. EO@BC2 – A robust bunch length monitorComissioning and first results of a possible standard diagnostic toolLaurens WissmannBernd Steffen, Jonas BreunlinEO@BC2 – A robust bunch length monitorFLASH seminar, 2011-12-20
2. OutlineBasicsThe electro-optic effect, measurement setupsElectro-optic spectral decodingSetupSchematicLaser system and laser synchronisation Electro-optic frontend, electronicsResultsEstablishing OverlapData acquisition, time calibrationLong range scan, low charge capabilityBeam shape measurement vs. LOLA, resolution limitationUpgradeExchange of EO crystal, new results, summaryLaurens Wissmann – EO@BC2 – Page 2/18
3. Electro-Optic EffectElectric field of relativistic electron bunch: THz-pulse in laboratory frameTHz pulse changes refractive index in the EO crystal Polarisation of a copropagating laser pulse accordingly changesFor example: crossed polariser setting (CP) pictured hereAnalyser changes polarisation modulation in amplitude modulation Laurens Wissmann – EO@BC2 – Page 3/18
4. Measurement Setups EOS – electro-optic sampling Least complexmultishot techniquelow laser power necessaryEOTD – electro-optic temporal decodingMost complexsingle shotrequires ~100 µJ laser pulsesOther – EOSpD, Frequency mixing, etc.General temporal resolution limitations:EO crystal resonancesLaser pulse length Laurens Wissmann – EO@BC2 – Page 4/18
5. EOSD – Electro-Optic Spectral Decoding Short laser pulse has a broad bandwidth (0.1 ps at 1030 nm => 10 nm)Chirped pulse: not transform limited, frequency components sortedChirped laser pulse copropagates with the THz field in the crystalDifferent spectral components acquire different polarisation modulation Translation into amplitude modulation, readout via spectrometerMedium complex, single shot, requires large bandwidth laser pulses Resolution limited to Δ by frequency mixing Laurens Wissmann – EO@BC2 – Page 5/18
6. EO@BC2 Setup - SchematicAll components in tunnelLead shielded box forLaserElectronicsReadout camera (10 Hz) Remote control onCrystal-to-beam-positionAnalyser wave plate setting Laser statusLaser synchronisationLaser-to-bunch timingLaurens Wissmann – EO@BC2 – Page 6/18
7. EO@BC2 Setup – Ytterbium Doped Fibre Laser (YDFL)Specifications of the laser systemRepetition rate108.330.2 MHz (1.3 GHz / 12)Centre wavelength1030 nmBandwidth55 nmPulse energy1.5 nJ after boosterPulse lengthComp. to <100 fsInt. Timing jitter1k – 10M: < 30 fsSpecifications of the laser systemRepetition rateCentre wavelength1030 nmBandwidth55 nmPulse energy1.5 nJ after boosterPulse lengthComp. to <100 fsInt. Timing jitter1k – 10M: < 30 fsCommercial system (Menlo)Ytterbium-doped fibre laserVery robust designVirtually no maintainance Pulse length chirped to 7 ps Laurens Wissmann – EO@BC2 – Page 7/18
8. EO@BC2 Setup– YDFL SynchronisationTemperature stabilised laserCavity length adjustment Rough: Motor actuatorFine: Piezo fibre stretcherVME based digital control loopGood long term performance (days, weeks,…)Laurens Wissmann – EO@BC2 – Page 8/18
9. EO@BC2 Setup – The Electro-Optic FrontendDesigned at PSIInstalled during 2010 shutdownEquipped with all necessary bulk opticsRequires 20 cm beam pipeFibre coupled, motorisedDifferent dive-in depths without adjusting opticsWave plates motorisedEO crystal: 0.5 mm GaPLaurens Wissmann – EO@BC2 – Page 9/18
10. EO@BC2 Setup – Electronics, Trigger, and Readout BoxLead shielded box withYDFL, spectrometer and InGaAs CamRF electronics, AOM Power supply unit with piezo driverVME crate wih RF lock control running on a DSP, delay cards, ADC`s, trigger enhancement board, AOM driver board Laser power supply unit95/5 Coupler, Photodiode, RF amplifiers, other stuffSRS DG535 for Gate generationFibre length to optical front end: 2 mLaurens Wissmann – EO@BC2 – Page 10/18
11. Measurements – Establishing OverlapFine timing: scan laser in steps of 1 ps w.r.t. bunch, look at camera and PDOnce found, timing does not change muchRough timing: compare pick-up antenna signal to laser pulse arrival timeSet correct timer value for AOM and CamLaurens Wissmann – EO@BC2 – Page 11/18
12. Measurements – Data Acquisition and Time CalibrationReference spectrum takenModulated spectrum takenPhase retardation is calculated from their relationPhase retardation is proportional to the THz field strengthTime calibration by shifting the laser with respect to the e-bunch-28 channel/ps (bunch head on the right)6.4 ps detector rangeLaurens Wissmann – EO@BC2 – Page 12/18
13. Measurements – Long Time Scan, Low Charge AbilitySubsequent sets of data, concatenated after requiryClearly visible artifact at 11 ps due to reflection in EO crystalRinging for several hundred psLaurens Wissmann – EO@BC2 – Page 13/18Signals for bunch charges as low as 50 pC have been measured
14. Measurements – Bunch Shapes: EO@BC2 vs. LOLAStraight through BC3, measure same bunchGood agreement in shape measurement of ordinary bunchesOscillations occur when a steep edge is producedLaurens Wissmann – EO@BC2 – Page 14/18
15. Measurements – Resolution LimitSteep edges -> Oscillations occurFrequency mixingSimulations have been done, here with gaussian bunchesLaurens Wissmann – EO@BC2 – Page 15/18
16. EO@BC2 – Upgrade (2011 Easter Shutdown) Crystal exchange0.5 mm GaP -> 5 mm GaPstronger phase retardation (larger signal)Shift of the reflection artifact from 11 ps to 110 psLonger optical fibre Stronger chirped pulses (7 ps -> 10 ps)Enhamncement of the detector rangeTrigger enhancement board integratedLess timing jitter for the optical gatingDecrease of amplitude jitterLaser had to be returned a second time Laurens Wissmann – EO@BC2 – Page 16/18
17. Hardware Monitor proved to work as plannedResolution sufficient for long bunches after BC2SoftwareAll measurements were taken with MATLAB scripts -> not user friendlyA Matlab GUI is available -> more user friendlyA dedicated DOOCS server is being developped -> operator toolFuture perspectiveUseful tuning tool, for example, for tailored bunchesMinor changes might be interesting (integrating a pulse compressor in the frontend)The frontend will be a part of the XFEL diagnostics, with a different laser systemEO@BC2 – Current Status and OutlookLaurens Wissmann – EO@BC2 – Page 17/18
18. Thank you for your attention. …any questions?The End