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  • 1. The Laser System at PETRA Wire G. A. Blair, Royal Holloway Univ. London ACFA Workshop, Mumbai 16 th December 2003 Accelerator-Related Session
    • Motivation for the project
    • Laserwire at PETRA
      • Environment at PETRA
      • Installation of Hardware
      • First measurements
    • Conclusions and Outlook
  • 2. Motivation
    • Maximise Luminosity performance of Linear Collider
    • Control of transverse beam size and emittance in the Beam Delivery System (BDS) and at the Interaction Point (IP)
    • Conventional techniques (wirescanner) at their operational limit
    • Development of standard diagnostic tool for LC and LC Test Facility operation based on optical scattering structures  Laserwire, Laser-Interferometer
    • Features
      • Resolution error smaller than 10%
      • Fast (intra-train) scanning
      • Non-destructive for electron beam
      • Resistant to high power electron beam
  • 3. Trans.+Long. Profiles Trans: 10-100  m Long: ~200  m Trans: 10-100 nm
  • 4. LC Layout and Parameters 535 5 335 4.5 196 4.5  x / n m  y / n m IP 20 to 150 1 to 25 7 to 50 1 to 5 3.4 to 15 0.35 to 2.6  x /  m  y /  m BDS TESLA NLC/GLC CLIC
  • 5. Optical Scattering Structures
    • Scanning of finely focused laser beam through electron beam
    • Detection of Compton photons (or degraded electrons) as function of relative laser beam position
    • Challenges
      • Produce scattering structure smaller than beam size
      • Provide fast scanning mechanism
      • Achieve efficient signal detection / background suppression
  • 6. Laserwire for PETRA
    • Positron Electron Tandem Ring Accelerator
    • Injector for HERA, upgrade to synchrotron light source
    • Long free straight section
    • Easy installation of hardware due to existing access pipe and hut outside tunnel area
    • Q-switch Nd:YAG with SHG
    • From CERN LEP polarimeter
    • Trans Mode: large M 2 ~9
    • Long Mode: stability ± 20%, beating  ps substructure
    • Homegrown timing unit for external triggering
    Laser parameter PETRA parameter 4.5 to 12 ~100 1 to 3 500 to 100 ~100 E/GeV  z /ps nC  x /  m  y /  m Energy Bunch Length Charge/bunch Hor. beam size Ver. beam size 1064/532 250/90 10 30 ~7 0.7 l/nm E/mJ dt/ns f rep /Hz  x,y /mm  /mrad Wavelength Energy Pulselength Reprate Beam size Divergence
  • 7. Laserwire for PETRA
  • 8. Signal and Backgrounds
    • Signal: Compton scattering
    • Background sources:
      • Synchrotron radiation
      • Cosmic rays
      • Bremsstrahlung
    • Simulation with Geant4 plus
    • tool kits with realistic setup
  • 9. Setup at PETRA
  • 10. Installation at PETRA
  • 11. Installation at PETRA
  • 12. Lab Measurements at RHUL
  • 13. Installation at PETRA
  • 14. Detector
    • Requirements for detector material
      • short decay time (avoid pile up)
      • short radiation length
      • small Moliere radius
    • Cuboid detector crystals made of PbWO4
    • 3x3 matrix of 18x18x150 mm crystals
    • Energy resolution better than 5%
  • 15. Detector Calibration
    • Detector studies with DESY II testbeam
    • Beamline with electrons with energy from 450 MeV to 6 GeV
    • Ten detector crystals were calibrated using a single PMT
    • Combination of nine crystals in matrix
    • Resolution
      • High intrinsic resolution
      • Full matrix less good
  • 16. First Photons 31.07.03 Laser on Laser off Photodiode at IP Q-switch Calorimeter
  • 17. First Beam Profile Scans
    • Positron beam in PETRA
    • Beam energy: 7 GeV
    • Bunch pattern: 14 x 1 bunch evenly filled
    • Average current: 12 mA
      • Bunch charge = avg. current / (reprate * Nbunches) = 6.5 nC
    • Laser energy measured: 40 mJ (specs 90 mJ), P L = 4 MW
    • Optimization: qswitch delay, timing of ADC sample point
    • Vertical and horizontal orbit bumps to steer positron beam
      • Closed symmetric bumps using four steerers
      • Bump length: 50 m, max offset: 10 mm
    • Operation of fast piezo scanner
  • 18. The Laser
    • The laser has been given to us by B. Dehning from CERN. It has been used at LEP to measure beam polarization
    • It’s a Nd:YAG Q-switched system, running with 30 Hz
    • pulse energy measured: 40 mJ, power: 4 MW
    • synchronization to PETRA beam by triggering the Q-switch Pockels-cell
    • transverse beam quality is modest (multimode)
    • measured spot size at IP: σ L = (80 ± 10) μ m
  • 19. Measurement of the longitudinal Profile
    • The longitudinal profile has been measured with a streak camera: FESCA 200 from Hamamatsu
    • largest window of the camera: 500 ps with a resolution of 5 ps (fwhh)
    • The camera was triggered with the laser via a fast photo diode
    • Problem: stability of the trigger probably not better than 0.5 ns
  • 20. Averaged Profile
    • Measured averaged profile: fits to gaussian with a width of 12.5 ns (as expected)
  • 21. Structure in the Longitudinal Profile
    • Example of a single shot measurement of the profile 500 ps window, resolution 5 ps
    60 ps 66 ps
  • 22. Unfortunately, the structure is not stable
    • The longitudinal structure is due to longitudinal mode beating – this was expected
    • The beating changes from shot to shot
    30 ps 79 ps
  • 23. Laser Transverse Profile Units – number of CCD pixels
  • 24. Laser Summary
    • As expected for a this type of laser, the longitudinal profile shows substructure due to mode beating
    • The spikes have a width of 30 to 60 ps and a distance of 60 to 80 ps
    • Unfortunately, the structure is not stable and changes from shot to shot
    • To overcome this, the laser has to be equipped with a frequency stabilized seed laser or eventually with an Etalon
    • Hot spots a problem
  • 25. Orbit Scan
    • First scan with signal on scope
    • Then sampling of peak using ADC
    • Moving beam orbit up and down with vertical orbit bump
    • 5k counts at each orbit position
    • 3 min for each spectrum
    • 40 min for complete scan
    • Background with 20k counts
      • Mainly synchrotron radiation and bremsstrahlung
      • Rate changed by factor 10
    • Signal rate expected at peak
      • 200 γ s x 380 MeV avg Energy
  • 26. Result Orbit Scan
    • Gaussian approximation of beam shape
    • σ m = (0.175 ± 0.020 stat ± 0.038 sys ) mm
    • Vertical beam size
    • σ e = sqrt( σ m - σ L )
    • laser σ L = (40 ± 10) μ m
    • σ e = (170 ± 23 ± 37) μ m
    • Result of fit sensitive to background modelling
    • Systematic error dominated by vertical orbit jitter
    • More measurements and understaning of bkg sources necessary
  • 27. Fast Scanner Operation
    • Next scan with remote controlled fast scanner
    • Orbit position stable
    • Scan range: ± 2.5 mrad
      • Scan line = range * f lens =
      • 0.625 mm (± 20%)
    • Change amplitude of scanner power supply (1-100V)
    • Take 5k counts
    • Record laser IP image with CCD
    • Move laser beam
    • Take 5k counts ...
  • 28. Data and Analysis
    • Seven scan points recorded
    • 5 min / point
    • 40 min for full scan
    • Positron beam position stable within ± 40 μ m
    • Moving low energy pedestal
    • No background model
    • Orbit stable  bkg const.
    • Simple pedestal cut instead
    • Sufficient background rejection
  • 29. New Setting 5.12.03
    • Positron beam in PETRA
    • Beam energy: 7 GeV
    • Posittron beam optics not as in October scans!
    • Bunch pattern: 14 x 1 bunch evenly filled
    • Low current: 7.1 mA, first bunch 0.458 mA
      • Bunch charge = avg. current / (reprate * Nbunches) = 3.9 nC
    • High current: 40.5 mA, first bunch 2.686 mA
      • Bunch charge = 22.3 nC
    • Vertical and horizontal orbit bumps to steer positron beam into laser beam
      • Closed symmetric bumps using four steerers
    • Scanning of laser beam using the fast piezo scanner
  • 30. Results 04.12.03 Data
    • Slopy Gaussian approximation of beam shape
    • σ m =(68 ± 3 ± 20) μ m at low current
    • σ m =(80 ± 6 ± 20) μ m at high current
  • 31. Conclusions and Outlook
    • Laserwire at PETRA produced first compton photons and measure vertical beam size Next steps:
    • Full characterisation of laser: beam size, divergence, and power (stability) with slot scans and imaging techniques
    • Update all readout software, merge BPM and PMT software
    • Do more systematic scans with the fast scanner
    • Go to smaller spot sizes and reduce error bars
    • Build second dimension scanner.
    • Start designing a complete laser-wire emittance measurement system for the LC BDS.
  • 32. Collaborators
    • DESY
    • BESSY (Thanks to T. Kamps for many of these slides)
    • UK: RHUL, UCL, RAL, (Oxford).
    • CERN: (Laser, plus collaboration)
    • Close contact with:
    • SLAC
    • KEK
  • 33. People
    • Thanks to PETRA and BKR shift crews !
    K Balewski, G Blair, S Boogert, G Boorman, J Bosser, J Carter, J Frisch, Y Honda, S Hutchins, T Kamps, T Lefevre, H C Lewin, F Poirier, I N Ross, M Ross, H Sakai, N Sasao, P Schmüser, S Schreiber, J Urukawa, M Wendt, K Wittenburg,