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