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Laser beamline and laser beam parameters measurements at FLASH

Mini-Workshop on Laser Pulse Shaping, Zeuthen 30-Nov/1-Dec-2007. Laser beamline and laser beam parameters measurements at FLASH. Overview of the laser system Discussion of Parameters Summary. The Photocathode RF Gun. L-band rf gun (1.3 GHz) Pulsed 5 or 10 Hz RF pulse length up to 900 µs

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Laser beamline and laser beam parameters measurements at FLASH

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  1. Mini-Workshop on Laser Pulse Shaping, Zeuthen 30-Nov/1-Dec-2007 Laser beamline and laser beam parameters measurements at FLASH Overview of the laser system Discussion of Parameters Summary

  2. The Photocathode RF Gun • L-band rf gun (1.3 GHz) • Pulsed 5 or 10 Hz • RF pulse length up to 900 µs • RF power 3.2 MW or 42 MV/m max gradient • Cs2Te cathode laser beam e- beam Cathode Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  3. General Design Issues of the Laser • L-band RF gun + requirement on e- bunches determines transverse and longitudinal shape of laser, synchronization • Photocathode: work function, QE determines wavelength, energy • Superconducting accelerator long bunch trains → Laser average power in the Watt range • Suitable type of lasermode-locked solid-state system (MOPA: synchronized oscillator + amplifiers + frequency converter to UV) Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  4. Laser System Overview Diode-pumped Nd:YLF Oscillator In cooperation DESY/Max-Born-Institute, Berlin Modulators (AOM EOM AOM) 108 MHz 1.3 GHz 13.5 MHz Piezo tuning of cavity length Faraday isolator Pulse picker Pockels cell Remote controlled mirror box fround trip= 27 MHz Epulse= 0.3 µJ Stabilized by quartz tubes Fiber-coupled pump diodes Diode pumped Nd:YLF amplifiers Epulse= 6 µJ Imaging to the cathode Fast current control Pulse picker Faraday isolator Flashlamp pumped Nd:YLF amplifiers Beam shutter LBO BBO Relay imaging telescopes Fast current control Remote controlled attenuator Epulse< 0.3 mJ IR→ UV Double pulse generator Epulse< 50 µJ Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  5. Modulators (AOM EOM AOM) 108 MHz 1.3 GHz 13.5 MHz Piezo tuning of cavity length fround trip= 27 MHz Stabilized by quartz tubes Fiber-coupled pump diodes Pulse Train Oscillator (PTO) • Mode-locked pulsed oscillator • Diode pumped (32 W) • Synchronized to 1.3 GHz master oscillator • 1.3 GHz EO modulator with 2 AOMs (108 + 13.5 MHz) • Phase stability < 300 fs rms • Pulse length 12 ps (fwhh) (IR) • Stabilized with quartz rods • Thermal expansion coefficientfused quartz = 0.59 ppm/K (Al = 24 ppm/K) • 27 MHz pulse train • Train length 2.5 ms, pulsed power 7 W Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  6. 5 m m Chain of Linear Amplifiers • 2 diode pumped and 2 flashlamp pumped single pass amplifiers • Fully diode pumped version is running here at PITZ • Laser diodes: 32 W pulsed, 805 nm end pumped through fibers energy from 0.3 µJ to 6 µJ/pulse Xe flashlampsNd:YLF rod, Ø 5 or 7 mm • Flashlamps pumped heads: cheap, powerful (pulsed, 50 kW electrical/head)current control with high power IGPT switches allows flat pulse trains energy up to 300 µJ (1 MHz), 140 µJ (3 MHz) small-signal gain = 20 extractable peak power 1.2 kW, duty cycle 2% Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen Fluorescence profile

  7. 1.2 nC Electron beam pulse train (30 bunches, 1 MHz) Resulting trains 800 µs After amplification (1 MHz) • 2 Pulse pickers, based on Pockels cell + polarizer → up to 3 MHz • Pre-amplification (diodes) of 1.2 ms long train • Power amplification with variable bunch pattern Amplitude Output of the laser oscillator (27 MHz) Time Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  8. Bunch Pattern and MPS • Change bunch pattern on user request • Number of bunches • Different bunch frequencies: 1 MHz, 500 kHz, 250 kHz, 100 kHz and others • Realized with an FPGA based controller producing the appropriate trigger for the Pockels cell • The controller is also the interface of the machine protection system to the laser Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  9. 1 MHz 599 bunches 50 kHz 100 kHz 500 kHz Lasing with long bunch trains • up to 600 bunches to beam dump • 1 MHz, 500 kHz, 100 kHz, and 50 kHz • Lasing with at least 450 bunches (limited by photon diagnostics) Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  10. Laser beamline • Relay imaging with spatial filtering • Hard edge aperture after diode pumped amplifiers • Aperture imaged to → amplifier heads → doubling crystals → cathode Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  11. joulemeter Variable attenuator Variable iris LBO BBO x2 Double pulse generator with variable delay photodiodes Laser beamline additions • Standard running with magnification of 10→ gives large spot on cathode (1 cm)→ together with iris at the vacuum window a quasi flat beam with fringes but without pointing jitter • Imaging also of overfilled iris on laser table to cathode • Adjustable delay to reduce fringes • Additional features: • Variable attenuator • Joulemeter, photodiodes • Double pulse generator Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  12. UV diagnostics/doubler joulemeter variable attenuator to rf gun Pulse shaping iris λ/2 wave plate polarizing splitter telescope x2 Photodiode (phase) LBO/BBO prism UV Photo-diode Double Pulse Generator Streak Camera Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  13. Virtual cathode and iris aperture RF gun Iris Mirror Box Virtual cathode (Ce:YAG crystal, CCD camera Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  14. 4.4 mm 1.3 mm 4.0 mm Transverse Laser Pulse Shape • Transverse shape of the UV laser pulses is not TEMoo • Intentionally closer to flat hat to avoid spatial hole burning along the pulse train 1.6 mm Exit amplifers (IR) exit BGO on cathode (no iris) magnification ~ 5 PITZ shaping (on cathode) Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  15. Large iris (~open) Slices With iris in front of vacuum window • With additional magnification by 2 (total ~10) • Very good pointing stability, paid with fringes (~20% modulation) Nominal iris 2.5 mm diameter Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  16. Table Iris on cathode • Imaging the overfilled iris on laser table • Problem: larger pointing jitter than iris in front of vacuum window • Jitter about 0.5 of diameter min/max Distance iris – telescope wrong by ~1 cm Distance iris – telescope correct Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  17. Transverse Profile Stability • Transverse profile not really flat hat • Still noticeable pointing jitter (~10 % of spot size) • Achieved good pointing stability with an additional iris in front of vacuum window (70 cm from cathode) • Paid with interference fringes (20 % modulation) • For the present FLASH running scheme: stability is more important than perfect beam shape Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  18. Measured (UV, 262 nm) 120 Flat Top Fit FWHM = 19-24 ps rise/fall = 7-9 ps 100 80 60 amplitude 40 20 0 -20 0 10 20 30 40 50 60 Time (ps) Photon density (a.u.) 0 10 20 30 40 50 Time (ps) Laser Pulse Length and Shape • R&D laser at PITZ:Longitudinal flat-hat shape • Works fine in ‘lab environment’, not mature for a user facility • Present laser technology does not allow shorter rise/fall times • New development at MBI ongoing Longitudinal shape is Gaussian • Measurement with a streak camera (FESCA 200) sL= 4.4 ± 0.1 ps (at 262 nm) Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  19. Pulse Stacker Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  20. Stacker for Four Stacked Pulses Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  21. Pulse Stacker set-up on the laser table Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  22. Example of a Measured Profile Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  23. Series of Measured Profiles Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  24. Temperature Laser Room Temperature stability < 0.02 dgC- if people do not access laser room 0.05 dgC 3.5 days 6 months Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  25. Charge Stability Stability = 1.4% rms Problem:to maintain a stability < 2 % rms fine tuning of phase matching angle of the frequency conversion crystal green to UV (BBO) often required If not tuned frequently, stability drops to 5 % rms Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  26. RF Gun llrf with SIMCON • Amplitude and phase regulation by calculating the vector sum of forward and reflected power • FPGA based controller in operation since Dec 2005 • Phase stability of 0.14 dg of 1.3 GHz or 300 fs achieved 1 dg of 1.3 GHz = 2.1 ps Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  27. Cathode • Cathode: Cs2Te film on a molybdenum plug • RF contact with silver coated Cu-Be spring Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  28. QE maps • QE map: scan with small laser spot (200 µm) over the cathode (step size 300 µm) • useful to diagnose status of cathode uniformity • Example of a non-uniform cathode → We need to consider the uniformity of both, the laser beam profile and the cathode QE • May need a feedback based on beam profiles… 78.1 73.1 78.1 Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  29. End of lifetime QE < 0.5% Cathode lifetime • Initially high QE, drops slowly to levels in the percent range • Lifetime ends if QE < 0.5% • Typically 6 months, early 2006 we observed shorter lifetimes of 2 months, now longer again End of lifetime QE < 0.5% • Lifetime depends much on the vacuum condition in the gun Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  30. Long Term Running • TTF phase 1: 12/1998 to 12/2002→ 4 years of running (36,000 h or 13•107 s) • oscillator 25,000 h or 9.2•107 shots, amplifiers 16,000 h or 6.4•107 trains @ 1 Hz • Total on-time oscillator 70%, amplifiers 50% • TTF phase 2: upgrade end of 2003: diode pumped oscillator and preamplifiers • FLASH: Running since 3/2004 with 2 and now 5 Hz • about 3•108 y pulse trains with in average 20 pulses (av.) delivered for beam • Total on-time == always, laser runs even during the maintenance days, off only during maintenance weeks (~6 per year) Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  31. Conclusive remarks • Good experience with the present laser system • Design philosophy pays off • As simple as possible (avoid components which require frequent adjustments) • ‘Ready-to-sell’ finish and robust • Use well-known technology (laser material, pump sources) • Fully integration into the control system as a must • Capability of long running in accelerator environment is proven • Laser room and laser development are separated: laser is part of the machine (access only for maintenance/repair) • Diagnostics as complete as possible to assist operators Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

  32. Summary of measured parameters • Temperature stability < 0.02 dgC • Laser pulse energy: IR up to 300 uJ, UV up to 50 uJ • Sufficient for some nC for present QE of cathodes • Pulse length 12 ps (fwhh) (IR), gaussian shape, sigma=4.4 ± 0.1 ps (UV) • Charge stability 1 to 2 % rms (single bunch) • Bunch train up to 800 us long, 1 MHz, 500, 250, 200, 100, 50, 40, 20, 10 kHz implemented • Phase stability < 300 fs rms • Up to now 3•108 trains (in average with 20 pulses) delivered (upgraded laser) • Transverse pulse shape: potato with fringes (20 % modulation) • Very good pointing stability Siegfried Schreiber, DESY * Mini-Workshop Laser Pulse Shaping * 30-Nov-2006 to 1-Dec-2006, Zeuthen

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