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Drive-Laser Experience at FLASH

LCLS Injector Commissioning Workshop (ICW) October 9-11, 2006. Drive-Laser Experience at FLASH. Overview of the laser system Running experience Remarks. 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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Drive-Laser Experience at FLASH

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  1. LCLS Injector Commissioning Workshop (ICW) October 9-11, 2006 Drive-Laser Experience at FLASH Overview of the laser system Running experience Remarks

  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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  4. Laser System Overview Diode-pumped Nd:YLF Oscillator Not a commercial system, cooperation DESY/Max-Born-Institute, Berlin Conservative design: operationable in a ‘user’ environment 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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  6. 5 m m Chain of Linear Amplifiers • 2 diode pumped and 2 flashlamp pumped single pass amplifiers • Fully diode pumped version is running 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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 Fluorescence profile

  7. 1.2 nC Electron beam pulse train (30 bunches, 1 MHz) Pulse Trains 800 µs After amplification (1 MHz) • 2 Pulse pickers, based on Pockels cell + polarizer → up to 3 MHz • Preamplification (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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  8. Bunch Pattern and MPS • Change bunch pattern on user request • Number of bunches • Different bunch frequencies: 1 MHz, 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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  9. 50 kHz 1 MHz 599 bunches 100 kHz 500 kHz Lasing with long bunch trains • This year: up to 600 bunches to beam dump • 1 MHz, 500 kHz, 100 kHz, and 50 kHz • Lasing with at least 450 bunches • Problems: • Toroid protection system not yet in operation, emulated by software • Improvements required for photon diagnostics • Beam loading compensation was at limit for ACC2/3 and ACC1 for 800 µs flat top • Activation of beam line components due to darkcurrent, mostly from rf gun Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  10. Laser beamline • Relay imaging with spatial filtering • Hard edge aperture after diode pumped amplifiers • Aperture imaged to → amplifier heads → doubling crystals → cathode 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 Compensation of path length addition in double pulse generator by telescope Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  11. UV diagnostics/doubler variable attenuator telescope x2 Movable mirrors/ splitters λ/2 wave plate polarizing splitter to rf gun prism UV Photo-diode Double Pulse Generator Streak Camera joulemeter Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  12. Virtual cathode and iris aperture RF gun Iris Mirror Box Virtual cathode (Ce:YAG crystal, CCD camera Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  13. 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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  14. On cathode • With additional magnification by 2 (total ~10) and remote controlled iris aperture Large iris (~open) Nominal iris 3.0 mm diameter Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  15. 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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  16. Vacuum mirror • Best would be to have high quality dielectric mirror • However, darkcurrent may charge it up • Charging and discharging effects beam orbit and destroys the mirror → after some tries with Al shielding we have now a good quality solid aluminum mirror (R=90%) → no long time experience with long pulse trains yet (heating due to absorption, ablation) TTF1 dielectric mirror with discharge traces Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  17. Controls • Laser is fully integrated into the control system – say almost, still pieces missing (rf phases, BBO, diodes) • Controlled by a Sparc cpu running Solaris in a VME crate, mixed standard (timing, ADCs) and special equipment (ns delays, …) • Server running real time operations and controls • Server handing over parameters to controls • Some other servers for specific tasks • Interlocks controlled by an SPS system with an interface to controls (cpu, cooling water flow, temperatures, door contacts etc) • SPS controlled special mode to allow laser in tunnel while having access • Operators have easy control of the laser with options of detailed access to parameters • Easy: number of laser pulses, bunch frequency, attenuator • Expert: timing, pump parameters, feedback parameters etc Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  18. 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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  19. 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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  20. 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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  21. Cathode • Cathode: Cs2Te film on a molybdenum plug • RF contact with silver coated Cu-Be spring Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  22. 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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  23. Alignment of Laser • Initial alignment of laser beam onto the cathode using a scintillator mounted into the plug • Fine adjustment with • Beam based alignment (electron beam position as a function of rf phase for low charge, solenoids off)→ time consuming (2 shifts) and difficult, but yields center in respect to rf, precise • Scan laser over cathode→ scan is fast, but assumes that cathode film is centered in respect to the rf, correction less accurate • Virtual cathode helps to recover alignment and to monitor movements and pointing • However, for high QE cathodes we need an intensified and gated camera to have a permanent monitoring darkcurrent ring may also be used for rough alignment Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  24. Quantum Efficiency • Example of a QE measurement:charge at rf gun exit as a function of laser pulse energy • QE = 4.8 % in this case Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  25. End of lifetime QE < 0.5% Cathode lifetime • Lifetime 6 months, early 2006 we observed shorter lifetimes of 2 months • Problem for the laser: large change in output required End of lifetime QE < 0.5% • Variable attenuator useful in order not to change laser operational parameters of the laser too much Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  26. 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 shots/trains with 20 pulses av. delivered for beam • Total on-time == always, even mostly during the maintenance days (4 %), off during maintenance weeks (~6 per year) • Backup system, fully diode pumped soon Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  27. Maintenance • All components need maintenance and have to be replaced from time to time • Flashlamps: life time 1 to 5 107 shots (1 month = 1.3 107) • Pockels cell drivers: roughly once per year • Flashlamp power supplies: once per 3 years • Laser crystals: once per 5 years • LBO/BBO: once per 5 years • Mirrors: no change required yet • Control hardware: frequent changes, cpu, ADCs, other boards • Diode lasers: no exchange yet (20,000 h now) • Cooling water/equipment: often • Component checks once per week, occasional adjustments of timing and other parameters Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

  28. Conclusive remarks • Good experience with the present laser system • Advantage of collaboration with one laser institute: permanent support and ongoing development to your needs, no overhead • Risk: no quick alternative if cooperation ceases • However, similar argument holds for companies in case of complex laser systems • Design philosophy • 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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006

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