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

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 The Photocathode RF Gun laser beam e- beam Cathode 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 Cs 2 Te cathode

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 Actual specs synchronization<1 dg of RF cycle<2 GHz< 1 ps rms longitudinal and transverse size ~5 dg RF → ~10 ps field uniformity → ~mm length 20 ps, Ø = 3 mm charge ~1 nC per bunch Cs 2 Te cathode QE ~ 1…10% (UV) ~1 factor of ~10 overhead long trains of pulses with low rep rate trains 800 µs long with up to 7200 pulses (9 10 Hz 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 Laser System Overview Not a commercial system, cooperation DESY/Max- Born-Institute, Berlin Conservative design: operationable in a ‘user’ environment f round trip = 27 MHz Modulators (AOM EOM AOM) 108 MHz 1.3 GHz 13.5 MHz Faraday isolator Piezo tuning of cavity length Stabilized by quartz tubes Fiber-coupled pump diodes Pulse picker Pockels cell Faraday isolator Fast current control Diode-pumped Nd:YLF Oscillator Diode pumped Nd:YLF amplifiers LBOBBO Flashlamp pumped Nd:YLF amplifiers IR→ UV Relay imaging telescopes Pulse picker E pulse = 0.3 µJ E pulse = 6 µJ E pulse < 0.3 mJ E pulse < 50 µJ Remote controlled attenuator Double pulse generator Beam shutter Imaging to the cathode Remote controlled mirror box

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 Pulse Train Oscillator (PTO) f round trip = 27 MHz Modulators (AOM EOM AOM) 108 MHz 1.3 GHz 13.5 MHz Piezo tuning of cavity length Stabilized by quartz tubes Fiber-coupled pump diodes Mode-locked pulsed oscillator  Diode pumped (32 W) Synchronized to 1.3 GHz master oscillator  1.3 GHz EO modulator with 2 AOMs ( MHz)  Phase stability < 300 fs rms  Pulse length 12 ps (fwhh) (IR) Stabilized with quartz rods  Thermal expansion coefficient fused 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 Chain of Linear Amplifiers 2 diode pumped and 2 flashlamp pumped single pass amplifiers Fully diode pumped version is running at PITZ 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% Laser diodes:  32 W pulsed, 805 nm  end pumped through fibers  energy from 0.3 µJ to 6 µJ/pulse 5 mm Fluorescence profile Xe flashlamps Nd:YLF rod, Ø 5 or 7 mm

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 Pulse Trains 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 Time Amplitude 800 µs Output of the laser oscillator (27 MHz) After amplification (1 MHz) Electron beam pulse train (30 bunches, 1 MHz) 1.2 nC

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 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 Lasing with long bunch trains 1 MHz 599 bunches 50 kHz 100 kHz 500 kHz 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 Laser beamline Compensation of path length addition in double pulse generator by telescope 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

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

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

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 Transverse Laser Pulse Shape 4.0 mm 4.4 mm on cathode (no iris) magnification ~ 5 exit BGO 1.6 mm 1.3 mm Transverse shape of the UV laser pulses is not TEM oo Intentionally closer to flat hat to avoid spatial hole burning along the pulse train PITZ shaping (on cathode) Exit amplifers (IR)

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

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 Laser Pulse Length and Shape Longitudinal shape is Gaussian Measurement with a streak camera (FESCA 200)  L = 4.4 ± 0.1 ps (at 262 nm) 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 Time (ps) Photon density (a.u.)

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 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 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 Controls

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 Temperature Laser Room Temperature stability < 0.02 dgC - if people do not access laser room 3.5 days 0.05 dgC 6 months

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 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 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 Cathode Cathode: Cs 2 Te 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 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…

Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006 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 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 Cathode lifetime End of lifetime QE < 0.5% Lifetime 6 months, early 2006 we observed shorter lifetimes of 2 months Problem for the laser: large change in output required 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 Long Term Running TTF phase 1: 12/1998 to 12/2002 → 4 years of running (36,000 h or s)  oscillator 25,000 h or shots, amplifiers 16,000 h or 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 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 Maintenance All components need maintenance and have to be replaced from time to time  Flashlamps: life time 1 to shots (1 month = )  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 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