Photoinjector Lasers for Ultra-Bright Electron Sources

1. Photoinjector Lasers forUltra-Bright Electron Sources • Graeme Hirst • STFC Central Laser Facility

2. Background The CTF3 photoinjector laser (developed with Marta Divalland Ian Ross - picture showsIan Musgrave andGabor Kurdi, 2006) The ERLP photoinjector laser (with Marta Divall, Gary Markey and Fay Hannon 2005)

3. Low-Emittance PI Laser Requirements Laser beams can be characterised in terms of four parameters: WAVELENGTH (photon energy, tunability) POLARISATION (may be dictated by technology choices) TEMPORAL PROFILE (pulse shape, time-structure of pulse train, timing jitter) TRANSVERSE PROFILE (intensity distribution, ‘pointing’ stability, could, perhaps, be dynamic ?)

4. Low-Emittance PI Laser Requirements Cs:GaAs Cs2Te Mg Cu WAVELENGTH OPA Photon energy shouldexceed the photocathodework function by as littleas possible. Ti:S Yb NLO allows energy multiplication (2w, 3w ...) Nd and full tunability via OPA. 1 2 3 4 5 Photon energy (eV) But it adds complexity, reduces efficiencyand can compromise stability, beam quality and reliability. POLARISATION Doesn’t affect photoelectron production so is, in principle a free parameter.But in practice NLO is polarisation-sensitive and cathode absorption may be too.

5. Low-Emittance PI Laser Requirements TEMPORAL PROFILE Repetitive picosecond/femtosecond pulses are generated by phase-lockingthe discrete frequency-domain modes of an optical cavity. Fourier relates the pulse shape to theindividual modes’ amplitudes and phaseswhich are limited by the laser medium’sgain profile but which are alsoindependently controllable. n Rapid changes in the pulse need broadspectral bandwidth from the laser. Low emittance electron bunches may needunusually short drive laser pulses. Mode control hardware can be complex andchallenging but is effective for pulse shaping,even if NLO is involved. On a picosecond timescale pulse shaping bydivision, delay and stackingis also effective. Data from M. Danailov, 2007

6. Low-Emittance PI Laser Requirements TEMPORAL PROFILE Pulse shaping systems are now becoming commercially available. Dazzler AOphase andamplitudemodulatorfrom Fastlite Coherent’s‘Silhouette’ providesfeedback control of spectral amplitude and phase.

7. Low-Emittance PI Laser Requirements TEMPORAL PROFILE But in the end there is no point in temporally shaping the laser pulseon timescales much faster than the response time of the photocathode.

8. Low-Emittance PI Laser Requirements TRANSVERSE PROFILE Laser oscillators tend to deliver Gaussian profile beams Amplifier saturation tends to ‘square off’ beams in the near field provided: • Gain media are uniformand pumping is stableand well-profiled • Transport optics and mediaare good-quality and clean • Diffraction is managed • NLO self-focusing is managed Data from M. Danailov, 2007 NLO frequency conversion efficiency is sensitive to beam direction which may be rapidly varying near a high-intensity focus and tends to reverse the squaring Optical squaring (refractive shaping or simple clipping) must balance inefficiency,limited depth of field, chromaticity and sensitivity to input beam variations

9. Low-Emittance PI Laser Requirements TRANSVERSE PROFILE Laser oscillators tend to deliver Gaussian profile beams. Amplifier saturation tends to ‘square off’ beams in the near field provided: • Gain media are uniformand pumping is stableand well-profiled • Transport optics and mediaare good-quality and clean Data from C.S. Chouet al, 2009 • Diffraction is managed • NLO self-focusing is managed NLO frequency conversion efficiency is sensitive to beam direction which may be rapidly varying near a high-intensity focus and tends to reverse the squaring Optical squaring (refractive shaping or simple clipping) must balance inefficiency,limited depth of field, chromaticity and sensitivity to input beam variations Transporting sharp-edged beams requires large numericalaperture and benefits from e.g. adaptiveoptics and image-relaying

10. Low-Emittance PI Laser Requirements TRANSVERSE PROFILE Laser oscillators tend to deliver Gaussian profile beams. Amplifier saturation tends to ‘square off’ beams in the near field provided: Data fromD.H.Dowellet al, FEL09, Paper WEOA032009 • Gain media are uniformand pumping is stableand well-profiled • Transport optics and mediaare good-quality and clean • Diffraction is managed • NLO self-focusing is managed NLO frequency conversion efficiency is sensitive to beam direction which may be rapidly varying near a high-intensity focus and tends to reverse the squaring Optical squaring (refractive shaping or simple clipping) must balance inefficiency,limited depth of field, chromaticity and sensitivity to input beam variations Transporting sharp-edged beams requires large numericalaperture and benefits from e.g. adaptiveoptics and image-relaying

11. Low-Emittance PI Laser Requirements TRANSVERSE PROFILE Laser oscillators tend to deliver Gaussian profile beams. Amplifier saturation tends to ‘square off’ beams in the near field provided: Data fromD.H.Dowellet al, FEL09, Paper WEOA032009 • Gain media are uniformand pumping is stableand well-profiled • Transport optics and mediaare good-quality and clean • Diffraction is managed • NLO self-focusing is managed NLO frequency conversion efficiency is sensitive to beam direction which may be rapidly varying near a high-intensity focus and tends to reverse the squaring Optical squaring (refractive shaping or simple clipping) must balance inefficiency,limited depth of field, chromaticity and sensitivity to input beam variations Transporting sharp-edged beams requires large numericalaperture and benefits from e.g. adaptiveoptics and image-relaying

12. Low-Emittance PI Laser Requirements TRANSVERSE PROFILE Laser oscillators tend to deliver Gaussian profile beams. Amplifier saturation tends to ‘square off’ beams in the near field provided: Data fromD.H.Dowellet al, FEL09, Paper WEOA032009 • Gain media are uniformand pumping is stableand well-profiled Projected emittance(microns rms) • Transport optics and mediaare good-quality and clean • Diffraction is managed • NLO self-focusing is managed NLO frequency conversion efficiency is sensitive to beam direction which may be rapidly varying near a high-intensity focus and tends to reverse the squaring Optical squaring (refractive shaping or simple clipping) must balance inefficiency,limited depth of field, chromaticity and sensitivity to input beam variations Transporting sharp-edged beams requires large numericalaperture and benefits from e.g. adaptiveoptics and image-relaying

13. Low-Emittance PI Laser Requirements TRANSVERSE PROFILE Laser oscillators tend to deliver Gaussian profile beams. Amplifier saturation tends to ‘square off’ beams in the near field provided: Data fromD.H.Dowellet al, FEL09, Paper WEOA032009 • Gain media are uniformand pumping is stableand well-profiled Projected emittance(microns rms) • Transport optics and mediaare good-quality and clean • Diffraction is managed • NLO self-focusing is managed If required laser designers can generate transverse profiles which arebetter controlled than the ‘standard’ commercial product But in the end there is no point in spatially shaping the laser pulse tomake it much more uniform than the QE profileof the photocathode.

14. Requirements for Practical PI Lasers RELIABILITY AND UPTIME Favours design simplicity, mature technologies, commercial laser systems, over-specification, low photon energy, high thermal efficiency AVERAGE POWER Proportional to average beam current and to photocathode QE, affects costand reliability, removing heat from the cathode may be an issue1mA with 1% QE requires 6×1017 ph/s which is 0.25W (green) or 0.5W (UV)~10W (IR) short pulse lasers are commercially availableMilitates against the use of low-QE cathodes STABILITY AND CONTROL For low emittance the laser must stay within a very small parameter space, requiring high intrinsic stability plus a multi-parameter feedback control system (timing jitter, temporal pulse shaping, adaptive beam shaping and pointing,environmental control e.g. temp, vibration, utilities (power, cooling, gas purge))applied to the whole optical transport system, not just the laser.Individual FCS’s are commercially availablebut integrated suites are not.

15. Laser System Options Nd:crystal (YAG, YLF, YVO4 ...) Pros: High power, mature, commercially available, diode or flash pumped, compatible with fibre systems Cons: Slow temporal response (<1ps), thermal beam quality issues, low hn FLASH amplifier chain Nd:YLF photoinjector lasersare in use e.g. at FLASH,PITZ and CERN CTF3and Nd:YVO4 at ALICE PITZ

16. Laser System Options Ti:S Pros: Fastest temporal response of conventional lasers (~10fs), mature, commercially available, some tunability, higher hn Cons: Complex, thermally inefficient, laser pumped, noisy (broad bandwidth, sensitive modelocking, short tupper), needs CPA SPARC Ti:S photoinjectorlasers are in usee.g. at SPARC, LCLSand FERMI@Elettra LCLS FERMI

17. Laser System Options Yb:glass, crystal (YAG, SFAP ...) or ceramic Pros: High power, diode pumped, can be largely fibre-based, efficient, quite fast temporal response (~100fs) Cons: Less mature but with some commercial availability, low hn, may need cryo cooling, may need CPA Clark-MXR Impulse Commercial Yb:fibre laser20Wave, 100mJ/pulse, 250fs FWHMRecently deployed at HZB BESSY(but not for photoinjection) Yb:fibre photoinjector laser is in use at Cornell ERL test facility and Yb:YAG is in use with SC Pbcathode at HZB (T Kamps)

18. Laser System Options OP(CP)A driven by Nd or Yb or Ti:S Pros: Tunable hn (real-time tuning not generally required) Cons: Inefficient, complex, can be noisy, can be prone to optical damage, temporal control is less mature, low QE may demand very high power STFC CLF ULTRA OPA systems are in wideuse for spectroscopy and selectiveprocessing and are being soldinto industry and medicine RIKEN

19. Conclusions • Photoinjector lasers have been developed over many years and arenow driving guns with state-of-the-art electron bunch brightness • Further improvements to increase the brightness are likely to involve control of at least three of the lasers’ fundamental parameters: • Wavelength (fine-tuned close to work function) • Temporal profile (shortened and/or shaped) • Spatial profile (smoothed and shaped to reduce electrons’ transverse momentum) • There is ‘headroom’ left to do this • Keeping the laser’s performance inside the necessarily small parameterspace will require both high intrinsic stability and tight feedback control • As well as delivering the technical advances photoinjector laser scientistsneed to satisfy demanding operational needs. The inevitable conflictcomplicates the choice between commercial andnon-commercial vendors