Photoinjector Lasers forUltra-Bright Electron Sources

Graeme Hirst

STFC Central Laser Facility

Background

The ERLP photoinjector laser(with Marta Divall,Gary Markey and

Fay Hannon 2005)

The CTF3 photoinjector laser(developed with Marta Divalland Ian Ross - picture showsIan Musgrave andGabor Kurdi, 2006)

Laser beams can be characterised in terms of four parameters:

WAVELENGTH (photon energy, tunability)

TRANSVERSE PROFILE (intensity distribution, ‘pointing’ stability,could, perhaps, be dynamic ?)

POLARISATION (may be dictated by technology choices)

TEMPORAL PROFILE (pulse shape, time-structure of pulse train, timing jitter)

Low-Emittance PI Laser Requirements

WAVELENGTH

Low-Emittance PI Laser Requirements

Photon energy shouldexceed the photocathodework function by as littleas possible.

1 2 3 4 5Photon energy (eV)

NLO allows energymultiplication (2, 3 ...)

Nd

Yb

Ti:S

Cs:GaAs Cs2Te Mg Cu

OPA

and full tunability via OPA.

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.

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.

Rapid changes in the pulse need broadspectral bandwidth from the laser.

On a picosecond timescale pulse shaping bydivision, delay and stackingis also effective.

Mode control hardware can be complex andchallenging but is effective for pulse shaping,even if NLO is involved.

Data from M. Danailov, 2007

Low-Emittance PI Laser Requirements

Low emittance electron bunches may needunusually short drive laser pulses.

Coherent’s‘Silhouette’

providesfeedback control

of spectral amplitude and phase.

TEMPORAL PROFILE

Pulse shaping systems are now becoming commercially available.

Low-Emittance PI Laser Requirements

Dazzler AOphase andamplitudemodulatorfrom Fastlite

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.

Low-Emittance PI Laser Requirements

TRANSVERSE PROFILE

Laser oscillators tend to deliver Gaussian profile beams

Low-Emittance PI Laser Requirements

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

Optical squaring (refractive shaping or simple clipping) must balance inefficiency,limited depth of field, chromaticity and sensitivity to input beam variations

NLO frequency conversion efficiency is sensitive to beam direction which maybe rapidly varying near a high-intensity focus and tends to reverse the squaring

Data from M. Danailov, 2007

TRANSVERSE PROFILE

Laser oscillators tend to deliver Gaussian profile beams.

Low-Emittance PI Laser Requirements

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

Optical squaring (refractive shaping or simple clipping) must balance inefficiency,limited depth of field, chromaticity and sensitivity to input beam variations

NLO frequency conversion efficiency is sensitive to beam direction which maybe rapidly varying near a high-intensity focus and tends to reverse the squaring

Data from C.S. Chouet al, 2009

Transporting sharp-edged beams requires large numericalaperture and benefits from e.g. adaptiveoptics and image-relaying

TRANSVERSE PROFILE

Laser oscillators tend to deliver Gaussian profile beams.

Low-Emittance PI Laser Requirements

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

Optical squaring (refractive shaping or simple clipping) must balance inefficiency,limited depth of field, chromaticity and sensitivity to input beam variations

NLO frequency conversion efficiency is sensitive to beam direction which maybe rapidly varying near a high-intensity focus and tends to reverse the squaring

Transporting sharp-edged beams requires large numericalaperture and benefits from e.g. adaptiveoptics and image-relaying

Data fromD.H.Dowellet al, FEL09,Paper WEOA032009

TRANSVERSE PROFILE

Laser oscillators tend to deliver Gaussian profile beams.

Low-Emittance PI Laser Requirements

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

Optical squaring (refractive shaping or simple clipping) must balance inefficiency,limited depth of field, chromaticity and sensitivity to input beam variations

NLO frequency conversion efficiency is sensitive to beam direction which maybe rapidly varying near a high-intensity focus and tends to reverse the squaring

Transporting sharp-edged beams requires large numericalaperture and benefits from e.g. adaptiveoptics and image-relaying

Data fromD.H.Dowellet al, FEL09,Paper WEOA032009

Data fromD.H.Dowellet al, FEL09,Paper WEOA032009

TRANSVERSE PROFILE

Laser oscillators tend to deliver Gaussian profile beams.

Low-Emittance PI Laser Requirements

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

Optical squaring (refractive shaping or simple clipping) must balance inefficiency,limited depth of field, chromaticity and sensitivity to input beam variations

NLO frequency conversion efficiency is sensitive to beam direction which maybe rapidly varying near a high-intensity focus and tends to reverse the squaring

Transporting sharp-edged beams requires large numericalaperture and benefits from e.g. adaptiveoptics and image-relaying

Data fromD.H.Dowellet al, FEL09,Paper WEOA032009

TRANSVERSE PROFILE

Laser oscillators tend to deliver Gaussian profile beams.

Low-Emittance PI Laser Requirements

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

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.

Requirements for Practical PI LasersRELIABILITY AND UPTIMEFavours design simplicity, mature technologies, commercial laser systems,over-specification, low photon energy, high thermal efficiency

STABILITY AND CONTROLFor 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.

AVERAGE POWERProportional 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

Laser System Options

Nd:crystal (YAG, YLF, YVO4 ...)Pros: High power, mature, commercially available, diode or flash pumped,

compatible with fibre systemsCons: Slow temporal response (<1ps), thermal beam quality issues, low h

Nd:YLF photoinjector lasersare in use e.g. at FLASH,PITZ and CERN CTF3and Nd:YVO4 at ALICE

FLASH amplifier chain

PITZ

Ti:S photoinjectorlasers are in usee.g. at SPARC, LCLSand FERMI@Elettra

SPARC

LCLS

FERMI

Ti:SPros: Fastest temporal response of conventional lasers (~10fs), mature,

commercially available, some tunability, higher hCons: Complex, thermally inefficient, laser pumped, noisy (broad bandwidth,

sensitive modelocking, short upper), needs CPA

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 h,

may need cryo cooling, may need CPA

Laser System Options

Commercial Yb:fibre laser20Wave, 100J/pulse, 250fs FWHMRecently deployed at HZB BESSY(but not for photoinjection)

Clark-MXR Impulse

Yb:fibre photoinjector laser is inuse at Cornell ERL test facilityand Yb:YAG is in use with SC Pbcathode at HZB (T Kamps)

OP(CP)A driven by Nd or Yb or Ti:SPros: Tunable h (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

Laser System Options

OPA systems are in wideuse for spectroscopy and selectiveprocessing and are being soldinto industry and medicine

STFC CLF ULTRA

RIKEN

Conclusions• Photoinjector lasers have been developed over many years and are

now 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