A Scalable Design for a High Energy, High Repetition Rate, Diode-Pumped Solid State Laser (DPSSL) Amplifier Paul Mason, Klaus Ertel, Saumyabrata Banerjee, Jonathan Phillips, Cristina Hernandez-Gomez, John Collier Workshop on Petawatt Lasers at Hard X-Ray Light Sources 5-9th September 2011, Dresden, Germany paul.mason@stfc.ac.uk STFC Rutherford Appleton Laboratory, Centre for Advanced Laser Technology and Applications R1 2.62 Central Laser Facility, OX11 0QX, UK +44 (0)1235 778301

Motivation Next generation of high-energy PW-class lasers Exploitation Multi-J to kJ pulse energy Multi-Hz repetition rate Multi-% wall-plug efficiency Exploitation Ultra-intense light-matter interactions Particle acceleration Inertial confinement fusion High-energy DPSSL amplifiers needed Pumping fs-OPCPA or Ti:S amplifiers Drive laser for ICF Pump technology for HELMHOLTZ-BEAMLINE Beamline Facility HELMHOLTZ- BEAMLINE

Amplifier Design Considerations Requirements Pulses from 10’s J to 1 kJ, 1 to 10 Hz, few ns duration, efficiency 1 to 10% Gain Medium Ceramic Yb:YAG down-selected as medium of choice Amplifier Geometry Long fluorescence lifetime Higher energy storage potential Minimise number of diodes (cost) Available in large size Handle high energies Good thermo-mechanical properties Handle high average power Sufficient gain cross section Efficient energy extraction Low quantum defect Increased efficiency & reduced heat load High surface-to-volume ratio Efficient cooling Low (overall) aspect ratio Minimise ASE Heat flow parallel to beam Minimise thermal lens

STFC Amplifier Concept ~175K Diode-pumped multi-slab amplifier Ceramic Yb:YAG gain medium Co-sintered absorber cladding for ASE suppression Distributed face-cooling by stream of cold He gas Heat flow along beam direction Low overall aspect ratio & high surface area Operation at cryogenic temperatures Higher o-o efficiency – reduction of re-absorption Increased gain cross-section Better thermo-optical & thermo-mechanical properties Graded doping profile Equalised heat load in each slab Reduces overall thickness (up to factor of ~2)

Modelling Laser physics Thermal & fluid mechanics Assumptions Target output fluence 5 J/cm² Pump 940 nm, laser 1030 nm Efficiency & gain Optimum doping x length product for maximum storage ~ 50% Optimum aspect ratio to minimise risk of ASE (g0D < 3) of ~1.5 Extraction Extraction efficiency ~ 50% Thermal & fluid mechanics Temperature distribution Stress analysis Optimised He flow conditions 50% 3.8 pump region Cr4+:YAG Yb:YAG

Scalable Design HiPER HiLASE / ELI / XFEL Extractable energy ~ 1 kJ Aperture 14 x 14 cm 200 cm2 5 x 5 cm 25 cm2 Aspect ratio 1.4 1.2 No. of slabs 10 6 Slab thickness 1 cm 0.7 cm No. of doping levels 5 3 Average doping level 0.33 at.% 0.79 at.% HiPER HiLASE / ELI Prototype DiPOLE Extractable energy ~ 1 kJ ~ 100 J ~ 10 J Aperture 14 x 14 cm 200 cm2 5 x 5 cm 25 cm2 2 x 2 cm 4 cm2 Aspect ratio 1.4 1.2 1 No. of slabs 10 6 4 Slab thickness 1 cm 0.7 cm 0.5 cm No. of doping levels 5 3 2 Average doping level 0.33 at.% 0.79 at.% 1.65 at.%

DiPOLE Prototype Amplifier Cr4+ Yb3+ Ceramic YAG disk with absorber cladding Design sized for ~ 10 J @ 10 Hz Aims Validate & calibrate numerical models Quantify ASE losses Test cryogenic gas-cooling technology Test (other) ceramic gain media Demonstrate viability of concept Progress to date Cryogenic gas-cooling system commissioned Amplifier head, diode pump lasers & front-end installed Full multi-pass relay-imaging extraction architecture under construction Initial pulse amplification tests underway Diode pump laser

Optical Gain Material 4 x co-sintered ceramic Yb:YAG disks Circular 55 mm diameter x 5 mm thick Cr4+ absorbing cladding Two doping concentrations (1.1 & 2.0 at.%) Cr4+ Yb3+ 35 mm 55 mm Pump 2 x 2 cm² Fresnel limit ~84% PV 0.123 wave 940 nm 1030 nm

Amplifier Head Design Schematic CFD modelling Vacuum Disks He flow 40 m3/hr ~ 25 m/s @ 10 bar, 175 K Pump Vacuum Disks pressure windows vacuum Uniform T across pumped region ~ 3K He flow

Diode Pump Laser Built by Consortium Two systems supplied Ingeneric, Amtron & Jenoptic Two systems supplied 0 = 939 nm, FWHM < 6 nm Peak power 20 kW, 0.1 to 10 Hz Pulse duration 0.2 to 1.2 ms Uniform square intensity profile Steep well defined edges ~ 80 % spectral power within  3 nm Good match to Yb:YAG absorption spectrum @ 175K Measured 20 mm

DiPOLE Laboratory Cryo-cooling system Amplifier head 2 x 20 kW diode pump lasers Amplifier head

Polarisation switching waveplate Front-end Injection Seed Free-space diode-pumped MOPA design Built by Mathias Siebold’s team @ HZDR Germany Cavity-dumped Yb:glass oscillator Tuneable 1020 to 1040 nm  ~ 0.2 nm Fixed temporal profile Duration 5 to 10 ns PRF up to 10 Hz Output energy up to 300 µJ Multi-pass Yb:YAG booster amplifier 6 or 8 pass configuration Output energy ~ 100 mJ nsec oscillator Booster pump diode Amplifier crystal 100 mJ output Polarisation switching waveplate x3 or x4

Initial Pulse Amplification Results Simple bow tie extraction architecture 1, 2 or 3 passes Limited by diffraction effects Injection seed Gaussian beam expanded to overfill pump region Energy ~ 60 mJ Pump Seed Amplified beam

Spatial Beam Profiles @ 100K, 1 Hz Gain  8 Gain  6 E = 2.6 J @ 10 Hz

Pulse Energy v. Pump Pulse Duration 3 passes @ 1 Hz Relay-imaging multi (6 to 8) pass extraction architecture is required to allow >10 J energy extraction at 175K 5.9 J Onset of ASE loss

Conclusions Cryogenic gas cooled Yb:YAG amplifier offers potential for efficient, high energy, high repetition rate operation At least 25% optical-to-optical efficiency predicted Proposed multi-slab architecture should be scalable to at least 1 kJ generating ns pulses at up to 10 Hz Limit to scaling is acceptable B-integral DiPOLE prototype amplifier shows very promising results Installation of relay-imaging multi-pass should deliver 10 J @ 10 Hz Strong candidate pump technology for generating high energy, ns pulses at ~ 1 Hz for HELMHOLTZ-BEAMLINE

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