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Motivation

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

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Motivation

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  1. 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

  2. Motivation • Next generation of high-energy PW-class lasers • 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 BeamlineFacility • HELMHOLTZ- BEAMLINE

  3. 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

  4. 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)

  5. Modelling Cr4+:YAG 50% pump region • Laser physics • 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 3.8 Yb:YAG

  6. Scalable Design

  7. DiPOLE Prototype Amplifier • 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 Ceramic YAG disk with absorber cladding Yb3+ Cr4+ Diode pump laser

  8. Optical Gain Material Cr4+ Pump2 x 2 cm² • 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.%) 55 mm 35 mm Yb3+ Fresnel limit ~84% PV 0.123 wave 1030 nm 940 nm

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

  10. Diode Pump Laser • Built by Consortium • 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 absorptionspectrum @ 175K Measured 20 mm 20 mm

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

  12. Front-end Injection Seed Amplifier crystal nsecoscillator • 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 boosteramplifier • 6 or 8 pass configuration • Output energy ~ 100 mJ Booster pump diode x3 or x4 Polarisation switching waveplate 100 mJ output

  13. 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 Seed Amplified beam Pump Pump

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

  15. Pulse Energy v. Pump Pulse Duration Onset of ASE loss • 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

  16. 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 toat 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

  17. Thank you for your attention! Any Questions?

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