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Diode Pumped Cryogenic High Energy Yb -Doped Ceramic YAG Amplifier for Ultra-High Intensity Applications

Diode Pumped Cryogenic High Energy Yb -Doped Ceramic YAG Amplifier for Ultra-High Intensity Applications . P. D. Mason , S. Banerjee, K. Ertel, P. J. Phillips, C.Hernandez-Gomez, J. Collier ICUIL 2010 Conference September 26 th to October 1 st 2010, Watkins Glen, NY, USA

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Diode Pumped Cryogenic High Energy Yb -Doped Ceramic YAG Amplifier for Ultra-High Intensity Applications

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  1. Diode Pumped Cryogenic High EnergyYb-Doped Ceramic YAG Amplifier forUltra-High Intensity Applications P. D. Mason, S. Banerjee, K. Ertel, P. J. Phillips, C.Hernandez-Gomez, J. Collier ICUIL 2010 Conference September 26th to October 1st 2010, Watkins Glen, NY, USA paul.mason@stfc.ac.uk R1 2.62 Central Laser Facility STFC, Rutherford Appleton Laboratory, OX11 0QX, UK +44 (0)1235 778301

  2. Motivation • Next generation of high-energy PW-class lasers • Multi-Hz repetition rate • Multi-% wall-plug efficiency • Applications • Ultra-intense light-matter interactions • Particle acceleration • Intense X-ray generation • Inertial confinement fusion • High-energy DPSSL amplifiers needed • Pumping fs-OPCPA or Ti:S amplifiers • Drive laser for ICF BeamlineFacility

  3. Amplifier Design Considerations • Requirement • Pulses up to 1 kJ energy @ 10 Hz, few ns duration, overall > 10% • Gain Medium • Amplifier Geometry

  4. Amplifier Concept • Ceramic Yb:YAG gain medium (slabs) • Best compromise to meet requirements • Possibility of compound structures for ASE suppression • Distributed face-cooling by stream of cold He gas • Heat flow along beam direction • Low overall aspect ratio & high surface area • Coolant compatible with cryo operation • Operation at cryogenic temperatures • Reduced re-absorption, higher o-o efficiency • Increased gain cross-section • Better thermo-optical & thermo-mechanical properties • Graded doping profile • Reduced overall thickness (up to factor of ~2) • Lower B-integral • Equalised heat load for slabs

  5. Amplifier Parameters • Quasi-3 level model • 1D, time-dependent model • Spectral dependence (abs.) included • Assume Fmax = 5 J/cm2 forns pulses in YAG • Results • Optimum doping x length product  maximum storage efficiency ~ 50% • Optimum aspect ratio to ensure g0D  3  minimise risk of ASE • Highly scalable concept • Just need to hit correct aspect ratio & doping # Aperture / length

  6. Amplifier Design Parameters

  7. DiPOLE Prototype Cr4+ Pump2 x 2 cm² • Diode Pumped Optical Laser for Experiments • 10 to 20 Joule prototype laboratory test bed • 4 x co-sintered ceramic Yb:YAG slabs • Circular 55 mm diameter x 5 mm thick • Cr4+ cladding for ASE management • Two doping concentrations 1.1 & 2.0 at.% • Progress to date • Ceramic discs characterised • Amplifier head designed & built • CFD modelling of He gas flow • Pressure testing • Cryo-cooling system completed • Diode pump lasers being assembled • Lab. refit near completion 55 mm 35 mm Yb3+

  8. Ceramic Yb:YAG Discs • Transmission spectra • Uncoated, room temperature • Transmitted wavefront Fresnel limit ~84% PV 0.123 wave 1030 nm 940 nm

  9. Amplifier Head • CFD modelling • Predicted temperature gradient in Yb:YAG amplifier disc • Head layout Vacuum vessel 1.1% Uniform T across pumped region ~ 3K Pump 2 cm Pump 2.0% He flow He flow

  10. Cryo-cooling System Vacuum insulated transfer lines Helium cooling circuit Amplifierhead Cryostat

  11. Diode Pump Laser • Built by Consortium • Ingeneric: Opto-mechanical design & build • Amtron: Power supplies & control system • Jenoptic: Laser diode modules • Specifications • 2 pump units – left & right handed • 0 = 940 nm, FWHM < 6 nm • Peak power 20 kW • Pulse duration 0.2 to 1.2 ms • Pulse repetition rate variable 0.1 to 10 Hz • Other specs. independent of PRF

  12. Diode Pump Laser Spatial profiles (Modelled) • Beam profile specification • Uniform square profile • Steep profile edges • Low (<10°) symmetrical divergence • Demonstrated performance • Square beam shape • Low-level intensity modulations • Steep edge profiles • 20 kW peak output power • High confidence that other specifications will be demonstrated shortly Near Field Far Field Preliminary measurement

  13. Lab Layout LN2 tank Cryo-cooling system Optical tables Floor area ~30 m2 Amplifier

  14. Next Steps • Short-term (3 to 6 months) • Complete lab. refit • Install & test cryo-cooler & diode pump lasers • Characterise amplifier over range of temperature & flow conditions • Spectral measurements (absorption, fluorescence) • Thermo-optical distortions (aberrations, thermal lensing etc.) • Opto-mechanical stability • Small signal gain & ASE assessment • Long-term (6 to 12 months) • Specify and build front-end system • Shaped seed oscillator & regen. amplifier • Complete design of multi-pass extraction architecture (8 passes) • Amplify pulses • Demonstrate >10 J, 10 Hz, >25 % o-o efficiency

  15. Any Questions ?

  16. Yb-doped Materials

  17. Yb:YAG Energy Level Diagrams • Cryogenic cooling (175K) Quasi-3 Level 4 Level-like • Room temperature (300K) 2F5/2 2F5/2 Low quantum defect (QD)p/ las~ 91% Re-absorption loss 940 nm 1030 nm 940 nm 1030 nm Significantly reduced re-absorption loss f13=4.6% f13=0.64% 2F7/2 2F7/2 Yb3+ Yb3+

  18. Temperature Dependence Operating fluence T=175K Storage Efficiency (%) Small Signal Gain T=300K Pump Fluence (J/cm²)

  19. Doping Profile (1 kJ Amplifier)

  20. Pump Absorption Absorption + Pump Spectra Efficiency vs. lpump 175 K 300 K Pump, FWHM = 5nm 175 K, 10 kW/cm2 300 K, 20 kW/cm2

  21. Absorption Spectra Factor of 2 940 nm 1030 nm

  22. Ceramic YAG with Absorber Cladding Sample of Co-Sintered YAG (Konoshima) Reflection at Interface? Camera Laser Cr4+:YAG a b c ? Yb:YAG Yb:YAG Cr4+:YAG c b a Nothing!

  23. Beamline Efficiency Modelling • Beamline parameters • 2 amplifiers, 4-passes • 1% loss between slabs, 10% loss after each pass (reverser & extraction) • Losses in pump optics ignored Amp 2 Amp 1 Extraction Beam Transport Reverser 2 Reverser 1 Injection Injection Amp1 + 2 Reverser 1 Amp1 + 2 Reverser 2 Amp1 + 2 Reverser 1 Amp1 + 2 Extraction 17.4 % (distributed) 17.4 % (distributed) 17.4 % (distributed) Losses: 17.4 % (distributed) 10 % 10 % 10 % 10 %

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