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 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

Motivation Next generation of high-energy PW-class lasers Applications 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 Beamline Facility

Amplifier Design Considerations Requirement Pulses up to 1 kJ energy @ 10 Hz, few ns duration, overall > 10% Gain Medium 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 High surface-to-volume ratio Efficient cooling Low (overall) aspect ratio Minimise ASE Heat flow parallel to beam Minimise thermal lens

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

Amplifier Parameters Quasi-3 level model Results 1D, time-dependent model Spectral dependence (abs.) included Assume Fmax = 5 J/cm2 for ns 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 Inputs Pump intensity (each side) 5 kW/cm2 Dlpump 5 nm FWHM Pump duration 1 ms Temperature 175 K Results Optimum doping x length 3.3 %cm Storage efficiency 50 % 5 J/cm2 stored Small signal gain (G0) 3.8 Optimum Aspect ratio# constant doping 0.78 graded doping 1.55 Inputs Pump intensity (each side) 5 kW/cm2 Dlpump 5 nm FWHM Pump duration 1 ms Temperature 175 K # Aperture / length

Amplifier Design Parameters HiPER HiLASE / ELI Extractable energy ~ 1 kJ ~ 100 J Aperture 14 x 14 cm 200 cm2 4.5 x 4.5 cm 20 cm2 Aspect ratio 1.4 1.3 No. of slabs 10 7 Slab thickness 1 cm 0.5 cm No. of doping levels 5 4 Average doping level 0.33 at.% 0.97 at.% HiPER HiLASE / ELI Prototype DiPOLE Extractable energy ~ 1 kJ ~ 100 J ~ 20 J Aperture 14 x 14 cm 200 cm2 4.5 x 4.5 cm 20 cm2 2 x 2 cm 4 cm2 Aspect ratio 1.4 1.3 1 No. of slabs 10 7 4 Slab thickness 1 cm 0.5 cm No. of doping levels 5 2 Average doping level 0.33 at.% 0.97 at.% 1.65 at.%

DiPOLE Prototype Progress to date 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 Cr4+ Yb3+ 35 mm 55 mm Pump 2 x 2 cm²

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

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

Vacuum insulated transfer Cryo-cooling System Vacuum insulated transfer lines Cryostat Amplifier head Helium cooling circuit

Diode Pump Laser Built by Consortium Specifications 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

Diode Pump Laser Beam profile specification Demonstrated performance Spatial profiles (Modelled) Near Field Far Field 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 Preliminary measurement

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

Next Steps Short-term (3 to 6 months) Long-term (6 to 12 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

Any Questions ?

Yb-doped Materials Parameter (at RT) Glass S-FAP YAG CaF2 Wavelengths (pump/emission in nm) 940-980 / 1030 900 / 1047 940 / 1030 Fluorescence lifetime (msec) ~ 2.0 ~ 1.3 ~ 1.0 ~ 2.4 Emission cross-section (peak x10-20 cm2) 0.7 6.2 3.3 0.5 Gain Low High Medium Non-linear index (n2 x 10-13 esu) 0.1 to 1.2 1.5 2.7 0.43 Bandwidth > 50 nm OK? Availability of large aperture Good Limited OK (Ceramic) (Ceramic) Thermal properties (K in Wm-1K-1) Poor 1.0 OK 2.0 10.5 6.1

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

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

Doping Profile (1 kJ Amplifier)

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

Absorption Spectra Factor of 2 940 nm 1030 nm

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 a b c Nothing!

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 Beam Transport Reverser 1 Reverser 2 Injection Extraction Amp 1 Amp 2 Injection Amp1 + 2 Reverser 1 Reverser 2 Extraction Losses: 17.4 % (distributed) 10 %