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 R Central Laser Facility STFC, Rutherford Appleton Laboratory, OX11 0QX, UK +44 (0)

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 Beamline Facility

Amplifier Design Considerations Requirement –Pulses up to 1 kJ 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 sizeHandle high energies Good thermo-mechanical propertiesHandle high average power Sufficient gain cross sectionEfficient energy extraction High surface-to-volume ratioEfficient cooling Low (overall) aspect ratioMinimise ASE Heat flow parallel to beamMinimise 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

Inputs Pump intensity (each side)5 kW/cm 2  pump 5 nm FWHM Pump duration1 ms Temperature175 K Results Optimum doping x length3.3 %cm Storage efficiency 50 % 5 J/cm 2 stored Small signal gain (G 0 )3.8 Optimum Aspect ratio # constant doping0.78 graded doping1.55 Amplifier Parameters Quasi-3 level model –1D, time-dependent model –Spectral dependence (abs.) included –Assume F max = 5 J/cm 2 for ns pulses in YAG Results –Optimum doping x length product  maximum storage efficiency ~ 50% –Optimum aspect ratio to ensure g 0 D  3  minimise risk of ASE Highly scalable concept –Just need to hit correct aspect ratio & doping Inputs Pump intensity (each side)5 kW/cm 2  pump 5 nm FWHM Pump duration1 ms Temperature175 K # Aperture / length

HiPER HiLASE / ELI Prototype DiPOLE Extractable energy~ 1 kJ~ 100 J~ 20 J Aperture 14 x 14 cm 200 cm x 4.5 cm 20 cm 2 2 x 2 cm 4 cm 2 Aspect ratio No. of slabs1074 Slab thickness1 cm0.5 cm No. of doping levels542 Average doping level 0.33 at.%0.97 at.%1.65 at.% Amplifier Design Parameters HiPER HiLASE / ELI Extractable energy~ 1 kJ~ 100 J Aperture 14 x 14 cm 200 cm x 4.5 cm 20 cm 2 Aspect ratio No. of slabs107 Slab thickness1 cm0.5 cm No. of doping levels54 Average doping level 0.33 at.%0.97 at.%

DiPOLE Prototype Cr 4+ Yb mm 55 mm Pump 2 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 –Cr 4+ 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

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

Head layout Amplifier Head 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

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

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

Diode Pump Laser 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 Spatial profiles (Modelled) Near FieldFar Field Preliminary measurement

Lab Layout Optical tables Amplifier LN 2 tank Floor area ~30 m 2 Cryo-cooling system

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

Any Questions ?

Yb-doped Materials Parameter (at RT) GlassS-FAPYAGCaF 2 Wavelengths (pump/emission in nm) / / / / 1030 Fluorescence lifetime (msec) ~ 2.0~ 1.3~ 1.0~ 2.4 Emission cross-section (peak x cm 2 ) GainLowHighMediumLow Non-linear index (n 2 x esu) 0.1 to Bandwidth High > 50 nm LowOK? High > 50 nm Availability of large aperture GoodLimited OK (Ceramic) Limited (Ceramic) Thermal properties (K in Wm -1 K -1 ) Poor 1.0 OK 2.0 Good 10.5 OK 6.1

Yb:YAG Energy Level Diagrams Room temperature (300K) 940 nm Yb 3+ 2 F 7/ nm Quasi-3 Level 2 F 5/2 Re-absorption loss Low quantum defect (QD) p / las ~ 91% f 13 =4.6% Cryogenic cooling (175K) 940 nm Yb 3+ 2 F 7/ nm 4 Level-like 2 F 5/2 Significantly reduced re-absorption loss f 13 =0.64%

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

Doping Profile (1 kJ Amplifier)

Absorption + Pump Spectra 175 K 300 K Pump, FWHM = 5nm 175 K, 10 kW/cm K, 20 kW/cm 2 Efficiency vs. pump Pump Absorption

Absorption Spectra 940 nm 1030 nm Factor of 2

Ceramic YAG with Absorber Cladding Cr 4+ :YAG Yb:YAG Laser Camera ? Cr 4+ :YAG Yb:YAG Sample of Co-Sintered YAG (Konoshima) Reflection at Interface? a cb 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 1Amp 2 InjectionAmp1 + 2Reverser 1Amp1 + 2Reverser 2Amp1 + 2Reverser 1ExtractionAmp1 + 2 Losses:17.4 % (distributed) 10 % 17.4 % (distributed) 17.4 % (distributed) 17.4 % (distributed)