Concept for a kj-class DPSSL amplifier

Transcription

1 Concept for a kj-class DPSSL amplifier Klaus Ertel 1, Cristina Hernandez-Gomez 1, Ian Musgrave 1, Ian Ross 1, Tom Bradshaw 2, Tristan Davenne 2, Chris Densham 2, Mike Fitton 2, John Collier 1 (1) Central Laser Facility (2) High Power Targets Group, Technology Department STFC Rutherford Appleton Laboratory Chilton, Didcot, OX11 0QX, United Kingdom klaus.ertel@stfc.ac.uk

2 Applications Pumping of fs-opcpa or Ti:S Inertial fusion energy Internal code name DIPOLE

3 Requirements Energy Repetition rate Efficiency Pulse duration Suitable for ps (CPA) Scalable to multi-kj ~ 1 kj Up to 10 Hz Important Few ns Desirable Desirable

4 Gain Material for HEC-DPSSL Need: Long fluorescence life time Good thermo-mechanical properties Available in large size Sufficient gain (sufficiently low F sat ) Best choice: ceramic Yb:YAG But: Still too high F sat Quasi 3-level system At room temperature

5 Some Modelling: Efficiency + Gain Storage Efficiency Extractable Fluence Pump Fluence Yb:YAG Slab amplifier Face pumped Double sided pumping 1 ms pump duration l pump ~ 940 nm Dl pump = 5 nm FWHM Extractable Fluence Pump Fluence - Losses Losses Quantum Defect,Fluorescence, Unabsorbed Pump, Min. Inversion Gain Extractable Fluence Saturation Fluence Fluorescence and quantum defect limit storage efficiency to 58 %

6 Efficiency + Gain at Room Temperature Efficiency, gain too low Fluence too high Room Temperature

7 Solution: Cryogenic Cooling Reduces min. inversion higher efficiency Increases gain easier extraction Improves thermo-optical properties Spectral lines narrow No problem for ns-pulse generation No problem for pumping Not ideal for CPA

8 Efficiency + Gain at 175 Kelvin 175 K Room Temperature

9 Dimensions of Amplifier Energy 1 kj Fluence 5 J/cm 2 Beam size 14x14 cm 2 = 200 cm 2 Pump intensity (each side) 5 kw/cm 2 Dl pump Pump duration 5 nm FWHM 1 ms (2 kj pump) Temperature <175K Storage efficiency Optimum area doping density 50 % (1 kj stored) 3.15 % cm How thick should it be? Determined by ASE management

10 ASE Management+ Cooling Rule of thumb: transverse gain g o x D < 3 Need L > 18 cm, N dope < 0.18 % Much too thick for 1-sided cooling (active mirror) Need distributed cooling (Mercury-concept)

11 Amplifier Geometry Distributed cooling by cold He gas flow, similar Mercury (LLNL) He Gas (cold) Window Ceramic Yb:YAG Cr 4+ :YAG Pump + Extraction Pump + Extraction Stronger doping towards middle Yb:YAG Co-sintered ceramic slabs with absorber cladding for ASE management

12 Choice of thickness and doping concentration Parameter/Result No of slabs n 10 g o,max D in each slab 3 Slab thickness l Value 10 mm Min doping 0.18 at % Max doping 0.51 at % Graded doping: Reduces thickness by factor 1.8 Equalises heat load

13 Modelling of He Gas Cooling Model parameters Helium model = Ideal gas Helium average velocity 25 m/s (equivalent to 150g/s total for all channels) Helium inlet temperature = 150K Helium outlet temperature 164.6K Helium inlet pressure = bar Helium outlet pressure = bar Power input into Absorber = 1.17E+7 W/m 3 Power input into Pumped region = 8.5E+5 W/m 3 Equivalent power for complete stack = 20kW Helium Channel width = 1.5mm Geometry modelled 2 quarter slabs with symmetry planes (helium flow into the page) Symmetry planes Region modelled

14 Computational Fluid Dynamics modelling (no conduction from edges of YAG slab) - Conjugate Heat Transfer (CHT) Helium flow velocity between two slabs Temperature of helium streamlines Temperature contour at core and on surface of slab at slab centre line YAG surface temperature contours Pumped region 7K

15 Computational Fluid Dynamics modelling (edges of YAG slab held at 150K) - Conjugate Heat Transfer (CHT) YAG surface temperature contours Temperature contour at core and on surface of slab at slab centre line 3K Pumped region Temperature jump per pulse : in absorber = 0.73K in pumped region 0.05K

16 Conclusion + Outlook Presented Concept High gain, high efficiency Moderate extraction fluence Much attention paid to ASE Based on available material: ceramic Yb:YAG w Cr 4+ :YAG cladding Challenges Cooling system (modelling does look promising) Pump source: large depth of field Upcoming activities More modelling Test of individual components/concepts Construction of ~10 J prototype

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