Efficient operation of a pulsed diode pumped cryogenic gas cooled Yb:YAG multislab amplifier delivering 7.4 J at 10 Hz

Summary form only given. The next generation of ultra-intense laser facilities, currently being developed in European projects such as HiPER [1] and ELI [2], require the development of a laser amplifier technology capable of producing kJ-level pulses with nanosecond duration. These will need to operate at multi-Hz repetition rate and high wall-plug efficiency, which is only possible using diode-pumped solid-state laser (DPSSL) technology. The DiPOLE project at the Central Laser Facility (CLF) is developing a scalable and efficient high pulse energy DPSSL architecture based on a cryogenic gas cooled, multi-slab ceramic Yb:YAG amplifier concept, capable of generating kJ pulse energies. To test the viability of this concept, a scaled-down prototype amplifier has been built, designed to deliver 10 J pulses at 10 Hz repetition rate with an optical-to-optical (η_o-o) efficiency of 25% [4]. Recently, a new multi-pass relay-imaging extraction architecture, including spatial filtering, has been installed that allows up to eight passes through the amplifier head. This has enabled more efficient extraction at higher coolant temperatures, where gain is lower and the impact of ASE is reduced [4], as well as improving the spatial quality of the output beam.In this paper we report details of the new multi-pass architecture and present recent performance results for amplification of ns-pulses at 1030 nm in a six-pass configuration, operating at a coolant temperature of 125 K. In this configuration we have measured up to 9.5 J at 1 Hz and 7.4 J at 10 Hz, corresponding to Ko-o efficiencies of 24% and 23%, respectively, for seed energies of approximately 20 mJ. The measured dependence of conversion on pulse repetition frequency (PRF) is shown in Fig. 1 and Fig. 2. A preliminary assessment of output energy stability has also been undertaken at 10 Hz where stable operation has been achieved for periods of over 5 minutes with a measured rms variation in output energy o- 0.7%. It has been found that insufficient isolation between amplifier stages has a detrimental impact on output stability. Low frequency modulation in the output energy is observed but this is directly related to the temperature stability of the gas cooling system.These results confirm the validity of the amplifier design concept and give confidence that the target specification will be reached once greater seed energy is available and round trip losses are reduced further.