Development of an Actively CooledLarge Aperture Laser Amplifier atthe GSI Helmholtzzentrum

Abstract

The development of petawatt-class lasers during the last decades has resulted in a reduction of the size and cost of such systems. This development has lead to the adoption of petawattclass lasers at facilites world-wide for the study of matter in extreme states. The energy density achieved by such systems has lead to their use as direct drivers for particle acceleration, research on inertial confinement fusion and radiation therapy. Further uses for these systems were found in their application as secondary sources for X-rays, ions, electrons, protons and neutrons. This range of applications has lead to the use of petawatt-class lasers in national as well as university laboratories. The operation as direct and indirect sources requires high repetition rates to ensure the rapid reproducibility of respective results and increased statistics. While low energy petawatt lasers featuring pulse energies below 100 J are commercially available with repetition rates greater than 1 Hz, the repetition rate of high energy petawatt systems above 100 J pulse energy is limited by the cooling behaviour of their amplifier materials. Most high energy petawatt lasers utilize glass based gain media at apertures above 20 cm in their power amplifier elements. These large apertures are chosen to minimize the occuring intensities in the system and prevent damages to optical elements. However, the low thermal conductivity of glass leads to thermal equilibration times of the gain medium in the range of hours, thus featuring low repetition rates. The goal of this scientific work is the development of a glass based power amplifier element for high energy petawatt lasers featuring an active cooling scheme to reduce the thermal equilibration time of the gain medium to improve the repetitionrate of the laser. The targeted repetition rates are in the order of 1/5 min^-1 for a gain factor of 1.5 with a focus on the high quality and reproducibility of the transmitted wavefront. To this end, a concept was used that utilizes a laminar coolant flow between two discs of the gain medium to reduce the thermal load on the discs. A set of coolants was investigated to determine their thermal, kinematic, chemical and optical properties to qualify them for use in the prototype. Simulations of the cooling process in the prototype further lead to the discovery of a steady state between the heating and cooling of the gain medium between consecutive pulses. This steady state could be used to enable repeatable wavefront patterns at repetition rates of 1/5 min^-1 in the simulations. Experimental investigations of a full scale prototype model were further used to determine the leak tightness of the coolant seals as well as to conduct the first qualitative measurements of transmitted wavefronts.