PHELIX is a Petawatt High Energy Laser for Heavy Ion EXperiments. The laser is currently being built at the Gesellschaft für Schwerionenforschung in Darmstadt, the German Heavy Ion Accelerator Facility.
This laser can amplify laser pulses to an energy of 1 Kilojoule and a power of 1 Petawatt (1015 W). The architecture of PHELIX is based on Nd:Glass amplifiers with a clear aperture of 28 cm in the last stage. PHELIX will offer the world-wide unique combination of a high-current, high energy (GeV/u) heavy-ion beam with an intense laser beam. This will open the door to a variety of fundamental science issues in the field of atomic physics, nuclear physics and plasma physics.
Amplifying a laser pulse to such a high energy and power leads to a large amount of thermal wavefront aberrations. Wavefront aberrations lead to an increase of the focal spot size and therefore a reduction of the achievable intensity for the experiments.
Within a research assignment between the University of Applied Sciences Münster and the GSI we worked on a physical beam propagation model for PHELIX and calculated the influence of aberrations, including thermal, pump-shot, and chromatic aberrations. We implemented a closed-loop adaptive optics system in the amplifier chain, consisting of a small pre-compensating bimorph mirror with an aperture of 70 mm and an adaptive mirror for astigmatism control with a 30 cm clear aperture in the main amplifier stage. The figure shows an engineer mounting the large adaptive mirror. In March 2006 we could already demonstrate some compensation of thermally induced aberrations and thus extend the repetition rate from one shot in two hours to one shot every hour.