Computation of thermally distorted wave-fronts in high-power lasers: Comparison with LIL experiments

Thermo-mechanical deformation of Neodymium doped glass slabs during high-power laser shots strongly affects the beam wave-front. The thermally distorted phase profile is corrected with an adaptative mirror on the French Ligne d'Integration Laser (LIL), the 8-beam prototype of the Laser Megajoule (LMJ). Nitrogen cooling of the flash lamps reduces the delay needed to reach thermal equilibrium between two shots by optimizing the slab relaxation back to their original shape. The wave-front is measured before, during, and after a shot with a Shack-Hartmann wave-front sensor. To predict and control the phase profile during cooling, phase dynamics is computed from the thermo-mechanical properties of the slab. Numerical results and measurements are in good qualitative and quantitative agreement.