Physics and technology of liquid face-cooling for multi-kiloWatt Yb-YAG laser amplifiers

Combined amplification technologies are explored in the frame of the HORIZON project at CELIA, Bordeaux University, aiming to deliver Joule-range picosecond pulses at the kW level, from CW diode-pumped rotating Yb:YAG multi-slabs, face-cooled with water. After an overview of HORIZON, we report on laser-induced damage tests performed at CEA-CESTA, showing good to exceptional resilience of multilayer coatings to damage underwater. We then report on systematic thermal and hydrodynamical studies of water face-cooling of multi-slab amplifier, and on the various flow regimes, including laminar, weak turbulence and a thermo-hydrodynamic striation instability. We suggest that the deleterious effects on the laser wavefront of unstable flows should be mitigated by an optical bleaching method, called index-leveling. We eventually present results on CW amplification, that demonstrate the capability of these technologies to reach the kiloWatt level and beyond.