Thermal Approaches to Interpret Laser Damage Experiments

Laser-Induced Damage (LID) resistance of optical components is under considerations for Inertial Confinement Fusion-class facilities such as NIF (National Ignition Facility, in US) or LMJ (Laser MegaJoule, in France). These uncommon facilities require large components (typically 40 × 40 cm2) with high optical quality to supply the energy necessary to ensure the fusion of a Deuterium-Tritium mixture encapsulated into a micro-balloon. At the end of the laser chain, the final optic assembly is in charge for the frequency conversion of the laser beam from the 1053 nm (1ω) to 351 nm (3ω) before its focusing on the target. In this assembly, frequency converters in KH2PO4 (or KDP) and DKDP (which is the deuterated analog), are illuminated either by one wavelength or several wavelengths in the frequency conversion regime. These converters have to resist to fluence levels high enough in order to avoid laserinduced damage. This is actually the topic of this study which interests in KDP crystals laser damage experiments specifically. Indeed, pinpoints can appear at the exit surface or most often in the bulk of the components. This is a real issue to be addressed in order to improve their resistance and ensure their nominal performances on a laser chain. KDP crystals LID in the nanosecond regime, as localized, is now admitted to occur due to the existence of precursors defects (Demos et al., 2003; Feit & Rubenchik, 2004) present in the material initially or induced during the laser illumination. Because these precursors can not be identified by classical optical techniques, their size is supposed to be few nanometers. Despite the several attempts to identify their physical and chemical properties (Demos et al., 2003; Pommies et al., 2006), their exact nature remains unknown or their role in the LIDmechanisms is not clearly established yet. From the best of our knowledge, the main candidates to be proposed are linked to hydrogen bonds (Liu et al., 2003;?; Wang et al., 2005) which may induce point defects. Indeed, atomic scale defects such as interstitials or oxygen vacancies may be responsible for LID in KDP crystals. Also, point defects can migrate into structural defects (such as cracks, dislocations...) to create bigger defects (Duchateau, 2009). In the literature, many experimental and theoretical studies have been performed to explain the LID in KDP crystal (Demos et al., 2010; Duchateau, 2009; Duchateau & Dyan, 2007; Dyan et al., 2008; Feit & Rubenchik, 2004; Reyne et al., 2009; 2010). These studies highlight the 10