Predicting bulk damage in NIF triple harmonic generators

Recently reported experiments(1,2) have investigated the statistics of laser damage in KDP and KD*P, Automated damage tests have allowed cumulative failure and damage probability distributions to be constructed. Large area tests have investigated-the feasibility of on-line laser conditioning and damage evolution for tripler harmonic generation (THG) crystals on the National Ignition Facility (NIF). These tests have shown that there is a nonzero probability of damage at NIF redline fluence (14.3 J/cm(2), 351 nm, 3 ns) and that the damage pinpoint density evolves exponentially with fluence. In this paper, the results of these tests are used in conjunction with model spatial profiles of the NIF beam to predict the level of damage created in the THG crystal. A probabilistic calculation based on the overlap of the beam fluence and damage probability distribution shows that the overall damage probability is less than 3% for well-conditioned, high quality KDP/KD*P crystals of conventional or rapid growth. The number density of generated pinpoints has been calculated by mapping the damage evolution curves onto the NIF model profile. This shows that the number of damage pinpoints generated in high fluence portions of the NIF beam will be low for well-conditioned THG crystals. In contrast, unconditioned triplers of the same material will exhibit an increase in pinpoint density of greater than 20X. To test the validity of these calculations a 37 cm, conventionally grown KD*P tripler from, the Beamlet laser was: scatter mapped for bulk damage. The tripler had been exposed to NIF-like fluences during its operational lifetime on Beamlet and exhibited very low levels of bulk pinpoint damage, essentially supporting the predictions based on tests and modeling.