The results of pulse-scaling experiments on rapid-growth DKDP triplers using the Optical Sciences Laser at 351 nm

Results are reported from recently performed bulk-damage, pulse-scaling experiments on DKDP tripler samples taken from NIF-size, rapid-growth boule BD7. The tests were performed on LLNL's Optical Sciences Laser. A matrix of samples was exposed to single shots at 351 nm (3 omega) with average fluences from 4 to 8 J/cm(2) for pulse durations of 1, 3 and 10 ns. The damage sites were scatter-mapped after testing to determine the damage evolution as a function of local beam fluence. The average bulk damage microcavity (pinpoint) density varied nearly linearly with fluence with peak values of approximately 16,000 pp/mm(3) at 1 ns, 10,000 pp/mm(3) at 3 ns and 400 pp/mm(3) at 10 ns for fluences in the 8-10 J/cm(2) range. The average size of a pinpoint was 10(+14,-9) mum at 1 ns, 37 +/- 20 mum at 3 ns and similar to 110 mum at 10 ns, although all pulse durations produced pinpoints with a wide distribution of sizes. Analysis of the pinpoint density data yielded pulse-scaling behavior of t(0.35). Significant planar cracking around the pinpoint as was observed for the 10 ns case but not for the 1 and 3 ns pulses. Crack formation around pinpoints has also been observed frequently for Zeus ADT tests at similar to8 ns. The high pinpoint densities also lead to significant eruption of near-surface bulk damage. Measurements of the damage site area for surface and bulk gave ratios (A(surf)/A(bulk)) of 2:1 at 1 ns, 7:1 at 3 ns and 110:1 at 10 ns. Maximum aperture averaged transmission losses on the order 15 percent have been measured by photometry for the worst damage at 1 and 3 ns for beam fluences in the 8-10 J/cm(2) range. Analysis of this data yielded a pulse-scaling behavior of t(0.25) for the obscured area. It was also determined that the crystals used in this test would survive unconditioned exposure to 4 J/cm(2) shots on the NIF laser and still meet the obscuration requirement of 0.1%.