Pulse length dependence of laser conditioning and bulk damage in KD2PO4

An experimental technique has been developed to measure the damage density rho(phi) variation with fluence from scatter maps of bulk damage sites in plates of KD2PO4 (DKDP) crystals combined with calibrated images of the damaging beam's spatial profile. Unconditioned bulk damage in tripler-cut DKDP crystals has been studied using 351 nm (3 omega) light at pulse lengths of 0.055, 0.091, 0.30, 0.86, 2.6, and 10 ns. It is found that there is less scatter due to damage at fixed fluence for longer pulse lengths. The results also show that for all the pulse lengths the scatter due to damage is a strong function of the damaging fluence. It is determined that the pulse length scaling for bulk damage scatter in unconditioned DKDP material varies as tau(0.24 +/- 0.05) over two orders of magnitude of pulse lengths. The effectiveness of 3 omega laser conditioning at pulse lengths of 0.055, 0.096, 0.30, 0.86, 3.5, and 23 ns is analyzed in term of damage density rho(phi) at 3 omega, 2.6 ns. The 860 ps conditioning to a peak irradiance of 7 GW/cm(2) had the best performance under 3 omega, 2.6 ns testing. It is shown that the optimal conditioning pulse length appears to lies in the range from 0.3 to 1 ns with a low sensitivity of 0.5 J/cm(2)/ns to the exact pulse length.