Laser conditioning study of KDP on the optical sciences laser using large area beams

Abstract

Considerable attention has been paid over the years to the problem of growing high purity KDP and KD*P to meet damage threshold requirements on succeeding generations of inertial confinement fusion lasers at LLNL. While damage thresholds for these materials have increased over time, the current National Ignition Facility (NIP) maximum fluence requirement (redline) for KD*P frequency triplers of 14.3 J/cm2 at 351 nm, 3 ns has not been reached without laser (pre)conditioning. It is reasonable to assume that. despite the rapid increase in damage thresholds for rapidly grown crystals, ·a program of large scale conditioning of the 192 NIF triplers will be required. Small area ramp (R/1) tests on single sites indicate that KDP damage thresholds can be raised on average up to 1.5X the unconditioned values. Unpublished LLNL 3m raster conditioning studies on KDP, however, have not conclusively shown that off-line conditioning is feasible for KD*P. Consequently, investigating the feasibility of on-line conditioning of NIF triplers at 3m has become a high priority for the KDP damage group at LLNL. To investigate the feasibility of on-line conditioning we performed a series of experiments using the Optical Sciences Laser (OSL) on numerous samples of conventional and rapid growth KDP and KD*P. The experiment entailed exposing sites on each sample to a range of ramped shot (Nil) sequences starting at average fluences of -2 J/cm2 (in a 7 mm "top hat" beam @ 351 nm, 3 ns) up to peak fluences of approximately 13 J/cm2• Test results indicated that the most effective conditioning procedure entailed a 7-8 shot ramp starting at 2 J/cm2 and ending at 12-13 J/cm2• The pinpoint onset fluence for the 8/1 tests was 1.4 times that of the unconditioned site. Damage evolution appears to be exponential as a function of increasing fluence. When damage occurs after conditioning however, pinpoint density evolution exhibits a greater slope than less conditioned sites. The overall reduction in the total pinpoint number can be as high as 300X. Despite laser conditioning , the pinpoint onset for the samples considered is below the NIF redline fluence of 14.3 J/cm2• In addition, the exponential pinpoint evolution curves indicate that damage levels at NIF redline fluences will be on the order of 1 Q4 pinpoints/mm3• 1bis suggests that there will be significant damage in NIF triplers, however, substantial damage has not been observed in the large Beamlet tripler (conventionally grown KD*P) under similar exposure conditions. By applying the OSL damage evolution curves to model NIF THG output spatial profiles it is possible to show damage in NIF triplers will be slight, consisting of isolated clusters with a few pinpoints at high fluence portions of the beam. This prediction has been verified by scatter mapping the 37 cm Beamlet tripler crystal. These results will be discussed in a future memo. These results indicate the feasibility of on-line conditioning for the NIF laser. Keywords: KDP, DKDP, KD*P, bulk laser damage, laser conditioning