Results are reported from recently performed bulk-damage, pulse-scaling experiments on DKDP tripler samples taken fi-om 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 mn (3co) with average fluences tlom 4 to 8 J/cm2for pulse durations of 1, 3 and 10 ns. The damage sites were scatter-mapped after testing to determine the damage evolution as a fimction of local beam fluence. The average bulk damage microcavity (pin~oint) density varied nearly linearly with fluence with peak values of approximately 16,000 pp/mm3 at 1 ns, 10,000 pp/mm at 3 ns and 400 pp/mm3 at 10 ns for fluences in the 8-10 J/cmz range. The average size of a pinpoint was 10(+14,-9) pm at 1 ns, 37+20 pm at 3 ns and-110 pm at 10 ns, although all pulse durations r~duced pinpoints with a wide distribution of sizes. Analysis of the pinpoint density data yielded pulse-scaling behavior of t. 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 ti-equently for Zeus ADT tests at-8 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,.ti/Ab.lk) 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 photome~for the worst damage at 1 and 3 ns for beam fluences in the 8-10 J/cm2 range. Analysis of this data yielded a pulse-scaling behavior of t0"25 for the obscured area. It was also determined that the crystals used in this test would survive unconditioned exposure to 4 J/cm* shots on the NIF laser and still meet the obscuration requirement of O.1%.
An experimental technique has been developed to measure the damage density ρ(φ) variation with fluence from scatter maps of bulk damage sites in plates of KD 2 PO 4 (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 (3ω) 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 τ 0.24±0.05 over two orders of magnitude of pulse lengths. The effectiveness of 3ω 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 ρ(φ) at 3ω, 2.6 ns. The 860 ps conditioning to a peak irradiance of 7 GW/cm 2 had the best performance under 3ω, 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.
A large aperture, kJ-class, multi-wavelength Nd-glass laser system has been constructed at Lawrence Livermore National Lab which has unique capabilities for studying a wide variety of optical phenomena. The master-oscillator, power-amplifier (MOPA) configuration of this "Optical Sciences Laser" (OSL) produces 1053 nm radiation with shaped pulse lengths which are variable from 0.1-100 ns. The output can be frequency doubled or tripled with high conversion efficiency with a resultant 100 cm 2 high quality output beam. This facility can accommodate prototype hardware for large-scale inertial confinement fusion lasers allowing for investigation of integrated system issues such as optical lifetime at high fluence, optics contamination, compatibility of non-optical materials, and laser diagnostics.
Considerable attention has been paid over the years to the problem of growing high ptity KDP and KD*P to meet damage threshold requirements on succeeding generations of inertial confinement tilon lasers at LLNL. While damage thresholds for these matmiak have increased overtime, the current National Ignition Facility (NW) maximum fluence requirement (nMine) for KD*P frequency triplers of 14.3 J/cm2 at 351 nm, 3 m has not been nxched without laser (preconditioning.It is reasonable to assume th@ 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 (M) tests on single sites indicate that KDP damage thresholds can be raised on average up to 1.5X the unconditioned values. Unpublished LLNL 30) raster conditioning studies on KDP, however, have not conclusively shown that off-line conditioning is feasible for KD*P. Consequently, investigating the feasibtity of on-line conditioning of NfF triplers at 3a 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 samplea of conventional and rapid growth KDP and KD*P. The experiment entailed exposing sites on each sample to a range of ramped shot (N/l) 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/cm 2. Test results indicated that the most effective conditioning procedure entailed a 7-8 shot ramp starting at 2 J/cmz and ending at 12-13 J/cm2. l%e 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 ND? redline fluence of 14.3 J/cm2. In addition, the exponential pinpoint evolution curves indicate that darnage levels at NIP redline fluences will be on the order of ld pinpoints/mm3. This 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 NTF triplers will be sligh4 consisting of isolated clusters with a few pinpoints at high fluence portions of the beam. This prediction has been veriiled 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 NIP laser.
In this paper we present the results of bulk damage experiments done on Type-IT DKDP triple harmonic generator crystals that were raster conditioned with 35 1-355 urn wavelengths and pulse durations of 4 and 23.2 ns. In the first phase of experiments 20 different scan protocols were rastered into a sample of rapid growth DKDP. The sample was then rastered at damage-causing fluences to determine the three most effective protocols. These three protocols were scanned into a 1 5-cm sample of conventional-growth DKDP and then exposed to single shots of a 1-cm beam from LLNL's Optical Sciences Laser at fluences ranging from 0.5 -1.5X of the 10% damage probability fluence and nominal pulse durations of 0.1, 0.3, 0.8, 3.2, 7.0 and 20 ns. The experiment showed that pulse durations in the 1-3 ns range were much more effective at conditioning than pulses in the 1 6.3 ns range and that the multiple pass "peak fluence" scan was more effective than the single pass "leading edge" scan for 23.2 ns XeF scans.
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