The effect of condensates and inner coatings on the performance of vacuum hohlraum targets

Experiments on the OMEGA laser system [Boehly et al., Opt. Commun. 133, 495 (1997)] using laser-driven vacuum hohlraum targets show distinct differences between cryogenic (<20 K) and warm targets. The cryogenic targets have 15% lower peak radiation temperatures, and the temporal profile of those temperatures is quite different than in warm targets. The cryogenic targets reflect significantly more (3%–7%) of the laser drive than the warm targets (<1%). The temporal and spectral features of the reflected light from the cryogenic targets show a significantly longer duration and more spectral features than the warm hohlraums. Warm hohlraum targets coated with 2 μm of CH replicate the behavior of cryogenic targets. This indicates that the cryogenic hohlraums are affected by the condensation of background gases on the cold hohlraum surface. The most important effect of low-Z material in the hohlraums is that they significantly reduce the x-ray conversion efficiency, resulting in lower hohlraum radiation temperature. The coatings (both CH and condensates) produce long-scale-length, low-Z plasmas that moderately reduce the absorption of laser light in the hohlraums. This causes higher reflectivity and produces hot electrons that generate hard x rays (hν>20 keV), both of which are detrimental to the performance of hohlraum-driven inertial confinement fusion targets.