Illumination uniformity of laser-fusion pellets using induced spatial incoherence

Abstract

We have examined the possibility of using a realistic laser system to produce uniform illumination of a direct drive laser-fusion pellet. This laser system consists of a short wavelength laser (KrF, λ=0.25 μm) with the induced-spatial-incoherence method. A numerical analysis describes the laser beams propagating through the pellet plasma, energy deposition by inverse bremmstrahlung absorption, and energy conversion to ablation drive pressure. The results indicate that highly uniform (ΔP/〈P〉<2%) distributions can be produced while a large fraction of the laser beam energy (>75%) is simultaneously absorbed. The spatial structure of the distribution is also analyzed and found to contain primarily low spatial frequencies with maximum magnitudes less than 0.5%. Misalignment of the laser beams increases the overall nonuniformity by about 1.5% for every 1% increase in the relative misaiming error (measured as a percentage of the pellet radius). A maximum misalignment error on the order of a few microradians in the final optics is suggested by this analysis.