Distributed Absorption and Inhibited Heat Transport

Abstract

High-gain laser-fusion pellets must be irradiated by1 long (pulse width, tl ≥ 10ns) pulses of moderate to high intensity (incident laser energy flux, Io≥5x 1015 W/cm2) laser light. These pulses produce large scale length plasmas that are cool enough that inverse bremsstrahlung is very strong, so that laser light absorption is not localized: rather, absorption is distributed2,3 over densities from well below the critical density to the critical density. Distributed absorption has several important consequences for laser fusion. The temperature and density profiles impact the threshold for parametric instabilities that can degrade pellet performance. In addition, the ablation pressure is reduced due to distributed absorption, because laser light is absorbed at lower densities that are farther from the ablation surface. Hydrodynamic computer codes4 such as LASNEX are used to study strongly absorbing plasmas, but it is difficult to use such complex codes to test alternative models for heat transport and laser light energy deposition. These considerations motivate the development of analytical models to improve our understanding. However, most previous models5-9 are not applicable to strongly absorbing plasmas because the absorbed laser energy was deposited at one location in the plasma (usually the critical surface). Recently we presented2 a model that incorporated distributed absorption, but it was assumed that the conduction region was in steady-state ( as in previous models ). As was shown in Ref. 2, these steady-state models only apply to low to moderate laser powers for typical laser pulse lengths.