FAST IGNITER - FLUID-DYNAMICS OF CHANNEL FORMATION AND LASER-BEAM PROPAGATION

The concept of fast ignitor is intimately connected with the fundament al phenomenon of ultra-intense light beam propagation through dense ma tter in which kinetic effects combine with radiation pressure dominate d hydrodynamics to form a complex scenario of extremely nonlinear phys ics. In this paper, the fluid dynamic aspect of channel formation in a highly over-dense plasma is studied and possible attenuation mechanis ms of the propagating pulse are evaluated in one dimension. Under the assumption that mass ablation reaches a quasistationary state, the rad iation-assisted ablation pressure, the speed of the bow shock, and the density steepening around the critical point are determined self-cons istently from the 1D fluid conservation relations and the electromagne tic wave equation. Due to ponderomotive profile steepening, the ablati on pressure is reduced by 40% in the subsonic region and is dominated by the radiation pressure in the supersonic domain. Channel lengths ar e calculated for various intensities and pellet compression ratios. Li kewise, the nonlinear propagation of a superintense electromagnetic wa ve in an underdense plasma channel is investigated for the 1D case wit h the help of a relativistic fluid model. Here the numerical calculati ons show that the growth of parametric instabilities like that of stim ulated Raman scattering and the partial reflection of the light beam e fficiently saturate by phase mixing due to subsequent breaking of regu lar wave structures. In this context, the propagation in a stochastic medium has also been studied, where light reflection again is found to be small. Thus, TD parametric instabilities do not prevent laser ligh t from being efficiently coupled into the target.