Fast ignitor: Fluid dynamics of channel formation and laser beam propagation

The concept of fast ignitor is intimately connected with the fundamental phenomenon of ultra-intense light beam propagation through dense matter in which kinetic effects combine with radiation pressure dominated hydrodynamics to form a complex scenario of extremely nonlinear physics. In this paper, the fluid dynamic aspect of channel formation in a highly over-dense plasma is studied and possible attenuation mechanisms of the propagating pulse are evaluated in one dimension. Under the assumption that mass ablation reaches a quasistationary state, the radiation-assisted ablation pressure, the speed of the bow shock, and the density steepening around the critical point are determined self-consistently from the 1D fluid conservation relations and the electromagnetic wave equation. Due to ponderomotive profile steepening, the ablation pressure is reduced by 40% in the subsonic region and is dominated by the radiation pressure in the supersonic domain. Channel lengths are calculated for various intensities and pellet compression ratios. Likewise, the nonlinear propagation of a superintense electromagnetic wave in an underdense plasma channel is investigated for the 1D case with the help of a relativistic fluid model. Here the numerical calculations show that the growth of parametric instabilities like that of stimulated Raman scattering and the partial reflection of the light beam efficiently saturate by phase mixing due to subsequent breaking of regular 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 light from being efficiently coupled into the target.