Dark-field study of rear-side density structure in laser-accelerated foils

Abstract : A dark-field, laser-probing diagnostic has produced the first high- resolution photographs of density structure on the rear side of laser- accelerated foils. This diagnostic allows the preferential sampling of the steep-gradient region of an expanding plasma and permits two-dimensional, multiple-time recordings on a single photograph. The studies are aimed at understanding the early-time physics of target implosions for inertial- confinement fusion. Both long (500 psec) and short (150 psec) probe pulses were used to study the rear-side plasmas of thin foils accelerated by the rocket-like reaction to a hot plasma ablated from the front side by the laser radiation. The longer pulse results, both for angular scatter and the life-time of small, transverse structure, imply a relatively cold (1 eV) rear side plasma. The short pulses provide high resolution photographs of the complete structure. One of these was a vortex-like structure, suggestive of the remnants of a hydrodynamic instability. These observations are relevant to two of the basic requirements of inertial-confinement fusion: cold fuel isentrope and implosion symmetry.