Direct-Drive Shockwave-Timing Experiments in Planar Targets

Summary form only given. Inertial-confinement-fusion target designs use multiple shock waves to condition the target material for optimal performance. These designs require that the shock waves be accurately timed to coalesce at a particular point in the target. It is essential that the propagation and dynamics of these multiple shocks be understood and correctly modeled. The OMEGA laser facility at the University of Rochester is used to perform direct-drive experiments that measure the propagation and coalescence of two laser-driven shock waves propagating in planar targets made of CH or cryogenic D₂. Laser pulses with various temporal shapes are used to generate two primary shocks that propagate in these transparent targets. The velocity and self-emission profiles of these shocks are temporally resolved and clearly show the first shock wave propagating through the material and then being overtaken by the second shock wave. The coalescence of these shocks forms a single, stronger, shock that eventually arrives at the rear surface of the target. The measured velocity and emission profiles exhibit distinct features that include: the decay rate of unsupported shocks, the coalescence and shock breakout times, and curvature of the shock fronts. These results are presented for a number of drive conditions and compared to 3-D and 2-D hydrodynamic codes. The simulated velocity profiles and coalescence times are in good agreement with experimental observations as are many of the two-dimensional effects. The effect of preheat by X-rays on these experiments is also discussed