Experimental and numerical study of thermal transport in 24-beam ultraviolet irradiation of spherical targets

A series of thermal transport experiments using spectral‐line signature and ion expansion measurements has been performed in spherical geometry using the 24 UV‐converted beams of the Omega laser system at the Laboratory for Laser Energetics of the University of Rochester. Comparison with previous experiments employing only six beams shows a much better agreement in the mass ablation rate, when using 24 beams, between experimental results obtained from charge collectors and code simulations using a flux limiter of 0.1. The influence of nonuniformities in the irradiation, x‐ray line reabsorption, and ionization time lag (non‐LTE effects) were studied with the help of a collisional‐radiative model. These processes were found to alter the falloff rates (determined mainly by the radical profile of the temperature) of the relative x‐ray line intensity with overcoat thickness. With these effects included, agreement between calculated and measured burnthrough curves is greatly improved.