TIME-RESOLVED X-RAY-IMAGING OF HIGH-POWER LASER-IRRADIATED UNDERDENSESILICA AEROGELS AND AGAR FOAMS

This article presents the results of experiments in which a high-power laser was used to irradiate low density (4-9 mg/cm(3)) silica aerogel and agar foam targets. The laser-solid interaction and energy transpo rt through the material were monitored with time-resolved imaging diag nostics, and the data show the production and propagation of an x-ray emission front in the plasma. The emission-front trajectory data are f ound to be in significant disagreement with detailed simulations, whic h predict a much more rapid heating of the cold material, and the data suggest that this discrepancy is not explainable by target inhomogene ities. Evidence suggests that energy transport into the cold material may be dominated by thermal conduction; however, no completely satisfa ctory explanation for the discrepancies is identified, and further exp erimental and theoretical research is necessary in order to resolve th is important problem in laser-plasma interaction physics. (C) 1995 Ame rican Institute of Physics.