Radiation energetics of a laser-produced plasma

Abstract

The energy transport in a laser-heated thin foil is investigated with the use of a detailed radiation-hydrodynamic model. The calculation is performed for a long laser pulse (3 nsec full width at half maximum) at relatively low irradiance (${10}^{13}$ W/${\mathrm{cm}}^2$), incident on an 8-μm-thick aluminum foil, and results confirm earlier experimental hypotheses that the dense plasma at the rear side of the foil is heated predominantly by radiation. The model couples a one-dimensional, planar hydrodynamic calculation with a detailed description of the radiation-ionization dynamics in a totally self-consistent manner, thereby assuring that the dominant physical processes determining the energy transport are characterized accurately. The specific radiative mechanisms responsible for energy transfer from the front to the back surface of the foil are found to be quite complex and are described in detail. The front and rear spectral emission are presented and discussed from the point of view of both energetic and diagnostic considerations. Shifting absorption edges due to ionizing plasma are shown to be responsible for many interesting phenomena affecting the energy transport. Time integration of the $K$-shell spectral features is also studied and its effects on the temperature and density diagnostics are analyzed. Finally $K\alpha$ emission lines are shown to be a promising new diagnostic in determining the time development of the temperature near the foil ablation surface.