MODELING AND EXPERIMENTS OF X-RAY ABLATION OF NATIONAL-IGNITION-FACILITY FIRST WALL MATERIALS

This paper discusses results of modeling and experiments on the x-ray response of selected materials relevant to the NIF target chamber desi gn. X-ray energy deposition occurs in such small characteristic depths (on the order of a micron) that thermal conduction and hydrodynamic m otion significantly affect the material response, even during the typi cal 10-ns pulses. The finite-difference ablation model integrates four separate processes: x-ray energy deposition, heat conduction, hydrody namics, and surface vaporization. Experiments have been conducted at t he Nova laser facility in Livermore on the response of various materia ls to NIF-relevant x-ray fluences. Samples of fused silica, silicon ni tride, boron carbide, boron, silicon carbide, carbon, aluminum oxide, and aluminum were tested. The response was diagnosed using post-shot e xaminations of the surfaces with scanning electron microscope (SEM) an d atomic force microscope (AFM) instruments. On the basis of these obs ervations, judgments were made about the dominant removal mechanisms f or each material. The relative importances of these processes were als o investigated with the x-ray response model.