Computational investigation of x-ray impulse generation in solid materials*

The generation of impulse in solid materials by x-ray absorption is difficult to model and relevant to many applications in high-energy-density physics. In these systems, x-rays are deposited within the material to a depth dictated by the material's opacity. This heated surface layer may blow off, imparting impulse to the remaining material. The thermal and mechanical dynamics of such systems are complex, and in general require full-physics simulation with appropriate models to account for all blow-off mechanisms ( eg. spallation, jetting, plasma ablation) that contribute to the impulse of the bulk material. Existing analytical models require closure of the final energy distribution of the blow-off material, which is not known a priori. 

We utilize the codes Mercury and Ares to simulate x-ray deposition and the resulting blow-off process in 1-D. We validate these simulations using experimental data for a variety of materials and x-ray spectra from the National Ignition Facility. Furthermore, we use simulations to quantify the sensitivity of impulse to the source spectrum, as well as to inform closure of analytical models that allow for simple and intuitive calculation of impulse for a given material and x-ray spectrum. These quantities are important when designing robust experiments quantifying impulse generated by x-ray illumination, particularly when considering uncertainty in the measured fluence and spectral distribution of the x-ray source.