Nonisentropic Release of a Solid Shocked via Laser-Plasma Ablation

Shock release is the fundamental process that takes place when a material rapidly decompresses from a high-pressure state. The conventional treatment of shock release assumes that the process is isentropic. To test this assertion, we performed an experiment at the Matter in Extreme Conditions instrument, wherein tantalum foils were compressed to a megabar shock state via laser-plasma ablation, and subsequently probed as they unloaded with an x-ray free-electron laser. From the resulting diffraction patterns, the post-release temperatures of the foils were deduced from their thermal expansion, and were found to exceed significantly the isentropic release temperature. These results are corroborated by large-scale molecular dynamics simulations of tantalum crystals in shock and release, which demonstrate that heating occurs due to the colossal plastic work that must be expended to overcome the crystal's extreme strength during rapid release.