Experimental investigation of solid-vapour phase transitions in metals during shock release

This work presents the results of an experimental investigation into the solid-gas phase transitions that occur when strong shocks in metals release to a low pressure state, on reflection from an interface with a low pressure gas. Such processes occur frequently in ICF targets, and this work was motivated primarily by a need to validate our equations of state in and around the phase change boundary.

In this study, shock waves are created in targets by the impact of a Cu tipped projectile from a two-stage light gas gun. Conditions in the released material are then varied from shot to shot by the modification of both the projectile velocity (between 5 and 7 km/s), and the pressure of the gas surrounding the material. Point projection X-ray imaging is performed approximately 100 ns after the incident shock has reflected from the rear face of the target; the subsequent image can then be Abel inverted to form a map of the density of the released material.

X-rays are generated for the backlighting source using a 100 kA X-pinch. This provides a broadband X-ray source, with energies up to 30 keV, a 200-300 um source size and a 20 ns pulse length. This enabled us to achieve a spatial resolution of 50 to 100 um, in a 30 mm field of view.