Laser-Based Dynamic Compression of Geological Materials to Ultrahigh Pressures

Abstract:

Laser-based dynamic compression provides new opportunities to study the structures and properties of geological materials to ultrahigh pressure conditions reaching 1 terapascal and beyond. By controlling the shape and duration of the incident laser pulse, either shock or ramp (shockless) loading can be produced. Samples can be compressed for ~10s of nanoseconds on spatial scales of ~1 millimeter. Diagnostics include velocity interferometry from which the stress-density response of the material can be determined and X-ray diffraction from which lattice-level structural information is obtained. Our experiments are being carried out at a suite of facilities including the Omega Laser (U. of Rochester), Linear Coherent Light Source (LCLS), and National Ignition Facility (Livermore). Using ramp compression we have constrained pressure-density states in a variety of materials including iron, magnesium oxide, and carbon. X-ray diffraction has been used as a diagnostic to probe the B1-B2 phase transition in MgO under both ramp and shock loading to multi-megabar pressures. We have also examined this same phase transition at more modest pressures on sodium chloride at the LCLS, observing both the B1-B2 transition upon compression and its back transformation upon release. X-ray diffraction measurements have also been used to study the melting curves and high-pressure phase stability of transition metals and alloys, including compositions relevant to the cores of Earth and super-Earth planets.