Melting of Tantalum at Multimegabar Pressures on the Nanosecond Timescale

Tantalum was once thought to be the canonical bcc metal, but is now predicted to transition to the Pnma phase at the high pressures and temperatures expected along the principal Hugoniot. Furthermore, there remains a significant discrepancy between a number of static diamond anvil cell experiments and gas gun experiments in the measured melt temperatures at high pressures. Our in situ x-ray diffraction experiments on shock compressed tantalum show that it does not transition to the Pnma phase or other candidate phases at high pressure. We observe incipient melting at approximately 254±15  GPa and complete melting by 317±10  GPa. These transition pressures from the nanosecond experiments presented here are consistent with what can be inferred from microsecond gas gun sound velocity measurements. Furthermore, the observation of a coexistence region on the Hugoniot implies the lack of significant kinetically controlled deviation from equilibrium behavior. Consequently, we find that kinetics of phase transitions cannot be used to explain the discrepancy between static and dynamic measurements of the tantalum melt curve. Using available high pressure thermodynamic data for tantalum and our measurements of the incipient and complete melting transition pressures, we are able to infer a melting temperature 8070_{-750}^{+1250}  K at 254±15  GPa, which is consistent with ambient and a recent static high pressure melt curve measurement.

Nonisentropic Release of a Shocked Solid

We present molecular dynamics simulations of shock and release in micron-scale tantalum crystals that exhibit postbreakout temperatures far exceeding those expected under the standard assumption of isentropic release. We show via an energy-budget analysis that this is due to plastic-work heating from material strength that largely counters thermoelastic cooling. The simulations are corroborated by experiments where the release temperatures of laser-shocked tantalum foils are deduced from their thermal strains via in situ x-ray diffraction and are found to be close to those behind the shock.

Bremsstrahlung x-ray generation for high optical depth radiography applications on the National Ignition Facility

We have tested a set of x-ray sources for use as probes of highly attenuating, laser-driven experiments on the National Ignition Facility (NIF). Unlike traditional x-ray sources that optimize for a characteristic atomic transition (often the n = 2 → n = 1 transition in ionized, He-like atoms), the design presented here maximizes the total photon flux by optimizing for intense, broadband Bremsstrahlung radiation. Three experiments were performed with identical targets, including a uranium x-ray source foil and a tungsten substrate with a narrow (25 μm wide) collimating slit to produce a quasi-1D x-ray source. Two experiments were performed using 12 beams from the NIF laser, each delivering approximately 46 kJ of laser energy but with different laser spatial profiles. This pair yielded similar temporal x-ray emission profiles, spatial resolution, and inferred hot electron temperature. A third experiment with only 6 beams delivering approximately 25 kJ produced a lower hot electron temperature and significantly lower x-ray flux, as well as poorer spatial resolution. The data suggest that laser pointing jitter may have affected the location and intensity of the emitting plasma, producing an emission volume that was not well centered behind the collimating slit and lower intensity than designed. However, the 12-beam design permits x-ray radiography through highly attenuating samples, where lower energy line-emission x-ray sources would be nearly completely attenuated.

Femtosecond X-Ray Diffraction Studies of the Reversal of the Microstructural Effects of Plastic Deformation during Shock Release of Tantalum

We have used femtosecond x-ray diffraction to study laser-shocked fiber-textured polycrystalline tantalum targets as the 37-253 GPa shock waves break out from the free surface. We extract the time and depth-dependent strain profiles within the Ta target as the rarefaction wave travels back into the bulk of the sample. In agreement with molecular dynamics simulations, the lattice rotation and the twins that are formed under shock compression are observed to be almost fully eliminated by the rarefaction process.