Grain-size-independent plastic flow at ultrahigh pressures and strain rates.

A basic tenet of material science is that the flow stress of a metal increases as its grain size decreases, an effect described by the Hall-Petch relation. This relation is used extensively in material design to optimize the hardness, durability, survivability, and ductility of structural metals. This Letter reports experimental results in a new regime of high pressures and strain rates that challenge this basic tenet of mechanical metallurgy. We report measurements of the plastic flow of the model body-centered-cubic metal tantalum made under conditions of high pressure (>100 GPa) and strain rate (∼10(7) s(-1)) achieved by using the Omega laser. Under these unique plastic deformation ("flow") conditions, the effect of grain size is found to be negligible for grain sizes >0.25 µm sizes. A multiscale model of the plastic flow suggests that pressure and strain rate hardening dominate over the grain-size effects. Theoretical estimates, based on grain compatibility and geometrically necessary dislocations, corroborate this conclusion.

Focusing of an explosive plasma expansion in a transverse magnetic field.

The dynamics of a laser ablation plasma expanding in an external magnetic field have been investigated with imaging interferometry and shadowgraphy. The diagnostics reveal a new interaction mechanism, namely, the redirection of the explosive plasma expansion into a converging flow. A comparison with three-dimensional ideal magnetohydrodynamic simulation results supports the observation that the efficient lateral plasma confinement causes the plasma to converge on the axis and initiate a directed flow. The resulting collimated flow propagates across the magnetic field in a kinetic regime, which cannot be modeled within the same framework.

Invited article: Relation between electric and magnetic field structures and their proton-beam images.

Proton imaging is commonly used to reveal the electric and magnetic fields that are found in high energy density plasmas. Presented here is an analysis of this technique that is directed towards developing additional insight into the underlying physics. This approach considers: formation of images in the limits of weak and strong intensity variations; caustic formation and structure; image inversion to obtain line-integrated field characteristics; direct relations between images and electric or magnetic field structures in a plasma; imaging of sharp features such as Debye sheaths and shocks. Limitations on spatial and temporal resolution are assessed, and similarities with optical shadowgraphy are noted. Synthetic proton images are presented to illustrate the analysis. These results will be useful for quantitatively analyzing experimental proton imaging data and verifying numerical codes.