The coupling of high-power nanosecond optical lasers to FELs has seen a rapid growth in the study of solid-state matter compressed to high pressures via laser-plasma ablation. Femtosecond diffraction of the x-rays during the compression process can give information on the formation of both new phases and the deformation processes that occur. To date almost all experiments are performed with optical laser spots larger than the micron-scale target thickness, resulting in uniaxial compression. This in turn implies significant plastic flow must occur, with a resultant heating of the target. Significant heating will occur when the target is compressed via a shock wave, but less if more slowly (but still on nanosecond timescales) via so-called ramp or ‘quasi-isentropic’ compression. This latter approach should in principle allow us to reach pressures with the DIPOLE laser at the HED instrument that exceed those attainable in standard diamond anvil cells, approaching the TPa regime, but generally still keeping the target below the melt curve. However, several important problems remain. Firstly, our current knowledge of plasticity at these high strain rates is in most cases insufficient to make meaningful predictions of the temperatures reached. Secondly, methods of measuring temperature under compression are in their infancy, and yet to be proven in situ. Thirdly, there is a significant body of evidence that the phase boundaries under dynamic compression are radically different from those that pertain in the static case for many classes of materials. All of the above factors should engender a degree of caution in those that claim that their high-pressure FEL experiments are of direct relevance to planetary science. In this talk, I aim to outline some of the modest steps we have been taking to address a few of the above issues.
Sakura Pascarelli / Gabriella Mulá-Mathews