Laser-driven quasi-isentropic compression experiments and numerical studies of the iron alpha-epsilon transition in the context of planetology

The iron alpha-epsilon transition is one of the most studied solid-solid phase transitions. However, for quasi-isentropic compression, the dynamic influences of this transition on the high pressure states of iron are still unknown. We present experimental results and numerical simulations to study these effects. Experiments performed at LULI2000 and the Janus laser facility (LLNL), using two different ramp shapes and different compression rates allowed us to study the dynamics of the alpha-epsilon transition. We have observed the transition at particle velocities ranging from 0.25 km/s to 0.52 km/s depending on the compression rate. Depending on the ramp, either a shock formation was observed (high compression rate) at the transition or a flat plateau whose duration is function of compression rate. This plateau results from the interaction of the phase transition dynamic with a small shock precursor at the beginning of the laser ramp. Increasing the compression rate leads to a smaller plateau duration. These results are important for reproducing Earth and Super-earth core conditions (2-15Mbar, 5-15000K) in the laboratory where the quasi-isentropic compression is the most promising experimental scheme.