Preliminary results on the EOS of water in the Megabar range

In recent years, shock waves driven by high power lasers have become a reliable tool for the study of the equation of state (EOS) of materials in the Megabar pressure range, where they become strongly correlated, degenerate, dense plasmas. The EOS of water at very high pressures plays a very important role in astrophysics. In particular, the large magnetic field measured on Uranus and Neptune by Voyager 2 is assumed to be generated in the "ice" layers (whose main constituents are ammonia, water and methane) where pressures up to 6 Mbar are reached. Recent calculations predict an insulator-metallic phase transition of water in this regime. Here, we present some preliminary results from a recent experiment on water EOS performed with the laser PHEBUS and funded by the European Union in the framework of the "Access to Large Scale Facilities" programme. In the experiment, structured targets made of an Aluminium step followed by a water layer are irradiated by the laser at an intensity ≃ 4.10¹⁴ W.cm^-2 to generate a shock wave. Velocities are measured in the two materials using a VISAR interferometric diagnostic for water, and a streak camera to measure target emission for Al. EOS points for water are obtained with the impedance mismatch method using Al EOS as a reference. Water reflectivity was also measured.