Observation of Metallic Helium: Equation of State and Transport Measurements Under Astrophysical Conditions

Summary form only given. The equation of state and opacity of warm dense helium (1<p<10 g/cm³, 0.5<T<5 eV) is essential for addressing a variety of astrophysical problems, such as the cooling rate of white dwarfs or the miscibility of H/He in the interior of giant planets. High-pressure experimental data on dense helium are sparse, and in particular none exist in the region of direct astrophysical relevance: models used by the astrophysical community have been calibrated on a small number of gas-gun measurements much below 1 g/cm³. It has recently been shown that by coupling static- and dynamic-compression techniques, it is becoming feasible to recreate the conditions of giant planetary interiors in laboratory. We present accurate pressure, temperature, density, and reflectivity measurements of helium using quartz as a reference material for impedance matching. We compressed helium to over 1.2 g/cm³, and reflectivity data at these conditions show that helium is not a clear dielectric fluid but reflects like a metal. The pressure for this transition is almost independent of temperature, as would be expected for pressure-induced ionisation, but it occurs at pressures 1-2 orders of magnitude lower than theoretically expected for the T=0 K solid or fluid. These measurements also have implications for the phase diagram of helium, including the presence of a maximum on the melting line or the existence of a plasma phase transition