Observation of metallic helium: Equation of state and transport measurements under astrophysical conditions

The equation of state and opacity of warm dense helium (1 $< \rho < $ 10 g/cm$^3$, 0.5 $<$ T $<$ 5 eV) is essential for addressing a variety of astrophysical problems. High-pressure experimental data on dense helium are sparse; models used by the astrophysical community have been calibrated against a small number of gas-gun measurements much below 1 g/cm$^3$. Using coupled static- and dynamic-compression techniques it is becoming feasible to recreate the conditions of giant planetary interiors in laboratory; in recent experiments we have compressed helium to over 1.2 g/cm$^3$. We present measurements of pressure, temperature, density, and reflectivity of compressed helium using quartz as a reference material for impedance matching. 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 ionization, but it occurs at pressures 1-2 orders of magnitude lower than theoretically expected for the T = 0 K solid or fluid.