Role of ionization potential depression for generation of strongly coupled plasmas in high-pressure supercritical fluids

The photosphere, one of the Sun's inner layers, and white dwarfs exist as plasmas with high electron density and relatively low temperature, satisfying the strongly coupled condition that the Coulomb coupling parameter exceeds unity. While strongly coupled plasmas are prevalent throughout space, it is difficult to produce and sustain such states in the laboratory with a sufficiently long lifetime for investigation of basic transport processes. To surmount these obstacles, we have developed a laser-produced plasma experiment in supercritical fluids of He, Ar, and Kr. For helium at 100 bar, a nanosecond laser pulse generates a strongly coupled plasma with an electron density, estimated from ionization potential depression (IPD) modeling, of approximately [Formula: see text] [Formula: see text], and a temperature of about 1 eV, corresponding to a Coulomb coupling parameter of 4. Systematic experiments with other species show that the pivotal factor for achieving strongly coupled plasmas is the degree of IPD. This study shows the potential to facilitate precise measurements of thermodynamic transport parameters and equation of state for dense plasma states.