Equation of state and the phase diagram of dense fluid helium in the region of partial ionization

Abstract

The plasma composition, equation of state, and phase diagram of dense helium plasma were calculated for temperatures of 104...105 K, total atom densities of 1015... 1025 cm−3, and pressures up to 102 TPa, including the region of partial ionization and strong Coulomb coupling. The basic thermodynamic potential was chosen to be the free energy density with contributions due to Coulomb interaction, hard-core repulsion, and van der Waals-like attraction for a mixture of differently charged atoms and free electrons. For the first time, we show the potential occurrence of a sequence of plasma phase transitions. In helium, they correspond to the ionization steps He0→He+ and He+→He++ respectively. The properties of the coexisting phases were determined by a Maxwell construction based on the combined chemical potential.