We have developed a completely semi-analytic model for the free energy of a fully ionized, weakly coupled plasma consisting of a H-He mixture, which is valid in the whole temperature-density regime and applicable to arbitrary mixtures. For the excess free energy of the electron fluid at finite temperature, we provide an analytic approximation based on quantum-statistics calculations. For the excess free energy of the classical ionic OCP, we adopt the present formula in the framework of the N-body HNC theory. Finally, we have taken into account the electron-ion interaction by using the extended Debye-Hückel theory with hard-sphere correction under the weakly coupled and weakly degenerate limits. Our formulae are compared with other available approximations, and provide quick and accurate computation of the thermodynamic functions of fully ionized and weakly coupled plasmas.
A more elaborate equation of state aimed at describing the thermodynamic properties of partially ionized plasmas and ionization equilibrium will be discussed in a later paper.
© European Southern Observatory (ESO) 2000
Online publication: December 15, 2000