Ionization equilibrium of hot hydrogen plasma
  The hydrogen plasma is studied at temperatures T ~ 10^4 - 10^6 K using the free energy minimization method. A simple analytic free energy model is proposed which is accurate at densities up to 1 g/cc and yields convergent internal partition function of atoms. The occupation probability formalism is modified for solving the ionization equilibrium problem. The ionization degree and equation of state are calculated and compared with the results of other models. 
A.Y.Potekhin (Ioffe Phys.-Tech. Institute, http://www.ioffe.rssi.ru)