Average atom model of partial ionization, revisited

The average atom model of partial ionization discussed by Zeldovich and Raizer provides an approximate solution to the Saha equation. As originally derived the model contains several approximations and ad hoc assumptions and violates one of the Maxwell equations of thermodynamics. In this paper, an alternate derivation of the model is presented which is thermodynamically correct and which reduces the number of assumptions and approximations. Several variations on the basic model are then discussed wherein accuracy can be traded for ease of computation. (U.S.)