STATISTICAL-MECHANICS OF HIGHLY-CHARGED ION PLASMAS IN LOCAL THERMODYNAMIC-EQUILIBRIUM

The screened-hydrogenic average-atom model is well suited to describe multicharged ion plasmas in local thermodynamic equilibrium (LTE) for in-line plasma physics calculations. Using general principles of stati stical mechanics, this model is shown to be properly defined and therm odynamically consistent. The grand canonical partition function Z(G) o f the bound electrons is written as a multidimensional integral. Its s addle-point evaluation gives the intuitive average-atom equations. Usi ng this formalism, a method for accounting the various ionization stag es of a LTE plasma is proposed. It can be used to estimate the integer charge stage distribution in this type of medium from any average-ato m model. Once the model is well established, simpler formulas, mon sui table for fast computations, are derived in the framework of the class ical theory of fluctuations. Numerical results are presented and discu ssed.