Lowering of the Ionization Energy for a Plasma in Thermodynamic Equilibrium

In a plasma the ionization energy is decreased due to the presence of the microfield. In the past several attempts have been made to calculate this effect. These calculations, which use statistical and thermodynamical procedures, give different results. They produce either a ``polarization term'' or a ``lattice term'' or both of them. Moreover there are quantitative discrepancies. The lowering of the ionization energy is here derived by a statistical method, which is physically conceivable. The results cover a wide density range below and above the so-called critical density. The new results are compared with the results of all earlier calculations and reveal the cause of their discrepancies.