Modified Fokker-Planck approach to steady-state distributions in plasmas

A diffusion-equation approach to the collisional radiative recombination and ionization processes in plasmas is improved in order to be applicable to low density plasmas. Dense excited levels are assumed to be quasicontinuous and thus treated by Fokker-Planck equations in the nearest neighbor (nn) approximation, and a few levels lying below the bottleneck are retained explicitly as a discrete set. These coupled equations are solved iteratively, which in turn allows incorporation of the corrections to the nn approximation. The method has been tested for the simple hydrogen plasma in a steady state. The numerical results agree with the exact solutions. Extensions of this approach to nonequilibrium plasmas and complex plasmas of heavy ions are discussed