Dense plasmas, screened interactions, and atomic ionization

There now exist many laboratory programs to study non-equilibrium plasmas in which the electron interparticle spacing n(e)(-1/3) is no more than a few Bohr radii. Among these are short-pulse laser heating of solid targets, where n(e) similar to 10(23) cm(-3), and inertial confinement fusion experiments, where n(e) > 10(25) cm(-3) can be achieved. Under such extreme conditions, the plasma environment is expected to have a strong influence on atomic energy levels and transitions rates. Investigations of atomic ionization in hot, dense plasmas have been motivated by the fact that the instantaneous degree of ionization is a key parameter for the modeling of these rapidly evolving physical systems. Although various theoretical treatments have been presented in the literature, here we focus on the "random field" approach, because it can readily incorporate (quasi-static) level shifts of the target ion as well as dynamic plasma effects. In this approach, the stochastic perturbation of the target by plasma density fluctuations is described in terms of the dielectric response function. Limiting cases of this description yield the familiar binary cross-sectional model, static screening collision models, and the more general dynamical screening models. Screening of the target ion is treated here with several static screening potentials, and bound state level shifts of these potentials are explored. Atomic oscillator strength densities based on these different models are compared in numerical calculations for ionization of He+ and Ar+17. Finally, we compile a list of atomic/plasma physics issues that merit future investigation. (C) 1998 Elsevier Science B.V. All rights reserved.