Plasma modeling and atomic reaction rates

The perturbative effects of plasma electrons and ions on the effective capture and ionization rates in hydrogen plasma with carbon impurities are analyzed. (1) A simple model of hydrogen plasma is first constructed to extract the salient features of the system. (2) The influence of electron-density fluctuations on atomic reaction rates is studied in a hydrogen plasma. We show that the effective ionization rates are extremely sensitive to deviations of the electron density from the local thermal equilibrium (LTE) distribution, while the effective recombination rates are relatively stable against such fluctuations. (3) A diffusion equation approach to the collisional radiative recombination and ionization processes in plasmas is improved, so as to be applicable to low density plasmas. Dense excited levels are assumed to be quasi-continuous and thus are 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. (4) A low density, low temperature plasma formed by two merged beams of electrons and ions at near zero relative velocity is studied by solving a set of time-dependent rate equations. In particular, we investigate the role played by the radiative recombination (RR) and three-body recombination (TBR) on the population of excited states during the initial stage of plasma rearrangement and relaxation. (5) The effects of $H\sp-$ and its resonant states $(H\sp-)\sp{\*\*}$ in hydrogen plasmas are investigated. When $H\sp-$ and $(H\sp-)\sp{\*\*}$ are introduced, the population distribution at equilibrium is not affected much, but the effective rates change appreciably as the electronic density increases and temperature decreases. (6) We formulate a unified approach to the problem of the plasma density effect, and apply it to carbon impurities in a hydrogen plasma. Both the plasma field distortion of atomic states and the corresponding rates by the plasma ions and stochastic plasma collisional transitions caused by the plasma electrons are included. The latter effect is estimated by constructing an effective collisional transition operator, and the electron-ion recombination processes are explicitly evaluated. It is shown that these two effects of the ionic field distortions and electronic collisions tend to cancel each other, resulting in many cases in reducing the overall effect of the plasma density on the ionization balance.