Dynamical screening effects on rate coefficients for dense plasmas

Usually collisional rate coefficients in plasmas are calculated from cross sections for the respective processes averaged with the electron distribution function. The cross sections are taken from scattering theory or scattering experiments referring to processes of isolated particles. In dense plasmas, however, there have to be taken into account many-particle effects like screening, lowering of the ionization energy and Pauli-blocking. These can be included into the theory if one starts from quantum kinetic equations for nonideal systems with chemical reactions. Statistical expressions can be derived for the rate coefficients now taking into account many-body effects. Also in-medium cross sections can be calculated which are dependent on the density. However, the concept of a cross section breaks down in dense plasmas in some cases. This shows that the ionization process, for instance, cannot be treated further as a process in which one electron collides with an atom, but is a collective one. Numerical results are presented for the coefficients of impact ionization, three-body recombination, excitation and deexcitation in nonideal hydrogen and carbon plasmas. These coefficients art strongly dependent on the density. Results obtained in the cases of statically and dynamically screened potentials are compared in the latter case, higher ionization and recombination rates are obtained in the dense plasma region. (C) 1997 Elsevier Science Ltd. All rights reserved.