Über das unterschiedliche Ionisationsgleichgewicht wasserstoff- und alkali-ähnlicher Ionen in optisch dünnen Plasmen

Abstract

There are many plasmas in which the populations of the various energy states of ions and electrons assume a steady state, but complete local thermal equilibrium is prevented because, on the one hand, radiation absorption is absent and, on the other, the electron density is too low. Calculations for a plasma without radiation absorption show that: Where hydrogen-like ions are involved it is nearly in order to dispense with detailed calculations and described the ionization equilibrium with the assumption that the collisional ionization processes from the ground state can be equated with the radiative recombination processes to the ground state. For sufficiently low electron densities this leads to the Corona formula. In the case of other ions, however, neglecting the excited states may result in serious errors. This is because these energy states — unlike those for hydrogen-like ions, which are relatively near the ionization limit — are distributed much more uniformly between the ground state and ionization limit. The implications of this behaviour are discussed with reference to alkali-like ions. A model for the term systems and the collision and radiation coefficients is used to derive the population densities and approximative ionization formulae. According to these the ratio of the densities of the lithium-like O VI ions and the next higher level of ions (O VII), for instance, may differ (for electron densities ne = 5 × 1017 cm–3) from the result of the Corona formula by a factor of 20.