Temporal relaxation of excited-level populations of atoms and ions in a plasma: Validity range of the quasi-steady-state solution of coupled rate equations

Abstract

By solving the coupled rate equations for excited level populations for an abrupt change of plasma parameters, we have examined the transient characteristics of these populations which approach the steady-state values with time. For hydrogen atoms under the ionizing plasma condition the transient time of the levels lying lower than Griem’s boundary, i.e., the level at which the radiative and collisional depopulation rates are equal, is determined by their natural lifetimes, and that of all the higher-lying levels is given by the relaxation time of the boundary level. Under the recombining plasma condition the transient time of the levels lying higher than the boundary is determined by their own relaxation times, and that of the lower-lying levels is given by that of the boundary level. Thus, the overall response of excited level populations is determined by the relaxation time of Griem’s boundary level. The effective rates for ionization and recombination are also examined.