The temporal decay of the diffusion-determined afterglow plasma of the positive column

Theoretical and experimental investigations of the afterglow decay in the diffusion-determined positive-column plasma of a neon discharge are presented. In the frame of the model the time-dependent electron Boltzmann equation, the rate equations for the neon atoms excited in the (metastable) and (resonance) level and an appropriate equation for the electric field are self-consistently solved. A radially averaged treatment has been used for the cylindrical, axially homogeneous column plasma. A detailed description of the model is given. The temporal behaviour of the decaying plasma has been calculated up to the millisecond-range. The results concerning the electron and heavy-particle kinetics have been comprehensively discussed. The analysis of the results shows that the chemo-ionization processes have an important impact. The isotropic part of the electron velocity distribution is subject to large structural changes in the temporal evolution of the decaying plasma. For higher kinetic energies two plateaux are caused as a result of the change from an active discharge plasma, driven by the electric field, to a state which is strongly determined by the action of the chemo-ionization processes. The dominant processes in the power balance of the electrons change drastically and, in the early afterglow, a slight increase of the electron density, mainly due to chemo-ionization processes, is found. The metastable neon atoms act as a power and charged-particle reservoir. In addition, probe measurements of the velocity distribution function, the density and the mean energy of the electrons have been performed in the microsecond-range of the afterglow. The experimental results are discussed and compared with the results of model calculations. The agreement between calculated and measured results is very good at steady state and satisfactory during about the first of the decay. In the further afterglow period increasing discrepancies between the results arise, which are evaluated critically with regard to various possible reasons.