DYNAMICS OF CHARGED SPECIES IN THE AFTERGLOW REGION OF A LOW-PRESSUREMICROWAVE PLASMA

A detailed model of the behaviour of charged species in the afterglow of a nitrogen plasma in a tubular geometry has been developed. The axi symmetric model for the species concentrations (N-k(+), where k = 1, 2 , 3 and 4) accounted for radial and axial ambipolar diffusion, homogen eous recombination, homogeneous ionization, ion-neutral species rearra ngements and recombination reactions at the sheath boundary at the wal l. Comparison of the model results with experimental measurements show ed agreement to within the experimental uncertainties. By systematical ly varying the terms in the model, it was possible to identify the imp ortant reactions in the afterglow. In terms of total ion concentration s, the homogeneous ionization reactions were a significant source term , particularly near the entrance of the afterglow, and recombination a t the sheath boundary layer was the dominant sink term (homogeneous re combination was negligible). Indeed, the rate of recombination at the sheath boundary was so fast that the overall process is limited by the rate of radial diffusion of ions to the sheath. The ion-neutral speci es reactions, being charge-conserving, had little effect on the total concentration of ions; however, their importance in determining the re lative concentrations was identified. It was also demonstrated that a fully two-dimensional model of the afterglow was necessary to predict the experimental measurements accurately. Truncation of the model to o ne dimension produced results that disagreed with experiment by severa l orders of magnitude.