Time-dependent kinetics model for a helium discharge plasma

Measurements of electron density and electron temperature at various times during a cathode discharge in helium have been recorded with a Langmuir probe in the UCLA large plasma device. The measured electron temperatures were then used in a time-dependent collisional-radiative model to propagate the electron density as a function of time for plasma conditions corresponding to population densities far from equilibrium. The initial gas density was extracted from pressure measurements and used in the calculations so there were no adjustable parameters in the model. The calculated and measured densities are in good agreement for three different radial positions in the plasma. The results show that the plasma formation becomes dominated by bulk thermal electrons early in the discharge. The detailed model is compared with several simplified models to study the role of metastable levels and excited states in the plasma kinetics. The relative importance of the various atomic processes in the calculation of plasma formation is also discussed.