Neutral gas distribution in a helicon plasma source

Summary form only given. High density helicon plasma sources have demonstrated the phenomenon of neutral pumping. The high electron density of a wave generated plasma column in an axial magnetic field ionizes an appreciable fraction of the neutrals in the column. The neutrals then become concentrated at the ends of the source, where the loss flux is concentrated. The axial profile of argon neutral pressure at a radius of 3.5 cm has been measured using a capacitive manometer in a helicon source operated at 1000 G, 200-700 W. The plasma radius (based on electron density half-width-half-maximum) was approximately 2 cm. The data indicate a decrease in neutral pressure from a 20 mtorr fill pressure to 2-3 mtorr over most of the column length, except near the ends. A 1-D analysis of the neutral particle balance, including plasma ionization, radial and end wall loss/recombination, and nonuniform ion neutral-ion diffusion coefficients has been developed to model the helicon source. Experimentally measured values of power, magnetic field, electron temperature and end loss current are used as inputs to the code. The calculated neutral density axial profiles are in good agreement with experimentally measured neutral pressure profiles. The results of the calculation indicate the role of neutrals in particle confinement of the helicon source; in particular, neutral density gradients at the ends appear to restrict axial ion flow and hence improve plasma confinement.