Characterization of microwave discharges produced by a slotted helical antenna in a mirror machine

This paper presents a detailed study on plasma production using an optimally designed slotted helical antenna, excited by a conical horn, in a mirror machine. For maximizing plasma production efficiency, the helical antenna is designed so that only its slow-wave mode (SWM) is excited and the fast wave is eliminated completely. The horn also excites efficiently the dominant field components of the SWM of the helical antenna. It is shown that this type of aperture excitation leads to much higher ionization efficiencies [≈23-90%] than could be obtained in earlier studies (A. Ganguli et al., IEEE Trans. Plasma Sci., vol. 19, p. 433, 1991) using coaxial feeds [≈5-20%]. As a consequence, the present experiments yield higher bulk plasma densities, broader radial profiles, and larger plasma volumes. The study also confirms that the bulk plasma in these experiments is produced by a warm electron population by impact ionization of the neutral gas atoms