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 opt imally designed slotted helical antenna, excited by a conical horn, in a mirror machine, For maximizing plasma production efficiency, the he lical antenna is designed so that only its slow-wave mode (SWM) is exc ited and the fast wave is eliminated completely, The horn also excites efficiently the dominant field components of the SWM of the helical a ntenna, It is shown that this type of aperture excitation leads to muc h higher ionization efficiencies [approximate to 23-90%] than could be obtained in earlier studies (A, Ganguli ct al., IEEE Trans, Plasma Sc i., vol, 19, p. 433, 1991) using coaxial feeds [approximate to 5-20%], As a consequence, the present experiments yield higher bulk plasma de nsities, broader radial profiles, and larger plasma volumes. The study also confirms that the bulk plasma in these experiments is produced b y a warm electron population by impact ionization of the neutral gas a toms.