Helicon wave excitation with rotating antenna fields

Abstract

By using phased bifilar antennas, helicon waves have been excited by applying fields which rotate either in space or in time, or both simultaneously. The direction of rotation is made to favour either m=+1 or m=-1 waves, where m is the azimuthal mode number, and m is measured directly. Rotation in time is found to be more effective than rotation in space and makes possible a direct comparison of excitation. An m=-1 structure was seen only in the antenna near field, while m=+1 modes propagate far downstream. Up to 2 kW of RF power, density profiles and antenna loading measurements show that m<0 (left-hand) waves are poorly coupled, and m=+1 (right-hand) waves are necessary for good plasma production. Loading results indicate that antennas also couple to absorption mechanisms unrelated to helicon waves.