A helical antenna for exciting azimuthally-asymmetric Alfven waves.

A new coupler for exciting azimuthally-asymmetric Alfven waves has been experimentally investigated. The coupler, which consists of four symmetrical helical straps wound around the vacuum vessel, is driven by a four-phase rf source. Waves excited by the coupler vary azimuthally as e^jm0, where m can be chosen to be either +1 or -1. The coupler exhibits a strong propagation directionality and is capable of launching waves whose wavelengths differ considerably from its own. Loading measurements indicate an equivalent coil impedance of 1.2 - 2.0 ohms in vacuum, and 1.2-4 ohms with plasma present. This impedance is dependent on the excitation frequency and the sign of m. The m = -1 wave was observed to propagate above and below wci and was identified as the fast wave. The m = +1 wave wci was observed only below o) . as the slow wave. No m = +1 wci fast wave or m = -1 slow wave was observed.