Helical antenna for exciting azimuthally asymmetric Alfvén waves

Abstract

A new coupler for exciting azimuthally asymmetric Alfvén 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 exp(jmϑ), 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, with a plasma load, of 1.2–3.9 Ω, which depends on the excitation frequency and the sign of m. The m=−1 wave was observed to propagate above and below ωci and was identified as the fast wave. The m=+1 wave was observed only below ωci as the slow wave. No m=+1 fast wave or m=−1 slow wave was observed.