Doubling off-axis electron cyclotron current drive efficiency via velocity space engineering
Xi Chen, C. C. Petty, J. Lohr, David Su, R. Prater, M. Cengher, Max Austin, C. T. Holcomb, L.L. Lao, R. I. Pinsker, B. Victor, L. Zeng
Abstract
Abstract For the first time, experiments on the DIII-D tokamak have demonstrated electron cyclotron current drive with more than double the conventional efficiency by tailoring the wave–particle interactions in velocity space using a novel ‘top launch’ geometry. Steering the EC waves to propagate nearly parallel to the resonance drives current more efficiently by (1) selective damping on electrons with higher parallel velocity v ||, and (2) longer absorption path to compensate for inherently weaker absorption at higher v ||. Experiments using a fixed-injection top launch system find an optimal velocity space interaction for maximum current drive efficiency at ρ ∼ 0.5 where the ease of drawing out a high v || electron tail is balanced by sufficient absorption.