Abstract
Abstract For the first time, experiments on the DIII-D tokamak have demonstrated electron cyclotron current drive (ECCD) 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.
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