Design Considerations and Research and Development of a Comb-line Traveling Wave Antenna for Helicon Current Drive in DIII-D

摘要

In the search for efficient methods of non-inductive current drive for tokamak reactors, attention has turned to current drive using fast waves in the lower hybrid range of frequencies, referred to as whistler waves or, more recently, helicons [1]. Early experiments on this were carried out soon after the successful demonstration of current drive with the other propagating cold-plasma wave in this frequency range, the quasi-electrostatic slow lower hybrid wave. The biggest challenges in these experiments, reviewed in [2], were (1) to obtain sufficiently strong single-pass damping, essentially by producing a plasma with sufficiently reactor-like electron beta, and (2) to efficiently excite a wave with high enough parallel index of refraction n_(||) with a wave launcher at the edge of the plasma. Both of these obstacles have been in principle overcome in the DIII-D tokamak recently. At least one operating regime has been identified [3] with simultaneously high density and electron temperature at mid-radius (p~0.5) to yield predicted full first-pass absorption of 0.5 GHz helicons launched at n_(||)~3. The launcher concept, developed by General Atomics in the early 1990s [4,5] and demonstrated at 0.2 GHz on JFT-2M [6], is the 'comb-line' traveling wave antenna. As viewed from the plasma, this launcher looks like a wide phased array of many conventional Faraday-screened poloidally-oriented current-carrying straps, while the feature that makes a large number of radiating elements practical is that rf power is fed only to one end of the array (the 'upstream' end); power not radiated into the plasma from the fed element couples to the adjacent element via mutual reactance. If any power remains at the 'downstream' end of the structure, it is coupled out of the vacuum vessel via a feed structure identical to the one at the other end. Ideally, the lack of any internal reflections, or reflections from the downstream end means that a high degree of directivity is achieved, and t