Alternating current ohmic heating in a toroidal stellarator

An experimental study of high frequency inductive heating in a toroidal stellarator is reported. It is demonstrated that high frequency power (omegatau/sub s/ > 1, where tau/sub s/ is the resistive skin time and omega is the frequency) can be efficiently coupled to the plasma by a mechanism that circumvents the limitations of the classical skin effect. This mechanism, described generally as modulation of the geometry of the magnetic surfaces, is studied for two magnetic field configurations: a classical stellarator and a doublestar - a stellarator with two large magnetic islands. The doublestar configuration is obtained by superimposing an axially symmetric multipole magnetic field on the stellarator field. The plasma is produced by breaking down the hydrogen gas with the use of the same toroidal rf electric field which is employed for plasma heating. The plasma response to the rf field is different in the two magnetic field configurations. In the stellarator, the plasma current exhibits a characteristic reversal with the current flowing in the direction of the rf coil current on the low-field side of the torus and in the opposite direction of the high-field side of the torus. In the doublestar, the current is distributed more uniformly. The different response is attributed to the different geometry of the magnetic surfaces and to reconnection of the magnetic flux in the doublestar configuration