The Influence of the Spatial Harmonics on the Rotating Magnetic Field Current Drive

Abstract

In rotating magnetic field (RMF) current drive experiments, the RMF is usually generated by means of a two-phase radio frequency (r.f.) system which feeds a pair of orthogonal coils. The magnetic field generated in this manner consists of the desired RMF as well as the (undesired) odd spatial harmonics. The (2j-l)th spatial harmonic field rotates about the axis at an angular speed (-1)-i-lwl(2j-1), where w is the angular frequency of the r.f. current. In order to evaluate the effect of these spatial harmonics on the performance of the RMF current drive experiments, we have developed a simplified theoretical model for the plasma. In this model, the ions are assumed immobile and the motion of the electron fluid in the azimuthal direction is assumed to be a rigid rotation. It has been found using this model that the presence of the spatial harmonics makes the RMF current drive less efficient. It has also been found that the effect of the spatial harmonics can be made negligible by (a) making the width of the coils sufficiently larger than the diameter of the plasma, (b) carefully designing the coil configuration to eliminate the fifth harmonic or (c) using a three-phase system to generate the RMF.