Oscillating Field Current Drive in the Reversed Field Pinch.

摘要

Oscillating Field Current Drive (OFCD) was originally proposed as a means of maintaining a quasi steady-state plasma current in a RFP without building up the magnetizing flux threading the central hole of the torus. The MHD dynamics of OFCD is analyzed using a combination of a 1-D transport code, a linear resistive MHD stability model, and Kadomtsev nonlinear global reconnection model. Using Faraday's law, Ohm's law and on-axis regularity conditions, it is shown that m = 1 modes are necessary for current drive, within the resistive MHD model. A class of m = 1 tearing modes that nonlinearly generate poloidal flux (a necessary condition for current drive) has been identified. These modes are destabilized by off-axis current peaks. A scoping study of the F-theta space has been undertaken to determine quantitatively the effectiveness of this class of tearing modes as a current drive mechanism. Results indicate that equilibria do exist, in regions of the F-theta space that are easily accessible experimentally, that can yield significant amounts of poloidal flux generation without seriously deteriorating the confinement. A scenario of the OFCD oscillations has been developed in terms of the evolution of the safety factor profile, by considering the stability of the RFP to current-driven tearing modes. The alternating compression and expansion of the plasma during the oscillations have been associated to two different classes of m = 1 tearing modes: the compression to modes driven by off-axis current peaks, that generate poloidal flux; the expansion to instabilities driven by on-axis current peaks, that generate toroidal flux. 76 refs., 34 figs., 5 tabs. (ERA citation 14:023785)