Radio Frequency Current Drive Considerations for Small Aspect Ratio Tori

Abstract

Noninductive current drive is required during plasma initiation and for current sustainment in the National Spherical Torus Experiment (NSTX). In this paper, the physics of high harmonic fast waves (HHFW) and the design of an antenna system for NSTX are considered using numerical models. For high current discharges in NSTX, the static magnetic field component in the poloidal direction varies widely during the discharge and can become comparable to the toroidal component in NSTX. Therefore, they calculate the plasma loading for a broad range of antenna and plasma geometries in a three-dimensional model, so that the results can be used to influence the antenna design. Two-dimensional calculations of the wave propagation and absorption in the core plasma indicate that the theoretical current drive efficiency for HHFW can be high, and a general survey of parameters gives a good target for the antenna design. The current drive efficiency calculation is sensitive to the equilibrium model because finite beta effects can substantially alter the calculation of the trapped particle fraction. Traditional methods of toroidally phasing an antenna array as well as poloidal phasing are studied to optimize the current drive efficiency for a range of equilibria. Non-zero poloidal model excitation is also found to affect the antenna performance and flexibility. Performance expectations for a preliminary antenna design are given.