Error-field induced electromagnetic torques in a large aspect-ratio, low-β, weakly shaped tokamak plasma

The toroidal electromagnetic braking torques exerted at the various internal rational surfaces of a large aspect-ratio, low-beta, weakly shaped, tokamak plasma by a nonaxisymmetric error field are investigated using a semianalytic approach. It is found that there is an optimal error-field spectrum for exerting a torque at a given rational surface. This spectrum is dominated by the resonant harmonic, but also contains sideband harmonics induced by plasma toroidicity, pressure, ellipticity, and triangularity. These sidebands couple back to the resonant harmonic in such a manner as to reduce its amplitude. Provided that there is significant coupling to a (stable) ideal external kink mode which is close to its marginal stability boundary, the optimal error field predominately contains sideband harmonics whose poloidal mode numbers are more positive than the resonant mode number (which is assumed to be positive), and also tends to balloon on the outboard side of the plasma.