Normal curvature, local magnetic shear and parallel current density in tokamaks and torsatrons

Reversed magnetic shear is associated with large negative surface averaged parallel current densities that increase in magnitude towards the plasma edge and guarantee stability at large beta in tokamaks. The surface averaged parallel current density is smaller and decreases in magnitude radially in conventional shear tokamaks. This results in much lower magnetohydrodynamic (MHD) stable beta. The parallel current density in torsatrons is dominated by the Pfirsch-Schluter currents, the global shear contributes negligibly to the local shear and the beta limits are comparable to that of a conventional tokamak. A simplistic evaluation of the local sign of the driving terms in the energy principle does not provide a useful guide to even qualitatively describe the MHD stability of a device. Ballooning instabilities concentrate in regions where the curvature is most destabilizing. Their localization, does not appear to depend on the sign of the interaction between the parallel current density and the local magnetic shear contribution to the driving term. (C) 1997 American Institute of Physics.