Effects of a non-resonant m=0, n=1 perturbation driven by coupling to the m=2, n=1 mode in an elongated tokamak

In a tokamak the coupling of resonant tearing modes with the plasma shape effects like toroidicity, elongation, and triangularity produces sidebands harmonics. In particular an m=0, n=1 nonresonant sideband can be driven by a m=2, n=1 tearing mode in an elliptic cross section tokamak. It is shown that this is an important effect. The m=0 mode is expected to produce a neoclassical toroidal viscous force which could explain the dramatic braking of the bulk plasma toroidal velocity observed during the onset of locked modes driven by resonant helical error fields or by spontaneous magnetohydrodynamic activity.