A toroidally localized vertical magnetic field produced by an external coil (the 'n=1 coil') is applied to a rotating tokamak plasma driven by co-injected tangential neutral beams and in the high confinement H-model. The toroidal plasma rotation is greatly slowed by magnetic braking, with consequent reduction and reversal of the core radial electric field Er and the shear in Er; the plasma maintains a negative edge Er and a negative edge Er shear, despite the large changes, and remains in the H-mode with insignificant changes in global confinement, density profile and temperature profiles. The experimental observations are consistent with theories predicting that the high confinement H-mode is produced and maintained by an edge region where strong radial shear in the velocity, v⊥ = Er × B/B.B, suppresses density microturbulence and resultant transport loss, and that in regions where the radial shear in v⊥ ≈ Er/Bϕ is below a critical value for turbulence quenching, as in the core of DIII-D, large core changes (reductions) in rotation, Er and shear in Er have no effect on confinement
Toroidal mode structure in weak and reversed magnetic shear plasmas and its role in the internal transport barrier