Effect of magnetic braking of the plasma rotation on the H-mode radial electric field and energy confinement in the DIII-D tokamak

Abstract

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