Core flow shear as the cause of very high confinement in the DIII-D tokamak

The leading hypothesis for the cause of the very high confinement observed during VH mode in DIII-D is that a spontaneous increase in the core flow shear suppresses microturbulence which reduces transport loss and, for example, further increases flow, in a positive feedback cycle. This is confirmed by using 'magnetic braking' as an independent control of the toroidal rotation contribution to the flow; when the toroidal rotation shear of a VH mode discharge is reduced, the turbulence and transport increase back to the level observed during 'standard' H mode.