Confinement and stability of VH mode discharges in the DIII-D Tokamak

A regime of improved H mode energy confinement, the VH mode, was obtained following boronization of the DIII-D Tokamak vacuum vessel. The gradual confinement improvement in VH mode is associated with expansion of the H mode pedestal and reduction of thermal diffusivity in a region just inside the pedestal. Disappearance of density fluctuation bursts is associated with a confinement improvement in the latter region. The VH mode is terminated in a rapid global energy loss which is initiated by a mode with toroidal mode number, n approximately 5, consistent with calculated instability to an edge localized ideal kink. The improved confinement in VH mode is consistent with the extension of the region of the E*B velocity shear turbulence suppression zone further in from the plasma boundary. Expansion of the ideal ballooning second stability region and of the region with drift reversal stabilization of trapped particle modes are also considered as possible explanations for the confinement improvement