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

A regime of very high confinement (VH-mode) has been observed in neutral beam-heated deuterium discharges in the DIII-D tokamak with thermal energy confinement times up to ∼3.6 times that predicted by the ITER-89P L-mode scaling and 2 times that predicted by ELM-free H-mode thermal confinement scalings. This high confinement has led to increased plasma performance, nD (0)Ti(0)τE = 2 x 1020 m-3 keV sec with Ip = 1.6 MA, BT = 2.1 T, Zeff ≤ 2. Detailed transport analysis shows a correspondence between the large decrease in thermal diffusivity in the region 0.75 ≤ ρ ≤ 0.9 and the development of a strong shear in the radial electric field in the same region. This suggests that stabilization of turbulence by sheared E x B flow is responsible for the improved confinement in VH-mode. A substantial fraction of the edge plasma entering the second regime of stability may also contribute to the increase in confinement. The duration of the VH-mode phase has been lengthened by feedback controlling the input power to limit plasma beta