Energy confinement in auxiliary heated divertor and limiter discharges in the DIII-D tokamak

The scaling of the H-mode power threshold with various plasma parameters has been determined; the roughly linear increase with plasma density and toroidal field is particularly significant. The data indicate that the H-mode transition is associated with the sudden reduction in anomalous, fluctuation connected transport across the outer midplane of the plasma. It is found that the edge localized modes (ELMs) are triggered when the pressure gradient at the edge of the plasma exceeds the threshold given by ideal ballooning mode theory. The energy confinement time increases linearly with the plasma current until the safety factor at the 95% flux surface q95 drops below three; at q95 < 3, it is independent of the current and, at constant q95, depends linearly on the toroidal field. Controlling the effects of ELMs has allowed nearly steady state plasmas with H-mode durations up to 4.4 s. The energy confinement time in H-mode plasmas created with Ohmic heating alone shows a factor of two improvement over standard Ohmic plasmas. The confinement time in 2.0 MA H-mode plasmas with neutral beam heating can significantly exceed the standard Ohmic value also. Local transport studies have shown that the thermal diffusivity in the outer half of the plasma in the neutral beam heated H-mode is a factor of three lower than the L-mode diffusivity at the same plasma density. (author). 32 refs, 8 figs