Understanding and control of transport in Advanced Tokamak regimes in DIII-D

Transport phenomena are studied in Advanced Tokamak (AT) regimes in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomics Energy Agency, Vienna, 1987), Vol. I, p. 159], with the goal of developing understanding and control during each of three phases: Formation of the internal transport barrier (ITB) with counter neutral beam injection taking place when the heating power exceeds a threshold value of about 9 MW, contrasting to co-NBI injection, where $P_{\mathrm{threshold}}\mathrm{<2.5 MW}.$ Expansion of the ITB is enhanced compared to similar co-injected discharges. Both differences are believed to arise from modification of the $\mathrm{E\times B}$ shear dynamics when the sign of the rotation contribution is reversed. Sustainment of an AT regime with ${\beta }_N{H}_{89}\text{=9}$ for 16 confinement times has been accomplished in a discharge combining an ELMing H-mode (edge localized, high confinement mode) edge and an ITB, and exhibiting ion thermal transport down to 2–3 times neoclassical. The microinstabilities usually associated with ion thermal transport are predicted stable, implying that another mechanism limits performance. High frequency magnetohydrodynamic (MHD) activity is identified as the probable cause.