Diii-D Tokamak Concept Improvement Research

Recent results in tokamak performance optimization and understanding through shape and plasma profile control have brought DIII-D closer to its goal of advanced tokamak (AT) demonstration (H = tau(E)/tau(ITER89P) up to 4, and beta(N) = beta(T)/(I/aB) up to 6, simultaneously in steady-state). A high performance regime characterized by negative central magnetic shear has resulted in a high BNH product of up to 18, and Q(d-equivalent) = 0.32. Strong plasma shaping coupled with the control of current and pressure profiles is responsible for the improvement in MHD stability, and stabilization of microturbulence by sheared ExB flow plays a dominant role in the reduction of transport. Density control and helium exhaust are demonstrated with divertor pumping, and reduction of heat flow to the divertor by an order of magnitude through radiative cooling suggests an effective heat exhaust scheme compatible with AT operation. Active current profile control using a combination of a neutral beam system, bootstrap current and a fast wave system has been initiated. Achievement of long-pulse AT operation is the future focus of the DIII-D Program.