High fusion performance in Super H-mode experiments on Alcator C-Mod and DIII-D

Abstract

The 'Super H-Mode' regime is predicted to enable pedestal height and fusion performance substantially higher than standard H-Mode operation. This regime exists due to a bifurcation of the pedestal pressure, as a function of density, that is predicted by the EPED model to occur in strongly shaped plasmas above a critical pedestal density. Experiments on Alcator C-Mod and DIII-D have achieved access to the Super H-Mode (and Near Super H) regime, and obtained very high pedestal pressure, including the highest achieved on a tokamak (p ped ∼ 80 kPa) in C-Mod experiments operating near the ITER magnetic field. DIII-D Super H experiments have demonstrated strong performance, including the highest stored energy in the present configuration of DIII-D (W ∼ 2.2-3.2 MJ), while utilizing only about half of the available heating power (P heat ∼ 7-12 MW). These DIII-D experiments have obtained the highest value of peak fusion gain, Q DT,equiv ∼ 0.5, achieved on a medium scale (R < 2 m) tokamak. Sustained high performance operation (β N ∼ 2.9, H98 ∼ 1.6) has been achieved utilizing n = 3 magnetic perturbations for density and impurity control. Pedestal and global confinement has been maintained in the presence of deuterium and nitrogen gas puffing, which enables a more radiative divertor condition. A pair of simple performance metrics is developed to assess and compare regimes. Super H-Mode access is predicted for ITER and expected, based on both theoretical prediction and observed normalized performance, to allow ITER to achieve its goals (Q = 10) at I p < 15 MA, and to potentially enable more compact, cost effective pilot plant and reactor designs.