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 (Pheat ~ 7–12 MW). These DIII-D experiments have obtained the highest value of peak fusion gain, QDT,equiv ~ 0.5, achieved on a medium scale (R < 2 m) tokamak. Sustained high performance operation (βN ~ 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 Ip < 15 MA, and to potentially enable more compact, cost effective pilot plant and reactor designs.
Highlights
In a tokamak, the edge transport barrier, or ‘pedestal,’ region plays a critical role in both fusion performance and compatibility with desired divertor conditions
Experiments on Alcator C-Mod and DIII-D have achieved access to the Super H-Mode regime, and obtained very high pedestal pressure, including the highest achieved on a tokamak in C-Mod experiments operating near the ITER magnetic field
Pedestal pressure up to ~80 kPa has been achieved on C-Mod at toroidal and poloidal field near the ITER value, extending tests of the EPED model nearly to the ITER predicted pped
Summary
The edge transport barrier, or ‘pedestal,’ region plays a critical role in both fusion performance and compatibility with desired divertor conditions. EPED predicts the H-mode pedestal height and width in high performance regimes, including regimes with ‘Type I’ edge localized modes (ELMs) and regimes with a quiescent (‘QH Mode’) edge and edge harmonic oscillations (EHOs) [2], based upon criticality to these two constraints These calculations are performed on realistic model equilibria, with self-consistent bootstrap current in the pedestal region, to enable pedestal predictions for future experiments and future devices. We describe more recent SH experiments on Alcator C-Mod and on DIII-D (experiments on C-Mod during its final month of operations in September 2016, and experiments on DIII-D, with co-current NBI and ELMs, from June 2017–April 2018) These experiments were aimed at further testing of EPED model predictions, as well as realization of very high pedestals, strong fusion performance, and investigation of dissipative divertor operation.
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