Extension of the JT-60U plasma regimes toward the next-step experimental reactor

In order to establish the physics basis for the sustainment of the high integrated performance required in the next-step experimental reactor, JT-60U has been optimizing the discharge control scenarios of high confinement plasmas and extending the operation regimes by utilizing a variety of heating and current drive systems, including the new Electron Cyclotron Heating/Electron Cyclotron Current Drive (ECH/ECCD) system, and the W-shaped pumped divertor. For the integration, current and pressure profile controls are essential and high triangularity is beneficial. The range of plasma current with high values of H-factor, N and bootstrap fraction was extended to 1.5 MA with nearly full non-inductive current drive by Negative-ion-based Neutral Beam (N-NB) injection into the high-p H-mode with Edge Localized Modes (ELMy H-mode). High current drive efficiency of 1.3 × 1019 A W-1 m-2 was demonstrated for N-NB. High triangularity (0.4-0.5) operation extended the long pulse (~3 s) high N (2.5-2.7) region to low-q95 (~3). For the reversed shear (RS) mode, feedback control of neutron production rate and stored energy enabled reproducible achievement of the DT equivalent fusion gain QeqDT>1 and sustainment of QeqDT~0.4-0.5 for ~1 s. Using the RS mode and the high high-p ELMy H-mode, the high confinement region has been extended to higher Te/Ti and also to higher density. Electron heating by Lower Hybrid Range of Frequency (LHRF) and N-NB extended the high confinement region to Te/Ti>1. Argon gas puffing improved confinement in the high density regime. With the W-shaped pumped divertor, the threshold heating power for the L-H transition was reduced by 30% as compared with the open divertor. Using the pumped divertor, a multiple parameter feedback control including both core and divertor plasma parameters was demonstrated.