Study of improved confinement modes with edge and/or internal transport barriers on the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U)

Abstract

Two types of advanced tokamak scenarios, namely, high βp H-mode and reversed shear (RS) mode, are developed and studied extensively in Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) [S. Ide and the JT-60 Team, Phys. Plasmas 7, 1927 (2000)]. These plasmas have both an internal transport barrier (ITB) and an edge transport barrier (ETB). The goal of the recent studies was therefore to get a deeper insight into the physics of ITB and ETB and to increase the discharge performance under a controlled manner based on the physics understanding. In the studies of ETB in an extended high triangularity operation with pellet injection from the high-field side, we have clarified the linkage between main parameters determining the pedestal structure. In the studies of type I edge localized modes (ELMs), simultaneous fast ELM measurements revealed four different phases in an ELM event. It was also established that the density collapse is poloidally asymmetric and is localized at the low-field side. In the studies of ITB, the response of the ion thermal diffusivity to the radial electric field (Er) shear and the effect of electron heating on ITB have been investigated. The importance of nonlocality and the effective Er shear are discussed. In the case of the RS plasma with a strongly reversed current profile, it has been found that there is no plasma current in the central region (called the “current hole”), and it is impossible to drive current in either the positive or negative direction by an electron cyclotron wave inside the current hole. Based on the new understanding of ETB and ITB, the control of the transport barriers is also discussed briefly.