TH/1-6 · Profile Formation and Sustainment of Autonomous Tokamak Plasma with Current Hole Configuration
N. Hayashi1), T. Takizuka1), T. Ozeki1)
1) Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Ibaraki, Japan
Abstract: The tokamak plasma with the current hole (CH) configuration is considered an attractive candidate for the steady-state reactor due to its high confinement and high bootstrap current fraction. We have investigated the profile formation and sustainment of tokamak plasma with the CH by using 1.5D time-dependent transport simulations. A model of the current limit inside the CH on the basis of the Axisymmetric Tri-Magnetic-Islands (ATMI) equilibrium is introduced into the transport simulation. We found that a transport model with the abrupt suppression of anomalous transport in the reversed-shear (RS) region can reproduce the time evolution of profiles observed in JT-60U experiments. The transport becomes neoclassical-level in the RS region, which results in the formation of profiles with internal transport barrier (ITB) and CH in RS plasmas. The CH plasma has an autonomous property because of the strong interaction between a pressure profile and a current profile through the large bootstrap current fraction. The energy confinement inside the ITB determined by the neoclassical-level transport also agrees well with the energy confinement scaling based on the JT-60U data. The scaling means the autonomous limitation of stored energy in the CH plasma. It is found from this scaling that the plasma with larger CH has higher normalized beta value and higher bootstrap current fraction. The plasma with the large CH is sustained with the full current drive by the bootstrap current. In this plasma, the CH size and the normalized beta value are self-determined. The plasma with the small CH and the small bootstrap current fraction shrinks due to the penetration of inductive current. This shrink is prevented by adding an appropriate external current drive.
Keywords: advanced tokamak, bootstrap current, current hole, internal transport barrier, reversed magnetic shear,