Vertical positional instability in JT-60SU

JT-60SU has been designed as an experimental device to establish an integrated scientific basis of steady-state tokamak operation. The high elongation of plasma cross section is employed to keep the safety factor at the plasma surface high for high current operation. The vertical instability becomes very dangerous for this high elongation configuration, and, among other MHD instabilities, it must be avoided first to obtain stable steady-state operation. The vertical positional instability of JT-60SU is investigated and it is shown that the growth rate of vertical instability can be reduced to 30–50 Hz, and stability margin can be increased to 0.5 for a standard steady-state configuration of JT-60SU by adding the baffle plate close to the plasma surface. It is also shown that the triangularity has a stabilizing effect on the vertical instability for the standard equilibrium configuration of JT-60SU, because the absolute decay index value becomes smaller for higher triangularity.