Studies of MHD Behavior in JT-60U
T. Ozeki and JT-60 team
Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki-ken 311-0193, Japan
Abstract.
Several critical topics of MHD behavior in JT-60U are presented. An extremely hollow current profile was found for the first time in JT-60U with very small plasma current in the central region, which is called the Current Hole. Analyses of equilibrium and stability of the current hole discharge show that, as βp increases, the Pfirsh-Schluter current increases, which reduces the poloidal field in the inboard side. It may cause the equilibrium limit. Beta limits due to ideal MHD instability in the current hole do not change very much comparing to that of the weakly reversed shear plasma. In high β long pulse plasmas, it is important to suppress the resistive MHD instability, neo-classical tearing mode (NTM), induced by the lack of bootstrap current inside the magnetic island. Analyses of two typical JT-60U discharges with and without NTM show that the polarization current due to the plasma rotation is a plausible candidate to suppress the NTM though it is very sensitive to the value of mode rotation. To establish an advanced control of NTM, real time contol system, which can identify the island by ECE measurement and control EC ray to the island by ECE measurement and control EC ray to the island by a steerable mirror, was prepared in JT-60U. Instability due to fastt ions is an impotant subject for sustaining the fusion reaction. In JT-60U, slow frequency sweeping (slow-FS) modes and bursting MHD modes in the frequency range of the toroidicity-induced Alfven eigenmodes (TAEs) are observed by injections of negatively-charged ion based neutral beam of ~ 360 keV. Analyses by the HINST code show that the resonant TAE (RTAE) mode is a candidate of the slow FS mode. A bursting mode was observed to change its frequency by 10-20kHz in 1-5 ms (fast frequency Sweeping mode) and evolve explosively in ~ 400 μs (A brupt Large amplitude Event). Analyses of the bursting mode show that the nonlinear deformation of the energetic ion distribution may cause the repetition of bursting phenomena and the recovery of lost energetic ions.