Studies of MHD behaviour in JT-60U

Several important topics relating to MHD behaviour in JT-60U are presented. An extremely hollow current profile was found for the first time in JT-60U with a 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–Schlü;ter current increases, which reduces the poloidal field on the inboard side. It may cause the null point of the magnetic field. β limits due to ideal MHD instability in the current hole do not change very much compared 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, a real-time control system, which can identify the island by ECE measurement and control EC ray to the island by a steerable mirror, was prepared in JT-60U, and it successfully stabilized the NTM. Instability due to fast ions is an important 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 Alfvén 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 mode is a candidate of the slow-FS mode. A bursting mode was observed to change its frequency by 10–20 kHz in 1–5 ms (fast FS mode) and evolve explosively in ∼400 μs (abrupt large amplitude event). Analyses of the bursting mode show that the non-linear deformation of the energetic ion distribution may cause the repetition of bursting phenomena and the recovery of lost energetic ions.