Stabilization of neoclassical tearing mode by Electron Cyclotron Current Drive and its evolution simulation on JT-60U tokamak

Stabilization of an m=3/n=2 neoclassical tearing mode (NTM) by a local electron cyclotron current drive (ECCD) has been studied in the JT-60U tokamak. In the stabilization experiment, the EC injection timing is scanned from the 'before mode onset' phase to the mode saturation phase. We have demonstrated that the ECCD is more effective when it is applied before the mode onset, and the critical timing for effective ECCD is related to the mode excitation phase. An ELMy H-mode plasma of βN=2.9 has been sustained for 5 sec by suppressing the 3/2 NTM using the second harmonic X-mode ECCD of 2.4 MW. A numerical study has been made on the basis of the modified Rutherford equation coupled with a 1.5D transport code and an EC code. The simulation well reproduces the time evolution of the magnetic island both at the growing and stabilizing phases. In the early ECCD, the changes in the background profiles such as the bootstrap current and/or the classical tearing parameter are required to explain the decrease of the mode amplitude. (author)