Observation of the bootstrap current reduction at magnetic island in a neoclassical tearing mode plasma

Oikawa, Toshihiro; Suzuki, Takahiro; Isayama, Akihiko; Hayashi, Nobuhiko; Fujita, Takaaki; Naito, Osamu; Tsuda, Takashi; Kurita, Genichi; JT-60 Team

Evolution of the current density profile associated with magnetic island formation in a neoclassical tearing mode plasma was measured for the first time in JT-60U by. As the island grew, the current density profile turned flat at the radial region of the island, followed by an appearance of a hollow structure. As the island shrank, the deformed region became narrower, and it finally diminished after the disappearance of the island. In an MHD-quiescent plasma, on the other hand, no deformation was observed. The observed deformation in the current density profile associated with the tearing mode is reproduced in a time dependent transport simulation assuming reduction of the bootstrap current in the radial region of the island. Comparison of the measurement with a calculated steady-state solution also explains the temporal behaviors of the current density and safety factor profiles with reduction and recovery of the bootstrap current. From the experimental observation and simulations, we reach conclusion that the bootstrap current decreases within the island O-point.