Observation of current profile evolution associated with magnetic-island formation in tearing-mode discharges on JT-60U

Evolution of the current density profile associated with the magnetic island formation during a tearing mode was measured for the first time in JT-60U. With the island growth, the current density profile turned flat at the radial region of the island, followed by appearance of a hollow structure. As the island shrank, the deformed region became narrower, and it finally diminished after disappearance of the island. It was also observed that the local poloidal magnetic field from MSE fluctuated in correlation with a slow island rotation, confirming the existence of the perturbed current in the island O-point. The experimentally observed local hollow current density is reproduced in a time dependent simulation assuming reduction of the bootstrap current near the rational surface. Our observations are an experimental proof of reduction of the bootstrap current within the island O-point. (author)