(TH/P3-5) Explosive growth and nonlinear dynamics of the forced magnetic island

Y. Ishii1), M. Azumi2), A. Smolyakov3)
1) Japan Atomic Energy Agency, Naka, Japan
2) Japan Atomic Energy Agency, Ueno, Tokyo, Japan
3) University of Saskatchewan, Saskatoon, Canada

Abstract.  Novel features of the explosive growth and the nonlinear dynamics of the forced magnetic island subject of the suppression by plasma rotation are studied by nonlinear MHD simulations. The island formation is a critical issue severely affecting the performance of the tokamak plasmas. There are two main mechanisms for the origin of the magnetic island. One mechanism is related to the unstable tearing mode and the other one is due to the forced magnetic reconnection from the external perturbation. The latter process can also be an important source of the seed island for the neoclassical tearing modes (NTM), where the MHD event like as the sawtooth oscillation also acts as the external perturbation for the target mode through the toroidal mode coupling. So far the theoretical work has been done to understand the forced magnetic island suppression by the plasma rotation and to evaluate its threshold value, while less attention has been paid to the subsequent break up process and the long term behavior of the forced island. These latter processes are important for understanding the island effect on tokamak confinement. In this paper, we study the whole process of nonlinear dynamics of magnetic island due to the growing external perturbation in rotating tokamak plasmas. It was found that the magnetic island grows explosively with changing its structure to cause a new energy source. This new energy source appears to be localized plasma current around the X-point. Contrary to the standard magnetic reconnection theory, this localized plasma current causes the enhanced magnetic reconnection in the low resistivity regime. As the result, the long term evolution of the forced magnetic island, i.e. the seed island for NTM, is dominated by the secondary reconnection as the resistivity becomes small. The secondary island formation will affect the bootstrap current contributions to the NTM evolution.

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