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(TH/5-2) Long Time Scale Plasma Dynamics Driven by the Double Tearing Mode in Reversed Shear Plasmas

Y. Ishii1), M. Azumi1), Y. Kishimoto1), J.N. Leboeuf2)
1) Japan Atomic Energy Research Institute, Naka, Japan
2) University of California at Los Angeles, Los Angeles, USA

Abstract.  Linear stability and nonlinear behaviors of the double tearing mode (DTM) in reversed shear plasmas are numerically studied, and the onset of the nonlinear instability of DTM was newly found. Resistive MHD analysis for JT-60U reversed shear plasmas shows that the DTM is the most unstable in low beta region. By nonlinear simulations, it was found that the DTM can be nonlinearly destabilized after the Rutherford-type phase with resistive time scale, in the case that the two resonance surfaces are properly separated. The destabilization of DTM is originated by the formation of the highly localized current point and the x-point structure through the triangular deformation of the magnetic islands. The mode growth rate increases explosively and is almost independent on the resistivity. In toroidal plasmas, multi-helicity modes make the stochastic magnetic region between the resonance surfaces. However, it does not change the key physics of the nonlinear instability of DTM, that is, the formation of the x-point structure and the current point. This two time scale process of DTM seems to qualitatively agree with the experimental observations in JT-60U.

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IAEA 2003