Long Time Scale Plasma Dynamics Driven by the Double Tearing Mode in Reversed Shear Plasmas
Y. Ishii1), M. Azumi1), Y. Kishimoto1) and J.N.Leboeuf2)
1) Naka Fusion Research Establishment, JAERI, Naka, Ibaraki, 311-0193 Japan
2) Department of Physics and Astoronomy, University of California at Los Angeles, Mira Hershey Hall, Los Angeles, CA, USA
Abstract.
The new nonlinear destabilization process is found in the nonlinear phase of the double tearing mode (DTM) by using the reduced MHD equations in a helical symmetry. The nonlinear destabilization causes the abrupt growth of DTM and subsequent collapse after long time scale evolution in the Rutherford-type regime. The nonlinear growth of the DTM is suddenly triggered, when the triangular deformation of magnetic islands with sharp current point at the x-point around the outer rational surface exceeds a certain value. Such structure deformation is accelerated during the nonlinear growth phase. Decreasing the resistivity increases the sharpness of the triangularity and the spontaneous growth rate in the abrupt growth phase is almost independent on the resistivity. Current point formation is also confirmed in the multi-helicity simulation, where the magnetic fields become stochastic between two rational surfaces.