Nonlinear destabilization of double tearing modes in reverced magnetic shear plasmas
Yasutomo Ishii, Masafumi Azumi and Yasuaki Kishimoto
Naka Fusion Research Establishment, JAERI, Naka-machi, Naka-gun, Ibaraki-ken 311-0193 Japan
Abstract.
Recent tokamak experiments have demonstrated the formation of an internal transport barrier, which produces improved core confinement, in negative shear discharges. These improved confinement modes often encounter MHD activity, even in the case less than the ideal b limit. For an equilibrium profile obtained from a JT-60U negative shear plasma, it is preliminary shown by the linear analysis based on the full MHD model in toroidal geometry that the double tearing mode (DTM) is unstable in the low beta regime. Hence, it is important to investigate the linear and nonlinear features of DTM in order to avoid the low b collapse and establish a stationary operational path to reach high b plasmas.
In this paper, the linear and nonlinear behaviors of the tearing mode is systematically studied for non-monotonic q-profiles on the basis of the reduced MHD equations in cylindrical geometry and some new features about DTM are found, especially in a nonlinear phase1). The DTM is characterized by the distance between two mode rational surfaces, Dr, and the plasma resistivity, h. We found the new type of nonlinear phenomena in tearing mode regime, where Dr is enough large so that the mode grows through the Rutherford like regime. After the mode evolution in the resistive time scale, the nonlinear destabilizaion suddenly takes place in much faster time scale, so that the mode evolves rapidly by changing a magnetic and flow structures, i.e. the inner and outer island strongly interact in each other and the inner island is pushed out from the central region of the plasma. As a result, the process induces the minor collapse, leading the flattening of the q-profile, and a new relaxed state is established.
In this paper, we study the underlying physical mechanism of the nonlinear destablization2),3). Specifically, we investigate the evolution of power spectrum during the mode evolution. The coupling among the higher harmonics is significantly contribute to the nonlinear destabilization of DTM and essential for triggering this phenomenon.
1) Y.Ishii et al., to be published in Phys. Plasmas(2000).
2) P.H.Diamond et al., Phys. Fluids 27, 1449(1984).
3) M.Persson and R.L.Dewar, Phys. Plasmas 1, 1256(1994).