Yasutomo Ishii

Plasma rotation effects on magnetic island formation and the trigger of disruptions in reversed shear plasma

Plasma rotation effects on the trigger and the evolution of MHD activities in reversed shear profiles are studied by nonlinear MHD simulations. It is found that, in a rotating plasma, magnetic islands formed around the inner and outer magnetic resonant surfaces, which are stable for the tearing mode, by an external perturbation (driven magnetic island), evolve with different growth rates during an initial growth phase. After the initial growth phase, an outer magnetic island grows rapidly prior to the inner one and triggers a rapid growth of the inner magnetic island. At the final phase, enlarged magnetic islands flatten a q-profile in a wide radial region including the plasma centre. Though this final phase closely resembles a nonlinear destabilization of a spontaneous double tearing mode (Ishii et al 2002 Phys. Rev. Lett. 89 205002), this process can explain the time delay of a plasma edge oscillation in triggering an internal MHD event and disruption.

Long timescale plasma dynamics and explosive growth driven by the double tearing mode in reversed shear plasmas

A new nonlinear destabilization process is found in the nonlinear phase of the double tearing mode (DTM) by using reduced MHD equations with helical symmetry. The nonlinear destabilization causes the abrupt growth of the DTM and subsequent collapse after long timescale evolution in the Rutherford-type regime. The nonlinear growth of the DTM is suddenly triggered, when the triangular deformation of magnetic islands with a sharp current point at the X-point around the outer rational surface exceeds a certain value. Decreasing the resistivity increases the sharpness of the triangularity and the spontaneous growth rate in the abrupt growth phase is almost independent of the resistivity. Current point formation is also confirmed in multi-helicity simulations, where the magnetic fields become stochastic between two rational surfaces.

Nonlinear evolution and deformation of driven magnetic islands in rotating plasmas

Novel features of the rapid growth and the nonlinear dynamics of driven magnetic islands in rotating plasmas are studied by nonlinear magnetohydrodynamics simulations. The transition phase of the magnetic island evolution is found between the rapid growth phase and the Rutherford-like phase in the low resistivity regime. It is shown that the rapid growth of the magnetic island is associated with its deformation that leads to the secondary magnetic island formation around the original X-points. The suppressed state is characterized by the asymmetric deformation of the magnetic island caused by the non-monotonic torque profile within the island separatrix. In the regime of the high magnetic flux input rate, the critical magnetic island width shows weak dependence on viscosity while it noticeably depends on resistivity. It is conjectured that this is a result of the effect of the Alfven type interaction between the rotating plasma and the fast growing external magnetic perturbation.

Current point formation and magnetic reconnection process in nonlinearly destabilized double tearing modes

Magnetic reconnection process triggered by the nonlinearly destabilized double tearing mode (DTM) is studied in detail. A localized plasma current, which is referred as a current point, is formed at the X-type reconnection region of the outer magnetic island in the nonlinear growth phase of DTM [Phys. Rev. Lett. 89, 205002 (2002)]. Magnetic reconnection process with a current point shows new features different from those with the well-known Sweet–Parker type current sheet. In contrast to the latter one, the inverse aspect ratio of the plasma current at the reconnection region of the outer magnetic island, δ/Δ, increases in the Rutherford type phase and decreases after the nonlinear destabilization of DTM is triggered. The weak dependence of the temporal growth rate on the plasma resistivity in the explosive growth phase is due to this unique temporal evolution of the spatial structure of a current point. The simulations in toroidal geometry show that a current point with coherent character is also formed ...