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 [1]. A current point is formed during the Rutherford type growth phase and by a global structure of the magnetic island. This current point causes the explosive growth with the weak dependency on the resistivity. Hence, the accelerated magnetic reconnection by a current point is roughly consistent with a time scale splitting phenomenon observed in a JT-60U reversed shear discharge [2]. Magnetic reconnection process with the current point shows new features different from those with the well known Sweet-Parker type current sheet. In contrast to the Sweet-Parker type current sheet, 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 the current point, in which the current density is almost inversely proportional to the resistivity.

A current point is formed by the higher harmonics of magnetic perturbations with the same helicity as the main harmonics. In toroidal geometry, however, higher harmonics with different helicities can be excited by the toroidal coupling effect and cause magnetic islands with various helicities. Hence, there is a possibility that these magnetic islands make the magnetic field stochastic and prevent a current point formation. In order to investigate a current point formation process under stochastic magnetic fields, simulations in toroidal geometry are done and show that the current point with coherent character is also formed even under the stochastic magnetic field.

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