Current Point Formation and Magnetic
Reconnection Process in Tokamak Plasmas

Y. Ishii (JAERI), M. Azumi and Y. Kishimoto

Magnetic reconnection process caused 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 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,
$\delta/\Delta$, 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 the current point with coherent character is also
formed even in the stochastic magnetic field resultant from the
overlapping of magnetic islands with different helicity.
We will also show the electron inertia effects, which may be important
in recent high performance tokamak plasmas, on a current point
formation.