Intrinsic propagation of magnetic island with finite width

Full text: The modified Rutherford equation is used as a model equation which is describing temporal evolution of the neoclassical tearing mode (NTM). The often-used polarization term plays a stabilizing role on the NTM when the magnetic island rotates in the direction of the ion diamagnetic drift, therefore it is important to determine the direction of island propagation to estimate the threshold of island width for the NTM onset. The NTM is essentially the nonlinear instability, hence it is useful to perform numerical calculations for determining the propagation direction of island with finite width where nonlinearity becomes to be important. In this work, the dependence of island width on the direction of island propagation is investigated in two dimensional slab geometry by using a two-fluid model including both diamagnetic drifts of ions and electrons. When equilibrium current profile is unstable for the tearing mode, it is found that the magnetic island rotates toward the direction of the electron diamagnetic drift in so far as the island width is small comparable to the ion Larmor radius ρi. The threshold where the propagation direction turns into the direction of ion diamagnetic drift is found to large comparable to the island saturation width. We also found that the threshold becomes lower as the temperature ratio between ion and electron fluid becomes large or the poloidal beta becomes lower. The island is found to rotate toward the direction of the ion diamagnetic drift as the viscosity becomes small. The physical mechanism of determining the propagation direction is discussed in the presentation. (author)