Fast reconnection due to localized anomalous resistivity

A three-dimensional resistive magnetohydrodynamic code has been used to model the reconnection process at the m=1, n=1 surface, in periodic cylindrical geometry. Large current densities are expected at this reconnection layer and an enhancement of the transport properties is expected if the local drift speed exceeds a critical velocity, such as some multiple of the local sound speed. This effect is modeled in these simulations by the local enhancement of the resistivity coefficient where the criterion for micro-turbulence is satisfied. It is found that the reconnection times for this type of simulation are comparable to the reconnection times for a plasma where the resistivity is enhanced everywhere, implying that the reconnection is dominated by the local resistivity value and not its gradient. An analytic scaling law of the reconnection rate for the case when the local electron drift velocity is Limited to a multiple of the sound speed is presented. This model predicts that when this multiple is (m(i)/m(e))(1/2), reconnection times are close to experimental values in large tokamaks. Under these conditions, electron inertia and electron viscosity can be shown to be unimportant. The onset of micro-turbulence acts as a trigger for the reconnection process, and partial reconnection can occur if the conditions for micro-turbulence cease. (C) 1998 American Institute of Physics. [S1070-664X(98)02309-X].