FAST RECONNECTION DUE TO LOCALIZED ANOMALOUS RESISTIVITY

A three-dimensional resistive magnetohydrodynamic code has been used t o model the reconnection process at the m=1, n=1 surface, in periodic cylindrical geometry. Large current densities are expected at this rec onnection layer and an enhancement of the transport properties is expe cted if the local drift speed exceeds a critical velocity, such as som e multiple of the local sound speed. This effect is modeled in these s imulations by the local enhancement of the resistivity coefficient whe re the criterion for micro-turbulence is satisfied. It is found that t he reconnection times for this type of simulation are comparable to th e reconnection times for a plasma where the resistivity is enhanced ev erywhere, implying that the reconnection is dominated by the local res istivity value and not its gradient. An analytic scaling law of the re connection rate for the case when the local electron drift velocity is Limited to a multiple of the sound speed is presented. This model pre dicts that when this multiple is (m(i)/m(e))(1/2), reconnection times are close to experimental values in large tokamaks. Under these condit ions, electron inertia and electron viscosity can be shown to be unimp ortant. The onset of micro-turbulence acts as a trigger for the reconn ection process, and partial reconnection can occur if the conditions f or micro-turbulence cease. (C) 1998 American Institute of Physics. [S1 070-664X(98)02309-X].