EFFECTS OF PLASMA-FLOW ON ERROR FIELD ISLANDS

The subject of magnetic islands, due to error fields in devices such as tokamaks, is of importance for understanding energy and angular momentum confinement observed in existing devices and for a rational determination of the accuracy required for the coils of future large devices. Simple arguments show that, for parameter values of interest, the viscous drag on plasma inside an island, caused by a toroidal flow of plasma outside the island, can affect the islands significantly. This subject is studied using a numerical implementation of a simple, almost ideal magnetohydrodynamical, two-dimensional model. A simplified version of the model is also studied analytically. The study suggests that the islands shrink somewhat as the drag force is applied and that there is an upper limit to the total drag force that a surface of islands can sustain; when this maximum drag force is approached, large gradients in the current density are formed, which may give rise to instabilities that make invalid the two-dimensional assumption.