Local magnetic divertor for control of the plasma-limiter interaction in a tokamak

An experiment is described in which plasma flow to a tokamak limiter is controlled through the use of a local toroidal divertor coil mounted inside the limiter itself. This coil produces a local perturbed field Be approximately equal to the local unperturbed toroidal field B r ~ 3 kG, such that when Be adds to Br the field lines move into the limiter and the local plasma flow to it increases by a factor as great as 1.6, and when Be subtracts from Br the field lines move away from the limiter and the local plasma flow to it decreases by as much as a factor of 4. A simple theoretical model is used to interpret these results. Since these changes occur without significantly affecting global plasma confinement, such a control scheme may be useful for optimizing the performance of pumped limiters.