An equilibrium model for helicity injector operation

Abstract

The Helicity Injected Tokamak (HIT) experiment is a low aspect ratio (A = 1.5) tokamak formed and sustained by helicity injection. A critical issue facing its operation is the efficiency of the helicity injector. In order to study this, an equilibrium model of the injector has been developed which assumes that the open field-lines in the injector and the edge region are in force-free equilibrium with a uniform resistivity ($\eta).$ This resistivity may include a dynamic as well as a classical component. This model yields good agreement with measurements of edge poloidal magnetic field and injector current, with as few as one fitting parameter. This model has been extended to shots with significant amounts of closed flux. The calculated equilibria are found to be consistent with measurements of edge electron temperature using a triple Langmuir probe. The equilibria are also found to be consistent with helicity conservation to within the accuracy of the measurements.