Obstacle Induced Convection in a Toroidal Octupole

The effects of hoop supports and other obstacles penetrating the plasma containment region of a toroidal octupole were determined. Obstacles produced large scale convective cells which move plasma toward the field boundary at the azimuth of the obstacles and toward the separatrix at other azimuths. Under certain obstacle bias conditions, 3 V floating potential steps were observed across a dummy support along with 100% density steps. The density step showed an exponential dependence on the obstacle bias potential. A model is discussed which predicts the observed properties of the potential and density steps. Electron cooling at the obstacle was also detected.