Studies of DC biasing of internal ring on plasma rotation and transport in a toroidal geometry

Abstract

We report results from experiments performed to study how a DC bias with respect to the plasma potential is affecting the plasma states in the toroidal geometry of the Blaamann device in Tromso. In the experiments discussed here, a ring with smaller diameter than the limiter was centered inside the bulk of the plasma and its bias varied with respect to the plasma potential. In the electron saturation current regime of the ring, a significant reduction of the fluctuation levels was observed, and a shear in the poloidal velocity occurred at the lowfield side of the ring. This shear was positioned at the same radial position as the maximum of the radial transport when the ring was left floating or biased in the ion saturation current regime. For the latter ring biases, the poloidal velocities had no shear on the low-field side of the ring. Without the velocity shear, the radial transport was similar to previous experiments without ring. With the velocity shear, the radial transport was destroyed. This plasma state, with very low radial transport and fluctuation levels as well as plasma densities around 1017 m-3, is believed to provide a suitable plasma for wave propagation studies in a magnetized plasma with curved magnetic field lines without end effects.