Plasma bias experiments on TORTUS

Bias experiments have been performed on the TORTUS tokamak, using a small biased electrode 3 mm radially outside the main poloidal limiter. The bias voltage, V_b, with respect to the vessel, was varied between +350 V and -350 V. The line-averaged electron density increases with both positive and negative biasing and the metallic impurity level is reduced by negative biasing. With positive biasing, the floating potential, V_f, increases in proportion to the bias voltage, but it does not decrease significantly with negative biasing. Magnetic fluctuations in the plasma edge increase with positive biasing and decrease with negative biasing. Negative biasing reduces the magnetic fluctuations.