Magnetohydrodynamic activity during limiter biasing on the CT-6B tokamak

Magnetohydrodynamic phenomena in the CT-GB tokamak based on Mirnov oscillat ions have been investigated by applying the limiter biasing potentials and changing the vacuum chamber gas pressure and plasma displacement. The resul ts show that setting up a radial electric field at the plasma edge could dr ive electromagnetic instabilities in the tokamak plasma. Magnetic oscillati on frequency upon application of a positive bias decreases by about 10-15% and then after a delay time, tau (d) = 2.5 - 3 ms, increases by about 20-25 % with respect to their value without biasing. In the negative bias regime, the oscillation frequency increases by about 10% in 1 ms after the applica tion of the bias pulse. The poloidal rotation velocity changes during two s teps are related to its link with the radial electric field and the timesca le of the density gradient. The frequency of oscillations increases with th e increasing chamber gas pressure and decreases with the increasing the out ward plasma displacement.