Magnetohydrodynamic Activity During Limiter Biasing on the CT-6B Tokamak

Magnetohydrodynamic phenomena in the CT-6B Tokamak based on Mirnov oscillations have been investigated by applying the limiter biasing potentials and changing the vacuum chamber gas pressure and plasma displacement. The results show that setting up a radial electric field at the plasma edge could drive electromagnetic instabilities in the tokamak plasma. Magnetic oscillation frequency upon application of a positive bias decreases about 10-15% and then after a delay time, τ_d = 2.5 - 3 ms increases about 20-25% with respect to their value without biasing. In the negative bias regime the oscillation frequency increases about 10% in 1 ms after the application of the bias pulse. The poloidal rotation velocity changes during two steps are related to its link with the radial electric field and the time scale of the density gradient. The frequency of oscillations increases with the increasing chamber gas pressure and decreases with the increasing the outward plasma displacement.