Rotation and Structure of Low-Frequency Oscillations inside the ST-Tokamak Plasma

Low-frequency oscillations, which can lead into the violent disruptive instability which sets the $\mathbf{q}$ (current) limit of tokamak operation, are investigated with a heavy-ion beam probe. Direct measurements of the space potential and the perturbation of the electron density inside the hot tokamak plasma resolve the electric field contribution to the mode rotation and the spatial structure of the mode.