Fluctuations in the Plasma of the Model Stellarator

Low m-number fluctuations of floating potential, density, and magnetic fields have been investigated in Ohmically heated discharges in the C stellarator. The azimuthal velocity of the perturbations reverses direction near vD/vi = 1 (vD is the electron drift velocity due to current and vi is the ion thermal velocity). For vD/vi ≳ 1, the wave moves at about the electron diamagnetic drift velocity and detailed probe measurements are consistent with the mode being the current-driven universal instability: the amplitude and relative phasing of the fluctuations indicate that the low-m fluctuations can cause the plasma loss. For vD/vi ≲ 1, the wave moves close to the plasma Er × B0 velocity and measurements are consistent with the resistive-g mode: measurement accuracy prevents a definite statement on the relationship between the fluctuations and the plasma loss.