Effects of Radial Electric Field and Its Shear on Low-Frequency Fluctuations in the QT-U Device

Radial potential profiles are precisely controlled to vary both radial electric field Er and its shear by using a 10-segmented endplate in an ECR-produced plasma. Observed frequencies and intensities of flute-mode and drift-mode fluctuations depend on the potential profile. The frequencies are Doppler shifted by E × B drift. The flute-mode fluctuation is identified as Kelvin-Helmholtz type instability which is destabilized by strong E × B flow shear. The drift-mode fluctuation is destabilized in the region of small and negative electric field. When the E × B rotation frequency shear is increased with Er being fixed, the drift-mode fluctuations increase once in a weaker shear region, attain its peak at a certain shear and then decrease in the strong shear region. This behavior suggests that the rotation frequency shear of net ion drift which is determined from both E × B drift and diamagnetic drift is important for stabilizing the drift mode.