Measurement of fluctuations in the supersonic poloidal flow driven by a hot cathode

The density and potential fluctuations were measured in hot-cathode biasing plasma at the Tohoku University Heliac. In the improved mode, high-frequency fluctuations (> 100 kHz) appeared in the density signal. On the other hand, low-frequency fluctuations (< 100 kHz) in the density and potential signals were suppressed. The characteristics of high-frequency fluctuation were compared with three kinds of instability, and they were consistent with those of the flute instability driven by the supersonic poloidal rotation. The suppression of low-frequency fluctuations in improved mode is considered the effect of E x B poloidal rotation or its shear. The profile of the anomalous particle flux was estimated by analysing the low-frequency fluctuation signals. The flux decreased in the improved mode in most of the region, although the decrease in flux was small near the rational surface (n/m = 5/3).