Localized plasma-density disturbance propagating along B in the interior of the ZT-40M reversed-field pinch

Using a ten-chord interferometer, we have measured a field-aligned (k∥≊2 m−1; k⊥≳25 m−1) plasma-density disturbance propagating along B with a speed in the ion acoustic range. The propagation is purely in the electron drift direction and is observed only when the drift parameter (electron drift speed/electron thermal speed)≳0.1. A novel spatial-filter technique resolves this localized mode, which otherwise would be hidden by more robust global disturbances present along the lines of sight.