Measurement of peripheral plasma turbulence using a fast camera in Heliotron J

Using a fast camera with a tangential view, filamentary structure in a peripheral region was observed during L and H-mode, and the apparent motion of filamentary structure across the magnetic field was also observed. In the L-mode the cross-field scale of the filamentary structure was smaller than that in the H-mode. In the L-mode the apparent motion was counterclockwise in a camera view. During L–H transition the apparent motion was stopped, and in the H-mode the apparent motion changed direction, and the speed of the apparent motion nearly doubled. If this apparent motion was due to Er × B drift, Er should be positive in the L-mode and Er should be negative in the H-mode. To get the density threshold for the H-mode a directional gas puff was used, and it was found that the dithering phase appeared after low frequency MHD activity (1–2 kHz), even though the density was higher than that of the H-mode threshold.