Estimation of three-dimensional structure on peripheral fluctuation using fast camera images and magnetic field calculation in Heliotron J

Abstract

In Heliotron J, peripheral plasma fluctuation and/or turbulence have been measured by using a fast camera for over 10 years. Many image data are provided as two-dimensional information of time series by a fast camera, and they represent the spatial shape of fluctuation and its complex behavior. From experience in Heliotron J experiment, most of the peripheral fluctuations have elongated spatial structure along magnetic field lines. Because the movement of electrons and ions along the field lines is much faster than their vertical motion, this is natural; the filamentary structure along the field lines should be formed as a fundamental of many types of fluctuations in Heliotron J. In the image of the fast camera, if the fluctuation having a filamentous structure along the magnetic field line is observed as a brighter region than its surrounding plasma, the image of this fluctuation should coincide with the projection image of the magnetic field line passing through the position of this fluctuation. That is, by comparing the projected image of magnetic field lines and the image of fluctuation images, three-dimensional information on peripheral plasma fluctuation can be obtained. Therefore, with the help of geometry of magnetic field lines, to extract three dimensional information about the fluctuation in Heliotron J from the two-dimensional image was tried. As a result, in the analysed range, the filament was along the magnetic field line; it appeared to occur from the O-point in the field of view, and disappear at the X-point. Apparent speed in the poloidal cross-section was approximately 500 m/s.In the future, if this analysis method is applied, more detailed data on fluctuation will be obtained in Heliotron J. Keywords: Peripheral fluctuation, Fast camera, Magnetic field line, Heliotron J