Gas puff imaging system for edge plasma fluctuation measurements in large helical device

A gas puff imaging system has been developed to measure edge fluctuations in large helical device. The optical system splits the image of the plasma into four wavelengths, H_α/D_α (656 nm), HeI (1s2p-1s3d; 587.6; 1s2p-1s3d, 667.8; and 1s2p-1s3s, 706.5 nm), enabling simultaneous measurement of the spatial distribution of line ratios of He I. The image of the plasma is amplified with an image intensifier and recorded with a fast-framing camera. The measurement area has a diameter of 20 cm just outside of the last closed flux surface. The spatial resolution of the optical system is about 3 mm, and the frame rate is 100 kHz for acquisition of the four wavelength images. Signal-to-noise ratio is evaluated for the system, and further improvement is discussed. Clear images are obtained for all wavelengths and a slightly different pattern is recognized, depending on the wavelength. A singular value decomposition analysis can decompose the image clearly to one perpendicular and parallel to the magnetic field lines.