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

Abstract

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, Hα/Dα (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.