Visible imaging of edge fluctuations in the TFTR tokamak

Images of the visible light emission from the inner wall region of the TFTR tokamak [M. G. Bell et al., in Plasma Physics and Controlled Nuclear Fusion Research 1988, Proceedings of the 12th International Conference, Nice, France (IAEA, Vienna, in press)] have been made using a rapidly gated, intensified TV camera. Strong ‘‘filamentation’’ of the neutral deuterium Dα light is observed when the camera gating time is <100 μsec during neutral-beam-heated discharges. These turbulent filaments vary in position randomly versus time and have a poloidal wavelength of ≊3–5 cm, which is much shorter than their parallel wavelength of ≊100 cm. A second and new type of edge fluctuation phenomenon, which is called a ‘‘merfe,’’ is also described. Merfes are a regular poloidal pattern of toroidally symmetric, small-scale marfes that move away from the inner midplane during the current decay after neutral beam injection. Some tentative interpretations of these two phenomena are presented.