Visible radiation sliced image emission velocimeter interferometer fluctuation diagnostic

An instrument designed to measure k-filtered plasma fluctuations has been used to look at edge fluctuations in Alcator C-Mod tokamak plasmas. The instrument utilizes a sieve technique, whereby an optical image is split and filtered through two out-of-phase, striped transmission masks, and then imaged onto photodiode detectors. The difference signal is then digitized and analyzed to give the frequency characteristics of the fluctuations at a particular value of k, as determined by the wavelength of the mask stripes. Measurements have been made looking at the edge plasma with hydrogen Balmer-alpha filtering, and k both parallel and perpendicular to the magnetic field, with |k| in the range from 5 to 15 cm-1. Broad band (ν<500kHz) fluctuations are seen for k perpendicular to B, and characteristics in ohmic and L-mode plasmas are described. copyright 1999 American Institute of Physics