Edge x-ray imaging measurements of plasma edge in Alcator C-Mod

Abstract

Recent results from the high resolution edge x-ray imaging diagnostic at Alcator C-Mod are presented. The diagnostic measures the soft x-ray emissivity using an array of 38 photodiodes viewing the plasma through a narrow aperture. A 10 μm beryllium foil located in the aperture filters away photons below 600 eV and has close to 100% transmission above 2 keV photon energy. The viewing chords are separated by about 1 mm in the plasma region and have minimal overlap. The absolute position of the chords with respect to the vessel has been determined within 1 mm. The chord integrated data are inverted into emissivity as a function of midplane radius assuming that the emissivity is constant on a flux surface. The full three-dimensional geometry is used in the inversion. The emissivity profiles have a radial resolution of 1.7 mm at the midplane and a temporal resolution of 12 μs. Results show that L-mode discharges have very low edge x-ray emission. During H mode, the edge x-ray emissivity is an order of magnitude higher than in the L mode, and has a pronounced pedestal shape with widths in the range 1.5–8 mm. The width is determined as a function of time by fitting a modified tanh function to each profile. Results for width scalings with local parameters such as ρpi and ρi, as well as time resolved edge localized mode data will be presented.