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

Recent results from the high resolution edge x-ray imaging diagnostic at Al cator C-Mod are presented. The diagnostic measures the soft x-ray emissivit y using an array of 38 photodiodes viewing the plasma through a narrow aper ture. A 10 mu m 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 ha ve minimal overlap. The absolute position of the chords with respect to the vessel has been determined within 1 mm. The chord integrated data are inve rted into emissivity as a function of midplane radius assuming that the emi ssivity 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 mu s. Results sho w 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 mod e, and has a pronounced pedestal shape with widths in the range 1.5-8 mm. T he width is determined as a function of time by fitting a modified tanh fun ction to each profile. Results for width scalings with local parameters suc h as rho(pi) and rho(i), as well as time resolved edge localized mode data will be presented. (C) 1999 American Institute of Physics. [S0034-6748(99)5 2601-4].