Simultaneous soft x-ray emissivity profile measurements in poloidally separate locations of the Alcator C-Mod edge plasma

Two high resolution edge x-ray imaging diagnostics have been installed in t he Alcator C-Mod tokamak. One array measures the radial soft x-ray emissivi ty profiles at the top of the plasma with 1.2 mm radial resolution, mapped along flux surfaces to the midplane, whereas the other measures the radial soft x-ray emissivity profiles at the outboard edge with 1.5 mm radial reso lution mapped to the midplane. The two diagnostics measure the chord bright ness profiles, which are then inverted to get soft x-ray emissivity simulta neously with a 12 mu s sampling time. This allows us to determine if the so ft x-ray emissivity, and therefore the fluorine density, is constant on a f lux surface during steady state high confinement mode conditions, as well a s during fast transient edge events, such as edge localized modes or transi tions from the high confinement mode to the low confinement mode. Measureme nts are presented showing that the soft x-ray emissivity is not constant on a flux surface, but instead shows a large poloidal variation, contrary to what is assumed in the inversion routine. The effects of the poloidal varia tion on the inversion accuracy are estimated numerically. It is found that the emissivity is systematically overestimated at the top, and underestimat ed at the outboard edge, by less than 15%. The width of the x-ray emissivit y pedestal is accurate to within 15%, and the location of the pedestal is a ccurate to within 1 mm. Measurements showing a poloidal propagation delay f or the onset of the transition from high confinement mode (H mode) to low c onfinement mode (L mode) are also presented. (C) 2000 American Institute of Physics. [S0034-6748(00)00909-6].