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

Abstract

Two high resolution edge x-ray imaging diagnostics have been installed in the Alcator C-Mod tokamak. One array measures the radial soft x-ray emissivity 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 resolution mapped to the midplane. The two diagnostics measure the chord brightness profiles, which are then inverted to get soft x-ray emissivity simultaneously with a 12 μs sampling time. This allows us to determine if the soft x-ray emissivity, and therefore the fluorine density, is constant on a flux surface during steady state high confinement mode conditions, as well as during fast transient edge events, such as edge localized modes or transitions from the high confinement mode to the low confinement mode. Measurements 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 variation on the inversion accuracy are estimated numerically. It is found that the emissivity is systematically overestimated at the top, and underestimated at the outboard edge, by less than 15%. The width of the x-ray emissivity pedestal is accurate to within 15%, and the location of the pedestal is accurate to within 1 mm. Measurements showing a poloidal propagation delay for the onset of the transition from high confinement mode (H mode) to low confinement mode (L mode) are also presented.