Radial impurity transport in the H mode transport barrier region in Alcator C-Mod

Measurements of profiles of soft X ray emissivity with 1.5 mm radial resolu tion are combined with high resolution electron density and temperature mea surements in the edge region of the Alcator C-Mod tokamak to facilitate tra nsport analysis of medium-Z impurities during Il modes. Results from detail ed modelling of the radiation and transport of fluorine are compared with e xperimental measurements, yielding information about the transport coeffici ents in the H mode transport barrier region. Evidence is found for a strong inward impurity pinch just inside the separatrix. The region of strong inw ard pinch agrees very well with the region of strong electron density gradi ent, suggesting that the inward pinch could be driven by the ion density gr adient, as predicted by neoclassical theory. Simulations using the neoclass ical impurity convection profile agree very well with experiments. Transpor t modelling shows that the X ray pedestal width is largely determined by th e diffusion coefficient in the transport barrier. This allows diagnosis of changes in the edge diffusion coefficient on the basis of observations of X ray pedestal width changes. Significant differences in the edge diffusion coefficient are seen between different types of H mode. Several scalings fo r the edge diffusion coefficient in the enhanced D-alpha H mode are also id entified. This may help elucidate the physical processes responsible for th is attractive confinement mode.