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

Abstract

Measurements of profiles of soft X ray emissivity with 1.5 mm radial resolution are combinedwith high resolution electron densityand temperature measurements in the edge region of the Alcator C-Mod tokamakto facilitate transport analysis of medium-Z impurities during H modes. Results from detailed modelling of the radiation and transport of fluorineare compared with experimental measurements, yielding information about thetransport coefficients in the H mode transport barrier region. Evidence is foundfor a strong inward impurity pinch just insidethe separatrix. The region of strong inward pinch agrees very well with theregion of strong electron density gradient, suggesting that the inward pinchcould be driven by the ion density gradient, as predicted by neoclassicaltheory. Simulations using the neoclassical impurity convection profile agreevery well with experiments. Transport modelling shows that the X ray pedestal width islargely determined by the diffusion coefficient in the transport barrier.This allows diagnosis of changes in the edge diffusion coefficient on thebasis of observations of X ray pedestal width changes. Significantdifferences in the edge diffusion coefficient are seen between different types of H mode. Several scalings for the edge diffusion coefficient in the enhancedDα H mode are also identified. This may help elucidate the physical processes responsible for this attractive confinement mode.