Empirical scalings of cross-field heat diffusivities in the scrape-off layer of Alcator C-Mod from a 2-D interpretive model

Abstract

A new 2-D interpretive code for the tokamak edge plasma, EDGEFIT, has been developed. It infers the anomalous cross-field heat diffusivity, χ, in the scrape-off layer (SOL) of Alcator C-Mod from the experimental data solving the heat conduction equation in the real magnetic geometry. Measurements of temperature from the Langmuir probes on the divertor target plates are used for setting up the boundary conditions. Cross-field diffusivities are found by matching the cross-field temperature profiles measured by a fast-scanning Langmuir probe (FSP). The separatrix values of the extracted heat diffusivity, χ 0, are statistically investigated for a large number of Alcator C-Mod discharges. No correlation between χ 0, on one hand, and plasma temperature on separatrix, magnetic field strength and the safety factor, on the other hand, has been found although parameters vary significantly in the investigated data set (20–80 eV for the separatrix temperature, 2.8–7.9 Tesla for the toroidal field, 3.0–7.4 for the safety factor). However it has been found that χ 0 values have a weak but noticeable trend to get smaller as the midplane neutral pressure, the divertor pressure and the separatrix plasma density grow. The most significant factor found to affect χ 0 is a bypass neutral gas leak existing in the Alcator C-Mod divertor. It appears that χ 0 is smaller by a factor of 3 on an average when the bypass leak is closed. This can be possibly due to a perturbation of plasma temperature at the FSP location caused by the flow of neutral gas associated with the bypass, or a systematic error in the FSP measurements.