Measurements of neutral density based on emissivity in the Alcator C-Mod divertor

Abstract

Neutral density profiles have been derived along the outboard divertor target of Alcator C-Mod and inside the last closed flux surface (LCFS). Local emission rates are obtained by a tomographic inversion of measured brightness profiles. Electron temperatures and densities provided by a divertor Langmuir probe array are then used to calculate the local neutral density, , from the emissivity profile. For points inside the LCFS, electron cyclotron emission (ECE) and interferometry data were used for electron density and temperature, respectively. Along the divertor target, steep gradients in neutral density persist during regular ohmic attached discharges, with . Plasma detachment leads to values of that can reach up to several times in the bottom of the divertor with much shallower pressure gradients to the midplane. Modelling of the parallel plasma momentum decrease due to charge exchange (CX) and elastic ion - neutral collisions indicates that neutral densities well below those observed are sufficient to explain the measured drop in parallel plasma pressure. This suggests that significant momentum is carried by the neutrals in the divertor resulting in decreased efficiency of momentum transfer between ions and neutrals.