MEASUREMENTS OF NEUTRAL DENSITY BASED ON H-ALPHA EMISSIVITY IN THE ALCATOR C-MOD DIVERTOR

Neutral density profiles have been derived along the outboard divertor target of Alcator C-Mod and inside the last closed flux surface (LCFS ). Local H-alpha emission rates are obtained by a tomographic inversio n of measured H, brightness profiles. Electron temperatures and densit ies provided by a divertor Langmuir probe array are then used to calcu late the local neutral density, n(n), from the H-alpha emissivity prof ile. For points inside the LCFS, electron cyclotron emission (ECE) and interferometry data were used for electron density and temperature, r espectively. Along the divertor target, steep gradients in neutral den sity persist during regular ohmic attached discharges, with n(n) appro ximate to 2 x 10(19) m(-3). Plasma detachment leads to values of n(n) that can reach up to several times 10(20) m(-3) in the bottom of the d ivertor with much shallower pressure gradients to the midplane. Modell ing of the parallel plasma momentum decrease due to charge exchange (C X) 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 effi ciency of momentum transfer between ions and neutrals.