Recombination and ion loss in C-Mod detached divertor discharges

We present detailed profiles of the magnitude and location of volumetric recombination occurring in the Alcator C-Mod divertor region during detached divertor discharges. The recombination sink (for ions) is compared to the ion current collected at the divertor plates both as a function of location on the plate and across the entire divertor. We find that, depending on plasma conditions, volume recombination can account for the removal of 10-75% of the ions flowing on detached flux surfaces. A similarly important cause of the observed ion current loss appears to be the reduction in divertor ion sources upstream from the plate. Changes in the ion source rate are consistent with changes in power flowing from the SOL into the divertor region. The lowest levels of recombination are found in I-I-mode discharges where detachment is induced through puffing of N-2 gas. In these cases the observed ion current loss is due almost entirely to decrease in the ion source. This shows that recombination is not a necessary condition for detachment. (C) 1999 Elsevier Science B.V. All rights reserved.