Screening of recycling and non-recycling impurities in the Alcator C-Mod tokamak

The behavior of argon and nitrogen, injected into single null diverted Ohmic discharges, has been compared. The screening of the divertor and scrape-off layer plasma is studied by comparing the number of impurity atoms in the core with the number injected by a calibrated gas puff. The effect of the poloidal position of injection on screening has been investigated. The core nitrogen density is dependent of an poloidal position of injection while the argon density is not. The spatial distribution of low charge states in the divertor has also been studied using a multichord visible spectrometer. Initial results of nitrogen modeling with the Monte Carlo code DIVIMP are presented.