A comparison of impurity screening between limiter and divertor plasmas in the Alcator C-Mod tokamak

A series of experiments has been carried out on Alcator C-Mod to compare impurity penetration between similar limiter and divertor discharges. Known amounts of recycling impurities (Ar and Ne) and non-recycling impurities (N-2 and CH4) are injected by gas puffing, and the fraction ending up in the plasma is deduced from spectroscopic measurements. The poloidal location of the gas injection is also varied. It is found that during the most recent run campaign, limiter plasmas have 1-3 times higher impurity penetration than divertor plasmas which detached, but 5-20 times higher penetration compared to divertor plasmas which remained attached. During the previous run campaign, limiter plasmas had only 1-3 times higher penetration than attached divertor plasmas. These ratios are the same for both recycling and non-recycling species. There are strong dependencies on gas puff location as well. The reason for the difference in the two run campaigns is not understood, but may be related to gas leakage paths behind the divertor structure, which were plugged up between the two campaigns.