Control of gaseous impurities by pump limiters

Nitrogen has been puffed into the Impurity Study Experiment (ISX-B) tokamak as a test gas for evaluating the effect of pump limiters on impurity control. The gas is introduced either through a nozzle at the limiter tip or from a location at the wall of the vacuum chamber. The interior nitrogen content is monitored by charge-exchange excitation of N VII lines and depends only slightly on the location of the N2 source or on whether the pumps are activated or not. The edge radiation from NIV is also affected little by activating the pumps when the source is at the limiter tip, but is strongly influenced when the source is at the wall location. The spectral data suggest that the reservoir of nitrogen in the scrape-off layer is efficiently pumped while the nitrogen content of the main plasma is not affected. The implication is that, in the quasi-steady interval of a discharge, radial diffusion is very slow compared to parallel transport in the vicinity of the limiter tip.