IMPURITY SCREENING IN OHMIC AND HIGH CONFINEMENT (H-MODE) PLASMAS IN THE ALCATOR C-MOD TOKAMAK

A series of experiments has been carried out to determine the relative screening of the low recycling gaseous impurities, nitrogen and metha ne, in Alcator C-Mod tokamak [Phys. Plasmas 1, 1511 (1994)] discharges . The impurity density in the core plasma is directly proportional to the rate of impurity injection. Screening was found to vary weakly wit h plasma density but to be markedly dependent on the poloidal position of injection. Detachment of the plasma results in significantly less screening both in Ohmic and in high confinement (H-mode) discharges, p articularly when the gas is injected into the divertor or at the inner vessel wall. Evidence of impurity transport from the scrape-off layer into the divertor using bolometry and visible spectroscopy is present ed and modeling of the transport using a two-dimensional Monte Carlo c ode is described. (C) 1997 American Institute of Physics.