IMPURITY TRANSPORT IN ALCATOR C-MOD PLASMAS

Trace non-recycling impurities have been injected into Alcator C-Mod [ I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas in orde r to determine impurity transport coefficients. Subsequent impurity em ission has been observed with spatially scanning x-ray and Vacuum Ultr a-Violet (VUV) spectrometer systems. Measured time-resolved brightness profiles of helium- and lithium-like transitions have been compared w ith those calculated from a transport code which includes impurity dif fusion and convection in conjunction with an atomic physics package fo r individual line emission. During Low-Confinement-Mode (L-mode) plasm as, the transport can be characterized by pure diffusion, with coeffic ients similar to 5000 cm(2)/s, reflecting the similar to 20 ms decay i n the x-ray and VUV line brightnesses. During High-Confinement-Modes ( H-modes), the impurity confinement times are much longer, and the mode lling requires that there be a strong inward convection (of order 1000 cm/s) near the plasma edge, with greatly reduced diffusion (of order 100 cm(2)/s), also in the region of the edge transport barrier. These edge values of the transport coefficients during H-mode are qualitativ ely similar to the neo-classical values. Nitrogen has also been inject ed, and after the H- to L-mode transition, the inner shell satellite l ines of lithium-like nitrogen dominate in intensity the resonance line of helium-like N5+ in a thin shell near the plasma edge. (C) 1997 Ame rican Institute of Physics.