Impurity compression and enrichment studies on Alcator C-Mod

Impurity transport has been investigated in the reactor-like conditions of the Alcator C-Mod divertor (divertor plasma density greater than or equal t o 10(21) m(-3), parallel heat flux in the SOL less than or equal to 0.5 GW m(-2)). Trace amounts of the recycling impurity gases helium, neon, argon, and krypton were injected into plasmas. A residual gas analyzer that sample s the private flux region of the divertor was used for divertor impurity ne utral density measurements. Compression ratios (C-z) of up to 400 and enric hment factors (eta(z)) of up to 20 have been observed for argon in Alcator C-Mod discharge. Compression ratios increase and enrichment values are cons tant or slightly increasing with line-averaged density for argon in Ohmic d ischarges. Above the divertor detachment threshold density, C-z and eta(z) both decrease. During H-mode operation, compression and enrichment ratios f or argon decrease relative to their Ohmic values. For similar plasma condit ions, impurity compression ratios and enrichment values increase with the m ass of the impurity. This mass scaling may be partially explained by the de crease in the ionization mean-free-path with increasing impurity mass, (C) 1999 Elsevier Science B.V. All rights reserved.