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 ≥1021 m-3, parallel heat flux in the SOL ≤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 samples the private flux region of the divertor was used for divertor impurity neutral density measurements. Compression ratios (Cz) of up to 400 and enrichment factors (ηz) of up to 20 have been observed for argon in Alcator C-Mod discharges. Compression ratios increase and enrichment values are constant or slightly increasing with line-averaged density for argon in Ohmic discharges. Above the divertor detachment threshold density, Cz and ηz both decrease. During H-mode operation, compression and enrichment ratios for argon decrease relative to their Ohmic values. For similar plasma conditions, impurity compression ratios and enrichment values increase with the mass of the impurity. This mass scaling may be partially explained by the decrease in the ionization mean-free-path with increasing impurity mass.