The effect of ICRF on the Alcator C scrape-off layer plasma

Abstract

This paper presents a characterization of the Alcator C scrape-off layer (SOL) plasma during ICRF second harmonic hydrogen fast wave heating experiments. The SOL plasma parameters were measured using a multifunctional probe, Janus, which is capable of simultaneously measuring the ion and electron parameters both parallel and antiparallel with respect to the toroidal magnetic field. The probe data indicate, at low value of injected RF power, that there is direct edge heating and density increases at radii greater than that of the antenna Faraday shield. Increasing the injected RF power spreads both the temperature and density increases throughout the edge region, flattening the radial profiles. Varying the position of the resonance layer in the main plasma does not significantly change the effect of ICRF on the SOL parameters. Given this single spatial point characterization of the SOL, a crude estimate of power flow into and through the edge plasma indicate that ≈ 20% of the ICRF power launched from the antenna is absorbed directly in the SOL plasma. Additional observation of the impurity source rates confirms the conclusions of an earlier paper, which attributed increasing central densities of high-Z impurities to the increase in physical sputtering rate at both the ICRF antenna's Faraday shield.