ICRF specific impurity sources and plasma sheaths in Alcator C-Mod

In Alcator C-Mod, we have sought to identify ICRF specific impurity sources and estimate the lifetime of thin, low-Z films in the presence of high power ICRF heating. With ICRF, the erosion rate of low-Z films is estimated to be 15–20 nm/s indicating the eroding species energy is much higher than that normally found in the SOL. Using emissive probes, plasma potential measurements confirm the presence of an enhanced sheath with ICRF when the probe is magnetically linked to the active antenna. Plasma potentials were typically 100–200 V for ∼1.25 MW injected ICRF power. Furthermore, an RF sheath was unexpectedly present with insulating limiters and was dependent on the wall conditions and plasma confinement. These dependencies suggest that other scrape-off layer and plasma-surface characteristics are influencing the resulting RF sheaths.