The Role of Fast ICRF Waves in Enhancing the Plasma Potential in the SOL of Alcator C-Mod*

An extensive experimental survey of the plasma potential ($\Phi _{\mathrm{P}})$ enhancement in the SOL of ICRF-heated discharges on Alcator C-Mod reveals that plasma facing surfaces that do not magnetically map to the active ICRF antennas experience $\Phi_{\mathrm{P}}$ enhancement of \textgreater 100 V. The key implication of this result is that surfaces that generally do not affect the core plasma performance may become impurity sources. The study reveals that the fast ICRF waves have a strong influence on $\Phi_{\mathrm{P}}$. A mechanism for $\Phi _{\mathrm{P}}$ enhancement is proposed that involves a fast-to-slow wave coupling at conducting surfaces. Several features of this coupling theory are observed experimentally. Examples include the effect of the shape of the conducting surface and the strength of the fast wave fields on $\Phi _{\mathrm{P}}$. Overall, we observe an increase in the enhanced $\Phi _{\mathrm{P}}$ in unmapped SOL regions with the strength of the fast wave amplitude and the correlation is most pronounced in monopole-heated discharges. It is concluded that strong single-pass absorption regime is crucial to minimizing the impact of ICRF power on plasma-material interactions and core plasma performance.