The effect of a field-aligned ICRF antenna on enhanced sheath potentials in Alcator C-Mod

Radio frequency (RF) rectification of the plasma potential ($\Phi _{P})$ is a leading mechanism suspected of causing enhanced sputtering of high-Z metallic plasma facing components in ion cyclotron range-of frequencies (ICRF)-heated discharges on Alcator C-Mod. Previous emissive probe measurements on Alcator C-Mod showed that significant ICRF-enhanced $\Phi _{P}$'s ($>$100 V) are present on magnetic field lines that map and not map to active ICRF antennas. A ``field-aligned'' (FA) ICRF antenna has recently been installed in order to minimize these deleterious ICRF-driven effects. The FA configuration of the antenna minimizes oscillating electric fields parallel to the magnetic field, reducing RF rectification. Plasma potentials are measured with probes that directly map along a field line to the antenna. Gas puff imaging (GPI) of density fluctuation flows also infers radial electric fields (E$_{r}$ = -d$\Phi _{P}$/dr) in the SOL of ICRF-heated discharges. Initial GPI results show that E$_{r}$, and hence $\Phi _{P}$'s, are mitigated in discharges heated with the FA antenna and the reduction is the strongest in field-aligned discharges as compared to non-aligned ICRF antennas. Direct $\Phi _{P}$ measurements on field lines mapped to the active FA antenna will also be presented.