Characterization of SOL Plasma Flows and Potentials in Alcator C-Mod

Gas-puff imaging techniques are employed to determine the far SOL region radial electric field and the plasma potential in ICRF heated discharges in the Alcator C-Mod tokamak. The 2-dimensional velocity fields of the turbulent structures, which are advected by RF-induced $\mathbf{E\times B}$ flows, are obtained via the time-delay estimation (TDE) techniques. Both the magnitude and radial extension of the radial electric field $E_r$ are observed to increase with the toroidal magnetic field strength $B_\varphi$ and the ICRF power. In particular, the RF-induced $E_r$ extends from the vicinity of the ICRF antenna to the separatrix when $B_\varphi=7.9\,\mathrm{T}$ and $P_{\mathrm{ICRF}} \gtrsim 1\,\mathrm{MW}$. In addition, low-Z impurity seeding near the antenna is found to substantially reduce the sheath potential associated with ICRF power. The ICRF-induced potentials are also estimated in different antenna configurations: (1) conventional toroidally-aligned (TA) antenna versus field-aligned (FA) antenna; (2) FA monopole versus FA dipole. Results show that FA and TA antennas produce similar magnitude of plasma potentials, and the FA monopole induced greater potential than the FA dipole phasing.