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

## TL;DR

This study uses gas-puff imaging to analyze plasma flows and potentials in the Alcator C-Mod tokamak, revealing how RF power, magnetic field, and antenna configuration influence edge electric fields and sheath potentials.

## Contribution

It provides detailed measurements of SOL plasma flows and potentials, highlighting the effects of RF power, magnetic field strength, impurity seeding, and antenna configuration on plasma behavior.

## Key findings

- Radial electric field magnitude increases with magnetic field and RF power.
- RF-induced potentials extend from antenna to separatrix at high B and power.
- Impurity seeding reduces sheath potential associated with ICRF power.

## Abstract

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.