Impurity-ion rotation dynamics in C-2W

The C-2W experimental device (also called ``Norman'') produces and sustains advanced beam-driven~field-reversed configuration (FRC) plasmas in steady state. Edge biasing of annular electrodes in the divertor region is routinely utilized as a boundary control technique to stabilize the FRC in the confinement vessel. The potential difference between open-field lines in the scrape-off layer creates a local radial electric field near the FRC separatrix and, consequently, \textbf{\textit{E}}$\times $\textbf{\textit{B}} shear flow which suppresses the plasma turbulence. Recently deployed charge-exchange recombination spectroscopy (ChERS) diagnostics are used to measure rotation velocities of high charge-state oxygen impurity ions at 343.4 nm as well as deuterium main ions at 656 nm. Radial momentum balance gives \textbf{\textit{E}}$\times $\textbf{\textit{B}} velocities near the separatrix which dominates the rotation dynamics of the oxygen impurities and are in good agreement with measured main-ion rotation and results from an independent Doppler Backscattering diagnostic. Results show a~strong correlation between applied electrode bias voltage and inferred radial electric fields in the confinement vessel of C-2W.