Spectroscopic measurements of plasma flow in the SOL in C-Mod

We evaluate the limits of the spectroscopic measurement of scrape-off layer flows utilizing HeII excitation emission (486.6 nm) in the inboard, high-field, scrape-off region. Velocities are inferred from the Doppler shift of the HeII emission line for lower and upper single-null discharges, exhibiting differences of up to 20 km/s between the two. Unfortunately at high temperatures (>30 eV), the short ionization time, compared to momentum transfer times, makes the interpretation difficult for those plasma regimes. The inferred variations of plasma flows near the inner wall correlates with Langmuir Mach probe measurements in its dependence on magnetic topology. However the flow magnitudes are different. The discrepancy between probe and spectroscopic measurements is likely due a combination of two effects: the magnitude of gas injection needed to achieve a reasonable signal is found to be too perturbative and loss of high-energy He ions from the viewing volume.