Flow velocity measurements using ChERS in the HSX stellarator

A Charge Exchange Recombination Spectroscopy (ChERS) system has been used to measure the velocity, density, and temperature profiles of fully stripped carbon impurities in the Helically Symmetric eXperiment (HSX). Velocity measurements have been obtained by reversing the magnetic field between sets of shots in order to reverse the plasma flow velocity. This essentially doubles the Doppler shift of the measured photons. It also eliminates the need to accurately determine the exact wavelength of the unshifted emission line which can depend on plasma conditions. Because two viewing angles are used for each radial location, the velocity magnitude and direction can be determined. The intrinsic velocity is found to move primarily in the direction of quasihelical symmetry with a peak velocity ~20 km/s. The velocity increases with increasing ECRH power.