Carbon ion plume emission produced by charge exchange with neutral beams on National Spherical Torus Experiment

Emission from impurity ions excited by charge exchange with injected beam neutrals is widely used to provide local measurements of ion temperature, velocity, and density. Following the charge exchange process, hydrogenic impurity ions travel along magnetic field lines and may be excited by electron impact from the ground state before they are ionized, producing “plume” emission. This nonlocal emission from plume ions that drift into view can adversely affect the interpretation of charge exchange spectra. Carbon plume emission is observed in the background sightlines of the charge exchange recombination spectroscopy diagnostic of the National Spherical Torus Experiment. In plasmas with high rotation, superthermal flow of carbon ions produces a near Gaussian line shape for the plume emission. Modeling of the production, electron impact excitation, and ionization of the plume ions along the magnetic field lines yields line widths and line shifts consistent with observations. A radial shift between the measured and modeled plume brightness profiles is observed. Plume emission is observed on sightlines that equilibrium reconstruction indicates are not connected to the neutral beam volume, indicating a possible new constraint for the equilibrium reconstruction.