Effects of neutral-beam excited states on charge-exchange emission cross sections

Abstract

Charge-exchange recombination spectroscopy can be used as a diagnostic for local densities of fully stripped impurities. For example, impurity profiles are valuable for transport studies. A major impediment to analysis is the availability of cross sections which relate the observed spectroscopic emission to the impurity densities. In this work, this impediment is removed for boron ions. Emission cross sections for B+4 states excited by charge exchange between beams of neutrals and fully stripped B+5 impurities in high density plasmas are computed for beams with multiple energy components. We show that even the low-energy neutral beam components make significant contributions to the emission for high-density plasmas which are commonly generated in the laboratory. Excited states as high as n=3 are important. This implies that a detailed description of the beam components including density, excited state population, and penetration is required for prediction of intensities of spectral lines excited by diagnostic neutral beams in high-density plasmas. Plasma experiments performed on the Alcator C-Mod tokamak will be described which clearly show the efficacy of our analysis method. This technique can be extended to species other than boron.