Charge exchange recombination (CER) spectroscopy in the visible spectrum is used to measure the radial ion density distribution of impurities in the core plasma of DIII-D. The radial profile of the effective ionic charge, Zeff(r), is subsequently calculated from the impurity densities and the electron density of the plasma. The CER measured radial distributions rely on a calculated neutral beam attenuation radial profile, which is confirmed by independent measurement. This technique, which determines the deuterium density of the neutral beam by coupling measured beam Dα emissions with a time dependent collisional radiative calculation, will be described. The CER derived absolute density/concentrations of carbon are verified by comparisons with the spectrometrically measured visible bremsstrahlung emission of the core plasma, which is proportional to Zeff. Conversely, the seeded neon concentration is overestimated by a factor of 1.7 by CER. This correction is shown to be caused by the enhanced direct capture into the upper level of the measured visible neon transition (Ne X n = 11 to 10, 5249 Å) from excited (n = 2) beam atoms. Owing to several problems, including line radiation contamination of the spectral region of the diagnostic, the standard Zeff(r) derived from inversion of line integrated visible bremsstrahlung emissions does not provide reliable profiles, but rather a measure of the average impurity content. The Zeff profiles are found to vary considerably in shape and magnitude over different operational regimes, confirming the need for accurate profiles.
Spectroscopic characterization of H2 and D2 helicon plasmas generated by a resonant antenna for neutral beam applications in fusion