Comparison of Optical Emission Spectroscopy and Cavity Ring-Down Spectroscopy in Large-Scaled Negative-Ion Source

Optical emission spectroscopy (OES) and cavity ring‐down spectroscopy (CRDS) systems are installed in a 1/3‐scaled negative hydrogen‐ion source at the National Institute for Fusion Science testbed to investigate the dynamics of ${\mathrm{H}}^{-}$ ions in the extraction region near the plasma grid. The signal form of the ${\mathrm{H}}^{-}$ ion density rapidly drops after beam extraction on applying a low‐bias voltage. A similar signal drop appears in the intensity of the hydrogen Balmer‐line emission measured by OES and is caused by decreasing atomic hydrogen produced by mutual neutralization effects between ${\mathrm{H}}^{-}$ and ${\mathrm{H}}^{+}.$ Shot trend of the beam currents are similar to the ${\mathrm{H}}^{-}$ density and ${\mathrm{H}}_{\alpha }/{\mathrm{H}}_{\beta }$ in the extraction region, which increases twice as large immediately after Cs seeding. We observe a linear correlation between the ${\mathrm{H}}^{-}$ density and the inclination of ${\mathrm{H}}_{\alpha }/{\mathrm{H}}_{\beta }$ which allows for experimentally benchmarking the OES measurement with that of CRDS. Thus, this approach is used for estimating the ${\mathrm{H}}^{-}$ density by OES in negative‐ion sources for high‐energy neutral beam injector.