Spectroscopic measurements of chromium atom and ion densities in coaxial helicity injection plasmas on QUEST

The densities of chromium atoms and ions in coaxial helicity injection plasmas on QUEST, a spherical tokamak with a major radius of 0.68 m and a minor radius of 0.40 m, were measured using optical emission spectroscopy. Effective electron temperatures and densities were evaluated from the ratios of chord-integrated Balmer line intensities using a collisional–radiative model, yielding values of approximately 3 eV and 3.6 × 1019 m−3 near the electrodes during the early phase and 12 eV and 1.4 × 1018 m−3 in the confinement region during the closed flux surface (CFS) formation phase. Using these plasma parameters and the chord-integrated intensities of Cr I, Cr II, and C II lines, the chord-averaged densities of chromium atoms and Cr+ and C+ ions were estimated. The chromium atom density near the electrodes during the early phase was approximately 8 × 1012 m−3. The estimated ionization mean free path indicated that chromium atoms are ionized primarily near the electrodes. Cr+ ions were inferred to be transported into the confinement region by the upward plasma flow. The resulting Cr+ ion density during the CFS formation phase was approximately 1 × 1017 m−3, and the C+ ion density was of the same order of magnitude. These results provide insight into the ionization and transport of electrode-derived metallic impurities.