Observation of a Forbidden Line of Fe xx and Its Application for Ion Temperature Measurements in the Princeton Large Torus Tokamak

A spectrum line in the Princeton Large Torus (PLT) tokamak discharges, with wave-length measured as 2665.1 ± 0.3 Å, has been identified as the $2s^{2}2p^3{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}$ magnetic dipole transition in Fe xx ground configuration. A variety of localized spectroscopic diagnostics, e.g., ion temperature and density distribution measurements in the high-temperature interior of the plasma, are feasible by means of forbidden lines of this type. The 2665-Å line has been used to measure near-central ion temperature in a discharge with auxiliary neutral-beam heating.