Critical Review of Plasma Spectroscopic Diagnostics via MTE

The often used absolute, ratio, and off-axis peaking methods of LTE spectroscopy are examined using the Multithermal Equilibrium (MTE) multifluid model. The partial MTE model distinguishes between the upper level and total excitation temperatures of atom and ion (Texßa or Texßi and Texa or Texi, respectively) as well as different valued electron (Te) and heavy particle (Ta = Ti) temperatures. In complete MTE, all excitation temperatures are equal valued but may differ from kinetic temperature values. In this paper, the particular temperature approximated by common diagnostic methods is identified. The probable errors in the temperature values are presented in a figure as a function of probable errors in data, temperature and level energy difference. The results indicate that the ratio methods (neutral-neutral, ion-neutral, and continuumneutral) yield upper level excitation temperature values; the absolute neutral line and absolute continuum (at low Texa) yield total atom excitation temperatures; and the off-axis peaking method for the neutral line yields the total excitation temperature. Application of the methods are more complex than in LTE spectroscopy. Probable errors are smallest for the absolute and off-axis peaking methods. The electron temperature may be determined from the continuum-neutral ratio at very high excitation temperatures (Texa >> E∞/10k). The paper also comments on MTE diagnostic methods.