Electron temperature determination in low-density plasmas from the He I 3889 AA and 5016 AA line intensities

The possibility of determining electron temperature by helium spectroscopy in low-density (n e <10 20 m -3 ) plasmas is discussed. It is concluded that most lines can only be used at very low densities (n e <2*10 16 m -3 ) because the line intensities are highly influenced by secondary processes, such as electron-impact-induced transitions between excited levels or excitations from metastable levels. The density range where measurements are possible can be extended if the influence of these secondary processes on the line intensities can be determined. For most helium I lines this is impossible for lack of atomic data. However, there are two exceptions, the 3889 AA (3 3 P to 2 3 S) and the 5016 AA (3 1 P to 2 1 S) lines. The influence from secondary processes on these lines is calculated, and methods are developed which can be used for measurement of electron temperatures T e <100 eV in plasmas of densities n e <5*10 19 m -3 . The use of the methods is illustrated by an experiment where they have been successfully applied.