Electron temperature measurements in low-density plasmas by helium spectroscopy

Abstract

Methods of measuring the electron temperature in low-density plasmas by He spectroscopy are examined. These utilize either the relative intensities of singlet and triplet lines or the absolute intensities of single lines. Calculations from measured and theoretical data show that both methods are seriously influenced by secondary processes, the most important of which are excitation from the metastable levels 2 1 S and 2 3 S, and excitation transfer in electron-atom collisions combined with imprisonment of resonance radiation. The calculations give parameter limits for the validity of different methods and combinations of lines. Due to the secondary processes, the determination of T e from relative line intensities is restricted to low-density, short-duration plasmas (typically n e < 2 × 10 16 m -3, t ex < 5 × 10 -6 s) or to even lower densities that depend on the apparatus dimensions (typically n e < 3 × 10 15 m -3, L ≈ 0.1 m). For the determination of T e from absolute line intensities, the situation is more favourable and, with a suitable choice of lines, typical restrictions on n e and t ex are n e < 5 × 10 17 m -3, t ex < 10 -5 s or n e < 10 17 m -3, L ≈ 0.1 m for electron temperatures above 10 eV. For temperatures below 10 eV and degrees of imprisonment below 7% measurements are possible for electron densities up to 10 19–10 20 m -3, without any limits on t ex or L.