Electron Temperature Measurements in Low Density Plasmas by Helium Spectroscopy II - Parameter Limits for Validity of Different Methods

摘要

Different methods to measure electron temperature in low-density plasmas by He spectroscopy are examined. They are based either on the relative intensities of singlet and triplet lines or on the absolute intensities of single lines. Calculations from measured and theoretical data show that both methods are seriously influenced by secondary processes, the dominating being excitation from the metastable levels 2 exp 1 S and 2 exp 3 S, and excitation transfer in electron-atom collisions combined with imprisonment of resonance radiation. The calculations give parameter limits for validity of different methods and combinations of lines. Due to the secondary processes, T sub(e)-determination from relative line intensities is limited to low-density, short-duration plasmas (typically n sub(e) < 2.10 exp 16 m exp -3 , t sub(ex) < 5.10 exp -6 s) or to even lower densities that depend on the apparatus dimensions (typically n sub(e) < 3.10 exp 15 m exp -3 , L approximately 0.1 m). For T sub(e)-determination from absolute line intensities the situation is more favourable, and with a suitable choise of lines the restrictions on n sub(e) and t sub(ex) can be typically (n sub(e) < 5.10 exp 17 m exp -3 , t sub(ex) < 10 exp -5 s) or (n sub(e) < 10 exp 17 m exp -3 , L approximately 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 exp 19 or 10 exp 20 m exp -3 , and without any limits on t sub(ex) or L. (Atomindex citation 10:478002)