Spectroscopic investigations of electron heating in a high-density helicon discharge

Abstract

The evolution of a high-density helicon discharge in argon and argon–helium mixtures was studied by spectroscopic methods as well as standard diagnostics. Applying optical emission spectroscopy, the electron temperature was deduced from the ratio of different emission lines. The spectral data were evaluated using collisional-radiative (CR) models taking account of secondary processes like the transitions between excited levels and excitation from metastable niveaus. In addition, time-resolved laser absorption spectroscopy was applied to determine the temperature and the density of the Ar*(3P0) metastable atoms from the absorption profile of the 772.42 nm argon line. Due to strong coupling by electron collisions, the temperature of the metastable atoms up to 1000 K reflects the gas temperature of the ground state atoms. The gas temperature and the gas density, which is readily obtained from the temperature, are needed as input parameters for the CR models. The spectroscopic results were compared with Langmuir probe measurements which partially utilized passive compensation. The main issue of this study is electron heating due to helicon wave absorption. Applying a double radio-frequency (rf) pulse technique with variable rf power in the second pulse, the growth of the electron temperature was measured as a function of rf power. To clarify whether or not fast electrons are generated in the helicon wave due to resonant electron heating, phase-sensitive emission measurements of the Ar II (480 nm) line with a short lifetime (τ ≈ 7 ns) were performed on the rf time-scale (period 73.7 ns). However, no modulation of the Ar II emission synchronous with the helicon phase was observed. Rather, the temporal growth of the electron temperature reveals the electrons to be predominantly heated in the bulk of the energy distribution function.