Time-dependent gas density and temperature measurements in pulsed helicon discharges in argon

Time-dependent measurements of the temperature and density of the Ar*(³P₀) metastable atoms in two high-density pulsed helicon discharges, deduced from the absorption profile of the 772.42 nm argon line, are presented. A single-mode tuneable diode laser is used for recording these profiles, and temperatures up to 1000 K are obtained from their Doppler width. As in high-density plasmas the metastable and ground-state atoms are strongly coupled by electron impact collisions, the temperature of the metastable atoms reflects the gas temperature. From the time variation of the Ar*(³P₀) density during the discharge pulse we were able to deduce the density of the ground-state argon atoms and found for the helicon regime that the neutral atoms can be depleted by more than a factor of 10. Over a wide range of plasma parameters, we examined the axial asymmetry that is characteristic for helicon discharges with helical antenna coupling. In particular, we analysed the plasma-induced emission of the Ar (750 nm) and Ar⁺ (461 nm) lines as well as the electron density in the beginning of the plasma pulse to better understand the evolution of the plasma and the transition to the helicon regime. The measurements of the gas temperature and the metastable density reveal the asymmetry to become pronounced when the discharge changes from inductively coupled plasma to the helicon discharge. In the helicon regime, the argon atom depletion of up to 90% and ionization degrees up to 65% were achieved in the antenna zone. In the afterglow, all excited states of argon, including the Ar*(³P₀) state, are fed by electron–Ar⁺ ion recombination. The measured very high temperature of Ar*(³P₀) atoms is partially related to the temperature of the Ar⁺ ions. Production of Ar⁺ in its excited states is also observed during several hundreds of microseconds in the afterglow. It is related to the electron–Ar²⁺ ion recombination and indicates the presence of a significant amount of doubly charged argon ions in the present helicon plasmas.