An investigation of the energy exchange mechanisms involving the 2 3S metastable level in an RF helium plasma

Abstract

The energy exchange mechanisms present in a pure helium and a helium-neon plasma were investigated using spectroscopic diagnostic techniques. The plasma was spatially resolved and only the volume element at the plasma centerline was considered in the energy- exchange analysis. The experiment was conducted with a constant total pressure of 0·7 torr, a fixed oscillator frequency of 4 MHz, and a constant input power of 1·8 kW. Emission line spectroscopy was used to determine the population densities of 16 levels in the n 3 S, n 3 P, and n 3 D series. Spatially resolved, self-absorption measurements of the 2 3 P-2 3 S transition were used to determine the 2 3 S metastable level number density. The electron number density of 3·3 × 10 13 cm -3 was determined from the spatially resolved H β blue wing profile, and a lower bound excitation temperature of 8800 °K was determined from a Boltzmann plot of the spatially resolved lower bound levels of the excited helium. The addition of 10% and 20% by volume of neon gas caused a measurable decrease in the population densities of the lower bound levels of helium, while the electron number density and lower bound excitation temperature remained unchanged. Three energy exchange models (local thermal equilibrium, corona, and collisional- radiative) were examined, and the collisional-radiative model was found to best describe the excitation processes for the 2 3 S level. This model was also appropriate for describing the helium-neon plasma at this level.