Investigation of a radio-frequency inductive- coupled-plasma discharge afterglow in noble gases

Abstract

It has been shown (Demidov et al 2005 Phys. Rev. Lett. 95 215002) that even a small number of nonlocal fast electrons, which do not significantly affect the overall mean electron energy, can dramatically change both the plasma and near-wall sheath properties. In this work, Langmuir probe measurements of the electron energy distribution function (EEDF) show the presence of fast electrons created due to collisions of pairs of metastable atoms and collisions of the second kind between metastable atoms and bulk electrons. These measurements were made in the afterglow of a 100% modulated radio-frequency inductive-coupled-plasma discharge in argon, neon and helium. It is shown that this fast component of the EEDF can be controlled independently of the slow electrons, which is a direct consequence of the EEDF nonlocality. Both EEDF and plasma emission spectroscopy measurements are presented for the helium afterglow. These data allow us to determine the absolute density of metastable atoms and their temporal decay during the afterglow. It is shown that under the experimental conditions, stepwise excitation of helium metastable atoms is the primary process for populating excited states and, therefore, decay of the excited atoms is governed by the decay of metastable states. The presence of a trace amount of nitrogen in the system, which does not significantly change the properties of the helium discharge, allowed us to independently measure the decay of helium metastables by monitoring the N+2(B–X) emission resulting from Penning ionization of N2(X) and confirmed the above conclusions regarding the presence and importance of metastable atoms.