Nitrogen vibrational excitation in a N2/He pulsed planar-ICP RF discharge

Abstract

A pure inductive regime of pulsed RF discharge in N2–He mixtures at 0.2 Torr has been investigated by time resolved Langmuir probe and optical emission spectroscopy. A planar coil ICP with a Faraday shield has been used. Mixture compositions of 5–100% N2 and duty cycles, TON = 1–15 ms and TOFF = 15 ms, have been explored. The Langmuir probe analysis evidences a Maxwell electron energy distribution in the discharge and post-discharge, characterized by electron density and temperature that depend on both duty cycle and composition. The vibrational excitation of N2(C) state is inferred by the N2 second positive emissions. Its kinetic analysis reveals the formation of a high vibrational excitation of ground state nitrogen. A vibrational temperature higher than 10 000 K can be reached at the end of the 5 ms discharge pulse on the levels v = 0–3 of the X state. Such a high vibrational temperature slows the post-discharge relaxation of the electron temperature.