Characterisation of a low-pressure oxygen discharge created by surface waves

Abstract

Low-pressure oxygen discharges created by surface waves are characterised. Three discharge characteristics, the electron-neutral collision frequency nu , the maintaining electric field Eeff and the power required to maintain one electron theta , are deduced from the measurement of the electron density. They are independent of the microwave power and are studied as functions of the tube diameter (8-52 mm), the pressure (0.05-2 Torr) and the excitation frequency (390 or 210 MHz). The concentrations of ground state and singlet molecular oxygen are measured by VUV absorption and the concentration ratio of singlet to ground state molecular oxygen is about 10%, whatever the microwave power and the pressure. The ground state atomic oxygen concentration is measured downstream from the discharge by VUV absorption. The dissociation rate increases with the microwave power but remains less than 5%. A power balance model in the discharge, i.e. calculation of theta , is performed and yields a relation between theta /N and Eeff/N (n being the total neutral density), which is in good agreement with the experimental results. Finally, similarity laws are shown in the form theta /N and Eeff/N (Nr), where r denotes the tube radius.