Experimental investigation and modelling of a low-pressure pulsed microwave discharge in oxygen

Abstract

A low-pressure (1-10 hPa) pulsed microwave discharge (2.45 GHz) in oxygen of power 100-1000 W operated in a cylindrical vessel is investigated by miscellaneous diagnostic techniques to determine space- and time-resolved concentrations of the most important species. The electron density is measured by monitoring the detuning of a Fabry-Perot resonator, which is in resonance at = 337 µm (HCN-laser). The same technique is used to detect those electrons generated when a sufficiently strong laser beam crosses laterally the intracavity HCN laser mode and destroys O- and O2- ions in the discharge by photodetachment. During the release of photodetached electrons the microwave field in the discharge chamber changes. This field alteration is utilized to determine the nO-/nO2- ratio, which is about 7 over the entire pulse. The concentration of rovibrational states of O2 and vibrational temperatures are measured by coherent anti-Stokes-Raman scattering (CARS) as well as O concentrations by two-photon-allowed laser induced fluorescence (TALIF). A global kinetic model has been developed to describe the production of various species such as oxygen atoms, metastable molecules, ozone and charged particles. The experimentally observed and theoretically predicted temporal behaviour of the oxygen atom density are in a good agreement, if wall losses of type O 1/2O2 with a probability of 0.004 are taken into account. Both experiment and modelling show a sharp enhancement of the electron density at the instant of the pulse switch-off. The effect has been explained to result mainly from the processes of collisional detachment of O- ions by collisions with metastable molecules O2(a) and oxygen atoms after switch-off.