Ignition phase of a pulsed microwave-excited oxygen plasma

A pure oxygen plasma has been investigated for continuous mode as well as for pulsed power operation. Microwave power is fed to the plasma by an annular waveguide with 10 slotted line radiators on the inner side (SLAN principle). Plasma diagnostics were performed using (i) electrostatic probe methods for electron diagnostics, (ii) two-dimensional optical emission spectroscopy for excited oxygen mapping, and (iii) two-photon laser-induced fluorescence LIF for the quantitative measurement of atomic oxygen. Time-resolved emission spectroscopy was possible with a fast gated CCD camera system equipped with suitable optical filters. Of special interest was the ignition phase of the discharge. The temporal and spatial density variation of excited atomic oxygen was monitored. A first signal is visible after 7 mu s after power application followed by an emission burst localized at a position nearby the quartz wall at about 15 mu s after gas breakdown. End-on images from the camera prove that the plasma then expands mainly in the azimuthal direction. After 150 mu s a symmetrical excitation zone with 10 emission maxima has formed and a homogeneous power coupling from all slot antennae is achieved. Electron densities measured by probes and atomic oxygen ground state densities determined py LIF show much slower temporal changes than the emission intensity. The experimental results are interpreted by considering the relevant channels leading to the production of atomic oxygen.