Ignition phase of a pulsed microwave-excited oxygen plasma

A pure oxygen plasma has been investigated for continuous mode as well as f or pulsed power operation. Microwave power is fed to the plasma by an annul ar waveguide with 10 slotted line radiators on the inner side (SLAN princip le). Plasma diagnostics were performed using (i) electrostatic probe method s for electron diagnostics, (ii) two-dimensional optical emission spectrosc opy for excited oxygen mapping, and (iii) two-photon laser-induced fluoresc ence LIF for the quantitative measurement of atomic oxygen. Time-resolved e mission spectroscopy was possible with a fast gated CCD camera system equip ped with suitable optical filters. Of special interest was the ignition pha se 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 p ower application followed by an emission burst localized at a position near by 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 dire ction. After 150 mu s a symmetrical excitation zone with 10 emission maxima has formed and a homogeneous power coupling from all slot antennae is achi eved. Electron densities measured by probes and atomic oxygen ground state densities determined py LIF show much slower temporal changes than the emis sion intensity. The experimental results are interpreted by considering the relevant channels leading to the production of atomic oxygen.