Microwave plasma initiation mechanisms in a circular tube with a radius below the cutoff value

Summary form only given, as follows. The mechanisms of microwave plasma initiation by short-pulse, high-power microwaves launched into a circular tube with a radius below the cutoff value is investigated for waves of dominant waveguide modes. The mechanism is based upon the fact that the wave of a finite amplitude penetrates into the tube and produces an electric field large enough to breakdown the gas even though the electric field decays in the axial direction. The spatial extent from the tube entrance of the electric field above the threshold value for breakdown, is estimated for fundamental waveguide modes, TE/sub 11/, TM/sub 01/ and TE/sub 01/. As an approach to actual experiments, with microwaves of a power of /spl sim/ 100 kW, a pulse duration of 0.05-1.2 /spl mu/s and a frequency of 3 GHz in a tube of length 37 cm and a radius of 2.87 cm, numerical simulation has been made by using a statistical treatment for collisions, and the degree of power absorption and the breakdown criterion have been obtained. It has been found that the spatial extent of breakdown is longest for TE/sub 11/ mode (about 6 cm at a pressure of I Torr). From the radial distribution of the electric field in the circular modes, it is inferred that the optimum position for breakdown is on the axis for TE/sub 11/ and TM/sub 01/ modes while it is situated half way to the wall for the TE/sub 01/ mode. The present results would be useful for an understanding of the mechanisms leading to plasma production in a narrow tube with a radial dimension below the cutoff value.