Production of microwave plasma in narrow cross sectional tubes: Effect of the shape of cross section

A microwave plasma is produced in a conducting tube with a cross section smaller than the cutoff value. Waves of 2.45 GHz are launched perpendicularly to the multicusp magnetic field formed by permanent magnets surrounding the tube. Circular and square cross sectional tubes are tested. Overdense plasmas with a density of (0.8–2.0)×1011 cm−3 are obtained in the range of 10−4 Torr for powers of 200–360 W. The electron temperature is 6–12 eV. Under the same experimental conditions, the plasma density and the electron temperature are higher for the circular cross section.