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 sm aller than the cutoff value. Waves of 2.45 GHz are launched perpendicularly to the multicusp magnetic field formed by permanent magnets surrounding th e tube. Circular and square cross sectional tubes are tested. Overdense pla smas with a density of (0.8-2.0) x 10(11) 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 electr on temperature are higher for the circular cross section. (C) 1999 American Institute of Physics. [S0034-6748(99)00308-1].