Abstract: A plasma producing device is furnished with a half height waveguide that transmits microwave energy through a window into an evacuated chamber, adjustable spacers controlling the distance between an opening of the waveguide and the window, and/or a return piece adjacent a permanent magnet producing the required magnetic field, preferably including sidewalls encompassing the permanent magnet. The improvements promote plasma uniformity at the workpiece by allowing smaller magnets to be employed, reducing stray magnetic fields outside the resonance zone, and facilitating transition of the microwaves from a rectangular transmission mode to a circular transmission mode outside the chamber. Facilitating transistion in this manner improves tuning or impedance matching.