Cylindrical cavity with impedance shifting by irises in a power-supplying waveguide

A plasma reactor comprising:

• a workpiece processing chamber including a workpiece support to hold a workpiece in a first plane;
• a cylindrical microwave cavity that overlies the workpiece processing chamber, the cylindrical microwave cavity including a floor that is parallel to the first plane and separates the cylindrical microwave cavity from the workpiece processing chamber and includes a plurality of apertures for microwave energy to pass into the workpiece processing chamber, a ceiling, a cylindrical sidewall, and exactly two microwave input ports in the cylindrical sidewall of said cylindrical microwave cavity, the two microwave input ports including a first input port and a second input port spaced apart by an oblique azimuthal angle;
• two monitoring antennas in the cylindrical microwave cavity;
• a microwave source having a microwave frequency and having a pair of microwave source outputs, wherein the microwave source is configured to generate microwave signals at the pair of microwave source outputs that are separated by an adjustable temporal phase difference;
• and
• a feedback controller configured to receive a target phase difference and two signals from the two monitoring antennas, determine a measured phase difference from the two signals, and determine an adjustment to the microwave source to bring the measured phase difference to the target phase difference.