A plasma processing system that includes a plasma chamber, an open resonator movably mounted within the plasma chamber, and a detector. The open resonator produces a microwave signal, and the detector detects the microwave signal and measures a mean electron plasma density along a path of the signal within a plasma field. Alternatively, the plasma processing system includes a plasma chamber, a plurality of open resonators provided within the plasma chamber, a plurality of detectors, and a processor. The processor is configured to receive a plurality of mean electron plasma density measurements from the detectors that correspond to locations of the plurality of open resonators.