The disclosure provides a plasma source, an excitation system for excitation of a plasma, and a method of operating an excitation measurement system. In one embodiment, the plasma source includes: (1) a coaxial radio frequency (RF) resonator including a first end, a second end, an inner electrode and an outer electrode, (2) a radio frequency interface electrically coupled to the inner and outer electrode and configured to provide an RF signal to the coaxial RF resonator, (3) a flange positioned at the first end of the resonator and defining a plasma cavity, and (4) a window positioned between the first end of the resonator and the flange, and forming one side of the plasma cavity, whereby the coaxial RF resonator is isolated from the plasma.