Observation of nonlinear generation of higher RF harmonics in hollow-cathode discharge

Two variants of high-power RF oscillators based on hollow-cathode discharge have been studied. The spectra of RF oscillations have been measured for two diameters and various lengths of the cathode cavity. In both variants, the generation of high (second and third) harmonics has been observed, which is apparently due to a nonlinear wave process in the discharge plasma. This phenomenon can be used for carrier frequency doubling in RF generators of this type.