Super Multi-Frequency Oscillations at Fundamental Harmonics With a Complex Cavity Gyrotron

Two resonators with different radii were separately excited in a complex cavity to demonstrate super multi-frequency operations of a gyrotron. A demountable gyrotron was constructed and driven by 15-kV, 0.4-A annular electron beams. The magnetic field strength was varied from 4 to 8 T to excite a series of transverse electric modes in each resonator. In the frequency range of 120 - 220 GHz, 13- and 9-mode oscillations at the fundamental harmonics were identified in the first and second resonators, respectively. Furthermore, backward wave oscillations of these modes allowed continuous frequency variation. Even if two resonators oscillate at the same time with different frequencies, most of them can be independently excited by regulating the velocity pitch and/or injection radius of the electron beam. As a result, 27% of frequencies from 120 to 220 GHz became available with a single gyrotron. This is the largest frequency occupation reported so far in this frequency range. The frequency gaps between neighboring modes were less than 10 GHz. The maximum output power obtained for each mode was more than 28 W.