Starting current of a 140 GHz/1.5 MW coaxial cavity gyrotron oscillator

Based on the gyrokinetics of free-electron masers, the starting current is derived for a coaxial-cavity gyrotron oscillator, where the misalignment between the inner rod and outer cavity is taken into account. It is found that the change of the cutoff wavenumber due to the existence of misalignment substantially influences the distribution of RF fields, consequently influences the interaction between the wave and relativistic electron beam. As a practical application, we present a numerical analysis of the 1.5 MW/140 GHz, TE_28,16,1 coaxial-cavity gyrotron oscillator constructed at the Research Center Karlsruhe (FZK). Results show that the misalignment substantially decreases the output power and increases the starting current, and the output power becomes sensitive to the beam energy due to the existence of the misalignment