Gyrokinetic description of the structural eccentricity influence on the starting current of a coaxial-cavity gyrotron

Based on the gyrokinetics of free-electron masers, the structural eccentricity influence on the starting current of a 1.5 MW/140 GHz, $TE28,16,1$ coaxial-cavity gyrotron oscillator is studied. Results show that the structural eccentricity substantially increases the starting current. The stable term of the instability, which results from the contribution of the guiding-center distribution, is of significance in a coaxial-cavity gyrotron, although it has usually been neglected in a cylindrical-cavity gyrotron. It is found that the influence of the structural eccentricity on the performance of the coaxial cavity gyrotron is more serious than that of the beam misalignment. The influences of the operating magnetic field and the beam energy on the starting current are also taken into account.