Experimental and numerical studies on eccentricity of annular electron beam in 1 MW gyrotron cavity

Summary form only given. In series production of new high power and high frequency gyrotrons the problem of beam alignment inside of cavity will become demanding, especially with the reduction of wavelength in future gyrotrons. In this work the effect of shifting of the electron beam the cavity is studied experimentally [1]. It is shown reduced overlapping between the annular beam and maximum amplitude of the cavity mode leads to reduction the gyrotron performance. At large offsets co- and counter rotating spurious modes are excited. The dependence of axial electron beam misalignment experimentally studied using a step-tunable gyrotron oscillating in the TE_22,8 mode at 140 GHz. It is shown misalignment of the beam of up to 0.5-0.6 mm, corresponding to λ/4 does not influence single mode oscillation, however the generated RF power is reduced up to 10%. The experimental results are compared with time dependent multimode numerical simulation using the code “GyroDyne” [2]. The model used in the code is based on the modal representation of the transverse electromagnetic wave field. The electron beam is represented as continuous flow of quasiparticles injected in the simulation domain. This approach allows to simulate gyrotron operation with annular beam shifted with respect to the cavity axis and with inclusion beam spreading effects in different parameters like pitch factor, guiding centers and energy. In agreement with experiment, the numerical study predicts the best performance at the optimal alignment, while with an increase of electron beam shift the output power is reduced. Similarly to the experiment at large beam shifts, excitation of spurious, counter rotating modes was observed which would lead to dangerous stray radiation inside the due to the helicity of the quasi-optical mode converter. The detailed comparison will be presented.