Influence of Annular Beam Displacement on the Performance of a High-Power Gyrotron

In this paper, we present the results of an experimental study on the influence of the lateral misalignment between the axes of the annular electron beam and the cavity on the performance of a 1-MW 140-GHz gyrotron. The subject of the study is of practical interest. On one hand, the discrepancy between numerical simulations of the gyrotron performance and experimental results could by partially attributed to the beam misalignment. On the other, it is in practice very difficult to reach the necessary tolerances in series production of high-power high-frequency (>100 GHz) gyrotrons. The problem actually will become more demanding with the increase of the operating frequency of the working mode because the required accuracy scales with the wavelength of the electromagnetic wave. It is shown that a large displacement of the annular beam from the cavity axis may lead to the excitation of a competing mod which is not excited under normal conditions. The excitation of the main mode is observed, however, at a beam-cavity displacement of up to 0.8 mm.