Effect of Beam Velocity Spread With Spatial Variation on Mode Excitation in Gyrotrons

The beam velocity spread and the guiding radius spread are important factors affecting the operation of the gyrotron, and they are typically considered to be independent. However, when the electric and magnetic fields near the cathode region are inhomogeneous, the beam velocity spread changes in space and becomes correlated with its radius spread. This results in a spatially varied velocity distribution, which is easy to occur but often overlooked. In this article, the formation, theoretical model, and effects of such spatially varied velocity distribution are investigated. By conducting simulations in an 800-GHz second harmonic (SH) gyrotron, the effects of this velocity distribution and its mechanism are analyzed. The difference between results considering spatially varied velocity distribution and typical unvaried one is also discussed.