Gap Coupling Factor of an Annular Electron Beam

This work studies the interaction of an annular electron beam with a cavity gap, which may represent a single-gap-cavity device such as an inductive output tube (IOT) or the output cavity of a klystron. The gap coupling factor for an annular beam under large-signal conditions is calculated using analytical theory as well as a disk model with space charge effects. It is known that, compared with the gap interaction with a solid electron beam, using an annular beam can significantly enhance the interaction of the same beam current. Here, we quantitatively assess this enhancement of interaction and provide a parametric scaling analysis for two configurations: keeping the cross-sectional area the same as a given solid beam and varying the inner and outer radius or keeping the outer beam radius the same as the solid beam and varying the inner beam radius. We calculate the output power from beam interaction with a single cavity gap at high frequencies, highlighting the performance advantages and efficiency improvements associated with the annular beam configuration. The results are verified against particle-in-cell (PIC) simulations.