Electron-Beam-Driven Devices With Synchronous Multiple Degenerate Eigenmodes

We provide here a comprehensive exposition of a novel dispersion engineering framework for high-power electron-beam-driven devices based on multiple degenerate eigenwave synchronization in slow-wave structures (SWSs). These degenerate eigenwaves, related to an exceptional point of degeneracy (EPD) in the parameter space of the SWSs, lead to an enhanced interaction with the electron beam associated with a vanishing group velocity, which depends on the EPD order. Therefore, a “supersynchronous” mechanism based on multiple degenerate eigenwaves, associated with any order of EPD, provides advantages in terms of power generation efficiency at microwave frequencies, millimeter-wave frequencies, and terahertz frequencies. We present a physical description of EPDs in SWSs using a generalized Pierce theory developed for multimodal interaction. We show unique characteristics related to EPDs in “hot” (coupled with the electron beam) SWSs with third- and fourth-order EPDs for the sake of demonstration, but this can be extended to any order of degeneracy. We demonstrate advantages of the proposed regimes compared to conventional single-mode high-power devices such as backward-wave oscillators and traveling-wave tube amplifiers. In addition to that we show a possibility of high-power oscillators at millimeter-wave and terahertz frequencies based on a fourth-order EPD. In particular, we demonstrate a degenerate band-edge oscillator at 638 GHz based on particle-in-cell simulations. Apart from the examples provided herein, the concept of EPD can be used to design a wide variety of devices including gyrotrons, free electron lasers, and high-power isolators.