High-Order Symmetric and Asymmetric Mode Control in Overmoded Relativistic Backward Oscillator

Relativistic backward oscillator (RBWO) has been developing toward the millimeter wave, which is easy to cause RF breakdown due to the reduction in device size. Using the overmoded slow wave structure (SWS) and selecting high-order mode to operate can effectively prevent RF breakdown. In this article, we analyzed the physical mechanism of the overmoded SWS dispersion curve, especially the asymmetric mode, and quantitatively obtained the variation laws of the dispersion curve and approximate methods by a concise mathematical formula. Based on these laws, we proposed methods for controlling the quality factor effectively and calculating the coupling impedance. The mode control method considering asymmetric mode is verified through 3-D PIC simulation.