Relativistic BWO with Gaussian beam extracted radially using an electromagnetic bandgap medium

Abstract

Summary form only given. Cerenkov high power microwave sources can be used to generate short, Gaussian microwave pulses that are attractive for many applications. The backward wave oscillator (BWO) is typically known to radiate the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0n</sub> -mode in circular waveguide. The microwave signal is usually extracted axially after reflecting the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> -mode from the upstream end of the SWS by using either a cutoff-neck or a cavity resonator reflector. In order to produce a Gaussian beam from BWOs, a mode converter is usually used. Even though an asymmetric mode can be generated in these devices, the cutoff-neck reflector is the only means to effectively reflect the wave for radiation. The disadvantage of the cutoff-neck reflector is the proximity to the electron beam and that leads to beam scrape-off and alignment difficulty. In this work, the idea of combining a periodic slow wave structure (SWS) with an electromagnetic band gap (EBG) structure for generating, coupling and extracting a Gaussian radiation pattern is presented. Since SWSs with period ~ λ/2 have been used for a long time to generate high power due to their mechanical strength and resistance to breakdown, it is best to use them for high power microwave generation with added improvements. The EBG structure follows a slot antenna concept where the slot array is cut at the SWS wall to perturb the surface currents. The slots are basically used for coupling between the interaction region and a concentric circular waveguide. After that the generated mode will be radiated axially by using a horn antenna. The EBG structure makes the BWOs more compact and allows more power to be extracted, not only for HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> -modes but also for TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0n</sub> -modes. The fully electromagnetic, relativistic particle-in-cell (PIC) code MAGIC and the fully electromagnetic software tool ANSYS-HFSS were used in this study.