Analysis and design of slow wave structure for backward wave oscillators

Thesis Type: Postgraduate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Engineering, Department of Electrical and Electronics Engineering, Turkey

Approval Date: 2019


Supervisor: ŞİMŞEK DEMİR


High power microwave is an emerging area that finds different applications in radar, directed energy weapons, plasma science, satellite communications, particle physics and medicine fields. Some examples of high power microwave devices are klystron, vircator, cyclotron, magnetron, travelling-wave tube amplifier (TWTA) and backward wave oscillators (BWO). In this work, we focus on backward-wave oscillator that is one of the high power microwave sources. Backward-wave oscillator is a real high power microwave sources that convert electron beam energy to waves. Itisalsocalled as Cherenkov devices because emission process is analogous to Cherenkov radiation which occurs when the electron velocity exceeds the light velocity in that medium. Beam-wave interaction is achieved in the part called as slow-wave structure. Wave velocity is reduced below the electron beam velocity in this part. This part can be designed with periodic obstacles, dielectric or metamaterial. However, using dielectric is not preferred due to dielectric breakdown for high power microwave devices. Instead of that metallic periodic obstacles or metamaterials are used. This part affects the operation frequency, conversion efficiency, output power and compactness. As a result of these, slow wave structure is the crucial part of the BWO. In this work, we analyzed and designed the slow-wave structure for high power backward-wave oscillators. First, the unit cell of the slow wave structure simulation is performed in terms of dispersion diagram using commercial 3D electromagnetic solver. Modes of the SWS are also investigated for both empty circular waveguide and SWS. Interaction impedance which is the parameter for conversion efficiency is extracted for the unit cell. Phase velocity and group velocity of the wave in this medium is also simulated. Eight unit cells are combined to measure the propagation of the wave in that structure. Combined 8 unit cells are fabricated and measured in terms of S parameters. This measurement validates the dispersion diagram of the unit cell since it resonates for the desired mode and frequency. Hot test simulation of the slow wave structure is also investigated using commercial 3D particle solver. In this simulation, annular electron beam is used and validated SWS is used for hot test simulation. In order to confine the annular electron beam analytical magnetic field is applied. At the end, we observed the operation frequency as the 2.49 GHz which is in the passband region of the validated SWS.