Electromagnetic sub-bands in an axisymmetric doubly rippled wall metallic slow wave structure

Slow wave structure (SWS) is the vital component of slow wave type high-power microwave (HPM) devices where the kinetic energy of an axially streaming electron beam is resonantly extracted to produce microwave radiation. The frequency of the generated microwave predominantly depends on the beam energy and the dispersion characteristics of the SWS. Many types of SWSs have been investigated and implemented in real experiments. Among them, cylindrical metallic structures with sinusoidally rippled inner wall have gained popularity in backward wave oscillators. To enhance the efficiency of the devices, prolonged and stable interaction of the EM wave with electron beam is important. One of the possible techniques to improve the performance of the devices could be the use of non-uniform SWS. In this study, a doubly rippled inner wall SWS is considered. The radius of the structure is varied sinusoidally with two periods. Using linear theory, the dispersion relation of the EM wave inside the structure in the absence of electron beam has been analysed numerically. It has been observed that a single band of the conventional sinusoidally rippled SWS is decomposed into several sub-bands depending on the ratio of the length of two periods. Some of the sub-bands are so flat that the frequency of the microwave radiation can be made insensitive to the beam energy fluctuation.