Numerical examination of the operation modes of weakly relativistic oversized backward wave oscillator

Backward wave oscillators (BWOs) have been studied as a candidate high-power microwave source. To increase the operation frequency, an oversized slow-wave structure (SWS) is used. The operation at reduced voltage is preferable for practical applications. This work is aimed at numerically examining the operation mode of a weakly relativistic oversized BWO. We examine not only the axisymmetric transverse magnetic mode but also the non-axisymmetric hybrid modes of the oversized SWS. Both of them are candidates for the operation modes. These modes are surface waves whose fields are concentrated near the SWS wall. They overlap in frequency and are not separated by stop-bands. For an efficient beam interaction, the injected electron beam needs to be controlledmore accurately than in the non-oversized SWS case