Single/dual band relativistic BWO using electron beam magnetic field decompression

For a relativistic backward wave oscillator (BWO) consisting of two cylindrical sections with sinusoidal profiles where one of them is axially symmetric and the other one has a combined left- and right-single-fold spiral corrugation, we demonstrate the possibility of generating the TE₁₁-mode with linear polarization at a single or dual frequencies using magnetic field decompression. Magnetic field decompression, as shown in Fig. 1, can be used to control the radial gap between the electron beam and the slow wave structure and that leads to controlling the microwave frequency at the output. In addition, using a cutoff neck and resonant cavity reflector improves the efficiency of the device. Using particle-in-cell computer simulations we show that generation with frequency 10 GHz is realized when the symmetric section operates as a BWO with the operating TM₀₁-mode and the combined spiral section operates as a Bragg reflector that converts the backward TM₀₁-mode to a forward TE₁₁-mode with linear polarization. Using the same electron beam parameters, generation with frequency 7 GHz is realized when the combined spiral section operates as a BWO with the operating linear polarized TE₁₁-mode followed by amplification in the next section which behaves as a TWT at this frequency. We also present the possibility to change the relation between the durations of generation at different frequencies within a microwave pulse by changing the electron beam current, and to change output frequencies by changing the value of the guide magnetic field near the region of cyclotron absorption.