A Solution for Backward Wave Oscillation in High-Order Mode Sheet Beam Slow-Wave Structures

This paper proposes a novel solution to suppress backward wave oscillation (BWO) in high-order mode (HOM) sheet beam (SB) slow-wave structures (SWSs) and designs an isolator between cavities based on a Bragg resonator. This method can cut-off the backward wave signal path without interrupting the operating signal path, thereby eliminating BWO while maintaining high circuit gain. Simulation results show that the S21 parameter of the isolator is less than −20 dB from 175 GHz to 228 GHz. To verify the method’s performance, particle-in-cell (PIC) simulation was conducted based on a HOM SB SWS—a T-slot coupled-cavity (TSCC) SWS. Results indicate that this method can effectively suppress BWO and shows significant improvement in gain and output power compared to traditional methods such as sever or lossy loading. Under operating conditions of 34.4 kV and 0.35 A, the circuit achieves a maximum output power of 527 W at 216 GHz, a maximum gain of 36.39 dB at 214.4 GHz, and a bandwidth of 3 GHz where the output power exceeds 300 W.