Application of Distributed Electron Collection to Increase Beam–Wave Conversion Efficiency in Low-Magnetic-Field Relativistic Backward Wave Oscillators

High-power microwave (HPM) sources with low guiding magnetic fields are urgently demanded for portable applications owing to their compact volume and low energy consumption. However, their efficiency remains limited due to the expanded electron beam envelope and degraded electron beam bunching. Here, we propose the concept of distributed electron collection to overcome this bottleneck. The key innovation lies in spatially distributed absorption of the electrons destined for substantial acceleration or having undergone severe deceleration by conductor walls within the beam–wave interaction region, as opposed to conventional terminal collection outside the interaction zone. This in situ electron removal at optimized positions effectively prevents energy flow from microwave to electrons and enhances the overall efficiency. Experimental validation in an X-band dual-mode relativistic backward wave oscillator (RBWO) with permanent magnet demonstrates a record-breaking efficiency of 49%, surpassing the previous benchmark of 40%. Theoretical calculation shows that ideal electron absorption could achieve 87% efficiency, revealing a transformative route toward ultrahigh-efficiency compact microwave sources that may extend to other beam–wave devices.