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New concepts to generate extremely high output power with ultrashort pulse duration are developed. At first, an oversized coaxial Cherenkov generator with a profiled and extended slow wave structure is used to generate a time-frequency-modulated pulse with power of several tens of GW and duration of several nanoseconds. Then this time-frequency-modulated pulse can be compressed into a sub-nanosecond pulse with power exceeding 100 GW via a circular waveguide compressor after mode conversion. As an example, particle-in-cell simulation demonstrates that a pulse is obtained with power of 35 GW, duration of 3.0 ns, and frequency sweeping from 8.9 GHz to 11.0 GHz almost linearly. Injecting this pulse into a circular waveguide as TE $_{\mathbf {{01}}}$ mode, it can be compressed into an ultrashort pulse with power of 168 GW and duration of 0.45 ns. Compared with previous method of producing superradiance pulse directly, the beam-wave interaction in the generator is not so highly unstable, and thus the whole system is more reliable and controllable.

It is reported on the design and experiment of a dual-mode (TM01-TM02) relativistic backward wave oscillator (RBWO) with a beam filtering ring (BFR) packaged with permanent magnet. The BFR makes use of the large transverse oscillation of electrons at low magnetic field. By absorbing part of the electron beam periodically, the amount of the electrons that would absorb power from the microwave in the slow-wave structure (SWS) is greatly reduced, so the beam-wave conversion efficiency is improved. Experimental results demonstrate that a 3.2 GW mixed-modes microwave at 9.87 GHz with an efficiency of 40% is obtained with a 675 kV, 11.8 kA beam guided by a 0.68 T magnetic field. The BFR has a considerable lifetime and can effectively improve the efficiency of the device with stable operation.