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We propose an oversized coaxial Cherenkov generator to generate ultrashort pulses with extremely high peak power exceeding 100 GW. The oversize factor of the device is about 5, which ensures sufficient power handling capacity for ultrashort pulses and a very high beam current, and thus the driving power. An optimized profiled slow wave structure (SWS) with squarely increasing corrugation amplitude, rather than linearly increasing function, and a uniform SWS with a slightly larger corrugation period are used to provide better beam-wave interaction and accomplish energy accumulation of electromagnetic wave from the gradually developed bunches precisely and continuously. The operation mode in the coaxial SWS is quasi-TEM mode, which is almost completely reflected by the coaxial reflector and coupled into the coaxial coupler with the mixed TEM mode and coaxial TM01 mode. This kind of single-pass output eliminates the power loss caused by conventional case as the pulse re-enters the SWS. Particle-in-cell simulations show that as the diode voltage is 760 kV and beam current is 78 kA, an ultrashort pulse of 0.4 ns with peak power of 129 GW and frequency range from 9.06-10.66 GHz is generated. The corresponding conversion factor is up to 2.18.

The initial experimental results of asymmetric mode competition in an X-band dual-mode relativistic backward wave oscillator (RBWO) operating at the low magnetic field are presented. In the experiment, the use of receiving antennas with different polarization directions allows direct determination of asymmetric mode, and the asymmetric mode with a frequency of 10.27 GHz is excited, which is in good agreement with the particle-in-cell (PIC) simulation results. Judging from that, the competition mode is asymmetric EH21 mode. The asymmetric mode competition mechanism in the device is investigated theoretically. It is shown that the drift length plays a key role in the suppression and excitation of the asymmetric mode, validated by the experiment and the PIC simulation.

We propose a novel cross-band high-power microwave (HPM) generator which is based on the relativistic magnetron (RM) with all cavity axial extraction and the radial three-cavity transit-time oscillator (TTO). A foil structure is used to isolate the output microwaves of the RM and the TTO. A regulating ring is introduced before the anode block of the RM to adjust the resonant frequency of the RM. A downstream endcap is added to collect the leakage current from the RM and delay the decrease in the output power. The particle-in-cell simulations show that as the diode voltage is 420 kV, the diode current is 11.4 kA, and the magnetic field is 0.22 T, an output power of 680 MW at the L-band from the RM and 1280 MW at the S-band from the TTO have been achieved, with a total beam–wave conversion efficiency of 41%. Moreover, the RM frequency ranges for about 10% by adjusting the position of the regulating ring, and the TTO frequency bandwidth is up to 68%, which covers the S- and C-band, by changing the cavity width of the radial TTO axially.

An investigation on the phase locking of S-band relativistic magnetrons with an all-cavity-extraction (ACE-RM) structure is carried out. In order not to disrupt the periodic distribution of electric fields in the resonant cavities, an azimuth-uniform coupling structure is introduced, which could achieve a transmission efficiency of over 98% at the operating frequency, with an amplitude difference of only ±3.3%. Furthermore, particle-in-cell (PIC) simulations demonstrate that when five ACE RMs are interconnected through four such coupling structures, phase locking can be achieved, and the relative phase jitters are controlled within ±1%. After establishing stable $\pi $ -mode oscillation in RMs, in each of the 20 ports, an average output power of over 400 MW can be obtained, corresponding to a total power of 8.5 GW and a power conversion efficiency of 45%.

To increase the beam–wave interaction and coupling output efficiency, we propose a sub-terahertz multiwave Cherenkov generator (MWCG) with a coaxial coupler. The coaxial coupler is a partially open coaxial waveguide, which locates interiorly the second slow wave structure (SWS). Inside this region, the well-modulated electron beam interacts with the axial electric fields of the quasi-TEM mode and the coaxial TM01 mode simultaneously. Meanwhile, the radial electric fields of the two modes are efficiently coupled into the coaxial coupler and sufficiently extracted from the coaxial output waveguide. Particle-in-cell (PIC) simulations show that when the diode voltage is 203 kV and the beam current is 1.08 kA, the output power reaches 67.3 MW, corresponding to an efficiency of 30.6%, and the frequency is 338 GHz.

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.

To further increase the beam-wave conversion efficiency, a super klystron-like relativistic backward wave oscillator (RBWO) with coaxial-premodulation (CPM) cavity is proposed. Compared with previous dual-premodulation (DPM) cavities, the CPM cavity intensifies the second and the third harmonic components, resulting in a larger fundamental harmonic current both in the slow-wave structure (SWS) and the extraction cavity, and, consequently, leads to an enhanced microwave power generation by the Cherenkov radiation and transition radiation. In the particle-in-cell simulation, an ${X}$ -band microwave with a power of 3.44 GW and record efficiency of up to 80% has been achieved.