An X-band dual-mode relativistic backward wave oscillator (RBWO) operating at low magnetic field is presented in this paper. Three new design principles are introduced in the device. First, the electron beam interacts with TM₀₁ mode and TM₀₂ mode simultaneously, rather than with a fixed single mode. Second, the device outputs with mixed modes, rather than with a pure mode. Third, an internal reflector inserted into the annular cathode, rather than a long resonant reflector before the slow-wave structure, is adopted to reflect part of the backward wave. Accordingly, the beam–wave interaction efficiency is increased significantly and the whole device is very compact. The particle in cell simulation results reveal that at a magnetic field of 0.64 T, the output microwave power is 4.8 GW and the conversion efficiency is up to 44%. In the experiment, at a guiding magnetic field of 0.66 T, a microwave pulse with power of 4.6 GW, frequency of 9.96 GHz, pulse duration of 42 ns, and efficiency of 42% was generated when the diode voltage was 880 kV and beam current was 12.5 kA, which agree well with the simulation results. Furthermore, as the diode voltage was 1.17 MV, a highest microwave power of 7.6 GW was achieved. This is a record of high efficiency and high power of microwave generation in an X-band RBWO operating at low magnetic field.

To realize power combination of two phase-locked relativistic backward wave oscillators (RBWOs), a compact power combiner is designed and investigated by 3-D particle-in-cell (PIC) simulation and experiment. The power combiner consists of two TM₀₁-TE₁₁ serpentine mode converters with a common output. When the two incident ports are fed with TM₀₁ modes with a relative phase of 180° and power of 2.5 GW at each port, the conversion efficiency from the incident TM₀₁ modes to the combined TE₁₁ mode is 95.2% at 9.3 GHz, and the maximum electric field in the combiner is 714 kV/cm. The PIC simulation shows that the output power from the common port is 4.2 GW when the power combiner is connected to the two RBWOs with input signals, both producing 2.2 GW microwave, corresponding to a combination efficiency of 95.4%. In the high power microwave test, a method is proposed to obtain the combination efficiency without breaking the vacuum, which is 94.1% when the two phase-locked RBWOs output 1.8 GW and 2.2 GW. The power capacity of multi-gigawatts has been demonstrated.

An investigation of a super klystron-like relativistic backward wave oscillator is presented in this article. To obtain high power and high efficiency, an additional anode cavity is used to promote electron-beam modulation. Additionally, two waveguide cavities at the end of the electron collector are employed to enhance the constructive resonance, which intensifies energy conversion from the modulated electron beam to the electromagnetic field. Because of the enhancement of centralized transition radiation, the power generated in the extractor constitutes over 95% of the total power. Consistent with the results from numerical simulations, microwaves with a power of 5.0 GW, a frequency of 4.4 GHz, and a pulsewidth of 36 ns were obtained in high-power microwave generation experiments with an external magnetic field of 0.42 T, in which the diode voltage and beam current were configured to be 815 kV and 15.5 kA, respectively, indicating a beam-wave conversion efficiency of 40%. The diode insulation was investigated experimentally, and an unfortunate shunting current of over 3 kA was observed when the working voltage was beyond 800 kV, which indicates an actual conversion efficiency of 33% in the diode region.

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.

It is reported on the design and experiment of a dual-mode (TM ₀₁ -TM ₀₂ ) 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.

This paper introduces the work undertaken to reliably use off-the-shelf differentiating voltage probes attached to coaxial transmission lines. The results obtained prove that indeed such probes are a valid and simple instrument for measuring nanosecond and subnanosecond voltage impulses. As a bonus, this paper also highlighted an important challenging phenomenon that appears whenever an attempt is made to measure fast voltage impulses with a differentiating probe positioned too close to the closing switch of a pulse forming line generator.