A compact and modular pulse forming network (PFN)-Marx generator with output parameters of 5 GW, 500 kV, and 30 Hz repetition is designed and constructed to produce intense electron beams for the purpose of high-power microwave (HPM) generation in the paper. The PFN-Marx is composed by 22 stages of PFN modules, and each module is formed by three mica capacitors (6 nF/50 kV) connected in parallel. Benefiting from the utilization of mica capacitors with high energy density and a mini-trigger source integrated into the magnetic transformer and the magnetic switch, the compactness of the PFN-Marx system is improved significantly. The structure of the PFN module, the gas switch unit, and the connection between PFN modules and switches are well designed for modular realization. Experimental results show that this generator can deliver electrical pulses with the pulse width of 100 ns and amplitude of 500 kV on a 59-ohm water load at a repetition rate of 30 Hz in burst mode. The PFN-Marx generator is fitted into a cuboid stainless steel case with the length of 80 cm. The ratio of storage energy to volume and the ratio of power to weight of the PFN-Marx generator are calculated to be 6.5 J/L and 90 MW/kg, respectively. Furthermore, utilizing the generator to drive the transit time oscillator (TTO) at a voltage level of 450 kV, a 100 MW microwave pulse with the pulse width of 20 ns is generated.

A V-band coaxial relativistic transit time oscillator is presented. Different from the traditional transit time oscillator, in order to improve the power handling capacity, we replace the traditional standing wave extractor with a traveling wave output structure. The operation characteristic of modulation cavity is also different from traditional transit time oscillators. The electron load conductance of modulation cavity is designed to be positive to reduce the maximum surface electric field. The device is investigated by particle-in-cell (PIC) simulation and preliminary experiment. In PIC simulation, an output microwave of 689 MW at 60.16 GHz is obtained with a diode voltage of 400 kV and beam current of 5.0 kA. The saturation time is 22 ns and efficiency is 34.5%. In experiment, the radiation pattern of the system is measured, and output power is calculated. A 135 MW V-band microwave at 60.20 GHz is obtained with a diode voltage of 405 kV and beam current of 5.1 kA.

A Ku-band relativistic transit-time oscillator with a three-cavity extractor and a distance-tunable reflector was proposed in our previous work [Ling et al., Phys. Plasmas 24(1), 013103 (2017)]. Recently, a new experiment was carried out on this device with an intense relativistic electron beam generator that produces electron beams with a duration of 55 ns and voltage of 0.4–1 MV. In preliminary experiments, mode competition was found with three main frequencies of which the reasons are analyzed theoretically and experimentally. The results show that plasmas generated from the collector's surface are most likely to cause this problem. To solve this problem, the collector structure is extended axially. With an input voltage of 600 kV and a current of 11.7 kA, a microwave output power 1.9 GW with pulse width 26 ns and frequency 12.42 GHz is obtained under a 0.7 T guiding magnetic field. These experimental results effectively confirm the potential of high power capacity of this proposed Ku band transit-time oscillator.

A novel dual-band nested transit time oscillator (TTO) is proposed in this paper, which consists of an outer X-band coaxial TTO and an inner Ka-band coaxial TTO. It is capable of generating dual-band microwaves simultaneously with three distinctive features. First, the emission processes of inner and outer annular electron beams are independent. Second, the inner and outer high frequency structures are spatially isolated. Third, the inner and outer annular electron beams share the guiding magnetic field. Particle-in-cell simulation is performed where a voltage of 450 kV and a guiding magnetic field of 0.6 T are used. Simulation results demonstrate that microwave output with a frequency of 8.4 GHz and a power of 1.72 GW is obtained for the X-band, corresponding to a conversion efficiency of 42% and a microwave saturation time of 30 ns. Meanwhile, microwave output with a frequency of 31.3 GHz and a power of 1.18 GW is achieved for the Ka-band, corresponding to a conversion efficiency of 40% and a microwave saturation time of 15 ns.

In order to make the high-power microwave sources packaged with a permanent magnet more compact and miniaturized, we design a novel compact and lightweight permanent magnet package based on a C-band coaxial transit time oscillator. Before conducting the microwave generation experiment, research on the emission and transmission is carried out. In the beam transmission experiment, when the compact and lightweight permanent magnet is applied, the transmission efficiency in the diode is only 68.1%, while a phenomenon of side emission of electron beams is observed. When a solenoid is used, no side emission occurs. We theoretically analyze this experimental phenomenon and conclude that the radial magnetic field generated by the permanent magnet aggravates the side emission on the surface of a cathode holder. To solve this problem, a novel diode with a gradient cathode holder is proposed. In the beam transmission experiment using the compact and lightweight permanent magnet, this diode can suppress the side emission of electron beams, and the transmission efficiency in the diode is increased to 80.7%. The potential of the diode proved to work efficiently with this permanent magnet package, laying the foundation for the experimental research of microwave generation with this permanent magnet.

Compactness and miniaturization are important development directions of high-power microwave sources at present. In the previous simulation research, we proposed a compact C-band transit-time oscillator with a low external guiding magnetic field. In order to verify whether the device can be better applied to practice, we have carried out experimental research on the device. With a diode voltage of 570 kV, current of 11.5 kA, and external guiding magnetic field of 0.35 T, the output microwave pulse of 1.3 GW is obtained at 4.3 GHz. The power conversion efficiency is about 20%. By observing the fluorescent screen imaging and the far-field pattern in the experiment, it can be seen that the working mode of the device is relatively stable. The experimental results show that the device has the ability to output GW-level C-band high-power microwave under a low external guiding magnetic field.