Even after 50 years of development, narrowband high-power microwave (HPM) source technologies remain the focus of much research due to intense interest in innovative applications of HPMs in fields such as directed energy, space propulsion, and high-power radar. A few decades ago, the main aim of investigations in this field was to enhance the output power of a single HPM source to tens or hundreds of gigawatts, but this goal has proven difficult due to physical limitations. Therefore, recent research into HPM sources has focused on five main targets: phase locking and power combination, high power efficiency, compact sources with a low or no external magnetic field, high pulse energy, and high-power millimeter-wave generation. Progress made in these aspects of narrowband HPM sources over the last decade is analyzed and summarized in this paper. There is no single type of HPM source capable of excellent performance in all five aspects. Specifically, high pulse energy cannot be achieved together with high power efficiency. The physical difficulties of high power generation in the millimeter wave band are discussed. Semiconductor-based HPM sources and metamaterial (MTM) vacuum electron devices (VEDs) are also commented on here. Semiconductor devices have the advantage of smart frequency agility, but they have low power density and high cost. MTM VEDs have the potential to be high power efficiency HPM sources in the low frequency band. Moreover, problems relating to narrowband HPM source lifetime and stability, which are the important determinants of the real-world applicability of these sources, are also discussed.

Triaxial klystron amplifier (TKA) is prom- ising for coherent power combining of high-power microwaves (HPMs) in high-frequency bands. In this letter, we present an improved X-band TKA to generate gigawatt long-pulse HPM radiations. By increasing the intrinsic quality factor of the input cavity, the seed microwave loss is reduced significantly. The beam loaded quality factor of the input cavity is increased to enhance the gap voltage and, moreover, to strengthen the electron beam modulation. In addition, a double-gap output cavity is designed on the one hand to increase the radio-frequency power capacity and on the other hand to improve the beam-wave power conversion efficiency. The improved TKA is demonstrated to be capable of producing the HPM X-band radiation typically with power of 1.1 GW and pulse duration of 105 ns in experiments. Furthermore, the phase of the generated microwave is locked by the seed signal over more than 90 ns.

The radio-frequency breakdown due to ultrahigh electric field strength essentially limits power handling capability of an individual high power microwave (HPM) generator and this issue becomes more challenging for high frequency bands. Coherent power combining therefore provides an alternative approach to achieve an equivalent peak power of the order of ∼100 GW, which consequently provides opportunities to explore microwave related physics at extremes. The triaxial klystron amplifier (TKA) is a promising candidate for coherent power combing in high frequency bands owing to its intrinsic merit of high power capacity, nevertheless phase-locked long pulse radiation from TKA has not yet been obtained experimentally as the coaxial structure of TKA can easily lead to self-excitation of parasitic modes. In this paper, we present investigations into an X-band TKA capable of producing 1.1 GW HPMs with pulse duration of about 103 ns at the frequency of 9.375 GHz in experiment. Furthermore, the shot-to-shot fluctuation standard deviation of the phase shifts between the input and output microwaves is demonstrated to be less than 10°. The reported achievements open up prospects for accomplishing coherent power combining of X-band HPMs in the near future and might also excite new development interests concerning high frequency TKAs.

We report the first experimental demonstration of coherent combining of phase-steerable high power microwaves (HPMs) generated by X-band relativistic triaxial klystron amplifier modules under the guidance of pulsed magnetic fields. Electronically agile manipulation of the HPM phase is achieved with a mean discrepancy of 4° at the gain level of 110 dB, and the coherent combining efficiency has reached as high as 98.4%, leading to combined radiations with equivalent peak power of 4.3 GW and average pulse duration of 112 ns. The underlying phase-steering mechanism during the nonlinear beam-wave interaction process is furthermore explored by particle-in-cell simulation and theoretical analysis. This Letter paves the way for high power phase array in large scale and may stimulate new interest in research of phase-steerable high power masers.

We present the simulation results of a Ku-band overmoded Cerenkov type high power microwave oscillator. A guiding magnetic field as low as 0.6 T has been operated in the device. Overmoded slow wave structures with gradually tapered vanes are used in order to increase power capacity and the efficiency of beam-wave interaction. The drift cavity is adopted to enhance the beam-wave interaction of the device. After numerical optimization, the designed generator with an output microwave power of 1.2 GW, a frequency of 13.8 GHz, and a power conversion efficiency as high as 38% can be achieved, when the diode voltage and current are, respectively, 540 kV and 5.8 kA. The power compositions of TM_0n modes of the output microwave have been analyzed, the results of which show that TM₀₁ mode takes over almost 95% of the power proportion.

A compact Ku -band coaxial relativistic klystron amplifier (RKA) with permanent magnets is proposed for the sake of producing repetitive high-power microwaves (HPMs) in this article. The improved input and output mode converters are designed to load the input signal at the end of this device to ensure the integrity of permanent magnets. Besides, the intense relativistic electron beam (IREB) is changed to be collected on the outer collector, which makes collector cooling feasible in the proposed device. The typical experimental results show that the Ku -band HPM with a power of 540 MW is generated, corresponding to a gain of 48 dB and an efficiency of 29%, when the guiding magnetic field is around 0.45 T produced by the permanent magnets. The result of continuous operation for 30 s at a repetition rate of 35 Hz indicates that the proposed coaxial RKA has the capability of operating at a high repetition rate.