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.

The burden of hepatocellular carcinoma (HCC) worldwide is increasing over time, while the underlying molecular mechanism of HCC development is still under exploration. Pseudogenes are classified as a special type of long non-coding RNAs (lncRNAs), and they played a vital role in regulating tumor-associated gene expression. Here, we report that a pseudogene peptidylprolyl isomerase A pseudogene 22 (PPIAP22) and its parental gene peptidylprolyl isomerase A (PPIA) were upregulated in HCC and were associated with the clinical outcomes of HCC. Further investigation revealed that PPIAP22 might upregulate the expression of PPIA through sponging microRNA (miR)-197-3p, behaving as competing endogenous RNA (ceRNA). PPIA could participate in the development of HCC by regulating mRNA metabolic process and tumor immunity based on the functional enrichment analysis. We also found a strong correlation between the expression levels of PPIA and the immune cell infiltration or the expression of chemokines, especially macrophage, C-C motif chemokine ligand 15 (CCL15), and C-X-C motif chemokine ligand 12 (CXCL12). Our findings demonstrate that the PPIAP22/miR-197-3p/PPIA axis plays a vital role in the progression of HCC by increasing the malignancy of tumor cells and regulating the immune cell infiltration, especially macrophage, through CCL15-CCR1 or CXCL12-CXCR4/CXCR7 pathways.

A cross-band frequency hopping high power microwave (HPM) oscillator with permanent magnet package was proposed. The oscillator comprises of an inner Ku-band hollow oscillator and an outer C-band coaxial oscillator. By comparing with other cross-band HPM oscillators, this device is quite compact and high efficiency, for the guiding magnetic field is as low as 0.43 T, which is provided by a 74 kg permanent magnet. Since the cathode can be adjusted telescopically to produce annular electron beams at different radius, different bands HPMs can be generated in the inner and the outer slow wave structure. Verified by particle-in-cell simulation, a HPM with a frequency of 7.8 GHz and a power of 3.1 GW is achieved for C-band, corresponding to a power efficiency of 45%. Respectively, a Ku-band HPM output with a frequency of 14.4 GHz and a power of 1.3 GW were obtained, corresponding to a power efficiency of 45%.

An improved small-signal theory of triaxial klystron amplifiers is proposed with a reduction factor and the beam coupling coefficient being taken into account. It may be concluded that the beam coupling coefficient amends the amplitude of the modulated current and the reduction factor mainly modifies the optimum drift distance, at which the modulated current is maximum. The modified theory is validated by means of PIC codes, which indicates that the improved theory predicts the first-order modulation current more accurately than the previous theory.

Suppression of the higher-order azimuthal mode competition is significant in the design of an X-band triaxial klystron amplifier (TKA), since coaxial waveguide cannot cut off the low-order coaxial TE modes. In this article, the generation mechanism of the higher-order azimuthal mode competition is analyzed, and a slotted coaxial waveguide is specially designed to suppress the coaxial TE₈₁ mode leakage. The results show that the reflection coefficient of the slotted coaxial waveguide can be optimized over 99%, and the reflection coefficient is sensitive to the variation of the slot length rather than the aperture angle or the thickness of the inner conductor. The combination of two identical slotted coaxial waveguides staggered azimuthally can effectively reflect the arbitrary polarized coaxial TE₈₁ mode. Together with the reflectors specially designed to suppress the coaxial TE₆₁ mode leakage, all of the asymmetric TE modes' competitions are restrained in this device. In the 3-D particle-in-cell simulation, the pulse-shortening resulting from the coaxial TE₈₁ mode leakage is successfully suppressed with the slotted coaxial waveguide. The output power of the TKA achieves 1.28 GW, and the efficiency and gain are 34.5% and 41.5 dB, respectively. The slotted coaxial waveguide together with the reflectors eliminates the utilization of attenuation materials.

A novel cross-band frequency hopping giga-watts (GWs) class high-power microwave (HPM) oscillator is proposed in this brief, which consists of an inner Ku -band coaxial oscillator, an outer $\textit{X}$ -band coaxial oscillator, a beam transfer channel, and an extraordinary external magnetic field system. Compared with ordinary design, the external magnetic field of this device is generated by three closely spaced solenoids. Through the adjustment of the current passing through each solenoid, two kinds of magnetic field distribution can be generated, each guides the beams into a separate frequency beam-wave interaction region. The preliminary particle-in-cell (PIC) simulation results were as follows: when the currents passing through the three solenoids were the same of 700 A, a HPM output with a frequency of 14.4 GHz, and a power of 1. 5 GW was obtained, achieving a power efficiency of 37%; when the three solenoid currents were 1800 A, 225, and 460 A, respectively, a HPM output with a frequency of 8.2 GHz and a power of 1.6 GW was obtained, achieving a power efficiency of 40%.