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This article introduces a radial staggered double vane (RSDV) backward wave oscillator (BWO) using a radial sheet electron beam (RSEB) to overcome the limitations of traditional Ka-band BWOs. Traditional BWOs face limitations due to a decrease in interaction space as frequency increases, leading to lower efficiency and considerable challenges in mitigating thermal stresses. In addition, these devices typically require an external magnetic field for beam focusing to counteract space charge effects, adding complexity to their fabrication. The RSDV slow wave structure (SWS) is designed, and its high-frequency characteristics are numerically analyzed. Particle-in-cell (PIC) simulations are conducted for the RSDV BWO using a converging RSEB without an external magnetic field, at a beam voltage of 20 kV and a current density of 22.43 A/cm2. The device demonstrates enhanced efficiency over traditional Ka-band BWOs, achieving an average power of 117.61 kW and an efficiency of 10.69% at 31.02 GHz. Furthermore, the performance using a diverging RSEB setup is evaluated and compared with that of a converging RSEB. Coupled particle-thermal simulation confirms the device’s capability to effectively manage thermal stresses. The device, consisting of 18 periods, is fabricated and tested; cold-test results indicate an ${S} _{{11}}$ parameter around −10 dB at desired frequencies, aligning with simulated results. Moreover, a comparison between analytical, simulated, and measured dispersion curves based on ${S} _{{11}}$ phase is provided.

A theoretical analysis and calculation for the two-stream instability in a dual-sheet electron beam (DSEB) system immersed in an infinite magnetic field in semi-infinite space are proposed in this article. The dispersion equation of the DSEB system is obtained by solving the Maxwell equations. By analyzing the dispersion relation of two electron beams with different velocities, the instability generated by the coupling between the space charge waves is found, which leads to the growth of waves. Then, the growth rate of the instability is calculated. Furthermore, the impact of system parameters, such as the number density of electrons, the distance between two beams, and the thickness of beams, on the instability is analyzed. At last, a special case is discussed to further illustrate the mechanism of the instability. This study provides theoretical guidance for the utilization or avoidance of two-stream instability in the design and fabrication of dual-sheet beam vacuum electron devices (DSB-VEDs).

A physics-informed neural network (PINN) is utilized to advance the research of broadband pillbox window (PW) for a terahertz traveling-wave tube (THz TWT). PINN integrates the physical laws of PW into the neural network training process, thereby enhancing the predictive speed and accuracy between the structure parameters of PW and its S-parameters. Subsequently, the global genetic algorithm (GA) is employed to obtain the structure parameter set best matching the desired S-parameter targets from the huge 5-D parameter-set space. Based on the PINN and GA, a general intelligent PW design method (IPWDM) is developed. With the help of IPWDM, a PW sample operating in 230–260 GHz is quickly obtained within only ~1 h. The predicted S-parameters of this PW sample by PINN clearly show excellent transmission characteristics, which are consistent with those simulated by CST. Experimental results of the PW sample provide strong support for the reliability of IPWDM. These facts demonstrate the superiority of IPWDM over time-consuming simulation softwares. This intelligent design pattern can ease researchers from extensive simulation work and lays a solid foundation for the future development of THz TWTs.

A broad-wall loaded photonic crystal waveguide (BWPCW) slow wave structure (SWS) for the terahertz traveling wave tube (TWT) has been presented in this article. The frequency-selective properties of photonic crystals are studied. The mode competition can be adequately suppressed by introducing photonic crystals into the SWSs. The parameters of the SWS and couplers are optimized to achieve a high-performance TWT at 220 GHz. The results of simulation of high-frequency characteristics show that the BWPCW-SWS has an interaction impedance of 1.02– $3.62 \Omega $ and an ohmic loss of 0.050–0.076 dB per period in the passband. The beam–wave interaction results indicate that the proposed BWPCW-TWT can produce a saturated output power of over 80 W ranging from 198 to 261 GHz when the operating voltage is 24.5 kV and the beam current is 150 mA. At 220 GHz, the maximum electron efficiency and saturated output power attain 4.35% and 160 W, respectively, with an input power of 0.55 W.

A magnetically insulated line oscillator (MILO) is a renowned crossed-field high-power microwaves (HPMs) device with an intrinsic property of self-insulation against electrical breakdown, which corroborates its gigawatt range operation. This article presents an idea of a reflector in MILO for efficiency enhancement. Reflectors are placed symmetrically in the beam dump region. Effects of reflector radius and positioning on output power are also studied, and it is observed that a matched reflector causes a drastic improvement in efficiency. The presented reflectors in conventional MILO enhance the axial electric field ( $\boldsymbol {E_{z}}$ ) in the slow-wave structure (SWS) region, which reinforces the beam–RF wave interaction. An eigenmode analysis is done to determine the dominant frequency of MILO. Simulation results show that the device is operating in the $L$ -band at 1.82 GHz. The dominant mode is $\boldsymbol {TM_{01}}$ with an average output power of 7.55 GW. The average power efficiency is 18.6%. The applied input voltage is 680 kV and current of 59.7 kA. While the conventional MILO only generates an average power of 5.2 GW at the same input power. PIC simulation results reveal that the efficiency of MILO with reflector configuration is 45% higher than that of conventional MILO with a commonly used beam stub position of $\boldsymbol {\lambda _{g}/4}$ from the extractor cavity.

The design, based on theory, simulation, and cold as well as hot tests, is proposed for a compact GW-level multibeam relativistic klystron amplifier (MB-RKA) with a low beam confining magnetic field in the ${S}$ -band, which can operate repeatedly and stably up to 8 min. For this purpose, graphene cathodes and a microchannel water cooling system were used. For the sake of compactness, the input Section of the MB-RKA was taken as a cylindrical conformal sectorial waveguide and the output Section as an inner cylindrical coaxial waveguide. The experiments were performed with a magnetic field of 0.39 T. In the single-pulse operation, the output power, pulsewidth, gain, and efficiency of the device were experimentally found as 1.047 GW, 68 ns, 56 dB, and 37% respectively, when the beam voltage, beam current, and injected power were taken as 530 kV, 5.30 kA, and 2.40 kW, respectively. In the multipulse operation at a pulse repetition rate of 20 Hz, over a period of 8 min, the MB-RKA experimentally delivered the mean output power of 0.889 GW of 42-ns pulsewidth, taking the mean beam voltage and current as 551 kV and 5.14 kA, respectively.

A terahertz (THz) broad compound tuning range extended interaction oscillator (EIO) operating at quasi-TM51 mode is proposed in this article. Based on the eigenmode analysis, the quasi-TM51 mode has a higher characteristic impedance ${R}/{Q}$ than the TM51 mode. Meanwhile, it can help to address the compatible mode competition issue brought on by increasing the gap number of traditional EIO devices, suiting to further power improvement. Moreover, it can work effectively and stably in multiple modes, so it can achieve a wider continuous tuning range. The results of particle-in-cell (PIC) simulation show that driven by a 28.3-kV and 25-mA electron beam, the novel quasi-TM51 mode EIO can generate a power of 2601 mW at 612.15 GHz and achieve a 3-dB bandwidth of 1.2 GHz. By tuning the beam voltage between 25.0 and 31.0 kV, the EIO source can produce more than 64-mW THz signals covering 607.21–616.29 GHz. In addition, by compound tuning, the frequency tuning range has been increased from 9.08 to 11.98 GHz. Also, the proposed EIO can produce more than 368-mW THz signals covering 607.07–619.05 GHz.

Increasing the input current is an important method to achieve the high-power output for the traveling wave tubes (TWTs) operating in the ${W}$ -band and higher terahertz (THz) band. For the folded waveguide (FW) TWT, increasing the height of the straight waveguide section of slow wave structure (SWS) can realize a double tunnel to increase the input current, but its electromagnetic characteristics, such as the operating passband, interaction impedance, and ohmic loss, become worse. Thus, a novel SWS, called folded rectangular-grating (FRG), with two tunnels for more input current, was proposed. Its two beam tunnels, where the electromagnetic wave in the operating band is cutoff, are located on the upper and lower edges of the grating, respectively, which makes the FRG-SWS have a wider operating passband, higher interaction impedance, and lower ohmic loss by freely adjusting its grating height. More importantly, its structural complexity is the same as that of FW-SWS. First, the electromagnetic characteristics, including dispersion, interaction impedance, and ohmic loss, are analyzed. Then, the particle-in-cell (PIC) simulation was carried out to verify that the saturated power, gain, and electron efficiency of TWT using FRG-SWS have been significantly improved compared with the TWT using FW-SWS. In the end, the cold test of transmission characteristics for FRG-SWS was done, and the experimental results are basically in good agreement with the simulation ones when the conductivity is set as $2.25\times 10^{{7}}$ S/m.