The Ka-band extended interaction klystrons (EIK) have been developed at the Institute of Electronics, Chinese Academy of Sciences (IECAS), to satisfy the requirement for the high-power millimeter-wave sources in scientific researches. In this work, two kinds of high compression electron gun with the solid or hollow beams, the derivative electron optics systems using the compact permanent magnet uniform focusing and the shared efficient interaction circuit based on the multigap ladder cavity, have been studied in detail and well designed through a mass of calculations. After the mechanical design, finely manufacturing, and a series of technical processes, the Ka-band EIK prototype tubes have been successfully built and tested. For the solid beam scheme, the maximum output power achieves 20 kW in the bandwidth of 60 MHz, and, correspondingly, the gain of 53 dB and the efficiency of 24% are observed. The measured -1 and -3 dB bandwidths are about 220 and 350 MHz, respectively. Meanwhile, the beam transmission is excellent, that is over 97% in dc state and over 90% in RF operation. For the hollow beam case, an over 91% dc transmission test has finished.

In this paper, a novel low-profile, dual-polarized reflect-transmit-array antenna is proposed to independently control the forward and backward beams. A single-layer polarization-dependent element is designed with $0.1~\lambda _{0}$ -thickness at 10 GHz, which comprises only two layers of metal patches printed on a dielectric substrate and two metallized vias. Through changing the polarization direction of the incident wave, the operating mode of the proposed array can be easily shifted among reflection mode, reflection-transmission mode, as well as transmission mode. To verify the design concept, a circular reflect-transmit-array ( $D =250$ mm) is designed, fabricated, and measured. The measured results indicate a 1 dB bandwidth of 17% (9.8-11.5 GHz) in reflection mode and a measured peak gain of 24.2 dB at 10.5 GHz. Moreover, a 1 dB bandwidth of 6% (9.7-10.3 GHz) can be achieved in transmission mode, and the measured peak gain is 24.6 dB at 10 GHz. The results prove that the proposed design is promising for bidirectional wireless communication applications.

A narrow coupling ladder-type extended interaction klystron (EIK) operating at 0.34 THz is proposed and analyzed. Compared with the traditional THz EIKs, the narrow coupling EIK can offer higher power capability and increased efficiency because such arrangement has higher characteristic impedance and a more uniform electric field in the beam tunnel. This article studies the electric field distribution of the narrow coupling circuits and traditional circuits. Moreover, the coefficient of variation is introduced to quantify the uniformity of the axial electric field in the beam tunnel. The operating stability is also ensured by adjusting the period length and number of cells. The 3-D particle-in-cell (PIC) simulation predicts the ideal EIK, driven by a 0.15-A, 17.5-kV electron beam confined in a 0.22-mm diameter beam tunnel, and generates an output power of 41 W. The corresponding gain, electronic efficiency, and 3-dB bandwidth are 28.03 dB, 1.56%, and 600 MHz, respectively. Finally, in order to verify the feasibility of the designed structure, we perform a sensitivity study on the EIK design for an eight-micron fabrication tolerance assuming normal distributions for all critical dimensions per each cell with PIC simulations.

The design approaches of the overlapping-mode extended interaction klystrons (EIKs) are presented to satisfy the requirement for broadband terahertz power amplifiers. Since all the cavities are designed and tuned to operate in multiple-cavity modes, the overlapping-mode EIKs can provide high output power over a wider frequency band. The circuit characteristics of the overlapping-mode EIKs are studied, including the dispersion curve, the coupling coefficient, and the frequency interval between cavity modes. Moreover, a six-cavity EIK operating at 0.34 THz is designed to demonstrate the broadband output capability of the overlapping-mode EIKs. The $2\pi $ -mode and its adjacent axial mode, the $\pi $ /13-mode, are chosen as the operating modes of the designed EIK. The 3-D particle-in-cell (PIC) simulation predicts a 3-dB bandwidth of 2.9 GHz for the designed power amplifier at a small-signal level or in saturation. Driven by an input power of 10 mW, the amplifier can provide the output power of 3.9 W, and the corresponding gain is 25.9 dB. When the input power is 150 mW, the amplifier is partially saturated and can provide the output power of 33 W with a corresponding electronic efficiency of 1.23%.

An integrated planar three-beam electron optics system is designed for our 220-GHz cascaded folded waveguide TWT. First, a planar three-beam gun is designed based on Pierce theory and Vaughan's synthesis method with simplification of the focus electrode and the anode to facilitate manufacture and assembly, which is characteristic with 20 kV, 3 x 0.1 A, and beam radius of 0.1 mm. Simulation results show that the gun model performs well. Next, a uniform magnetic focusing structure used for constraining the three-beam array with adjacent beam separation of 8 mm is designed. Magnetic saturation analysis of the pole piece and structure parameters optimization of the magnets is considered thoroughly, which may guarantee the transport stability of the three-beam array and keep down the volume of the magnet. Simulation results show that the transmission of three-beam array up to 18 mm is 100% with the maximum ripple of 11.2% in the designed focusing structure. And the integrated planar three-beam electron optics system is verified as a stable system with the beam characteristics analyses of matching error, beam voltage fluctuation, and variation of peak Bz.

In this work, exploitation of artificial intelligence algorithms for the design of forward-wave amplifier is proposed and discussed. The multilayer perceptron (MLP), a quintessential example of deep learning, is employed to get the cost function, i.e., the relationship between the geometrical parameter set and dispersion characterization from only a small proportion of the parameter sets. Due to the structural complexity, there did not exist a closed-form solution and the neural network shows its superiority compared to the time-consuming numerical calculation. Subsequently, the genetic algorithm (GA) is used to find the parameter sets best matching the goals that broadband beam-wave interaction at specific voltages and even specific phases or frequencies, from the huge 4-D parameter-set space. The intuitive goals can be easily accessible, as long as the goals are within the ability of the parameter-set space and converting the goals into the goal functions for GA with some attempts and adjustments. As a practical example, the recently proposed defect photonic crystal waveguide is utilized to demonstrate the design pattern. Through almost full-automatic design with little judgment and analysis, a response of 14-GHz 3-dB bandwidth and 25-dB gain at the 168.5-GHz central frequency has been realized, indicating the effectiveness of the design pattern. This design pattern will ease researchers from numerous simulation work and help to get deeper insight of the structure.