The stable operation of the high-order mode multibeam extended-interaction oscillator (EIO) with coaxial structure can effectively overcome the size limitation of the fundamental mode operating circuit at high frequency. The research on the transverse mode shows that the high-order TM₃₁ mode can establish a stronger axial electric field than the fundamental TM₀₁ mode in a large size coaxial cavity. Although the design of a coaxial extended interaction cavity is straightforward, the operating stability is greatly affected by the mode competition. Based on the interaction characteristic between the multiple beams and the multigap cavity operating in the high-order mode, this article proposes a simplified analysis method based on the equivalent circuit theory to study the high-frequency characteristics of the complex structure. The space-charge wave theory is combined with the dispersion curve to analyze the stability of the five-gap coaxial EIO operating in the TM₃₁ mode. Finally, the particle-in-cell (PIC) simulation has obtained a TM₃₁ mode EIO operating at 35.3 GHz with an output power of 290.3 kW. The device uses six beams with a total current of 25.8 A (evenly divided among six beams) at a voltage of 51 kV. The design provides a promising approach for making a stable high-power RF circuit applied in high-frequency EIOs.

The efficiency of the RF power sources is one of the major concerns in the large-scale high-energy accelerator projects such as the future circular collider (FCC) and the compact linear collider (CLIC). The new bunching method, the core stabilization method (CSM), can achieve high efficiency in the case of shorter interactive length. The multibeam klystron (MBK) could accommodate high RF power with a low operating voltage, compared with a single-beam klystron. This article presents the design and simulation of a 650-MHz, 0.9-MW continuous wave (CW) CSM MBK. The detailed design of a six-beam optics system is also elaborated in this article. The performance of such a device was validated by the 3-D particle-in-cell (PIC)-code computer simulation. With an operating voltage of 50 kV, PIC simulation predicts an efficiency of 81.8%.

A dual-mode five-beam extended interaction klystron (EIK) with a coaxial structure is proposed that can stably obtain high output power. The TM_mn mode with uniform angular distribution in the coaxial cavity can effectively interact with beams located at the peaks of the axial electric field. However, when the center radius of the cavity is enlarged to meet the cathode area requirements of high-power devices at high frequency, the interval between adjacent modes will rapidly decrease. Especially when combined with a multigap structure, the circuit will suffer from mode competition. This article describes a study of the beam-wave interaction characteristics of the multigap coaxial cavity driven by multiple beams, contributing to a stable circuit design. The results show that the appropriate mode selection for each cavity can increase the stability and efficiency of this device. A Ka-band EIK with four cavities has been designed to show the circuit capability. The output cavity operates in the TM₅₁-2π mode, but the other cavities operate in the TM₀₁-2π mode. The 3-D particle-in-cell (PIC) simulation predicts a power of 211.6 kW at 35.19 GHz with the beam parameters of a voltage of 42 kV and a total current of 15 A (3 A x 5). The corresponding gain, electronic efficiency, and 3-dB bandwidth are 53.26 dB, 33.5%, and 50 MHz.

In this paper, a W-band multi-beam extended interaction klystron operating in the coaxial high order mode (HOM) is proposed. The HOM coaxial cavity with large transverse dimensions is suitable for accommodating multiple electron beams, promising to achieve high output power in the millimeter-wave frequency band. However, the operating method also brings lower characteristic impedance and a higher risk of mode competition. Based on the angular boundary condition of the coaxial HOM electric field, we propose a coaxial multi-gap cavity with a ridge structure to solve these problems. The TM₅₁-2π mode is selected as the operating mode, of which the characteristic impedance can overwhelm that of the competing mode. The corresponding five-beam gun with a total current of 7 A (5 × 1.4 A) at a voltage of 25 kV is focused by a 0.65 T permanent magnet system. To demonstrate the capability of the device, the 3D particle-in-cell simulation predicts an output power of up to 30 kW at 94.4 GHz. The gain and electric efficiency are 54.8 dB and 17.2%, respectively.

In this article, the design study of a V-band klystron with an internal coupling cavity is presented. The effective characteristic impedance of the internal coupling cavity is approximately 25% higher than that of the external coupling cavity when both operate at $\pi $ mode. A V-band RF circuit of 35% efficiency has been designed based on the proposed internal coupling cavity. An RF circuit based on single-gap resonators was designed first with the help of genetic algorithms (GAs). Then, a five-gap internal coupling cavity replaced the last several resonators to improve circuit bandwidth and efficiency. Simulation results have shown that the proposed internal coupling cavity can significantly improve the efficiency and bandwidth of an RF circuit based on single-gap resonators. Driven by a 60-kV and 9-A pencil beam, the designed RF circuit can achieve a peak power of 188.5 kW, a gain of 47.8 dB, and a 1-dB bandwidth of 180 MHz. The matching high compression ratio beam optics is also elaborated in this article.

We propose a W-band circularly polarized reflectarray antenna in this article, which contains a single-layer reflectarray and a linearly polarized horn feed. To realize the proposed antenna, we designed a novel W-band multi-resonant element containing a Rogers RT5880 substrate with copper patches printed on its both surfaces. Then, a circular reflectarray with 12.25 λ₀ (39.1 mm) aperture diameter was designed based on the proposed multi-resonant element, whose center frequency is 94 GHz. We further fabricated the proposed circular reflectarray and tested its performance. In the measured results, we can see that the obtained 1 dB gain bandwidth is 19.1% (91~109 GHz) and the obtained 2 dB gain bandwidth can reach as wide as 27.6% (89~115 GHz). Moreover, the 3 dB axial ratio bandwidth is 13.8% (89~102 GHz). The measured gain of our proposed reflectarray antenna at 94 GHz is 29.1 dBi and the corresponding aperture efficiency can reach as high as 52.0%. Those results show that our proposed antenna may be prospective in wireless communication applications due to its strengths in broadband and high aperture efficiency.