In this article, a 1-THz traveling-wave tube (TWT) based on flat-roofed sine waveguide (SWG) slow wave structure (SWS) has been studied theoretically and experimentally, and parts of the component have been fabricated. First, the flat-roofed SWG SWS has been optimized for working at 1 THz, the normalized phase velocity is around 0.287, and the interaction impedance is over 3.5 Ω in the frequency range of 1.02-1.04 THz. Meanwhile, the performance of the designed TWT has been simulated, whose saturated output power is 720 mW and the corresponding gain is 28.57 dB. What is more, to decrease the transmission loss of the high-frequency structure, the input and output structures have been designed and a transition structure that transforms the waveguide to standard rectangular waveguide has been proposed. The simulation results show that the reflection coefficient of the novel proposed transmission system is less than -20 dB, and the insertion losses of the transition structure with the input and output waveguides are 2.45 and 1.68 dB, respectively. Finally, the designed SWS has been fabricated by deep reactive-ion etching (DRIE) and tested; S ₁₁ is generally less than -10 dB in the frequency range of 1020-1040 GHz.

In this paper, the analysis of field distributions and dispersion characteristics in an elliptical metal waveguide filled with a chiroferrite medium is presented. The electric and magnetic fields are expanded by Mathieu functions, using the separate variable method. It is shown that each of the eigenmodes consists of even and odd parts. The effects of constitutive parameters of chiroferrite material on the propagation behavior of hybrid modes are examined. Some new phenomena are discovered and compared with the circular chiral and chiroferrite waveguides.

To improve the performance of efficiency bandwidth product (EBP) and power capacity for G-band continuous-wave traveling-wave tube (CW TWT), a novel staggered U-shaped groove sine waveguide slow-wave structure (SUGSWG SWS) is proposed in this article. The theoretical analysis of high-frequency and beam–wave interaction characteristics indicates that the SUGSWG TWT has low insertion/reflection loss, broadband (3-dB power bandwidth of over 19 GHz), and high maximum output power (over 66 W), of which the EBP could reach whopping 103% $\cdot$ GHz (about three times the state of art). In the merit of its special geometry, the power capacity of this novel SWS is also about 1.6 times its existing peers. A prototype of a two-section SUGSWG TWT is designed, fabricated, and experimented for verifications. The hot test demonstrated that the peak power of 56 W, the 3-dB power bandwidth of 16 GHz, and the EBP of 75.4% $\cdot$ GHz were achieved, with the transmission rate practically tuned around 93%. Those measurement results are consistent with the theoretical predictions under the same transmission conditions. Those studies validate the remarkable potential of our proposed novel slow wave structure (SWS) in TWTs for THz applications.

The multibeam klystron (MBK) faces the challenge of miniaturization together with stringent requirements for high power and high efficiency (HE) in L -band and below frequency regimes. Inspired by the core stabilization method (CSM), this article proposes three types of hybrid-modes resonant cavities, which can operate simultaneously in two harmonic modes. The large-signal simulation and 3-D particle-in-cell (PIC) simulation results demonstrate that this novel CSM MBK equipped with two hybrid-modes resonant cavities, operating at both fundamental and second harmonics, could provide up to 11.5-MW output power with 65.9% electronic efficiency at a total circuit length of 592 mm. This L -band MBK is launched with six beamlets, of which the beam voltage is 110 kV and the total current is 168 A. Moreover, compared with the MBKs designed by the traditional approach with only fundamental cavities or the CSM approach with individual harmonic cavities, the new hybrid HE klystron proposal exhibits not only decent efficiency performance but also, more importantly, ultracompact dimensions, where the circuit length is significantly shortened by about 45% and 35%, respectively.

A phase shift that is caused by the machining errors of independent circuits would greatly affect the efficiency of the power combination in traditional multi-beam structures. In this paper, to reduce the influence of the phase shift and improve the output power, a multi-beam shunted coupling sine waveguide slow wave structure (MBSC-SWG-SWS) has been proposed, and a multi-beam overmoded flat-roofed SWG traveling wave tube (TWT) based on the MBSC-SWG-SWS was designed and analyzed. A TE₁₀-TE₃₀ mode convertor was designed as the input/output coupler in this TWT. The results of the 3D particle-in-cell (PIC) simulation with CST software show that more than a 50 W output power can be produced at 342 GHz, and the 3 dB bandwidth is about 13 GHz. Furthermore, the comparison between the single-beam sine waveguide (SWG) TWT and the multi-beam overmoded SWG TWT indicates that the saturated output power of the multi-beam overmoded SWG TWT is three times more than that of the single beam SWG TWT.

Klystrons designed for high-power, high-efficiency (HE) microwave amplification typically suffer from limited instantaneous bandwidth. Improving bandwidth is critical for enhancing device responsiveness and relaxing fabrication tolerances. This paper proposes an appealing mechanism to significantly broaden the instantaneous bandwidth by applying counterintuitive frequency downshifting strategy in selected gain cavities. Using the X-band 20-MW high-beam-perveance (2.2 μP) HE single-beam klystron (SBK) as an example, the conventional design achieved 50% conversion efficiency with −1 dB instantaneous bandwidth (under constant RF input drive) of only 40 MHz. By strategically downshifting the frequencies of specific gain cavities, the −1 dB instantaneous bandwidth is increased by fivefold up to 200 MHz without compromising its HE performance or prolonging the circuit. Furthermore, scaling this SBK to a mild-high-perveance (0.6 μP), the proposed mechanism still works well where its bandwidth could be improved by three times. Additionally, the study reveals that this novel mechanism also significantly lifts the oscillation threshold induced by backstreaming electrons, thereby improving device stability. This new mechanism is conceptually simple yet remarkably effective, without introducing any additional manufacturing complexity. It is, therefore, strongly recommended for future high-power HE klystron designs.