A high-efficiency microwave atmospheric pressure plasma jet (APPJ) has been developed in previous work. Long and stable plasma filament is excited directly in the open air with 2.45-GHz microwave power and double-channel argon flow by a double-coaxial structure. Unlike those plasma jets that strongly rely on a high flow rate to shape the plasma jet, this method produces a stable filamentous plasma jet at a low flow rate whose length and diameter are in a linear relationship with absorbed microwave power. This study aims to investigate the fundamental parameters of this special plasma filament under different power levels and gas flow rates by optical emission spectroscopy (OES), including the electron temperature and the spectrum. The electron temperature is determined to be 0.71-0.81 eV. It decreases slightly as the power increases, which is different from other high-voltage or radio frequency excited plasma jets under high power. The electron temperature does not change significantly with the change of microwave power and argon flow rate, more manifested as a change in the length, which indicates that this special plasma filament is related to the complex interaction of the microwave field and plasma. This study provides basic parameters for further investigation of this kind of plasma jet for theoretical and numerical simulation.

Motivated by some emerging high-frequency applications, a high-power frequency-tunable sub-THz quasi-optical gyrotron cavity based on a confocal waveguide is designed in this paper. The frequency tuning characteristics of different approaches, including magnetic field tuning, mirror separation adjustment, and hybrid tuning, have been investigated by particle-in-cell (PIC) simulation. Results predict that it is possible to realize a smooth continuous frequency tuning band with an extraordinarily broad bandwidth of 41.55 GHz, corresponding to a relative bandwidth of 18.7% to the center frequency of 0.22 THz. The frequency tunability is provided by varying the separation distance between two mirrors and correspondingly adjusting the external magnetic field. During the frequency tuning, the output power remains higher than 20 kW, which corresponds to an interaction efficiency of 10%. Providing great advantages in terms of broad bandwidth, smooth tuning, and high power, this research may be conducive to the development of high-power frequency-tunable THz gyrotron oscillators.

This paper proposes a taper cascaded over-mode circular waveguide TE_0n mode converter for the millimeter and terahertz wave gyrotron. The mode converter of this structure can effectively reduce the difficulty of high frequency mode converter in fabrication. This paper verifies the feasibility of this new structure from theory, simulation, and experiment. Based on coupled wave theory calculations, three TE₀₂-TE₀₁ mode converters with lengths of 65.43 mm (4 segments), 119.3 mm (6 segments) and 136 mm (8 segments) and a TE₀₃-TE₀₂ mode converter with a length of 92 mm (8 segments) are optimized. The conversion efficiency in the frequency band 215–225 GHz is 91.8–94%, 93–95%, 95–98.78% and 95–98.44%. Because the length of the mode converter is clearly limited, this paper selects the TE₀₂-TE₀₁ mode converter with a length of 65.43 mm (4 segments) and the TE₀₃-TE₀₂ mode converter with 92 mm (8 segments) for simulation and experimental verification. In the simulation software Computer simulation technology (CST), the TE₀₂-TE₀₁ and TE₀₃-TE₀₂ mode converters and their composed TE₀₃-TE₀₁ mode converters are selected for modeling and analyzing. The simulation results and theoretical calculation results of the three mode converters only have different degrees of frequency deviation, and the frequency deviation of the 4-stage TE₀₂-TE₀₁ mode converter can be ignored; the frequency deviations of TE₀₃-TE₀₂ mode converter and TE₀₃-TE₀₁ mode converter are 2 GHz and 3 GHz, respectively. The experimental system is a field scanning system based on a vector network analyzer (VNA), which scans the input and output of the mode converter respectively. The experimental result is that when the input mode purity is 92% in TE₀₁ mode, the output mode TE₀₃ mode has a mode purity of 82%, and it has lower transmission loss. In this paper, the results from theory, simulation and experiment are in good agreement. This type of mode converter is easy to prepare, which makes it an effective alternative for high frequency curvilinear waveguide mode converter.

In order to develop wide-band low-loss windows for W-band vacuum electronic devices and easily fabricate them, symmetric and asymmetric pillbox windows are investigated and reported in this paper. A symmetric pillbox window and an asymmetric pillow-box window were designed, simulation optimized, fabricated, and tested. The initial parameters for the two pillbox windows were designed by equivalent circuit theory. Computer simulation technology (CST) three-dimensional (3D) electromagnetic simulation software was used to verify and optimize the design. Because of the uncontrollability of welding during the experiment, this article provides two solutions. One is to measure and reprocess the symmetrical pillbox window with the dielectric sheet welded to reduce the influence of welding on the measurement results; the other is an asymmetrical box window which is designed to avoid the error caused by the welding of the box window. The best experimental results for the symmetric pillbox window were |S₂₁| close to 1 dB and reflection parameter |S₁₁| close to 10 dB in the frequency range of 77–110 GHz. The experimental results for the asymmetric pillbox window were |S₂₁| < 1 dB nearly in the frequency range of 76–109.5 GHz. The experimental results show that both solutions efficiently complete the design of broadband pillbox windows and would potentially be operated in the gigahertz millimeter-wave region.

A broadband sub-terahertz quasi-optical mode converter (QOMC) that converts a TE₀₆ confocal waveguide mode into a fundamental Gaussian mode has been realized and presented in this article. Simulation results predict that the QOMC could be operated in an extraordinarily broad bandwidth of 48 GHz (a relative bandwidth of 21.8% around 0.22 THz) when the output scalar Gaussian content is over 90%. Without phase correcting system, the vector Gaussian contents are larger than 75% in a frequency range of 24 GHz. In the low-power experiment, driven by a narrow-band confocal mode generator, the measurement results are consistent with the simulation results, demonstrating the feasibility of broadband operation. With the advantages of broad bandwidth, high conversion efficiency, and compact structure, this proposed QOMC can be applied to many kinds of confocal gyrotron devices.

Low-voltage compact gyrotron is under development at the University of Electronic Science and Technology of China (UESTC) for industrial applications. Due to the low operating voltage, the relativistic factor is weak, and interaction efficiency could not be high. Therefore, a magnetron-injection gun (MIG) with an extremely high-velocity ratio α (around 2.5) is selected to improve the interaction efficiency. As beam voltage drops, space charge effects become more and more obvious, thus a more detailed analysis of velocity-ratio α is significant to perform low-voltage gyrotrons, including beam voltage, beam current, modulating voltage, depression voltage, cathode magnetic field, and magnetic depression ratio. Theoretical analysis and simulation optimization are adopted to demonstrate the feasibility of an ultra-high velocity ratio, which considers the space charge effects. Based on theoretical analysis, an electron gun with a transverse to longitudinal velocity ratio 2.55 and velocity spread 9.3% is designed through simulation optimization. The working voltage and current are 10 kV and 0.46 A with cathode emission density 1 A/cm² for a 75 GHz hundreds of watts’ output power gyrotron.