The Langmuir probe is a feasible method to measure plasma parameters. However, as the reaction progresses in the discharged plasma, the contamination would be attached to the probe surface and lead to a higher incorrect electron temperature. Then, the electron density cannot be obtained. This paper reports a simple approach to combining the Langmuir probe and the optical emission spectrometry (OES), which can be used to obtain the electron temperature to solve this problem. Even the Langmuir probe is contaminative, the probe current–voltage (I–V) curve with the OES spectra also gives the approximate electron temperature and density. A homemade coaxial line microwave plasma source driven by a 2.45 GHz magnetron was adopted to verify this mothed, and the electron temperature and density in different pressure (40–80 Pa) and microwave power (400–800 W) were measured to verify that it is feasible.

A dual-frequency plasma source has many advantages in applications. In this paper, a dual-frequency microwave plasma source is presented. This microwave plasma source is based on a coaxial transmission line without the resonator, and it can be operated in a wide band frequency region. Two microwaves are inputted from two ports into the plasma reactor: one is used firstly to excite the plasma and the other one is used to adjust plasma characteristics. Based on the COMSOL Multiphysics simulation, the experiment is carried out. In the experimental investigation, the plasma electron density and electron temperature can be controlled, respectively, by feeding in different frequencies from the second port, causing the particles at different energy levels to present different frequencies. This exploratory research improves the operation frequency of dual-frequency microwave plasma sources from RF to microwave.

All-dielectric metamaterials (ADMs) have recently attracted much attention due to their ability to achieve wavefront control with high power capability. In this article, an investigation on quasi-optical mode converter (QOC) based on ADM for high-order vacuum electronics devices (VEDs) is presented. Specifically, four ADM QOCs are designed to convert the cylindrical waveguide TE01, TE21, TE31, and TM01 modes into linear polarized Gaussian beams at 35 GHz. Electromagnetic simulations show that the Gaussian mode contents of the output waves from four mode converters are all over 94% with high polarization contents. Furthermore, a Ka-band ADM mode converter with the input mode of circular waveguide TE01 mode is fabricated and measured. Experimental results show that the output fields with scalar Gaussian content over 90% can be obtained at 30–38.85 GHz, corresponding to a relative bandwidth of 25.3% around 35 GHz. The maximum scalar Gaussian mode content of 99.18% is observed at 34.56 GHz in the experiment. The investigation opens new avenues toward high-order mode conversion technology based on ADM.

This paper investigates a 220 GHz quasi-optical antenna for millimeter-wave wireless power transmission. The quasi-optical antenna consists of an offset dual reflector, and fed by a Gaussian beam that is based on the output characteristics of a high-power millimeter-wave radiation source-gyrotron. The design parameter is carried on by a numerical code based on geometric optics and vector diffraction theory. To realize long-distance wireless energy transmission, the divergence angle of the output beam must be reduced. Electromagnetic simulation results show that the divergence angle of the output beam of the 5.6 mm Gaussian feed source has been significantly reduced by the designed quasi-optical antenna. The far-field divergence angle of the quasi-optical antenna in the E plane and H plane is 1.0596° and 1.0639°, respectively. The Gaussian scalar purity in the farthest observation field (x = 1000 m) is 99.86%. Thus, the quasi-optical antenna can transmit a Gaussian beam over long-distance and could be used for millimeter-wave wireless power transmission.

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.

A dynamically reconfigurable, polarization-insensitive terahertz (THz) metamaterial absorber based on phase-change vanadium dioxide (VO2) is proposed and numerically investigated by full-wave simulations (CST Studio Suite). The absorber adopts a simple three-layer configuration: a patterned VO2 resonant layer, a Topas dielectric spacer, and a continuous Au ground plane. VO2 is chosen because its reversible insulator-to-metal transition near 68 °C changes the conductivity by about three orders of magnitude, enabling continuous, geometry-independent tuning of the absorption. In the metallic state, the patterned VO2 layer behaves as a lossy resonant meta-layer: its moderate conductivity supplies the ohmic loss required for near-perfect impedance matching, while the multiple closely spaced resonances of the four sector-shaped elements overlap, merging into a continuous absorption window exceeding 90% from 6.55 to 17.55 THz (absolute bandwidth 11 THz, relative bandwidth 91.3%). By tuning the conductivity of VO2 from 200 to 2 × 105 S/m, the absorption can be continuously modulated from 2.33% to 99.96%, and the rotationally symmetric geometry ensures polarization-insensitive, wide-angle absorption (stable up to 50°). Owing to its ultrabroadband absorption, wide tuning range, polarization insensitivity, and wide-angle stability, the proposed absorber is expected to serve as a promising platform for prospective THz applications, such as electromagnetic shielding, modulation, and sensing.