The design, performance, and characteristics of a low-temperature microwave-driven atmospheric pressure plasma jet (MAPPJ) are presented. The MAPPJ is based on a complex coaxial transmission line structure, and the gas is discharged by a continuous-wave microwave. By inputting two channel gas through inner and outer coaxial transmission lines, respectively, the gas flow can be restricted and the plasma jet open to atmospheric air could be long and straight without quartz tube. A 28.6 mm long straight low-temperature plasma jet is observed in the experiment. This MAPPJ device shows high efficiency in a wide operation range, and a maximum efficiency of 89.6% is measured in the experiment without a microwave matching network. This approach can be an initiation toward the commercialization of low-temperature MAPPJs.

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 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.

An investigation of sterilization by a low-temperature microwave-generated atmospheric pressure plasma jet (MAPPJ) is presented. Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), and yeast Saccharomyces species (Saccharomyces sp.) at 1.0 × 10³ and 1.0 × 10⁴ cfu/mL dilutions are investigated and fungi tested experimentally. The MAPPJ is generated by several watt continuous-wave microwaves and Ar gas, and direct treatment of the bacteria by a programmable planar XY motorized translation stage. After several minutes of plasma treatment, the number of bacterial colonies was sufficiently reduced and found to be nearly zero, and bare ozone and nitrogen dioxide were observed. This investigation shows that plasma sterilization would be a novel application for solid-state microwave power sources.

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.

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.