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.

To explore the approach for generating high-power frequency-tunable subterahertz (sub-THz) to THz radiation, a 220-GHz high-order axial modes (HOAMs) gyrotron is developed, and the proof-of-principle experiment is presented in this paper. Experimental results demonstrate that driven by an electron beam of high voltage and high current, HOAMs are successfully excited in a long cylindrical cavity. By tuning the operating magnetic field, a mode transition between the first four axial modes was experimentally observed with a stepwise frequency tuning range of 0.79 GHz around 219 GHz, while a 0.58-GHz frequency tuning range via beam voltage tuning by operating in the first three axial modes. During the process of frequency tuning, the output power was kept not less than 0.45 kW, while the maximum output power was 3.80 kW with an output efficiency of 6%. These experimental results of mode transition and mode competition between axial modes in high-power millimeter-wave gyrotron could be beneficial for the development of frequency-tunable gyrotron toward some up-and-coming novel THz applications.

This letter presents a novel compact impedance matching layer (IML) to improve the transmission efficiency of dual-band implantable medical devices (IMDs) operating in the wireless medical telemetry services (WMTS, 1.395-1.4 GHz) and industrial scientific and medical (ISM, 2.4-2.48 GHz) bands. This IML with a simple structure enables robust impedance matching at the skin-air interface, and enhances the transmission efficiency in both bands. To verify the performance enhancement induced by the IML, we designed a dual-band implantable antenna and studied lateral/angular misalignment on the gain with and without the IML. Furthermore, the analysis of varying implant depths suggests the potential applicability of the IML for deep implant scenarios. Finally, the prototypes of the IML and implantable antenna are fabricated, and then measured in minced pork to validate the simulation results. The inclusion of the IML increased transmission efficiency by approximately 3.7 dB at 1.4 GHz and 10.3 dB at 2.45 GHz.

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.