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.

To reduce mode competition in a high-harmonic terahertz gyrotron, the gyrotron with a quasi-optical confocal cavity is proposed. Theoretical and experimental investigations demonstrate that the confocal cavity has good-mode selection and is suitable for high-harmonic operation. However, due to the transverse nonuniformity of the radio frequency field, the beam–wave interaction efficiency is low in the confocal cavity. To mitigate this drawback, the double confocal cavity is investigated in this study. Through three-dimensional particle-in-cell simulation on a designed 330-GHz gyrotron with a double confocal cavity, the physical mechanism of beam–wave interactions in a double confocal cavity is analyzed and presented.

To generate high-power continuous-frequency tunable sub-terahertz radiation, a quasi-optical confocal waveguide is attempted as the interaction structure of a gyrotron. In experiments, a gyrotron operated in quasi-optical TE06 mode generated a maximum output power of 20 kW when driven by a 50-kV, 3.4-A gyrating electron beam, and achieved a continuous-frequency tuning band of 205.66–208.97 GHz by adjusting the operating magnetic field. The design, experimental and particle-in-cell simulation results of this gyrotron are presented.

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.