Recent hot test details and inspiring results of the W-band lossy-dielectric-loaded gyrotron traveling-wave tube (gyro-TWT) developed at the University Electronic Science and Technology of China (UESTC) are reported in this letter. Measurement of the gyro-TWT had achieved a pulse output power over 100 kW with a broad bandwidth of 8.5 GHz (92.0~100.5 GHz). For the first time as we know, the W-band gyro-TWT can stably operate with a 10% duty cycle and the average output power is over 10.3 kW in the frequency range. Excellent stability, pure spectrum and mode patterns are all verified in the hot test experiment.

In this letter, a 20-kW-average-power Ka-band TE01 mode gyro-traveling wave tube (gyro-TWT) with a cut-off waveguide section is presented and verified by hot test experiment for the first time. To improve the average output power, the new type gyro-TWT adds a cut-off waveguide in high frequency beam-wave interaction circuits. The large signal nonlinear theory calculations and particle in cell (PIC) simulations show that the new type gyro-TWT has broadband and high-power output performances. The hot test experiment shows that the measured maximum average power is 20.3 kW at 34 GHz, corresponding to a saturated peak power of 169.2 kW, a saturated gain of 52 dB, an efficiency of ~26.9%, and 3 dB bandwidth of 3.2 GHz.

A periodic dielectric meta-surface output window was cold and hot tested to extend the operating bandwidth and suppress the parasitic oscillations for a W-band wideband TE ₀₁ mode gyrotron traveling wave tube (gyro-TWT) in this letter. The fabricated beryllium oxide meta-surface dielectric window demonstrates a relative bandwidth of 37.84% with the reflection below −20 dB in the range of 75–110 GHz. The influences of engineering factors and machining errors on the performance are investigated and analyzed. Thermal analysis shows the average power handling capability of the window is over 33 kW within a wide frequency range. Hot test results demonstrate that the meta-surface window is excellent for oscillation suppression and the power handling capability is predicted larger than 20 kW.

Overall design and cold test of a G-band gyrotron traveling-wave tube (gyro-TWT) amplifier are presented. This gyro-TWT aims at achieving a goal of 10-kW pulse output power in the range of 210–220 GHz. It is operated in a circular TE ₀₁ mode at the fundamental cyclotron harmonics, which is driven by a 50-kV, 3-A gyrating electron beam. Low velocity spread diode-type magnetron injection gun (MIG), multichannel TE ₀₁ mode input coupler, broadband metasurface output window, and lossy material loaded beam–wave interaction circuit are simulated and partially measured. Particle-in-cell (PIC) simulation shows that the designed gyro-TWT can achieve a saturated output power over 10 kW in the range of 210–228 GHz with a beam velocity spread of 2.29%.

This article proposed an overmoded waveguide method (OWM) for more accurate measurement of dielectric properties in the subterahertz and terahertz band. Compared with the traditional standard waveguide method (SWM), the OWM is beneficial to alleviate the significant error caused by the air gap. Simulation predicts that for typical medium permittivity and loss materials, the relative errors of $\varepsilon _{r}$ and tan $\delta $ can be reduced from 10% to below 3% using the OWM with an overmoded factor of 3 when the air gap is 0.02 mm at W-band. When extending the overmoded factor over 5, the relative error can be reduced below 1.5%. The tolerance improvement was verified by measuring a polytetrafluoroethylene (PTFE) sample through SWM and OWM. Experimental results show the relative error of $\varepsilon _{r}$ is lower than 5% even when there exists a 0.24-mm air gap in the OWM.

In this article, a broadband W-band TE□₁₀-TE◯₀₂ mode converter is proposed and verified. To improve the output performance of the TE□₁₀-TE◯₀₂ mode converter, a TE□₁₀-to-TE□₄₀ intermediate mode conversion section with a Y-type power divider structure and a TEx₄₄-like-to-TE◯₀₂ intermediate mode conversion section with a novel cross-sector-shaped transition structure are adopted in the design (the superscript symbol □ represents the rectangular waveguide, ◯ represents the circular waveguide, x represents the cross waveguide). The simulation results show that the transmission coefficient (S₂₁ parameter) of the designed TE◯₀₂ mode converter is better than -0.18 dB in the whole W-band (75-110 GHz), and the corresponding mode conversion efficiency is larger than 96%. Cold test results show that the transmission coefficient is higher than -1.9 dB and the reflection coefficient (S₁₁ parameter) is less than -15 dB in the frequency range of 75-110 GHz.