A slotted piecewise sine waveguide (SPWSWG) is suggested as the slow wave structure (SWS) to develop the terahertz traveling wave tube (TWT). This letter is the first to report the SPWSWG SWS with the advantages of flat dispersion and large coupling impedance from our calculation. Based on this novel SWS, a wideband 220-GHz TWT has been designed, fabricated and tested. The cold test results show that the reflection parameters at the input and output port of the TWT are less than -15.2 dB in the frequency range of 209-235 GHz. Moreover, the hot test results demonstrate that the maximum saturated output power reaches 28.6 W at 215 GHz and the 3-dB bandwidth exceeds 15.5 GHz, which are basically consistent with the simulation results.

In this paper, a piecewise sine waveguide (PWSWG) is proposed as the slow-wave structure (SWS) to develop high-power terahertz (THz) traveling wave tubes (TWTs). The PWSWG is an improvement over the rectangular waveguide wherein its two E-planes simultaneously oscillate up and down along the longitudinal direction. The oscillation curve in the H-plane is a piecewise sine curve formed by inserting line segments into the peaks and troughs of the sine curve. The simulation analysis and experimental verification show that the PWSWG offers the advantages of large interaction impedance and excellent electromagnetic transmission performance. Furthermore, the calculation results of beam–wave interaction show that the TWT based on PWSWG SWS can generate a radiated power of 253.1 W at the typical frequency of 220 GHz, corresponding to a gain of 37.04 dB and an interaction efficiency of 6.92%. Compared with the conventional SWG TWTs, the PWSWG TWT has higher interaction efficiency and shorter saturation tube length. In conclusion, the PWSWG proposed in this paper can be considered a suitable SWS for high-power THz radiation sources.

In this article, we propose a low reflection folded waveguide (LRFWG) as the slow wave structure (SWS) for the development of a terahertz (THz) traveling-wave tube (TWT). Our simulation results show that the LRFWG SWS has lower reflection and larger coupling impedance compared to the conventional folded waveguide (FWG) SWS over a wide frequency range. Using this modified SWS, we have designed, simulated, and manufactured a wide-band high-power TWT operating in the G-band. The cold test results demonstrate that the reflection parameters at the input and output ports of the TWT are less than -13.8 dB from 200 to 240 GHz, confirming the effectiveness of our proposed structure. In addition, the hot test results show that the maximum output power of the 220 GHz LRFWG TWT amplifier reaches 31.2 W at 220 GHz, with a 3 dB bandwidth exceeding 23 GHz. The power-bandwidth product of the device is greater than 717.6 W $\cdot $ GHz, providing a demonstration of a wideband high power THz source.

A double flat-roofed sine waveguide (DFRSWG) slow wave structure (SWS) with low loss for 220-GHz traveling-wave tube (TWT) has been demonstrated in this letter. Through the simulation method, it is inferred that this structure has a large coupling impedance and excellent transmission properties. Besides, the experimental results demonstrate that the transmission loss of the circuit is only 0.62–0.75 dB/cm and the return loss is more than 19.3 dB in the frequency band of 210–250 GHz. Eventually, a two-section DFRSWG interaction circuit is designed to achieve stable device performance. Based on the particle-in-cell (PIC) simulation results, it can be predicted that the DFRSWG TWT can generate the maximum output power of 235 W at 220 GHz, which corresponds to a gain of 36.72 dB and an electronic efficiency of 6.06%.

In this article, the design and test results of an ${X}$ -band high-efficiency long-pulse gyrotron traveling wave tube (gyro-TWT) based on a lossy ceramic-loaded high-frequency (HF) circuit with an operation mode of TE11 are presented. In the design, a pill-box input window, a magnetron injection gun (MIG), a room-temperature coil magnet, and a double-disk output window are adopted. A single-stage depressed collector (SSDC) is applied to enhance the efficiency of this amplifier and to reduce the dissipated beam power. In the hot test, with the beam parameter of 70 kV and 5 A, and the depressed voltage of 20 kV, this ${X}$ -band gyro-TWT produces 104 kW, 1-s long-pulse radiations at 7.8 GHz, corresponding to a saturation gain of 32.2 dB and a maximum overall efficiency of 41.6%, the measured −3-dB power bandwidth exceeds 0.4 GHz.