The design method for ultrawideband amplification with low-gain fluctuation is explored for the terahertz folded-waveguide traveling-wave tube (TWT). Although the folded-waveguide circuit has a large cold bandwidth (~30%), effective amplification with a flat gain characteristic can only be achieved in a limited range due to sharply reduced interaction impedance. The situation further deteriorates in the design of a terahertz TWT where an oversized beam tunnel has to be used to facilitate the transport of the beam. Through careful analysis of the folded-waveguide (FWG) design characteristics, a modification to the geometry of the circuit has been proposed, leading to a significant enhancement in design capability. As a result, the interaction impedance is increased by 26% over a wide operation frequency band in comparison to the normal folded-waveguide circuit. This makes it possible to use the phase velocity tapering technology to increase efficiency and balance the gain at the same time. A design example in G-band is presented. The interaction circuit is capable of producing 18-W output power over 30 GHz from 202 to 232 GHz with a gain fluctuation of less than 1.5 dB.

Development of a G-band broadband continuous wave (CW) traveling wave tube (TWT) for wireless communications is described in this paper. This device provides the saturation output power over 8 W and the saturation gain over 30.5 dB with a bandwidth of 27 GHz. The maximum output power is 16 W and the bandwidth of 10 W output power is 23 GHz. The 3 dB bandwidth is greater than 12.3% of f_c (center frequency). The gain ripple is less than 10 dB in band. A pencil beam of 50 mA and 20 kV is used and a transmission ratio over 93% is realized. The intercept power of the beam is less than 70 W and the TWT is conduction cooled through mounting plate and air fan, which makes the device capable of operating in continuous wave mode. A Pierce’s electron gun and periodic permanent magnets are employed. Chemical vapor deposition diamond disc is used in the input and output radio frequency (RF) windows to minimize the loss and voltage standing wave ratios of the traveling wave tube. Double stages deeply depressed collector is used for improving the total efficiency of the device, which can be over 5.5% in band. The weight of the device is 2.5 kg, and the packaged size is 330 mm × 70 mm × 70 mm.

A modified folded waveguide (MFWG) slow wave structure (SWS), where the beam-tunnel wall in the conventional folded waveguide (CFWG) is partially removed, has been proposed for THz traveling wave tubes (TWTs). Compared with the CFWG, MFWG provides a wider operating bandwidth, lower circuit loss, and stronger longitudinal electric field, indicating higher interaction efficiency with significant bandwidth. Meanwhile, MFWG has stronger heat dissipation performance indicating high duty cycle and even continuous wave capability. Given these aforementioned advantages, the analysis, design, fabrication, assembly, and hot test of the MFWG-SWS were carried out for a TWT at G-band. With beam parameters of 22.4 kV and 58 mA, this TWT produces a maximum of 62.5W, around 217 GHz, corresponding to a saturation gain of 40.2dB. The measured 50 W power bandwidth exceeds 3 GHz. More importantly, the tube can work up to 40% duty cycle. The above experiments indicate excellent performance of the MFWG for application in THz TWTs.