A sheet beam traveling-wave tube (TWT) amplifier focused by periodically cusped-magnetic (PCM) fields has been successfully developed. The amplifier produces ~115 W of peak power at 212.4 GHz with a voltage of 25.8 kV. The output power is more than 100 W from 210.2 to 213.6 GHz. The measured transport current is about 106 mA accounting for 81.5% of the total current. The high-efficiency circuit design, along with the novel coupling structure, makes it possible to greatly reduce the required current, enabling the use of the PCM focusing. As a result, large power and a wide band have been achieved in a compact volume with an overall weight of less than 1.8 kg. The results demonstrate a significant breakthrough in developing the sheet-beam amplifier at terahertz frequencies. Although a single-section circuit without the attenuator is used at this stage, we have verified all the key technologies. The measured results are in good agreement with the design.

A novel scheme is proposed for the enhancement of the gain and output power of terahertz (THz) traveling-wave tube (TWT) amplifiers. The scheme combines a slow-wave structure (SWS) and multigap resonant cavities, resulting in a complementary hybrid of traveling-wave and standing-wave elements, thus offering a compact circuit length. The idea has been verified by a specific design of a 220-GHz TWT amplifier by means of particle-in-cell (PIC) simulations. For an electron beam with a voltage of 21 kV and a current of 200 mA, a maximum gain of 42.22 dB and a 3-dB bandwidth of 6.5 GHz has been achieved while the overall circuit length is only 38 mm. For an input power of 11.25 mW, the CW peak power is predicted to be 187.5 W at 220 GHz, corresponding to an electronic efficiency of 4.47%. These simulation results predict a performance that well exceeds that reported for conventional TWTs, along with a significant reduction of the input power and circuit length.

The focusing and transmission of a sheet electron beam with periodic cusped magnetic (PCM) fields were studied for the terahertz traveling-wave tubes (TWT). Due to the limitation on the width of the sheet beam from the slow wave structure (SWS), the resulting transmission current density is inevitably high and the width of the sheet beam is highly disproportionate to the magnet system. This defines a situation different from before, which challenges the validity of the traditional PCM scheme. Thus, the focusing of a sheet beam in the above scenario was theoretically analyzed to evaluate the focusing conditions. A practical magnetic field system was then designed based on the offset-pole-piece PCM scheme with the shortened pole pieces, which can meet the focusing requirements, especially on the side focusing. The theoretical analysis and the modified PCM scheme lead to a significant improvement in the focusing/transmission of the sheet beam. The 3-D particle simulation showed that a stable long-distance transmission over 100 mm can be achieved with the two-plane PCM focusing where an elliptical-cross section sheet beam is assumed with dimensions of 0.6 mm ×0.11 mm, voltage of 24.5 kV, and current of 0.18 A.

To gain higher power with a lower cathode current density and a simpler structure, a novel rectangular beam folded waveguide traveling-wave tube (RB-FW-TWT) operating at 220 GHz is proposed and analyzed in this paper and compared with the normal circular beam (CB) FW TWT. The dispersion characteristic was investigated based on an equivalent circuit model. The interaction impedance and the S-parameter of a RB-FW traveling-wave tube were analyzed by numerical simulations. A 3-D particle-in-cell code CST particle studio was introduced to analyze the performance of RB-FW TWT. The influence of the initial electron energy, frequency, input power, guiding magnetic field, and aspect ratio of the RB tunnel on the circuit performance was observed, and physical explanations were given. It reveals that the output power of RB-FW can reach 51.1 dBm, 8.4% dBm higher than a CB-FW, with a 3 dB bandwidth of 35 GHz when the beam voltage and current are set to 14.25 kV and 140 mA, respectively. An aspect ratio of 0.4 is recommended to optimize the high-frequency properties and circuit performance.

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.

The successful transport of a sheet electron beam under the periodic cusped magnet (PCM) focusing at the terahertz frequencies is reported. The sheet beam with a current density of 285 A/cm² is intended for the developing G-band sheet-beam traveling-wave tube (TWT) whose operating voltage is nominally 24.5 kV. A beamstick was developed to validate the design of the electron optics system, which is considered as the most challenging part for developing a sheet-beam device. A beam transmission ratio of 81% is achieved over a distance of 37.5 mm at a cathode voltage of −25.0 kV. The total current and the collector current were measured to be 125 and 102 mA, respectively. The experimental results are promising, demonstrating that the PCM scheme is capable of focusing a high-current-density sheet beam and hence can find use in the terahertz TWTs, offering the advantages of compact size and light weight.