Development of a Dual Beam-based Traveling Wave Tube at E-band

Abstract

In the realm of millimeter-wave (mm-wave) frequencies, the Traveling Weave Tube (TWT) emerges as a crucial vacuum electronic device (VED) renowned for its capability to produce high- power output within a compact system. Research endeavors have concentrated on developing diverse TWT systems for specific applications. In the pursuit of advancing functionality and compactness, a multibeam-based TWT system has been proposed to amplify multiple RF sources simultaneously within a small system. However, the practical realization of such a system poses notable experimental challenges. In the multibeam-based TWT system, the electron beam is inevitably positioned away from the manet’s center. As a result, the electron beam is no longer free from the radial magnetic field. This radial magnetic field disrupts the motion of the electron beam, resulting in a deterioration in its quality. Specifically, the electron beam deviates from its initial trajectory, leading to a decrease in its transmission through the slow-wave structure (SWS). This decrease in beam transmission substantially diminishes the efficiency of the TWT. Furthermore, the radial magnetic field lowers the axial velocity of electrons while increasing the spread in velocity and the beam emittance, complicating its matching with the SWS for effective interaction between the radio frequency (RF) wave and electron beam. These alterations in the properties of the electron beam also contribute to the decline in TWT efficiency. This paper presents the design and development of a dual beam-baed TWT system tailored for operation in the 81 – 87 GHz frequency range. All the related components for the dual beam-based TWT system including the dual-beam electron gun, folded waveguide (FWG) structure SWS, solenoid magnet, RF window, vacuum envelope, etc., were meticulously designed, manufactured, and subjected to testing. In the design process, the influence of the radial magnetic field on the electron beam was examined using the tracking solver of computer simulation technology (CST). Subsequently, the resulting impact on the TWT efficiency was analyzed utilizing the particle-in-cell (PIC) solver of CST. To address the decrease in beam transmission caused by the radial magnetic field, the beam tunnel position of the TWT circuit was adjusted by 0.19 mm based on the CST simulation results. This adjustment significantly improved beam transmission from 16.7 % to 66.9 % and increased the gain of the TWT system from 17.92 dB to 29.26 dB. To proceed with the amplification test based on this design, when optimal alignment with the solenoid magnet was achieved, beam transmission of the circuit without considering the radial magnetic field was about 3 %. By comparison, when accounting for the radial magnetic field, beam transmission increased to approximately 10 %. These results show the considerable challenge of aligning the dual beam-based TWT, exceeding expectations. Upon conducting the amplification test using the optimized TWT circuit with an 11 keV electron beam, the dual beam-based TWT achieves an output power of 1047 mW at 81.5 GHz, with a corresponding gain of 13.39 dB. Compared to the CST simulation results with similar conditions, while the center frequency has changed slightly due to the fabrication error, the gain closely resembles those of the simulation. Moreover, when the experiment was conducted with increased input power, it was observed that the output power also increased. The dual beam-based TWT described in this paper is anticipated to offer valuable insights into the obstacles presented by the radial magnetic field, thereby offering guidance for optimizing multibeam-based TWT systems.