Simulation of microfabricated folded waveguide traveling-wave tube as broadband terahertz amplifier

In this paper, electromagnetic field simulations and related results of a 400 GHz central frequency microfabricated folded waveguide traveling-wave tube were presented, for a broadband terahertz amplifier. 3-D cold-test (absence of real electron beam) and particle-in-cell simulations were carried out. Two models of different beam tunnel cross-section, i.e. cylindrical and rectangular, were simulated. It was found that the rectangular tunnel model is similar to the cylindrical tunnel model in cold-test properties, when the transverse square side length is 85% to the diameter of the cylindrical tunnel. The three-dB bandwidth is 40.6 GHz and the maximum gain is 15.8 dB at 393.3 GHz, for an 18.5 mm-long (60 periods) loss-free circuit, predicted by Pierce small signal theory. Particle-in-cell simulation also revealed the three-dB bandwidth of ~35 GHz and ~30 GHz for cylindrical-tunnel model and rectangular-tunnel model, respectively, for a 29.6 mm-long (96 periods) loss-free circuit with concentrated attenuator. It is also found that the gain of rectangular-tunnel model is about 5 dB lower than that of cylindrical-tunnel model, for 29.6 mm-long circuit. The loss-free saturated output power and the linear gain of cylindrical-tunnel model at 400 GHz are 6.9 W and 28 dB, respectively.