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An innovative complementary electric split-ring resonator metamaterial (MTM) structure applied as the slow-wave circuit for a cascade backward-wave oscillator (CBWO) operating in C-band is studied in this paper. The idea of a drift tube in a multiresonant cavity extended interaction klystron is borrowed to design a novel backward-wave oscillator (BWO). The construction of this device features two BWOs separated by a short cutoff waveguide for permitting the flow of the electron beam and stopping the electromagnetic wave. The high-frequency characteristics are analyzed and optimized by using a high-frequency structure simulator and computer simulation technology (CST). Meanwhile, the S-parameter retrieval approach is used to retrieve the effective permittivity and permeability. In addition, the CST code is adopted to investigate the performance of the MTM-based CBWO. The particle-in-cell simulation results show that the novel CBWO is capable of achieving over 51.77% electronic efficiency from 4.8344 to 4.8687 GHz. Meanwhile, the maximum electronic efficiency can reach 82.44%, corresponding to a peak output power of 14.51 MW at 4.8466 GHz. These results indicate that the MTM-based CBWO proposed in this paper has the characteristic of miniaturization, manufacturability, and high electronic efficiency.

3-D printed Ka-band sine waveguide (SWG) slow wave structures (SWSs) are presented in this paper. A kinds of 3-D printing technologies, rapid investment casting (RIC) with brass, have been considered. The VSWR of the assembled 3-D printed brass SWG SWS is less than 1.37, and the dissipative attenuation ranges between 0.025 and 0.054 dB/mm at 33-40 GHz. The 3-D printing technologies appear as a promising fabrication method for vacuum electron devices fabrication with the advantages of low-cost and rapid-prototyping compared with traditional machining processes.