Millimeter-wave New Twists And Turns Waveguide Slow-wave Structure

Traveling wave tubes (TWT) have significant application in radar, space communication, electronic counter measures, remote sensing and other domains as the millimeter-wave radiation sources with advantages for stability, credibility, broad band and considerable power. Slow-wave circuit, as the core part which amplifies the power of electromagnetic wave in TWT, decides its progress. Among the typical slow-wave amplifiers, power capability of the helix is not sufficient and can't be easily promoted when the frequency range come to millimeter wave though the band is broad, by contraries, the coupled-cavity produces high power with very narrow bandwidth, however, the folded waveguide (FW) not only has moderate bandwidth and power capability in the millimeter-wave frequency range, but also prevails for the simple structure, low-cost and precise fabrication, robust performance, simple input and output transitions.High power with good band performance is required as technology moving forward, but the low interaction impedance of the normal FW restricts the amplification of microwave power. Meanwhile, the gain is low per period, so the circuit length is considerably long if enough gain needed. Miniature circuit is a trend as well, if we want to obtain higher power with a shorter circuit, finding other novel structures become necessary.Based on normal FW, three kinds of novel FW are studied in this paper to satisfy the requirements above. The radio-frequency characteristics including the dispersion properties and interaction impedance are investigated by numerical simulation, the nonlinear large-signal beam-wave interaction of the novel FW TWT is also analyzed by a 3-D particle-in-cell code MAGIC3D. Compared to the normal FW, higher output power together with larger gain and electronic efficiency are predicted, good band performance and shorter circuits are promised as well. We take Ka band as an example. 1. Normal FW. The theory and research methods of radio-frequency property are introduced preparing for the study of novel FW.2. Rectangular grooves loaded FW. Thanks to the notably promoted interaction impedance, it produces more than 2 times continuous-wave power at 35 GHz with a much shorter circuit than the normal FW through simulation, together with larger gain and electronic efficiency. The band performance is quite well in above 2 GHz and broad enough for many requirements. Furthermore, the novel FW effectively improves the problem that gain of a normal FW decreases at higher working frequencies due to diminishing of interaction impedance.3. Curved grooves loaded FW. It has similar dispersion property but higher interaction impedance at 35 GHz and subsequently produces considerably higher power and efficiency with a wider 3-dB bandwidth than the normal FW through simulation. The circuit is shorter as well.4. Triangular grooves loaded FW. It notably improves the bandwidth of a normal FW, also provides higher power and efficiency, shorter circuit. The novel FW therefore promise high power, broad band and miniaturized circuit for millimeter wave TWT.