Study On Novel Folded Type Of Slow-wave Structure For Millimeter Wave Traveling Wave Tube

As a kind of significant vacuum device, millimeter wave traveling wave tube(TWT) has some advantages, such as wide bandwidth, large output power and small volume. It can be applied in microwave communication, radar, space detection, electromagnetic countermeasure(ECM) and so on. Slow-wave structure(SWS) is the core component of the TWT and largely determines the performance of the TWT, so the research and exploration on the SWS are very important. Traditional helix and coupling cavity SWS have been widely used in millimeter wave TWTs, but the contradiction between the power capacity and the operation bandwidth limits their development. As a novel all-metal SWS, folded waveguide(FWG), which has larger power capacity and relatively wide bandwidth, can overcome the disadvantages of the helix and coupling cavity. So the FWG becomes the research hot spot in the worldwide and has bright application prospects in millimeter wave, sub-millimeter wave and even terahertz frequency range.In this dissertation, several novel kinds of folded SWS are investigated based on the design and experiment of the W-band FWG TWT. With the combination methods of theory analysis, software simulation and experimental test, deep researches on these novel SWS are carried out. The main work and innovations of this dissertation are concluded as follow:1. To overcome the disadvantage of the lower coupling impedance of the normal FWG SWS, a novel ridge-loaded FWG SWS is employed to design a W-band continuous wave TWT. The radio-frequency characteristics and the large signal beam-wave interaction are simulated. The simulation results reveal that compared to the normal FWG SWS, the coupling impedances of the ridge-loaded FWG SWS are 20% larger and the output power of the TWT are 12.8% larger within the bandwidth. Simultaneously, the saturated length of the ridge-loaded FWG SWS is shorten about 50%. These results lay a foundation for the development of the large power miniaturized millimeter wave TWT.2. Based on the theory analysis and the simulation results above, the components used for W-band ridge-loaded FWG TWT is manufactured and tested. The simulation results indicate that the VSWR of each component and entire model can meet the requirement of the TWT fabrication. By breaking through some key technologies on the fabrication and assembly, a W-band ridge-loaded FWG continuous wave(CW) TWT is designed and manufactured. The experiment test results indicate that the CW output power of the TWT is larger than 25 W within the operation frequency bandwidth from 93.5 GHz to 95 GHz, which is leading in our country.3. A novel tapered ridge-loaded FWG SWS is proposed to overcome the disadvantage of the high reflection loss of the ridge-loaded FWG SWS. The simulation results indicate that compared to the ridge-loaded FWG SWS, the S11 parameters of the 40 period tapered ridge-loaded FWG SWS are 2dB lower and the S21 parameters are 0.4dB larger. The W-band and 140 GHz tapered ridge-loaded FWG TWTs are designed and simulated. The simulation results reveal that the large reflection loss is reduced and the fluctuation of the output power is well depressed. As a result, the operation stability of the TWT is well improved.4. Based on the E-field distribution characteristics of the FWG SWS, the FWG SWS with sheet electron beam is deeply investigated and an elliptical sheet beam is employed in the FWG TWT. The simulation results reveal that the FWG TWT with the elliptical sheet beam can make full use of relatively large electric fields and can improve the input power with the same current density. As a result, the output power and electron efficiency are improved. Then, a W-band FWG TWT with elliptical sheet beam is designed and simulated. Also, with the same beam current, the elliptical sheet beam can effectively reduce the current density of the TWT.5. To overcome the fabrication problems of the small beam tunnel of the FWG SWS in the high frequency range, a novel folded frame SWS with natural sheet beam tunnel is proposed motivated by the structure characteristics of the symmetric double V-shaped microstrip meander-line SWS. The simulation results of the radio frequency characteristics indicate that compared to the symmetric double V-shaped microstrip meander-line SWS, the average coupling impedances of the folded frame SWS are 25% larger on W-band and 20% larger on 140 GHz. Then, the W-band and 140 GHz folded frame TWTs are designed. The large signal interaction simulation results indicate the large improvement of the output power and electron efficiency. Simultaneously, the folded frame TWT can maintain the advantage of wide bandwidth. These results lay a foundation for the research of the millimeter wave TWT with large power and wide bandwidth.