| As a highly regarded terahertz radiation source,the traveling-wave tube（TWT）has been widely used in many important fields such as communications,radar,high-precision imaging,and biomedical applications,demonstrating its unique technical advantages and broad market prospects.Among many kinds of TWTs,the folded type all-metal slow wave structure（SWS）has a great prospect in terahertz because of the features of all-metal and the benefits of combing the broadband of the helix TWT with the high-power capacity of the coupled cavity TWT to a certain extent.The SWS,which is the main component of the TWT,will experience issues with lower coupling impedance and challenges in fabrication realization with the increase in operating frequency.In this paper,the folded-type SWS in the THz band are studied,including four novel SWSs such as circular beam quasi-sine waveguide（Q-SWG）and sheet beam Q-SWG that can be fabricated in the terahertz band by Nano-CNC technology,sine-folded waveguide（S-FWG）that can improve the output power in the terahertz band,and tunnel-opening folded waveguide（TO-FWG）that can expand the bandwidth.In addition,a novel multi-beam SWG with multi-beam and high-order mode operation is studied.The main work and innovations of this dissertation are given as follows:1.A SWS called Q-SWG that is appropriate for circular electron beam is proposed to address the issue that conventional SWG cannot be manufactured in the terahertz band with Nano-CNC（nano Computer Numerical Control）technology.The design of the structure makes it possible to be processed in the terahertz band by the Nano-CNC method.The high frequency and transmission characteristics of the structure are investigated,machining and cold measurement experiments are carried out.Only 0.61 dB separates the value of S21obtained from the test from the simulation result at 340 GHz,the experimental results agree well with the simulation results,which demonstrates the structure’s viability.The results of beam-wave interaction show that the maximum output power of the TWT based on the Q-SWG SWS can reach 17.4 W,and the bandwidth of 3 dB is about 11 GHz.In addition,the matching electronic optical system and pillbox window are designed,and the pillbox window has been fabricated assembled,welded.The tested S11 is less than-15 dB in the frequency range of 335～350 GHz,and S21at the 340 GHz frequency is about-2.5 dB.2.A S-FWG SWS is proposed,which makes it simpler to alter the dispersion properties by adjusting the sinusoidal height than the conventional FWG,in order to increase the coupling impedance,output power,and gain of the FWG SWS.The results show that the coupling impedance of a S-FWG is about 18%higher than that of a conventional FWG.According to the comparison data,under the same working conditions,the output power of sinusoidal TWT is about twice that of conventional TWT,and the saturation output power of TWT is about 16%higher than that of conventional TWT.The input power required to reach saturation is only 29%of the conventional FWG.Therefore,the S-FWG can significantly improve the output power and gain.In addition,a corresponding electronic optical system for the S-FWG SWS is designed.3.It would most likely cause increase of reflection near the operating frequency if the conventional FWG SWS has machining errors,which affects the performance of the TWT and limits the operating bandwidth.A TO-FWG SWS is suggested to avoid the risk and broaden the bandwidth of the TWT while ensuring output power.According to an analysis of its dispersion and transmission characteristics and a comparison with conventional FWG SWS,it can be found that the TO-FWG SWS can effectively avoid the risk of reflection near the operating frequency caused by incomplete symmetry.The beam-wave interaction results show that the 3 dB bandwidth of the TO-FWG TWT can reach 24 GHz,which is significantly broader than that of the conventional FWG TWT,and the output power is enhanced by about 10%.The slow wave structure has been fabricated.The test results show that the machining errors are all within the tolerance range.The tested S11 is less than-15 dB in the frequency range of 310～372 GHz,which meets the requirements of the TWT.4.A Q-SWG SWS suitable for sheet beam is proposed.The proposed structure has a bigger input current at the same current density compared with the circular electron beam Q-SWG SWS suggested in the previous chapter because of its larger beam-wave interaction region.The beam-wave interaction characteristics of high frequency system are studied.The results show that the output power of the sheet beam Q-SWG is about1.8 times that of the circular beam Q-SWG when the operating voltage and current density are the same.While the current density is only 0.75 times that of the circular beam Q-SWG and the output power of the circular beam-wave waveguide can be obtained.In addition,an electron-optical system that is compatible with the high frequency system of sheet beam Q-SWG is designed.5.The paralleling of TWT can improve the power level effectively,while the independence of each circuit may cause a phase difference between circuits,which greatly affects the power combining of TWT.To avoid the phase difference,a multi-beam SWG SWS is proposed,and its dispersion characteristics are calculated and compared with the single-beam SWG SWS.A TE10-TE30 mode converter for signal input is designed,which can be matched with the slow-wave structure.The transmission characteristics and beam-wave interaction characteristics are simulated.The power capacity of the multi-beam SWG TWT is obviously improved,and the output power of the TWT is about three times that of the single-beam SWG TWT.In addition,a multi-beam electron gun that can generate three electron beams is designed to match the high frequency system. |
