Research On Electron-optical System And Slow-wave Structure For 0.22 THz Traveling-wave Tubes

Terahertz（THz）technology has a wide application prospects in the fields such as high-speed communication,high-resolution radar,imaging,and material analysis.The THz sources are the core electron devices of THz technology with important research value.As one of the solutions for THz source,traveling-wave tube（TWT）is a linear beam vacuum electron device that uses the traveling-wave field to interact with the electron beam to exchange energy,which has the advantages of high power,wide bandwidth,high gain,and high efficiency.With the increasingly urgent demand for THz sources,THz TWTs have received a lot of attention in recent years.However,when the operating frequency of the TWTs is raised to the THz band,many difficulties restrict the development of THz TWTs,such as:（1）The key dimensions of the devices are drastically reduced,and the machining accuracy and assembly consistency of small-sized components are more difficult to be guaranteed;（2）The reduction of the electron beam tunnel leads to weakened transport capacity of electron beam current,which makes the devices extremely difficult to achieve high output power;（3）The skinning depth decreases and the slow-wave structure（SWS）loss increases.Compared with the pencil beam,the sheet beam shows a larger cross-sectional size and can transport larger beam current with the same current density,which is more helpful in terms of power enhancement of the TWTs.A lot of work has been carried out around THz TWTs,especially the 0.22 THz TWTs whose operating frequency is located in one of the atmospheric windows,and some progress has been made.However,the THz TWTs with sheet beam are not yet fully mature,and still need to be further studied in the depth.In order to further develop THz sheet beam TWTs,it is necessary to carry out research on THz sheet beam electron-optical systems（EOSs）and SWSs.Based on this,this dissertation investigates the EOS and SWS for the 0.22 THz sheet beam TWT utilizing theory,simulation,and experiment.The main research and innovation points of this dissertation are as follows.1.The simulation and experimental study of the sheet beam EOS for 0.22 THz TWT were carried out.The simulation designs of the high-current sheet beam electron guns for0.22 THz TWT are carried out,and the technical solutions of the sheet beam electron guns using elliptical and circular cathodes are given,and the emission performance tests of the elliptical and circular cathodes and the experimental verification of the sheet beam electron gun are completed.Based on the results of the above sheet beam electron gun,a novel tunable periodic cusped magnetic（PCM）focusing configuration is proposed for sheet beam focusing,and the simulation results show that the stable transport distance of sheet beam exceeds 60 mm.In addition,the experimental validation of the novel tunable PCM focusing configuration is carried out,and the test results of the axial magnetic field are in good agreement with the simulation results.Finally,the experimental verification of the EOS using the technology scheme of sheet beam electron gun with circular cathode is conducted,and the test results show that the beam current and transmission reached115 m A and 92.1%（at beam voltage of 22 k V）,respectively.2.A modified staggered double-vane SWS with wideband,high interaction impedance,and low transmission loss is proposed for 0.22 THz sheet beam TWT.The high frequency characteristics of the single-period modified staggered double-vane SWS are simulated,and the calculation results show that the average interaction impedance of the first spatial harmonic of the fundamental mode is about 10%higher than that of the conventional staggered double-vane SWS with the same parameters.Also,the simulation of the transmission characteristics for the modified staggered double-vane SWS with 85periods is performed,and the calculated transmission coefficient S₂₁ in the frequency band ranges from-4.7 to-3.9 d B,which is～0.5 d B better than that of the conventional staggered double-vane SWS.The machining,assembly,and verification of transmission characteristics of the 0.22 THz one-section modified staggered double-vane SWS are completed,and the measured transmission loss is less than 0.79 d B/cm in the frequency range of 0.211～0.26 THz,which is in good agreement with the simulation results.3.To develop high power and compact terahertz sheet beam TWT,an all-metal metamaterial-inspired SWS is proposed.This metamaterial-inspired SWS exhibits obvious advantages such as high interaction impedance,double beam tunnels,and compactness.The calculated average interaction impedance of the SWS for the first spatial harmonic of the fundamental mode is over 5Ωin the frequency range of 0.21-0.226 THz.Further,the beam-wave interaction of the metamaterial-inspired 0.22 THz TWT with double sheet beams is simulated.The maximum saturated output power is predicted to be 400 W with 3 d B bandwidth of 5.4 GHz when the input power is 100 m W.Most importantly,the total length of the interaction structure of the 0.22 THz TWT is only29.2 mm.The proposed metamaterial-inspired SWS provides an idea for novel high-power and compact 0.22 THz TWTs.4.A 0.22 THz sheet beam TWT using two-section modified staggered double-vane SWS with attenuators is proposed.The simulations of EOS,transmission and beam-wave interaction characteristics of the TWT are completed.A machining,assembly,and welding scheme for a 0.22 THz two-section modified staggered double-vane SWS with attenuator is presented,and experimental verification of transmission and reflection is carried out.The cold test results of the 0.22 THz two-section modified staggered double-vane SWS with an attenuator showed that in the frequency range of 0.202～0.26 THz,the reflection coefficient S₁₁ and transmission coefficient S₂₁ are less than-11 d B and-40 d B,respectively,which are closer to the simulated results.In addition,the cold tests are carried out on the 0.22 THz two-section modified staggered double-vane SWS with attenuators and input-output couplers.The results are as follows:（1）input port:the voltage standing wave ratio（VSWR）measured at 0.213～0.247 THz is less than 1.5;（2）output port:the VSWR in the frequency of 0.214～0.243 THz is below 1.5.