Study Of High-Gain Millimeter-Wave And Terahertz Traveling Wave Tubes

With the development of science and technology,electromagnetic devices and systems are operating at higher and higher frequencies,and the power available from signal sources is getting lower and lower.Hence,the gain performance of power amplifiers becomes even more important.At high-frequency bands,such as millimeterwave and terahertz bands,compared to solid-state power amplifiers,traveling wave tubes(TWTs)have obvious advantages in the respects of output power and efficiency.However,high-gain high-frequency TWTs are facing considerable processing problems,as the structure and assembly process of TWTs are quite complex.In general,attenuators or severs are essential in high-gain TWTs for splitting the slow wave structure into two or more sections to suppress oscillations.However,as the frequency increases,the feasibility of this method decreases and limits the development of high-gain millimeter-wave and terahertz TWTs.In addition,high-gain planar TWTs face another problem,which is the focusing of the sheet electron beam.With a certain gain per unit length,a TWT needs a longer interaction circuit to produce higher gain.However,the sheet-beam focusing system is not easy.The longer the slow-wave structure(SWS)is,the more difficult to obtain a high beam transmission.In order to solve these problems,this dissertation conducts detailed theoretical and experimental studies of high-gain millimeter-wave and terahertz TWTs.The aim is to improve the single-section gain capacity of folded waveguide TWTs and the gain per unit length of planar TWTs.The main content and innovations of this dissertation are listed as follows:1.A novel Modified Angular Log-Periodic(MALP)folded waveguide SWS is proposed.This dissertation theoretically derived the Quasi Synchronous Region(QSR)of the slow wave structure,illuminated MALP folded waveguide SWS’s ability to suppress oscillations,and explained the reason of its capacity of high single-section gain.An example design at Ka-band is presented.The Particle-in-cell(PIC)simulation results show that the single-section MALP folded waveguide SWS can support more than 40 d B without the help of attenuators or severs which are indispensable in conventional highgain TWTs.2.A Ka-band prototype with an MALP folded waveguide SWS is experimentally studied,including simulation,engineering design,machining and processing,assembly and testing,etc.The hot-test results show that the maximum gain of the designed singlesection Ka-band MALP folded waveguide TWT exceeds 35 d B,which breaks the physical limit of 26 d B in other conventional TWTs.3.In order to improve the electronic efficiency of MALP folded waveguide TWTs,this dissertation proposed a symmetrical MALP(SMALP)folded waveguide SWS.This dissertation completed the design,fabrication,and experiments of a V-band prototype TWT with an SMALP folded waveguide SWS.While maintaining the maximum gain of about 35 d B,the V-band SMALP folded waveguide TWT has a maximum electronic efficiency of about 7.5%.With the help of 4-stage depressed collector,the overall efficiency of the V-band prototype reaches about 45%.4.In order to reduce the fabrication difficulty of single-section high-gain TWTs at terahertz band,based on the concept of QSR,this dissertation proposed a symmetrical quasi-synchronized step-varied(SQSSV)folded waveguide SWS.This dissertation analyzed its oscillation conditions and proposed the simulation design of a 650 GHz prototype TWT.In addition,the design of 650 GHz overmoded diamond pillbox window is proposed,and the mode-conversion route in the window is analyzed.The PIC simulation results show that the maximum saturated gain is about 38 d B,corresponding to an output power of 2.5 W and an electronic efficiency of 0.67%,respectively.5.In order to improve the gain per unit length of planar miniaturized meander-line SWSs.This dissertation proposed Symmetrical Ridge-Loaded Conformal Microstrip Meander-Line(SRLCMML)SWS and Rectangular Coaxial Meander-Line(RCML)SWS.They have high interaction impedance and high gain per unit length without sacrifice in bandwidth.In addition,this dissertation discussed an assembly scheme for RCML SWSs.The PIC simulation results show the gains per unit length of the designed Q-band SRLCMML TWT and W-band overmoded RCML TWT are up to 13 dB/cm and8 dB/cm,respectively.