| The W-band traveling wave tube(TWT),operating at the atmosphere window of millimeter-wave,features wide frequency band,narrow beam and low loss.It is therefore known that such device has potential applications in the fields of high-resolution imaging,precision detection and high-speed communication.As a key device for the above application systems,the W-band TWTs provide the amplified signal for the transmitter of the systems,whose operating distances rely on the output power of the TWTs.In order to meet the requirement for long-range precision detection and high-resolution imaging,it is in an urgent need to improve performance of the existing W-band TWT,in particular its output power.This thesis describes studies on increasing the output power of a W-band folding waveguide(FWG)TWT,including theoretical analysis,circuit design and experimental study to provide some techniques for the design and development of novel millimeter wave and terahertz TWTs with substantially enhanced performance.The main research contents and achievements are follows:1.An ear-shaped cavity structure of FWG slow wave structure has been proposed.By adding an ear-shaped cavity structure at right angle position of the FWG,the axial coupling impedance of the FWG slow wave structure was increased by 20%,due to changes in the field strength distribution and electromagnetic wave transmission path at the waveguide connection.The theoretical analysis and calculation of the ear-shaped FWG were carried out by combining the equivalent circuit and software simulation,while the influence of the ear-shaped structure on the electromagnetic characteristics of the slow wave circuit was analyzed separately based on the 3D simulation software.Based on this novel FWG slow wave structure,the W-band TWT interaction circuit was designed and fabricated,showing an output power>200W in the bandwidth from 90.1GHz to 98.4GHz(maximum of 278W).The existing issue of low coupling impedance for the FWG slow wave structure has been overcome to a great extent.2.The phase velocity stepping technique of the FWG slow wave structure circuit was studied.The resulting output power and electronic efficiency of the W-band TWT were substantially improved by using the positive and negative phase velocity stepping technique,with the output power of 250W in 8GHz bandwidth,with the electronic efficiency maximum of 10%at 92.9 GHz.A new method of combining the non-simi-circular bending(NSCB)FWG with the phase velocity stepping was proposed and applied to the W-band TWT for the first time.Based on the effect of NSCB FWG’s inner arc and outer arc on the tuning of phase velocity,the FWG interaction circuit with phase velocity stepping was designed.The resulting fabricated TWT shows that the output power and electronic efficiency reach 647W and 13.4%,respectively.3.To analyze the power and spectrum performance of the TWTs’sever region,a four-port structure was proposed.The TWT with the four-port structure was developed successfully,with the power and spectrum characteristic at the sever region at each frequency point obtained and studied.For the improvement on the total efficiency of W-band TWTs,theoretical analysis and experimental research of multistage depressed collectors were conducted.The W-band TWT with the three-stage depressed collector was developed,attaining an improved efficiency of40.2%(maximum of 47.2%)in 5GHz bandwidth.4.A bidirectional bending FWG slow wave structure with relatively smaller size and higher coupling impedance was proposed.To further improve the TWT’s output power,the operating current needs to be increased.This results in a great challenge since the current magnetic field of the periodic permanent magnet(PPM)focusing system is not high enough to meet the requirements of the W-band TWT with larger current.The proposed four-port high-frequency structure and bidirectional bending FWG slow wave structure were designed to reduce the cross section of FWG and inner diameter of the magnetic field system.By doing so,the axial magnetic field amplitude of the focusing magnetic field was increased from 0.6T to 0.8T,resulting in greatly improved focusing capability of the PPM,whereas the working current would be increased by 80%with the same operating voltage and focusing conditions.Based on bidirectional bending FWG slow wave structure and four-port structure,the electron-optics and interaction circuit design of the W-band TWT at 21k V operating voltage and 350m A current was completed.The output power of the TWT reached more than 1040W with the same working voltage at 6GHz bandwidth.5.The theoretical analysis and experimental study of the double periodic focusing system were carried out to improve the focusing performance of W-band TWTs PPM focusing system for higher current and lower pulsation.First,the theoretical derivation of the double periodic focusing magnetic field was carried out to obtain the relationship between the magnetic field distribution curve and each dimension of the magnetic system,which provide a theoretical basis for the design of the double periodic focusing system.Then,the influence of the ratio of third harmonic to the first harmonic on the electron injection pulsation and stability zone of the double periodic focusing system was analyzed theoretically.In order to realize high harmonic multi-parameter regulation and improve the efficiency of magnetic field adjustability,a split-type double periodic focusing system was proposed.Based on the new structure,the electron optical system of the double periodic focusing was designed.According to the design results,the W-band beamstick tube with the double periodic focusing system was fabricated and tested.The actual measurement results show that the circulation rate is up to 98%at 21k V voltage and 289m A emission current.The micro-electronic tunnel’s radius and length are respectively 0.24mm and110mm and the electron beam density is improved from 132A/cm~2 to 160A/cm~2.The design method and experimental study verify the design feasibility of the double periodic focusing system,and lay a foundation for power enhancement studies of short millimeter wave and terahertz traveling wave tubes. |
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