| With many advantages such as high efficiency,high output power and suitability for repetitive frequency operation,the relativistic return tube is one of the most promising high-power microwave devices in current scientific research.In this thesis,theoretical studies and particle simulations are carried out on relativistic return tubes with low guided magnetic fields in the Ku-band.This thesis begins with a theoretical study of the operating principle of the return tube,including a study of the dispersion characteristics of the periodic slow-wave struc-ture,the expression for the strength of the guided magnetic field,the cyclotron resonance absorption effect,the relationship between microwave output power and magnetic field,and the coupling impedance.At the same time,the principles ofthe over-modeslow-wave structure and its transverse modeselection are presented.The dispersion relationship of the rectangular corrugated slow-wave structure is investigated using analytical methods and finite element simulations,and the effect of the structure size on the dispersion curve is explored.By selecting appropriate electron beam parameters and slow-wave structure parameters,the operating frequency point ofthe device is successfully made to be near theπ-mode of the TM01-mode,where the device has a high Q value and the starting current of the operating mode is small and more easily excited.The calculation of the coupling impedance in the slow-wave structure is developed,a graph of the variation of the cou-pling impedance with the electron beam radius is derived,and the electron beam radius is tentatively determined.Finally,the particle simulation software CHIPIC was used to build a preliminary model of the relativistic return tube and carry out particle simulations to obtain a certain output power.Secondly,the mechanism of action of the resonant reflector is theoretically anal-ysed,its S-parameter transmission characteristics curve is simulated,the influence of the structural dimensions of the resonant cavity on the S11-parameter transmission curve is in-vestigated,and the preliminary dimensions of the resonant reflector are determined.This was combined with extensive simulation to optimise the structural parameters such as the length and radius of the drift tube of the device,the ripple period and depth of the first slow wave structure,the electron beam radius and the diode parameters such as the elec-tron beam voltage and the guiding magnetic field strength to obtain an optimised structure.A microwave average output of 0.688 GW with a power of 12.5 GHz and a power effi-ciency of 39.91%was finally obtained at a diode voltage of 532 k V,a current of 3.24 k A and a magnetic field of 0.61 T.Finally,in order to grasp the influence of important parameters on the device per-formance,the influence of structural parameters such as the cathode radius,the length and radius of the drift tube,the ripple depth and radius of the first slow wave structure,and diode parameters such as the electron beam voltage and the guiding magnetic field strength on the output power and power efficiency of the Ku-band relativistic return tube were studied.The final pattern of variation is as follows:as the outer radius and ripple depth of the first slow wave structure increase,the output power increases and then de-creases;as the length and radius of the drift tube increase,the output power increases and then decreases;as the radius of the cathode increases,the output power increases and then decreases;as the guiding intensity increases,the device transitions from the low magnetic field region to the high magnetic field region,and the corresponding output power in-creases and then decreases As the electron beam voltage increases,the output power will always increase,but the power efficiency will first increase and then decrease. |
