Research On All-metallic Metamaterial Slow-wave Structure With Larger Space-charge-limited Current

Limited by the co-degree relationship between working wavelength and device size,the existing low-frequency high-power microwave sources are generally faced with problems such as insufficient compact structure,which cannot be adapted to the increasingly wide range of application scenarios.The metamaterial slow wave structure（MTMSWS）can be operated under the cut-off frequency and has high coupling impedance,which is expected to achieve the goals of miniaturization and high efficiency of high-power microwave sources.However,the existing metamaterial high-power microwave sources（>100 MW）have a low space-charge-limited current,which restricts the injection power.At the same time,the field distribution in the cavity is not uniform,which is not conducive to improve the beam-wave conversion efficiency and output power.Taking the application of metamaterials in high-power microwave sources as the theme and improving the injection power as the traction,an all-metallic metamaterial slow wave structure with larger space-charge-limited current is proposed in this paper.The relevant research is carried out in the aspects of electromagnetic characteristics,space-charge-limited current and preliminary particle simulation.Firstly,the electromagnetic characteristics of slow wave structures are analyzed theoretically.The factors affecting the space limit current are clarified,the‘double negative’characteristics of metamaterials are introduced,the extraction method of equivalent electromagnetic parameters is given,and the miniaturization principle of metamaterial slow wave structure is expounded,which provides a theoretical basis for the design of new metamaterial slow wave structure.Secondly,an all-metallic metamaterial structure is proposed.The mode analysis shows that the fundamental mode of the slow wave structure unit is quasi TM₀₁ mode,and the angular distribution fluctuation of the axial electric field is only 3.62%,the structure with the fundamental mode has negative equivalent permittivity and permeability.The reverse Cherenkov radiation and miniaturization can be achieved.Subsequently,the electromagnetic and transmission characteristics of slow wave structure are analyzed.The fundamental mode has a strong longitudinal electric field,which is conducive to the beam-wave interaction.The zero-order spatial harmonic of the fundamental mode has negative dispersion,which shows that the MTMSWS can work in the backward wave state.Compared with the traditional SWS,the coupling impedance is higher and the potential advantage of high efficiency is obvious.By analyzing the transmission characteristics of the MTMSWS with the fundamental mode,it is convenient to design and verify the working frequency band range,and effectively optimize the signal coupling structure parameters to ensure a high feedback mechanism.Through three-dimensional simulation,it is obtained that the space-charge-limited current of the SWS is 12 k A,which is more than twice that of the original MTMSWS.It is expected to increase the injection power and output power.Finally,a high-power microwave source loaded with the novel MTMSWS is preliminarily explored.Particle-in-cell simulation shows that under the conditions of voltage 700 k V,current 2 k A and guiding magnetic field 0.9 T,the microwave with average power of 681 MW and frequency of 1.50 GHz is obtained,and the beam wave conversion efficiency is 48.66%.By introducing the MTMSWS with larger space-charge-limited current proposed in this paper,the injected power level of metamaterial microwave source is effectively improved and higher power output is realized.