Study On The Novel Design Method Of High-Power Circular Waveguide Mode Converter

High-Power microwave mode converter is an important device in high-power microwave system,such as high-power radar,directed-energy weapon,etc.It converts the mode generated by the high-power microwave source to the wanted mode applies to specified application.Mode conversion efficiency and power handling capacity are two important design indicators.The common method is to select the optimum structure with high-power capacity among groups of structures with high-conversion efficiency by comparing the maximum electric field value.Design efficiency of this method is not good enough.Therefore,it is necessary to develop an efficient method that could design the structure of the mode converter directly according to the specified maximum electric field.An "analytic and iterative optimization" design method based on segmental structure is proposed in this paper,and has been successfully used to design a circular waveguide serpentine shaped TM01-TE11 mode converter.The research contents include:1.Focusing on the serpentine shaped mode converter,an efficient design scheme is proposed based on serpentine shaped structure.The relationship among the power ratio,the relative phase of TM01 and TE11 modes and the maximum electric field value of the combined field of these two modes in a circular waveguide is analyzed.Therefore,the change rule of the maximum electric field on every cross section of the mode converter along the axis is presented,and it is clear that the maximum electric field value of the mode converter is determined by the power ratio of the TM01 and TE11 modes at the crux position where the bending direction of the converter changes.Then,a segmental design strategy which could control the electric field distribution of the mode converter is proposed.That is,divide the mode converter at where the bend direction of the axis changes,specify the boundary conditions according to target of the maximum electric filed,optimize each segmental structure to realize the specified power conversion,and then,complete the design.2.A series of key points about the segmental design scheme are studied.Based on the analysis of the complex magnitude and power of each mode along the bending waveguide,the setting method of the boundary condition for segmental design scheme is given.It is confirmed that specifying the relative phase of TM01 and TE11 modes to be ? at the boundary could improve the conversion efficiency per unit length and minimize the structure.Based on the contour map of the maximum electric field,the power constraint at the boundary can be specified according to the relationship of the electric field value and the power ratio presented on the contour map when relative phase equals ?,which could achieve the fine control of the maximum electric field of each segmental structure.Furthermore,the value range of the specified maximum electric field and the selection gist of the number of segments have been discussed according to the symmetry of the contour map.An X band mode converter with small radius is designed to verify the effectiveness of the segmental design scheme.3.A segmental design method based on iterative algorithm is proposed for the design of mode converter.Phase self-adaption and phase normalization is adopted to avoid divergence caused by the mismatch of absolute phase in the boundary conditions,and the modified iterative formula for the curvature function of segmental structure is given.Based on this,the fine control of the process of mode conversion can be realized.Meanwhile,the optimization choice of arc length based on crude model and the realization method of the structure constraint are proposed,which extend the applicability of the segmental iterative design method.Using this method,an X band high-power mode converter with large radius is designed,two ports of the final structure are parallel,the conversion efficiency of the TM01 mode to the TE11 mode is higher than 98%and VSWR is about 1.02 at 9.7 GHz,power handling capacity is about 5 GW.The total time of iterative optimization design is less than five minutes on the computer with Intel Core 15-3032@2.6GHz.4.The test method of mode converter is studied.Test scheme and test system are proposed in this paper.A circular waveguide TM01 mode exciter with suppression structure loaded on a hybrid E-T waveguide is designed to assure the essential mode purity for the test system.Based on the time-domain gating function of VNA,a test scheme which combines frequency domain and time domain is proposed.The port reflection signals of exciters and mode converter can be separated and selected through the time domain test,which solve the problem that characteristics of the mode converter cannot be tested directly.Using this test system,the performance of the X band mode converter is obtained:the conversion efficiency of the TM01 mode to the TE11 mode is about 96%and VSWR is about 1.1 at 9.7 GHz,which is coincide with the design target.Finally,the power handling capacity of the mode converter is verified using an X band high-power microwave source,which provide scientific data basis for the integration of mode converter and high-power microwave source.In summary,focusing on the design of high-power circular waveguide serpentine shaped TM01-TE11 mode converter,a novel and efficient design technique is provided by adopting the segmental design scheme and iterative optimization method,which realize the fine control of the electric field distribution in the course of mode conversion.