Design Of A Ka-band Coaxial Relativistic Backward Wave Oscillator With Low Magnetic Field

Currently,with the development of high-power microwave technology,there is an urgent need for various equipments to carry out intense electromagnetic compatibility tests under high power electromagnetic pulse irradiation.Light-weighted and small-sized high-power millimeter wave sources are needed.The core of lightweight and compactness is to achieve low guiding magnetic fields,as most of the weight of a high-power millimeter wave source comes from the excitation power supply or permanent magnets.The paper proposes a low magnetic field coaxial relativistic backward wave oscillator(RBWO)operating in the Ka band.In this paper,the design of a low-magnetic-field,high-efficiency Ka-band RBWO is presented through both theoretical analysis and particle simulations.Firstly,a theoretical analysis is carried out for electromagnetic structure of the proposed device.The scattering matrix of the front reflector is derived by the use of the mode matching method,and the design method of the coaxial asymmetric TEM mode reflector is pointed out;the dispersion characteristics,coupling impedance and growth rate of the trapezoidal slow wave structure are investigated by theoretical solutions;on this basis,the coaxial double ripple slow wave structure(SWS)is designed,and the working mode is expected to be coaxial TM01 mode.Next,the overall design and physical characteristics of the whole tube were analyzed on the basis of the theoretical analysis.A two-stage double-wave SWS is adopted,in which the electron beam is velocity-modulated in the front section of the SWS and converted to density modulation through the drift section,and energy conversion is carried out in the rear section of the SWS to improve efficiency.The shape of the SWS is determined,a method for designing the dimensions of the coaxial SWS and the low magnetic field operating range is given,a diode structure that reduces the operating magnetic field is designed,and its effect on power efficiency and electron beam quality is investigated.Finally,detailed two-dimensional and three-dimensional particle simulation research is carried out.When the overmode ratio of the SWS is 3.7,the diode voltage is370 kV,the electron beam current is 3.3kA,and the guiding magnetic field is 0.7T,the2-D particle-in-cell(PIC)simulation results are: the output power is 468 MW with the efficiency of 38%;the microwave frequency is 30.2GHz.Asymmetric modes appear in3-D PIC simulation.The suppression of non-rotationally symmetric mode competition can be accomplished by adjusting the discontinuous structure beyond the SWS.