Investigation Of An X-band High-power And High-efficiency Relativistic Triaxial Klystron Amplifier

In high power microwave(HPM)area,the spatial coherent power combining is believed as an efficient route to achieve higher microwave radiation power.Considering the advantages of controllable frequency and phase of the output microwave,the relativistic klystron amplifier(RKA)was chosen as one of the key devices for spatial coherent power combining.Nevertheless,due to the the limit of power capacity,RKA with hollow structure is mainly applied in low frequency band,such as S-band at present.To pursuit a higher pf ~2 factor,the technical route of the triaxial klystron amplifier(TKA)is investigated in this paper.The research focuses on the significant issues to achieve GW-level output microwave.Such as:the design of coaxial cavities,the suppression of microwave coupling between the cavities,the establishment of microwave positive feedback between the cavities,the compactness design of the TKA,the operation of the TKA under low magnetic field conditions,and so on.The designed X-band TKA is demonstrated in experiments.The main contents and conclusions of this paper are listed as follows:1.The space charge limit current of the annular electron beam and the power capacity of the coaxial cavity are calculated by numerical method,which provide a reference for the size design of the TKA.The characteristic impedance,coupling coefficient,electron beam load conductance and fundamental harmonic current that characterize the modulation ability of the cavities,are also analized,which could provide guidances for the cavities designs.2.The design methods of the cavities are investigated,the influences of Q value,frequency and E-field distribution on each cavity are discussed.In the two-dimensional particle-in-cell simulation,the single-port re-entry injection cavity,the cascaded bunching cavities and the double-gap standing wave output cavity are designed.Finally,the modulation depth of the fundamental harmonic current of 110%is achieved,and the output power exceeds 2.5 GW.3.The parasitic modes coupling between the cavities is effectively suppressed.The designed TKA is simulated in three-dimensional particle-in-cell simulation.The generation and influence of TEM mode and asymmetric modes between the cavities are analyzed.Mode reflectors and microwave attenuation materials are investigated to effectively suppress the microwave coupling between the cavities.In the three-dimensional particle-in-cell simulation,when the electron beam voltage is 690 k V1and the current is 9.3 k A,a stable microwave with a power of 2.42 GW is obtained when the injection frequency is 8.40 GHz and the injection power is 25 k W.The conversion efficiency is 38%,and the gain is about 49.8 d B.Besides,the output phase and frequency are well locked.4.By adopting TEM mode coupling between the cavities,the microwave positive feedback is established in the TKA.High current modulation depth and high power microwave output are realized with a shorter length of the TKA.The conditions for establishing positive and negative feedbacks of TEM mode between the cavities are analyzed.Under the condition of positive feedback,the mode reflector upstream the bucher I is redesigned.The utilization of TEM mode and the cut-off of the non-operating modes are realized.When the total axial length of the device is reduced by 24%,the output microwave with a power of 2.25 GW is generated,and there is no spurious oscillation within 100 ns.5.By optimizing the structure of the diode,injection cavity and redesigning the reflectors,the normal operation of TKA at low magnetic field intensity of 0.3 T?0.4 T is realized in the simulation.The factors affecting the operation of TKA in low magnetic field are analyzed.The results show that the intrinsic mode oscillation of the reflectors under cyclotron effect are the key reason to limit the operation of TKA in low magnetic field.By optimizing the structures of the diode and the injection cavity and adopting the high-order mode reflector,the normal operation of TKA in 0.3 T?0.4 T magnetic field is realized.6.High power experiments of the designed TKA are carried out.When the injected microwave power is 0,no output microwave is detected,which proves that the TKA is operating in the amplifier state.When the electron beam voltage is 610 k V,the beam current is 9.1 k A,the guiding magnetic field is 0.77 T,and the injected microwave power is 40 k W,the TKA produces a high power microwave with a power of 1.7 GW,an efficiency of 31.8%,a gain of 46.5 d B,and a pulse width of more than 70 ns.The output microwave frequency is strictly locked at 8.40 GHz,and the relative phase difference is stable within±10 degree.In addition,the effects of electron beam voltage,current,the injected the microwave power,the injected microwave frequency,the Q value and the cavities frequencies on the output results have been investigated.