Investigation Of A Ku-band Radial-line Relativistic Klystron

Pursuing higher power,higher efficiency,higher frequency-band and longer pulse width are always the developing trends in the high power microwave(HPM)field.At present,when enhancing the operating frequency-band further(such as Ku-band),existent HPM sources almost all suffer from the problems of radio-frequnecy breakdown,pulse shortening,and collector erosion.And this issue becomes more serious under the condition of the long-pulse and repetitive operation.Additionally,the further development of HPM technology asserts a new claim for the size and weigth of the HPM sources.Pulled by the application background of HPM system,the compact and miniaturized devices have gained popularities.For this purpose,based on the radial-line high frequency structure,this paper presents a technical route of a radial line relativistic klystron,which is capable of combining the features of high power capacity and high beam-wave interaction efficiency.Meanwhile,it is characterized of weak space-charge effect and compcact axial size.Thus,it has an enhanced magnetic field using efficiency and a low magnetic field strength is required,which is beneficial to the miniature design of permanent magnet packaging.By means of theoretical analysis,particle-in-cell(PIC)simulation and experimental validation,this technical route was investigated systematically in this paper.Furthermore,we have broken through serval key technologies,such as the stable transmission of the radially-radiated intense electron beam,high efficiency beam-wave energy exchange and the design of the radial permanent magnet guiding system.It established the foundation of the further development of this novel technical route.The main research contents and conclusions are as follows:The physical problems of the transmission of radially-radiated electron beam are theoretically analyzed,which offers the theoretical basis for the generation of radial electron beam with high quality.The space-charge effect of the radial line structure is theoretically analyzed,and the space-charge limited current exhites a positive proportion relationship with the radius of the radial-line drift tube.The envelope equation of the magnetic-focusing radial electron beam is derived,and the beam trajectory expression is obtained.By establishing the model of the radial beam emission and transmission,the magnetic insultation effect in the diode area is analyzed.This magnetic insultation effect would result in the axial diviation of the radial beam,and the diviation amplitude is determined by the combination of the guiding manetic field strength,diode voltage and diode current.The related theories of the beam-wave interaction in radial line resonantor are analyzed,which privide the theoretical basis for the design of radial line HPM devices.By using the method of field matching,the resonsant characteristic of single radial line resonantor is derived,which proves the “volume wave” feature of radial TM01 mode.By adopting the motion theory of single particle,the beam coupling coefficient for the radial-line electromagnetic structure and the dispersion relation of space-charge waves on the radially-radiated electron beam are derived.Based on the transimission theory of space-charge waves,a small-signal theory for the radial line relativistic klystron amplifier(RL-RKA)is derived.And this small-signal theory is capable of predicting the foudamental harmonic current distribution in the RL-RKA.The Ku-band radial line relativistic klystron oscillator(RL-RKO)is designed.The 3?/4 mode is selected as the operating mode of the four-gap buncher based on the beam-loading conductance ratio.The gap period parameter of the three-gap extractor,operating at ? mode,is selected based on the beam-wave synchronization condition.In the PIC simulation,under the condition of voltage of 450 k V,current of 10 k A and guiding magnetic field of 0.4 T,the RL-RKO is capable of outputting microwabes with power of 1.6 GW,frequency of 14.82 GHz and efficiency of about 35%.The maixumal radio-frequency field on the surface of the high-frequency strucuture is controlled below 800 k V/cm.Meanwhile,the RL-RKO is testified by the 3-D PIC simulation,and the no rational symmetric mode interference is observed.The Ku-band RL-RKO is investigated by mean of experiment,which demonstrates the feasibility of the proposed technical route.After designing the radial pulsed magnetic field and microwave radiation system,the designed Ku-band RL-RKO is experimentally investigated.The radial intense electron beam is diagnosed by means of striking the witness target.Under the guiding of the magnetric field strength of 0.43 T,the beam is well confined with a depth of 2 mm.the POCO graphite is employed as the electron emission material in the experiment.A microwave beam,with a power of 1.5 GW,an effeiciency of about 24%,a frequency of 14.86 GHz and pulse duration of 16 ns,is generated under the exciation of diode voltage of 480 k V,diode current of 12.8 k A.The experimental results are basically consistent with the PIC simulation results.The miniaturation ideas of the radial line HPM sources are explored.A soft-magnetic cathode is proposed to decrease the electron emission radius.The guiding magnetic field strength is increased by one factor when loading the soft-magnetic cathode.In experiment,the electron beam is well focused when the electron emission radius is 3 cm.In addition,a permanent magnet(PM)focusing system is designed for the Ku-band RL-RKO.In the PIC simulation,the PM packaged RL-RKO is capable of generating HPMs with a power conversion efficiency of 39%.Besides,the whole device is weighted less than 50 kg.Changing the operating mechanism of the radial line relativistic klystron from the oscillation to the amplification,a Ku-band RL-RKA is proposed and designed,which provides a possible technical route for the high frequency-band HPM coherent combination.The high-frequency strcture,including an injecting system of the seed signal,a reentry input cavity,two groups of two-gap bunchers and a three-gap buncher,is designed by means of simulation.The TEM mode reflectors are adopted to isolate energy leakage between the adjacent resonant cavities.In the PIC simulation,the RL-RKA is capable of generating microwaves with a power of 445 MW,an efficiency of 37%,a gain of 46.4 d B under the excitation of voltage of 300 k V,current of 4 k A,seed power of 10 k W.The microwave frequency is locked at 14.25 GHz,which is consisitent with the seed signal frequency.Moreover,the relative phase jitter is less than ±5°.