Investigation Of A Ka-band Radial Transit Time Oscillator

As the application of the high power microwave(HPM)is expanding,investigation on high power millimeter-wave devices would cause the concern day by day.However,the limited power capacity and the mode competition prevent progress of the axial millimeter-wave devices.Since the power capacity in the radial generator is higher and the diode impedance is lower,the radial devices have potential to be employed to realize production of the Ka-band millimeter-waves.Besides,the transit time oscillator(TTO)has the advantages of high power,high efficiency and the there is only the working mode in the oscillator.Through theoretical analysis and particle-in-cell(PIC)simulation,a Ka-band radial transit time oscillator(RTTO)with high output power,high efficiency and high power capacity is proposed in this paper.Besides,the studies on power capacity,ohmic loss and characteristics of the oscillators with different radial overall sizes have been carried out.In the end,high output power of Ka-band microwaves has been realized in experiment.The research laid foundation of radial HPM devices'progress to millimeter-wave or higher bands.The main research contents are as below.1.Theoretical research on the Ka-band RTTO to provide the basis of design.Firstly,we studied the space charge effects of the radial intense relativistic electron beam to enhance the understanding of the transmission of radial electron beam and the mode selection characteristics of the radial cavities.And the results proves that as electrons move in the radial direction,the space charge effect will be weaken,the space charge limit current will increase.And if the distance between electron beam and the inner wall of the radial line decreases,the space charge limit current will grow.Investigation on the distribution of the field with TM₀₁ mode in the one-gap cavity,the results tell that the field is vacuum wave,which is benefit to realize effective beam-wave interaction.The theory of electron conductance ratio from small signal theory is used to analyze the mode selection characteristics of the buncher.For the N-gap buncher,the(N-1)?/N mode is more likely to be stimulated.As N grows,the beam-loading conductance ratio of the(N-1)?/N mode will increase,and there will be more transferred energy between electrons and the radial electric field.2.Design of the Ka-band transit time oscillator and PIC simulation.Based on the theory study,a compact Ka-band RTTO is proposed through PIC simulation.The field with the TM₀₁ mode is applied to realize effective beam-wave interaction,and radius of the beam-wave interaction region is less than 91 mm.If the diode voltage is 400 k V,the electron beam current is 7.5 k A and the radial guiding magnetic field is 0.8 T,the HPMs with the power of 1.12 GW,the frequency of 31.13 GHz,the efficiency of 37.3%can be outputted.And the 3-D simulation proves that there is almost no rotational asymmetric mode.Besides,the chamfering can improve the power capacity and the maximum radial electric field will decrease from 1.6 MV/cm to 1.2 MV/cm.The 3?/4 mode in the four-gap extractor can improve the beam-wave interaction efficiency,and the efficiency over 40%can be realized in simulation.In the end,the one-gap extraction cavity is proposed to realize multiple energy transfers between electrons and electric field to realize high power output.If the output power is over 1.1 GW,the maximum radial electric field in the RTTO with the one-gap extractor is only 850 k V/cm.3.Study on characteristics of the RTTO.Parameter sensitivity is very important for HPM devices,and the PIC simulation proves that to ensure steady operation of the impact RTTO with two-gap extractor,the diode voltage should range from 360 k V to480 k V and the diode impedance should be about 53?.When the guiding magnetic field is larger than 0.6 T,variation of the magnetic field will almost not influence operation of the RTTO.Since the microwave with TM₀₁ mode is volume wave,as the hickness of the electron beam ranges from 0.5 mm to 1.5 mm or the axial offset varies within 0.5 mm,the steady operation can be ensured.The ohmic loss will decrease the beam-wave interaction efficiency,and result in great energy loss on the surface of the cavities.These factors will decrease the efficiency of the RTTO and lengthen the saturation time.Setting the material to be stainless,the output power will decrease to 660 MW,and the corresponding decrease ratio reaches up to 41%.Changing the inner radius and the outer radius of the RTTO and making sure that diode distance and the other size will not change,this overall increase of the radial sizes will increase the output power of the RTTO,make the efficiency to be the same and increase power capacity effectively Considering the influence of the ohmic loss,the overall increase will weaken the influence of the ohmic loss and make efficiency of the RTTO increase to relative stable value.4.Experimental research of the RTTO.Stable movemwnt of the radial electron beam and the microwave extraction from the RTTO with large radial size are two main problems.When the diode voltage is 400 k V and the guiding magnetic field is 0.67 T,the microwaves with the frequency of 31.35 GHz can be outputted.Studying on the mode of the microwaves,results prove the mode is the rotational symmetric TEM mode,and the output power can be calculated to be 320 MW.As the guiding magnetic field increases,the electrodynamic force between coils will grow to increase instability of the output power.Setting the magnetic field to be 0.68 T,which can confirm radial movement of the electrons to increase the efficiency and the electrodymanic force will be relatively smaller.As the diode voltage increases from 350 k V to 430 k V,the output power will increase from 100 MW to 344 MW,which agrees with the simulation result.