基于新型多间隙谐振腔的W波段高次模式扩展互作用速调管设计.pdf

第 42 卷第 4 期2023 年 8 月红 外 与 毫 米 波 学 报J.Infrared Millim.WavesVol.42，No.4August，2023文章编号：1001-9014（2023）04-0504-06DOI：10.11972/j.issn.1001-9014.2023.04.012Design of W-band high-order mode extended interaction klystron based on novel multi-gap cavityXIE Bing-Chuan1，2，ZHANG Rui1*，WANG Huan-Huan3，WANG Yong1，2，GENG Zhi-Hui1，LIAO Yun-Feng1，YANG Xiu-Dong1（1.Key Laboratory of Science and Technology on High Power Microwave Sources and Technologies，Aerospace Information Research Institute，Chinese Academy of Sciences，Beijing 100190，China；2.School of Electronic，Electrical and Communication Engineering，University of Chinese Academy of Sciences，Beijing 100049，China；3.Center of Materials Science and Optoelectronics Engineering，School of Optoelectronics，University of Chinese Academy of Sciences，Beijing 100049，China）Abstract：A novel resonator has been proposed for the klystron that operates at TM310 mode.The electric fields of both the middle cavity and the side cavity are effectively enhanced by discretizing the field in the side cavity.The novel resonator demonstrates a higher characteristic impedance than the traditional barbell shape cavity when both cavities operate at TM310 mode.Sensitivity analysis has been performed on these two kinds of cavities.Moreover，an RF circuit in the W-band has been designed based on the proposed novel cavity.The RF circuit can achieve a high gain of 52.4 dB and a peak output power of 7.8 kW when 45 mW input power is injected.The circuit is driven by a 25 kV and 2 A pencil beam.The electronic efficiency and 3-dB bandwidth are 15.6%and 120 MHz，respectively.Key words：extended interaction klystron（EIK），high-order mode cavity，field discretizing，barbell shaped cavity，sensitivity analysis基于新型多间隙谐振腔的W波段高次模式扩展互作用速调管设计谢冰川1，2，张瑞1*，王欢欢3，王勇1，2，耿志辉1，廖云峰1，杨修东1（1.中国科学院 空天信息创新研究院 高功率微波源与技术重点实验室，北京100190；2.中国科学院大学 电子电气与通信工程学院，北京 100049；3.中国科学院大学 材料科学与光电技术学院，北京 100049）摘要：提出了一种工作在TM310模式下的用于高次模式速调管的新型谐振腔。通过对侧腔电场的离散化，可以有效地增强中间腔和侧腔的电场。当两腔均工作在TM310模式时，新型谐振腔的特性阻抗均高于传统的哑铃型腔。对两种腔体进行了尺寸敏感性分析。在此基础上，设计了一个W频段的射频电路。当向射频电路注入45 mW的输入功率时，获得了52.4 dB的高增益和7.8 kW的峰值输出功率。互作用电路由25 kV、2 A的圆形电子注驱动。该互作用电路的电子效率和3-dB带宽分别为15.6%和120 MHz。关键词：扩展互作用速调管；高次模腔；场离散化；哑铃型腔；敏感性分析中图分类号：TN122 文献标识码：AReceived date：2022 12 01，revised date：2023 03 21 收稿日期：2022 12 01，修回日期：2023 03 21Foundation items：Supported by the National Magnetic Confinement Fusion（MCF）Energy Research and Development Program（2018YFE0305100），and the Scientific Instrument Developing Project of the Chinese Academy of Sciences（YJKYYQ20180005）Biography：XIE Bing-Chuan（1997-），male，Henan，doctor.Research area involves high peak power microwave devices.E-mail：*Corresponding author：E-mail：4 期 XIE Bing-Chuan et al：Design of W-band high-order mode extended interaction klystron based on novel multi-gap cavityIntroductionKlystron is a kind of high-power microwave device that plays a critical role in remote sensing，imaging，high-power radar，and other applications.The extended interaction klystron（EIK）has high output power，high gain，tolerable bandwidth，and most importantly，excellent spectrum purity.EIK was first proposed by Chodorow and Kulke，et al1.The EIK combines the advantages of klystrons and traveling wave tubes（TWT）.The multi-gap cavity used in the EIK has a high characteristic impedance，which can effectively shorten the length of the interaction circuit.Therefore，it has received extensive attention in high frequency bands.In 30-300 GHz，the EIK has shown great power potential.In 2007，CPI（Communications&Power Industries Canada Inc）revealed a variety of EIKs that could operate at frequencies up to 280 GHz2.In the W-band，CPI has demonstrated an EIK that could generate an output power of 3 kW.However，as the frequency increases，the device becomes incredibly small.The power capacity decreases rapidly.The klystron using a sheet beam has been proposed to increase the total current that can be transmitted at a given voltage3.In the W band，two sheet beam klystrons have been successfully developed.The NRL（The Naval Research Laboratory）reported a W-band extended interaction klystron，which used a strong uniform magnetic field to focus sheet beam in 20144.The total current in the beam tunnel was 3.6 A and the beam voltage was 20 kV.The peak output power was 7.5 kW with an electronic efficiency of 10.4%.In addition，researchers from the University of California at Davis（UC Davis）have also demonstrated a high-power W-band klystron，which can generate a peak output power of 56.3 kW5，6.The klystron is driven by a 74 kV and 3.6 A sheet beam.The electronic efficiency is 21%.The sheet beam klystron exhibits great power potential.However，the optical design of the sheet beam is complex，and the focusing of the sheet beam is difficult7，8.Most importantly，the efficiency of the sheet beam klystron is unsatisfactory.Moreover，simultaneously arranging multiple pencil beams in the klystron can increase the output power and reduce the beam voltage9-11.However，in the W band，the device becomes very small，and the circular ring cavity used in the multi-beam klystron becomes difficult to fabricate.To accommodate modern high-precision planar machining techniques such as UV LIGA and WEDM，a klystron using a planar arrangement of multiple electron beams has been proposed12.However，the opticcal design of a planar multi-beam klystron is also a problem，and such a planar multi-beam design requires extremely high cathode loading（50-10 A/cm2）because of its extremely narrow electron beam spacing.Designing a klystron with a single pencil beam and planar resonators is more practical in high-frequency bands.In the high-frequency band，high-precision processing mainly relies on WEDM，X-Ray Lithography，and UV LIGA.These machining techniques can maintain precision machining up to 1.1 THz13.An extended interaction klystron with resonators operating in a high-order mode has been proposed to increase the total current in the klystron that operates at high-frequency bands14.In Ref.14，the designed 0.34 THz klystron can accommodate a total current of 0.3 A.However，the gain of the klystron is only 43 dB due to the various losses and the low characteristic impedance of the high-order mode cavity used.In this paper，a novel resonator has been proposed for the klystron that operates in a high-order mode.The resonator can operate in the true TM310-2 mode to generate a stronger field in interaction gaps.Therefore，the proposed resonator has a higher characteristic impedance.As a result，efficiency and gain are improved.An interaction circuit with 52.4 dB gain and 15.6%electronic efficiency has been designed.The novel cavity remains a planar structure，which is suitable for modern high-precision planar machining techniques.1 Study of novel cavity The conventional ladder type cavity used in high-order mode klystron is shown in Fig.1（a）.In Fig.1，the drift tube radius is represented as dr and the period of the gap is represented as p.The gap axial length is represented as g.The TM310 mode distribution of the ladder type cavity is shown in Fig.2（a）.The reverse field in the side cavity is joined together，which is a waste of energy in the cavity.In order to use the field in the cavity efficiently，we proposed a novel cavity see Fig.1（b）.The special structure designed in the novel cavity is to realize the real TM310 mode distribution.The TM310 mode distribution of the novel cavity is shown in Fig.2（b）.The reverse fields in both side cavities have been split into three peaks.The field becomes more concentrated.The two reverse field peaks are well confined in the side gaps see Fig.2（b）instead of filling the whole side cavity see Fig.2（a）.Figure 3 shows the axial field distribution at center gaps and side cavities of these two kinds of cavities.The novel cavity has a stronger field both in the central gaps and in the side cavities.Parameters of the two cavities are listed in Table 1.Figure 4 shows the transverse distribution of the axial electric field of the novel and traditional one.The characteristic impedance（R/Q）and coupling coefficient（M）of the resonator are important parameters in the design of klystron and can be calculated from equaFig.1Diagram of cavities（a）the traditional ladder type cavity，（b）the novel cavity图1腔体示意图（a）传统的梯形腔，（b）新型腔505红 外 与 毫 米 波 学 报42 卷tions（1）and（2），respectively6，where Ez is the axial field distribution，=2f is the angular frequency of the mode，Ws is the total energy stored in the resonator（set as 1W in CST），e is the propagation constant of the electron beam.The high-frequency characteristic parameters of two cavities defined in Table 1 are listed in Table 2.The novel cavity has almost the same coupling coefficient as the traditional cavity；and the characteristic impedance of the novel cavity is almost twice that of the traditional cavity.R/Q=(|Ez|dz)2/(2Ws),（1）M=Ezejzdz|Ezdz.（2）1.1Mode spacing studyFor multi-gap resonators，it is necessary to pay attention to the multiple axial modes caused by multiple gaps.N gaps bring N axial modes.These competitive modes need to be far away from the operating mode.During the study，we found that the height of the first side cavity（sh1）could control the mode separation and did not change the frequency of the operating mode.Figure 5 shows the effect of the height of the first side cavity（sh1）on the mode spacing.As the height of the side cavity decreases，the mode separation between the operating mode and the other two axial modes increases.Moreover，changing sh1 hardly changes the frequency of 2 Table 2High-frequency characteristics of the two cavities表2两个腔体的高频特性参数ParametersFrequencyMR/QM2R/QTraditional cavity94.64 GHz0.362744.50 5.85 Novel cavity94.61 GHz0.362684.6 11.12 Fig.2TM310 mode distribution in xOz plane of two different cavities，（a）the traditional ladder type cavity，（b）the novel cavity图2TM310模式在xOz平面上的分布示意图，（a）传统的梯形腔，（b）新型腔Fig.3Axial field distribution at center gaps and side cavities of two different cavities，（a）the traditional ladder type cavity，（b）the novel cavity图3两种腔体在中间间隙和边腔处的轴向场分布，（a）传统的梯形腔，（b）新型腔Table 1Parameters of the two cavities表1两个腔体的参数Parametersdrcwchswshpgsw1sh1sw2sh2sw3Traditional cavity（mm）0.32.41.82.112.80.90.25-Novel cavity（mm）0.31.21.8-0.90.250.82.50.72.00.72Fig.4Comparison of transverse electric field distribution between the traditional ladder type cavity and the novel cavity图4哑铃型腔和新型腔的横向场对比5064 期 XIE Bing-Chuan et al：Design of W-band high-order mode extended interaction klystron based on novel multi-gap cavitymode.However，smaller sh1 will also make the cavity smaller，which is bad for the heat dissipation of the cavity.In addition，the decrease of sh1 does not significantly improve the field in the central gaps，especially when sh1 is less than 2.5 mm.When sh1=2.5 mm，the mode interval of 2 mode and/2 mode is 1.7 GHz，which is sufficient to avoid mode competition.1.2Sensitivity analysisIn the W band，the cavity size becomes very small.Therefore，it is necessary to study dimension sensitivity when designing a cavity.Variations of 10 m and 5 m have been added to novel cavity models in simulation to see deviations in the resonant frequency.The result is shown in Fig.6（a）.It can be seen that the resonant frequency is the most sensitive to ch1，whereas other sizes are not.A similar sensitivity analysis has been performed on the traditional cavity defined in Table 1 to give a comparison.The traditional high-order-mode cavity has two sensitive sizes，which are ch and sw see Fig.6（b）.The most sensitive size is ch，which is the height of central gaps.For both the novel cavity and the traditional cavity，when the most sensitive dimension is changed by 5 m at the same time，the frequency change of the novel cavity is 140 MHz while that of the traditional cavity is 120 MHz.However，it is worth noting that the characteristic impedance of the novel cavity is almost twice that of the traditional cavity see Table 1；and the novel cavity has only one sensitive dimension while the traditional cavity has two.Moreover，the characteristic impedance of resonant cavities plays a key role in determining the circuit efficiency.The effect of fabrication tolerance on novel cavity characteristic impedance has been studied.The result is shown in Fig.7.In the variation range of 5 m，the characteristic impedance varies less than 1 ohm even for the most sensitive size.The 1-ohm variation in characteristic impedance has little effect on circuit efficiency.2 PIC simulation analysis An RF circuit operating at the W-band is designed based on the proposed novel multi-gap resonator.Distances between cavities are firstly determined as about 1/4 reduced plasma wavelength and then optimized manually to get the maximum output power.The 3D particle-in-cell（PIC）simulation is done by commercial software CST.In the PIC model，the oxygen-free high conductivity copper with a conductivity of 5.8 107S/m is assigned to the circuit boundary.A pencil beam with 0.2 mm radius is injected into the 0.3 mm radius drift tube.The operating voltage and current of the pencil beam are 25 kV and 2 A respectively.A uniform axial field of 0.9 T is defined in CST to confine the pencil beam.The magnetic field is twice as strong as the Brillouin field.The Brillouin magnetic field can be given by Bb=0.83 10-3(I0)1 2/(R0(V0)1 4)，where I0 is the beam current in Fig.5The resonant frequencies of the three axial modes as a function of sh图5三个轴向模式谐振频率随sh的变化曲线Fig.6Effect of dimensional deviation on resonant frequency of two kinds of cavities that operate at TM310 mode，（a）the novel cavity，（b）the traditional cavity图6不同尺寸偏差对工作在TM310模式的两种谐振腔谐振频率的影响，（a）新型谐振腔，（b）传统谐振腔Fig.7Effect of dimensional deviation on the characteristic impedance of the novel cavity that operates at TM310 mode图7不同尺寸偏差对工作在TM310模式的新型谐振腔特性阻抗的影响Fig.8Diagram of the interactive circuit图8互作用电路示意图507红 外 与 毫 米 波 学 报42 卷A，R0 is the beam radius in m，and V0 is the beam voltage in V.The designed interaction circuit is shown in Fig.8.The whole interaction circuit consists of five resonators.Each resonator is composed of three gaps to avoid mode competition caused by multiple gaps.The microwave energy input and output of the RF circuit are done by the WR-10 standard waveguides.The parameters of the individual cavities in the interacting circuit are listed in Table 3.The frequency and the period of the cavity should be carefully designed in order to avoid self-oscillation of the interaction circuit.The whole interacting circuit can be qualitatively analyzed by the equivalent circuit approach.The beam conductance Ge of each resonator is an important parameter to measure the interaction between the field in the resonator and the electron beam.The beam conductance can be given by（3）15，where G0=I0/V0 and e=q/ve.q is the reduced plasma angular frequency.M(e-q)and M(e+q)are coupling coefficients of fast and slow space charge wave respectively，and they can be calculated from（4）and（5）.E(z)is the axial field distribution.Ge/G0=18eq|M(e-q)|2-|M(e+q)|2,（3）M(e-q)=E(z)ej(e-q)zdz|E(z)dz,（4）M(e+q)=E(z)ej(e+q)zdz|E(z)dz.（5）The RF circuit should be d