Numerical Study of Mode Competition in Coaxial Cavity Gyrotrons with Corrugated Insert

A time-dependent self-consistent multimode code which allows to model the interaction between the modes of a coaxial cavity gyrotron operating at the fundamental and/or at higher harmonics of the cyclotron frequency is presented. The Method of Moments (MoM) is used to obtain a rigorous solution of the eigenvalue problem for a coaxial waveguide with corrugated insert. It is shown that the use of the Surface Impedance Model for the calculation of modes operating at higher harmonics leads to incorrect results in high-power coaxial cavity gyrotrons. However, the numerical simulations based on the MoM agree well with the available experimental results.     

Design of a 1-MW, CW coaxial gyrotron with two gaussian beam outputs

The design of a 170 GHz, 1 MW-CW gyrotron for electron cyclotron heating of nuclear fusion plasmas is presented. The designed gyrotron incorporates a coaxial cavity to reduce mode competition, and a coaxial electron gun to support the cavity inner conductor. A new mode converter splits the generated wave into two beams and radiates them in different directions. The radiated beams are transmitted to two output windows through two mirror systems, being transformed into Gaussian-like beams. A single-stage depressed collector improves the overall efficiency of the gyrotron and reduces the heat flux to the collector surface.     

双共焦波导结构二次谐波太赫兹回旋管谐振腔设计

准光共焦波导具有功率容量大、模式密度低的特点,能够有效地减少模式竞争对回旋管互作用的影响,有利于高次谐波太赫兹回旋管的设计.为提高太赫兹准光回旋管的互作用效率,在共焦柱面波导的基础上,研究了一种新型高频互作用结构——双共焦波导结构,设计了一种330 GHz二次谐波双共焦结构回旋管谐振腔并对其进行了理论分析和粒子模拟.研究结果表明,双共焦谐振腔中的高阶模式能够与高次电子回旋谐波发生稳定的相互作用,并且没有模式竞争现象,具备工作在太赫兹波段的潜力.相比普通共焦波导谐振腔,双共焦谐振腔能够增强准光回旋管的注波互作用强度,提高回旋管的输出功率和工作效率.此外,结果还表明双共焦波导中的电磁波模式是一种由两个独立的共焦波导模式叠加而成的混合模式.利用这种混合模式有望实现太赫兹回旋管的单注双频工作,为新型太赫兹辐射源的研究提供了新的途径.     

全三维110GHz和220GHz同轴腔双注回旋管数值模拟研究

为了打破传统单注回旋管只能产生较低功率的局限性, 本文基于自主研发的PIC粒子模拟软件CHIPIC 对110 GHz和220 GHz同轴腔双注回旋管进行全三维数值模拟研究. 在理论分析同轴双电子注电子枪设计模型和初始参数的基础上通过CHIPIC对其进行优化设计, 得到了具有横纵速度比为1.0, 最大速度零散约为5.4%的高性能电子束; 并将此优化后的双阳极双注电子枪取代传统回旋管数值模拟时采用的回旋 发射进行110 GHz和220 GHz双注回旋管整管的数值模拟, 并采用MPI四进程并行计算, 最终获得了具有双频分别为110 GHz和220 GHz、模式为TE₀₂模、平均输出功率约在70 kW、 效率达到8.75%的高性能双注回旋振荡管. 关键词： 同轴腔双注回旋管 同轴双电子注电子枪 横纵速度比 速度零散     

Multifrequency operation of a gyrotron

One specific example with the parameters close to the existing coaxial cavity gyrotron at the Forschungszentrum Karlsruhe (FZK) is analyzed in detail. Mode competition calculations are performed using the time dependent and self-consistent codes developed both at the FZK and the Institute of Applied Physics (IAP). The results are compared with the calculations performed earlier by one of the authors in the cold cavity approximation. Significant qualitative differences are found and the conclusion is drawn that it is absolutely necessary to use the self-consistent approach in analyzing advanced operation regimes of a gyrotron     

Experimental investigation of a 140-GHz coaxial gyrotron oscillator

We report experimental results on a megawatt power level, 140-GHz coaxial gyrotron oscillator. The gyrotron has an inverted magnetron injection gun (IMIG) designed for operation at up to 95 kV and 88 A. The IMIG has an inner grounded anode which extends from the center of the gun down through the entire length of the tube including the cavity and collector. The IMIG was tested at up to 105 kV and 93 A in 3 μs pulses, achieving an electron beam power of 10 MW. The output power from the coaxial gyrotron cavity was transported to an internal mode converter and a single mirror that coupled the power out transversely from the tube axis. A maximum output power of up to 1 MW was obtained in the TE_27,11 mode at 142 GHz at an efficiency of 16%, about one half of the design efficiency. The reduced efficiency was attributed to nonuniformity of the cathode emission and the sensitivity to the relative alignment of the electron gun, coaxial insert, and cavity. The cathode emission over the azimuthal angle was measured for two cathodes and was shown to be nonuniform due to both temperature and emitter work function nonuniformity. The gyrotron was also tested in two alternate configurations: 1) with the internal mode converter removed (axial output), and 2) with both the internal converter and the coaxial insert removed (empty cavity). In operation in the empty cavity configuration, which is equivalent to a conventional gyrotron oscillator, output power of up to 0.9 MW was observed     

Nonperturbative solutions to cylindrical resonant cavities with dissipative medium and imperfectly conducting walls

Cylindrical waveguides without end surfaces can serve as two-dimensional resonant cavities. In such cavities the electromagnetic oscillations corresponding to an eigenfrequency can always be taken as TM or TE modes even when the walls have a finite conductivity and the medium is absorptive. This paper obtains analytic solutions to the field equations when the cylinder has a circular cross section. Some nonperturbative conclusions are drawn from the eigenvalue equation. Approximate analytic results for the resonant frequencies are obtained when the absorption of the medium is small and the walls are good conductors. Stability of the eigen modes is discussed. Similar results for the coaxial line are presented.     

Gyrokinetics of Coaxial-Cavity Electron Cyclotron Maser

Based on the gyrokinetics of free-electron masers, the energy-transfer rate and starting current are derived for a coaxial-cavity electron cyclotron maser (gyrotron oscillator) with the misalignment of the inner-rod axis to the outer-cavity axis taken into account. The effect of the finite cavity length is included in the derivation. The stable term of the instability which results from the contribution of the guiding-center distribution is of significance, although it is usually neglected in a cylindrical-cavity gyrotron. It is found that the structure misalignment has more serious influence on the performance of the coaxial cavity gyrotron than the beam misalignment.     

回旋管同轴谐振腔模式的选择

 为了研究回旋管工作模式的选择机理,分析了圆柱波导开放式谐振腔、光滑同轴谐振腔和纵向内开槽同轴谐振腔相应波导的色散特性,进而研究了这些谐振腔内模式竞争的问题。对同轴结构波导色散方程作了相应的数值计算,结果表明,同轴谐振腔利用自身色散曲线与谐振腔内外半径比值的数值关系,能够使在开放式谐振腔内无法工作的高阶模式在其内部稳定工作。最后,以德国卡尔斯鲁厄理工学院(KIT)研制的同轴回旋管为例,进行了相应的分析计算,结果表明纵向内开槽同轴谐振腔相对于光滑同轴谐振腔更适合高阶模式的稳定工作。     

Coaxial resonator for a megawatt 280 GHz gyrotron

To provide the required mode selectivity for a megawatt 280 GHz gyrotron, a coaxial resonator operating in a high order TE mode is considered. Mode discrimination is achieved both by exploring the differences in the transverse structures of the competing modes and investigating a suitable geometry for the coaxial insert. For modes with close eigenfrequencies the associated diffractionQ factors can be widely different in value, thereby ensuring an effective mode selection. In the resonator studied here, the frequency separation between the design mode TE_26,10,1 and its nearest competing mode TE_20,12,1 is about 0.6% and the ratio of the correspondingQ factors is as high as 6.5. Unlike the coaxial resonator, in the hollow cavity without the inner conductor the fundamental spectrum of eigenfrequencies is more dense, and all TE modes within the frequency interval 271–288 GHz have approximately the sameQ factor.     

0.3 THz TM10, 1, 0模同轴耦合腔链

通过本征方程研究了工作在太赫兹(THz)频段的高次模同轴谐振腔,讨论了TM_{m, 1, 0}模,TM_{m, 2.0}模与TM_{m, 1, 1}模的谐振频率与腔体的几何参数之间的关系,并给出了工作模式的选择依据。在此基础上,提出了一种新型的0.3 THz TM_{10, 1, 0}模同轴耦合腔链,使用等效电路模型和CST-MWS软件对耦合腔链的色散特性、特征阻抗和电场分布等冷腔特性进行了分析和仿真,并着重分析和总结了耦合腔链的几何参数对色散特性和特征阻抗的影响。研究结果表明:对于工作在THz频段的高次模同轴耦合腔链,采用TM_{10, 1, 0}模为工作模式是合理的选择; 工作于2π腔模的0.3 THz TM_{10, 1, 0}模同轴耦合腔链具有较大的特征阻抗,但模式间隔较小,因此可将其应用于窄带太赫兹扩展互作用器件; 增大高次模耦合腔链的耦合槽张角是增大模式间隔的最佳途径。     

Design and simulation of a new type of 500 MHz single-cell superconducting RF cavity

This paper illustrates the design and simulation of a unique 500 MHz single-cell superconducting radio frequency cavity with a fluted beam pipe and a coaxial-type fundamental power coupler. The simulation results show that the cavity has a high r/Q value, a low peak surface field and a large beam aperture, so it can be a candidate cavity for high current accelerators. With the help of a fluted beam tube, almost all the higher order modes can propagate out of the cavity, especially the first two dipole modes, TE₁₁₁ and TM₁₁₀, and the first higher monopole mode, TM₀₁₁. The external quality factor of the coaxial fundamental power coupler is optimized to 1.2×10⁵, which will be useful when it is applied in the light source storage ring.     

Magnetic field tapering in the KfK coaxial gyrotron

Equations describing the magnetic field tapering in a gyrotron cavity with allowance for the dependence of the radius of the electron guiding centers on the longitudinal coordinate are derived. Influence of the shift of the magnet of the KfK coaxial gyrotron along the gyrotron axis onefficiency is studied. It is found that for the operating voltage 90kV the optimal alignment is such that the maximum of the magnetic field is placed in the middle of the straight section of the cavity. For 80kV the absolute values of the field must be reduced by factor 1.017, or the maximum of the field must be placed in the output taper of the cavity.     

缓变回旋管谐振腔研究

在高次模回旋管中, 模式竞争是亟待解决的问题. 本文基于广义传输线理论, 得到能准确描述缓变结构谐振腔模式耦合的一阶传输线方程组; 通过分析耦合系数, 优化得到了一种新型光滑渐变(缓变)谐振腔. 与直线连接的(折变)谐振腔相比, 该腔体有更强的模式抑制能力, 腔中非工作模式归一化幅值可达-100 dB, 输出行波段非工作模式归一化幅值可达-88.6 dB.     

170GHz兆瓦级同轴回旋振荡管的分析计算

回旋管是最有希望应用于正在实施的国际热核实验反应堆计划的微波源器件,然而研究设计符合要求的回旋管还存在很多困难需要解决.对170 GHz兆瓦级光滑同轴回旋管的注-波互作用进行了研究.选取模式谱相对稀疏的TE31,12作为工作模式,利用Matlab编制源程序,计算了同轴回旋管的注-波耦合系数、起振电流.在考虑电子速度零散、腔壁电阻率和单模近似的基础上,对光滑同轴谐振腔的优化设计和注-波互作用进行了仿真,给出了磁场、电压、电流和内导体倾角等参量与回旋管效率的关系.结果表明,电压和磁场对回旋管效率影响较大,电子速度零散对回旋管效率影响较小,因而可降低电子枪的设计要求.此外,优化内导体倾角和同轴谐振腔结构参数可提高注-波互作用效率,降低电子速度零散对互作用效率的影响,获得了约50%的电子效率及1.7 MW输出功率.     

Mode selective properties of coaxial gyrotron resonators

In this paper, the mode-selective properties of coaxial gyrotron resonators are discussed for different cavity configurations. Expressions are derived for the ohmic loading of a coaxial conductor with longitudinal corrugations where the RF field inside the corrugations is fully taken into account, and for the diffractive quality factor in coaxial cavities with tapered inner conductor relative to that of an empty cylindrical cavity. The competing action of a kind of “surface mode” that appears in corrugated structures is investigated by considering its diffractive and ohmic quality factors. Additional mode competition problems that can arise in coaxial cavities with corrugated inner conductor due to second cyclotron harmonic interaction are investigated and solutions to the problem are proposed     

同轴谐振腔高阶横磁模式参数的研究

理论计算和分析了微波圆柱同轴谐振腔高阶横磁(TM)模式的系列关联参数.研究发现：与圆柱腔相比, 同轴谐振腔的TM_n10模式有较大的模式间隔，即工作模式能够远离非工作模式的干扰，这有利于保持器件稳定的频率和功率；在保持腔长不变、腔横截面外半径取特定值时，腔内电场峰值位置的轴向特性阻抗随内径的变化存在极大值；在高频段可以采用任意大横截面的腔体结构和任意阶的模式.计算结果与仿真结果相一致. 关键词： 高阶横磁模式 圆柱同轴谐振腔 特性阻抗 多注速调管     

Influence of reflections on mode competition in gyrotrons

A general theory is presented for studying the influence of reflections on mode competition in gyrotrons. Strong effect is found in the illustrative example: the coaxial cavity TE_28,16 gyrotron operating close to the optimal point in the general parameter spare. In some cases, the fixed-field approximation and the self-consistent approach lead to qualitatively different predictions, which emphasizes the need for self-consistent calculations in the final stage of design of a gyrotron     

Are Coaxial Super Power Gyrotrons Feasible?

Limits on the power generated by coaxial gyrotrons are investigated. It is shown that among all the operating modes suggested recently for coaxial super power gyrotrons only modes with azimuthal index m lower than about 44 pass the spatial stochasticity test. Modes with higher azimuthal indices cannot be used as operating modes because gyrotron oscillations become chaotic in the azimuthal direction and efficiency is very low. It is argued that there exists a natural upper limit on power generated by gyrotrons which is about 4 MW.     

回旋管光子带隙谐振腔冷腔电磁模式分析

利用二维三角格子金属光子带隙谐振腔代替回旋管的传统柱形谐振腔,并对腔体进行了TE波模式的计算与分析.综合考虑腔体内外的结构特征,给出了光子带隙谐振腔冷腔模式理论的研究方法,发现腔内存在单模工作的可能与条件以及非角对称情况下的电磁模式分布特征.研究结果表明,利用光子带隙谐振腔代替回旋管的传统腔体,可使回旋管在不受腔体横向尺度限制的条件下实现单模工作.这对于提高回旋管的功率容量、有效实现高次单模与高次回旋谐波耦合条件下的注-波互作用、降低工作磁场并从物理上根本改变回旋管的工作状态提供了理论依据. 关键词： 回旋管 金属光子带隙 谐振腔