Three-dimensional modeling of AC space charge for large-signal TWT simulation

We describe a new model for calculating the ac space charge in a linear-beam traveling wave tube (TWT) large-signal simulation code when the true three-dimensional (3-D) geometry of the interaction circuit is taken into account. We use the 3-D electromagnetic simulation code CTLSS to characterize the ac space charge fields generated by a set of test currents placed inside the periodic interaction structure. This information, expressed in matrix form, is used by the large-signal simulation codes CHRISTINE-1D and CHRISTINE-3D to compute self-consistently the additional space charge field terms due to the structure in response to the beam evolution during the simulation. We present the formulation, and describe the implementation in both CTLSS and the large-signal codes. We validate the model by comparison with the analytical sheath helix model, and evaluate the importance of including this correction for modeling a number of helix and coupled-cavity TWT devices.     

螺旋线行波管三维CAD技术进展

介绍了螺旋线行波管三维CAD技术的国际国内研究现状；对目前主流的部分螺旋线行波管三维CAD软件(MICHELLE、COCA、CTLSS、CHRISTINE等)进行了简单介绍。展望了螺旋线行波管CAD技术的发展前景，对我国如何自主开发螺旋线行波管三维CAD软件提出了一些建议。     

基于速调管2.5维粒子模型的非线性注波互作用的研究

该文采用恰当的近似提出了速调管专用的2.5D粒子模型,并基于该模型研究速调管中复杂的非线性注波互作用过程。该模型采用粒子模拟的思想将电荷和电流恰当地分配到空间网格上,采用时域有限差分方法(FDTD)求解麦克斯韦方程组以精确解出电子注的空间电荷场,采用端口近似方法模拟高频谐振腔场对电子注的影响,最后基于洛伦兹力方程求解电磁场对粒子的推动作用。该模型首次将FDTD和端口近似方法结合起来,能够比传统的1维非线性模型更加精确地模拟互作用的物理过程,同时又比全3维模拟方法节约计算资源。基于该物理模型编制了国内首个速调管2.5D粒子模拟程序KLY2D,并仿真了一支S波段50 MW高峰值功率速调管。理论计算与实验结果一致,说明了物理模型和程序的可靠性,这对推动高功率速调管的设计和发展具有重要的推动作用。     

TWO-DIMENSIONAL NUMERICAL SIMULATION OF LOW FREQUENCY OSCILLATIONS OF SPACE CHARGE AND POTENTIAL IN THE GYROTRON ADIABATIC TRAP

The development of oscillations of the space charge and potential in the adiabatic trap of a gyrotron between the cathode and the cavity is studied numerically. The PIC method is applied, with the real two-dimensional distributions of the electric and magnetic fields taken into account. Secondary emission is included into the numerical model, as well as such factors as the electron beam space charge, thermal velocities of electrons, and emitter roughness. The value of the trapped space charge as a function of time is calculated. Time dependences of the potential in various cross-sections of the formation system are traced. The amplitudes of its variable component have been found and then the corresponding frequency spectra have been calculated. The process of cathode bombardment is investigated. The energy distribution and time dependence of the current in the electron beam coming to the operating space are found. Some ways to reduce the oscillations of the potential and the space charge are discussed.     

Simulation of noise-power ratio with the large-signal codeCHRISTINE

This paper describes simulations of the noise-power ratio (NPR) for a helix traveling wave tube (TWT) performed with the large-signal, one-dimensional (1-D), multifrequency code CHRISTINE. The results obtained with this code are in better agreement with measured values than are the more traditional values calculated by power series. We conclude that NPR simulations with large-signal codes have the potential to shorten the design phase of TWTs by eliminating the need for repeated build-test cycles to meet a required NPR     

倍频程螺旋线行波管2.5维大信号分析和设计

本文利用2.5维大信号分析程序和辅助分析程序,对一只7-18GHz带宽,2-2.5kW脉冲输出功率的螺旋线行波管进行计算机模拟,电子注感应电流基波分量在螺旋线慢波系统高频场中的相位分析和最佳工作点分析。在此基础上,探讨了色散曲线布局,衰减器配置,切断位置,再同步设计等倍频程螺旋线行波管的设计方法。     

Three-dimensional evaluation of energy extraction in output cavities of klystron amplifiers

An accurate three-dimensional analysis of the performance of output gaps in klystron amplifiers was carried out. The parameters investigated were: beam radius, perveance, output gap angle, output gap peak voltage, RF phase, magnetic field, and beam interception. A new, accurate method for computing space-charge forces between axially and radially deformable rings was developed. Rigorous expressions for electric and magnetic fields in the tunnel and the gap were used. The highest computed efficiency for an optimally bunched beam was 80.3 percent with 6 percent "ideal" interception. An important conclusion is that efficiency decreases with increasing normalized tunned radiusbeta_{e}aforb/aconstant in the examined range0.4 and probably beyond. Excellent correlation in efficiency and interception between a model klystron developed by E. Lien of Varian and the authors' program was established.     

Accurate parametric modeling of folded waveguide circuits for millimeter-wave traveling wave tubes

In this paper, results of different models are compared for calculating effective, cold-circuit (beam-free) phase velocities and interaction impedances of folded waveguide (FW) slow wave circuits for use in millimeter-wave traveling wave tubes (TWT). These parameters are needed for one-dimensional (1-D) parametric model simulations of FW traveling wave tubes (FWTWTs). The models investigated include approximate analytic expressions, equivalent circuit, three-dimensional (3-D) finite difference, and 3-D finite element. The phase velocity predictions are compared with experimental measurements of a representative FW circuit. The various model results are incorporated into the CHRISTINE1D code to obtain predictions of small signal gain in a 40-55 GHz FWTWT. Comparing simulated and measured frequency-dependent gain provides a sensitive, confirming assessment of the accuracy of the simulation tools. It is determined that the use of parametric 1-D TWT models for accurate, full band predictions of small signal gain in FWTWTs requires knowledge of phase velocity and impedance functions that are accurate to <0.5% and <10%, respectively. Saturated gain predictions, being approximately half as sensitive to these parameters, appear to require correct specification of phase velocity and interaction impedance to within /spl sim/1% and 20%, respectively. Although all models generate sufficiently accurate predictions of the interaction impedance, not all generate sufficiently accurate predictions of the effective axial phase velocity.     

Use of the BFCPIC and BFCRAY codes to describe space charge limited emission in electron guns for gyrotrons

Numerical modelling of an electron gun in the space charge limited regime requires determining the current density distribution as well as the electric fields and electron trajectories. This is a rather complicated self-consistent problem, since the space charge influences the electric field, which in turn influences the electron trajectories. Previous simulations of magnetron electron guns using the BFCPIC and BFCRAY codes used a simple emission model (constant current density) that is approximately valid for thermionic emission. The code has been modified to include space charge limited emission. Several different ways of doing this are considered. One of the models considered uses Gauss’s law to force the electric field on the emitter to vanish; it was used in the original version of BFCPIC for the simulation of ion diodes. A second is based on the use of Child’s law (locally), which may be more appropriate for extension to fully electromagnetic particle-in-cell (PIC) codes. Calculations were performed with both models, and the results compared with each other and with experiments performed at FZK.     

Temperature distributions in the human leg for VLF-VHF exposures atthe ANSI-recommended safety levels

Using a block model of 1532 cubical cells, temperature distributions are calculated for the lowest 21 cm of the human leg for electric fields recommended in the ANSI RF safety guideline. The thermal model uses inhomogeneous volume-averaged tissue properties: blood-flow rate, metabolism, thermal conductivity, specific heat, etc. The SARs are obtained using the impedance method. A modified finite-difference technique is used to solve the 3-D heat-conduction equation for the thermal model. Numerical results are obtained for RF currents at 3 and 40 MHz projected for the E fields recommended by the ANSI Standard (614 and 61.4 V/m, respectively) and also for power densities one-tenth of that level. Temperatures as high as 41.6°C are obtained for some internal cells for the higher E fields while relatively moderate temperatures on the order of 37°C are obtained for the lower E fields. Some of the calculated results for the surface temperature have been compared and found to be in good agreement with the experimental data for initial rates of heating     

MAGIC3D Simulations of a 500-W Ka-Band Coupled-Cavity Traveling-Wave Tube

The 3-D particle-in-cell (PIC) simulations of a Ka-band coupled-cavity traveling-wave tube (CCTWT) are shown. The computational analysis of the Ka-band coupled-cavity slow-wave structure was conducted through the use of an electromagnetic PIC code MAGIC3D. The choice of a double-slot staggered RF cavity circuit was made because of a wide frequency bandwidth, moderate interaction impedance, and excellent thermal dissipation properties. We investigated the large-signal and nonlinear beam dynamics of a Ka-band CCTWT using MAGIC3D. The center frequency of the Ka-band CCTWT can be tuned from 28.4 to 30 GHz by varying the cathode voltage. Hot-test simulations show that the 84-cavity Ka-band CCTWT produces 540 W of saturated output power at 29 GHz with an electronic efficiency of 8.1% and a gain of 28 dB when the beam voltage and current are set to 17 kV and 390 mA, respectively.      

W波段折叠波导慢波结构设计及三维注波互作用模拟

本文综合分析了折叠波导的几何尺寸和电子束参数，运用电磁场软件MAFIA的粒子模拟程序对三维折叠波导慢波结构进行了模拟．模型中，电磁波通过波导模式导入；为了克服较大空间电荷效应造成的电子注发散，使用了纵向聚焦磁场．基于三维互作用模型得到了W波段折叠波导的模拟结果，该结果可以对折叠波导慢波结构的三维互作用性能进行预测和分析．     

Low power digital TTL command for 1 bit 60 GHz RF MEMS phase shifter

This paper presents a circuit used to actuate Radio Frequency MicroElectroMechanical Systems (RF MEMS) based phase shifters for 60 GHz beam forming. Wireless transmission in the 60 GHz band show a dramatic free space loss, thus making it mandatory to use antennas with high gain in a desired direction. Beam forming is then necessary to optimize the link budget between communicating nodes. Antenna arrays can be used for beam forming. The work done at LAAS was aiming to provide RF MEMS based phase shifters working with a standard digital command. RF MEMS commonly have high actuation voltages usually obtained with charge pumps. This paper presents a full 1 bit phase shifter with a digital translation part, an analog DC–DC converter, and an RF MEMS based loaded line phase shifter for 60 GHz signals. This converter is built using boost converters as an alternative to the use of charge pumps for driving RF MEMS in the field of wireless communications. Boost converters, despite relatively high current peaks, allow shorter settling times compared to charge pumps.     

The Beam-Wave Interaction in a Ka-Band Relativistic Coupled-Cavity TWT

By the method of the eigen mode expansion, a full three-dimensional (3-D) model has been developed that can be used to investigate the beam-wave interaction in a high-power, Ka-band relativistic coupled-cavity traveling-wave tube (TWT). In the tube studied by us, a sever in the interaction section is used to restrain the oscillation of RF electromagnetic fields between the input and output end. In this case, the asymmetric hybrid HTM₁₁ mode has little impact on the main interaction process ^[1,2], so the RF electromagnetic fields mainly interacting with the modulated electron beam belong to the symmetric mode TM₀₁. The presented model includes three-dimensional RF fields, three-dimensional electron motions, and improved three-dimensional space-charge fields including dc and ac space-charge fields. Moreover, this model can also calculate backward radiation excited by modulated electron beam and the direct effect of the transverse electron motion on the energy exchange. Our calculation results show that the space-charge field has evident effect on the interaction process, the transverse electron motion has some, and the backward radiation has little.     

W波段带状注速调管多间隙腔高频结构及其特性

通过对W波段带状注速调管的哑铃型谐振腔特性进行分析,利用三维高频仿真设计软件CST-MSW建立了单间隙、三间隙、五间隙腔的物理模型,详细分析了不同结构尺寸对于高频系统相关冷参数的影响,优化得到了相应的高频结构及其特性参数,以此作为开发的二维注波互作用程序SBK.2D快速计算的高频输入参数.同时,为了结合注波互作用系统的热参数三维仿真设计,采用MAGIC-3D对多间隙腔的高频特性及参数进行了仿真计算,得到的结果与CST冷参数设计基本一致,为W波段带状注速调管注波互作用系统的设计提供了重要参数和依据.     

Discretized Gabor-based beam algorithm for time-harmonic radiation from two-dimensional truncated planar aperture distributions .II. Asymptotics and numerical tests

For pt.I see ibid., vol.50, no.12, p.1751-59 (2002). In this second part of the two-part sequence dealing with Gabor-based Gaussian beam (GB) representations for the excitation of time-harmonic three-dimensional (3-D) vector electromagnetic fields excited by two-dimensional (2-D) arbitrarily polarized truncated planar aperture distributions (set in a discretized [configuration (space)]-[spectrum (wavenumber)] phase space), we employ high-frequency asymptotic approximations to reduce the formal solutions developed in part I to efficient algorithms for implementation. The resulting explicit expressions for the 3-D GB propagators are applied to the species of narrow-waisted GBs, which possess ray-like features without the failures of ray fields in ray-optical transition regions. The potential utility of such GBs in the synthesis of wave interactions with complex environments has been discussed previously. The narrow-waisted GB algorithms for the aperture and radiated near-to-far zone fields are calibrated for robustness, accuracy and efficiency by comparison with numerically generated reference solutions in a series of tests involving coordinate-separable rectangular aperture distributions with cosine amplitude tapers.     

Improved three-dimensional model for the nonlinear analysis of helix travelling-wave tubes

Based upon Tien's disc model, a plastic three-dimensional (3D) super-particle model is proposed to improve the nonlinear analysis of helix travelling-wave tubes. The 3D space-charge forces are calculated, and 3D large-signal working equations are obtained. Numerical results employing a periodic magnetic focusing field are presented. The effects of the space-charge forces with respect to the different directions are investigated and a comparison of travelling wave amplifier (TWA) solutions obtained with the 3D model and the general two dimensional (2D) large-signal programs is given. The 3D electron motions are also analysed. Some useful conclusions, on both theory and experimental researches on TWAs, are obtained: (1) the 3D numerical results are obviously different from the 2D ones, in which the saturation gain decreases while the interaction length increases; (2) because the angular electron bunching results from axial spacial bunching in the helical 3D electron motions, there exists an angular space-charge force; despite the RF potentials and magnetic focusing fields being symmetrical around the axis; (3) the angular space-charge forces can be safely ignored whereas the radial and the axial space-charge forces must be included in the 3D calculation.     

变参量Wiggler FEL的三维非线性理论

本文建立了变参量Wiggler自由电子激光放大器的三维自洽非线性理论。考虑电磁波、电子注和Wiggler场三者均在三维空间中变化,忽略电子注的自身场和空间电荷效应,导出了带有变参量双绕螺旋Wiggler场和轴向导引磁场的矩形波导,轴对称电子注自由电子激光器的非线性互作用微分方程组。数值模拟表明:在适当位置逐渐减小Wiggler场的幅值或周期均能有效地提高电子注的能量转换效率。