M. Botton, T. Antonsen, S. Cooke, B. Levush, I. Chernyavskiy, A. Vlasov
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The ADI scheme is unconditionally stable, thus the time step can be larger than the one imposed by the CFL condition. This allows, in principal, high spatial resolution required for high frequency cavities without extending the calculation time. Nevertheless, the ADI scheme requires a solution of six tri-diagonal equations for the electric field followed by derivation of the magnetic field. This increased complexity somewhat impairs the efficiency of the calculations. The boundary conditions on the six faces of the beam tunnel are general, and can be specified either as perfect conductor, symmetry boundary conditions or arbitrary external specification. The electron beam is described by a set of trajectories. 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引用次数: 0
摘要
海王星(Non-stationary Excitation Nonlinear Evolution of Particle Trajectories Under Non-stationary Excitation)是一个正在开发的用于相干辐射源三维时间相关大振幅模拟的新程序。该代码主要描述电子束与电磁波相互作用发生的电子束隧道。电磁场是由复杂的包络来描述的,它允许几个载波频率,这些载波频率都是某个基频的谐波。以电子束为源,采用ADI格式自一致地计算了电场。ADI方案基于将时间步长分成两半。在每个子步骤中,麦克斯韦方程组在一个方向上隐式求解,在垂直方向上显式求解。ADI方案是无条件稳定的,因此时间步长可以大于CFL条件所施加的时间步长。原则上,这可以在不延长计算时间的情况下实现高频腔所需的高空间分辨率。然而,ADI方案需要六个电场的三对角线方程的解,然后推导磁场。这种增加的复杂性在一定程度上削弱了计算的效率。光束隧道的六个面边界条件是一般的,可以指定为完美导体、对称边界条件或任意外部规范。电子束用一组轨迹来描述。用于求解这些轨迹运动方程的冻结场近似简化了计算,但要求电子在相互作用域中的传递时间远短于空腔填充时间,这在大多数相干辐射源中都是如此。与传统的三维细胞内粒子编码相比,由此产生的编码预计将更高效,并且需要适度的计算能力。文中给出了将该代码用于介质加载波导作为慢波放大器的实例。
Neptune: An efficient time dependent 3D simulations of coherent radiation sources
NEPTUNE (Nonlinear Evolution of Particle Trajectories Under Non-stationary Excitation) is a new code under development for 3D time dependent large amplitude simulations of coherent radiation sources. The code deals mainly with the description of the beam tunnel where the interaction between the electron beam and the electromagnetic (EM) waves takes place. The EM fields are described by complex envelopes allowing several carrier frequencies which are all harmonics of some fundamental frequency. The fields are calculated self consistently with the electron beam as a source, using the ADI scheme. The ADI scheme is based on splitting the time step into two halves. In each sub-step Maxwell's equations are solved implicitly in one direction and explicitly in the perpendicular direction. The ADI scheme is unconditionally stable, thus the time step can be larger than the one imposed by the CFL condition. This allows, in principal, high spatial resolution required for high frequency cavities without extending the calculation time. Nevertheless, the ADI scheme requires a solution of six tri-diagonal equations for the electric field followed by derivation of the magnetic field. This increased complexity somewhat impairs the efficiency of the calculations. The boundary conditions on the six faces of the beam tunnel are general, and can be specified either as perfect conductor, symmetry boundary conditions or arbitrary external specification. The electron beam is described by a set of trajectories. The frozen-field approximation used for the solution of the equation of motion for these trajectories facilitates the calculations, but requires that the transit time of the electrons in the interaction domain is much shorter than the cavity fill time, as is the case in most of the coherent radiation sources. The resulting code is expected to be more efficient and requires modest computing power compared to conventional 3D particle-in-cell codes. Examples of the use of the code for dielectric loaded waveguide serving as a slow-wave amplifier are presented.