Author: Gjonaj, E.
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Recent Developments in Wake Field and Beam Dynamics Computation  
  • E. Gjonaj
    TEMF, TU Darmstadt, Darmstadt, Germany
  Funding: Work partially funded by DESY, Hamburg.
Wake potentials and beam coupling impedances can be calculated analytically only for simple structures and for special limiting cases. For the calculation of wake fields in ’real-world’ 3D accelerator structures, one has to rely on numerical electromagnetic field computation. Among the most successful numerical techniques for wake field calculations in the time domain are dispersion-free methods in the moving window. These techniques are particularly useful for short-range wake field calculations. Recently, this class of methods has been extended to include Surface Impedance Boundary Conditions (SIBC) based on the Auxiliary Differential Equation (ADE) technique. These boundary conditions allow the computation of resistive wall wake fields for 3D structures with arbitrary frequency dependent conductivity. An important application of this method is the calculation resistive wall wake fields in novel accelerator chambers with NEG and amorphous carbon coatings. Other developments to be discussed include the calculation of CSR-wakes in bunch compressors and undulator structures for x-ray sources. This task is computationally very difficult because of the curved bunch trajectory that leads to extremely high frequency and long-range wake fields. Time domain as well as frequency domain methods based on high order DG and FE discretization techniques for the electromagnetic fields computation in such structures will be presented.
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Exploring the Validity of the Paraxial Approximation for Coherent Synchrotron Radiation Wake Fields  
  • D. A. Bizzozero, H. De Gersem, E. Gjonaj
    TEMF, TU Darmstadt, Darmstadt, Germany
  Coherent synchrotron radiation (CSR) is an essential consideration in modern accelerators, yet is often computationally difficult to accurately model. A common approach used in simulating CSR effects uses the paraxial, or slowly-varying envelope approximation with a simple constant cross-section approximation of the geometry. While these approximations are often valid for the simulation of many accelerator components, we aim to more closely analyze the errors introduced by such approximations by comparing them with wake field solutions obtained by full-wave electromagnetic field simulations. The simulations are performed with CSRDG (Coherent Synchrotron Radiation with Discontinuous Galerkin), our GPU-enabled MATLAB code. Extended from earlier work [Coherent Synchrotron Radiation and Wake Fields With Discontinuous Galerkin Time Domain Methods, Proceedings of IPAC 2017, Copenhagen, Denmark], CSRDG evolves Maxwell’s equations the time domain after a curvilinear coordinate transformation and a Fourier series decomposition in a transverse direction.  
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REPTIL - A Relativistic 3D Space Charge Particle Tracking Code Based on the Fast Multipole Method  
  • S. A. Schmid, H. De Gersem, E. Gjonaj
    TEMF, TU Darmstadt, Darmstadt, Germany
  Funding: This work is supported by the DFG in the framework of GRK 2128.
Modern free electron lasers and high current energy recovery linacs accelerate electron beams with particle bunch charges reaching up to several nanocoulombs. Especially in the low energy sections, such as the photoinjector of the accelerator, space charge interaction forces are the dominating effect influencing the dynamics of the electron beam. A direct computation of space charge forces is numerically very expensive. Commonly used simulation codes typically apply mesh based particle-in-cell methods (PIC) to solve this problem. Our simulation tool, REPTIL, is a relativistic, three-dimensional space charge tracking code, which computes the interaction forces based on a meshless fast multipole method (FMM). The FMM based space charge solver is more flexible regarding the choice of the interaction model and yields maximum accuracy for the near field forces between particles. For this reason, the FMM is very suitable for the simulation of the influence of space charge on the particle emission process in high current photoinjectors. In this contribution, we present a numerical study of the efficiency and the accuracy of the method. Therefore, we perform a case study for the PITZ photoinjector used for the European XFEL at DESY. Furthermore, we compare the performance of REPTIL with commonly used PIC codes like e.g. ASTRA. Finally, we investigate a hybrid approach by using the FMM on a mesh. The latter method makes further increases in the particle number possible, which translates to a higher resolution in the phase space of the electron bunch.
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