Paper  Title  Page 

WEPLG01  Analysis of Emittance Growth in a Gridless Spectral Poisson Solver for Fully Symplectic Multiparticle Tracking  335 


Funding: This work was supported by the Director, Office of Science, Office of High Energy Physics, of the U.S. Department of Energy under Contract No. DEAC0205CH11231. Gridless spectral methods for selfconsistent symplectic space charge modeling possess several advantages over traditional momentumconserving particleincell methods, including the absence of numerical grid heating and the presence of an underlying multiparticle Hamiltonian. Nevertheless, evidence of collisional particle noise remains. For a class of such 1D and 2D algorithms, we provide analytical models of the numerical field error, the optimal choice of spectral modes, and the numerical emittance growth per timestep. We compare these results with the emittance growth models of Struckmeier, Hoffman, Kesting, and others. 

Slides WEPLG01 [11.804 MB]  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWICAP2018WEPLG01  
About •  paper received ※ 18 October 2018 paper accepted ※ 28 January 2019 issue date ※ 26 January 2019  
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WEPLG02 
REPTIL  A Relativistic 3D Space Charge Particle Tracking Code Based on the Fast Multipole Method  


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 particleincell methods (PIC) to solve this problem. Our simulation tool, REPTIL, is a relativistic, threedimensional 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. 

Slides WEPLG02 [2.443 MB]  
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WEPLG03  Theoretical and Computational Modeling of a Plasma Wakefield BBU Instability  341 


Funding: This work was supported in part by the Department of Energy, Office of Science, Office of High Energy Physics, under contract number DESC0018718. Plasma wakefield accelerators achieve accelerating gradients on the order of the wavebreaking limit, m c^{2} k_{p}/e, so that higher accelerating gradients correspond to shorter plasma wavelengths. Smallscale accelerating structures, such as plasma and dielectric wakefields, are susceptible to the beam breakup instability (BBU), which can be understood from the PanofskyWenzel theorem: if the fundamental accelerating mode scales as b^{1} for a structure radius b, then the dipole mode must scale as b^{3}, meaning that high accelerating gradients necessarily come with strong dipole wake fields. Because of this relationship, any plasmaacceleratorbased future collider will require detailed study of the tradeoffs between extracting the maximum energy from the driver and mitigating the beam breakup instability. Recent theoretical work* predicts the tradeoff between the witness bunch stability and the amount of energy that can be extracted from the drive bunch, a socalled efficiencyinstability relation . We will discuss the beam breakup instability and the efficiencyinstability relation and the theoretical assumptions made in reaching this conclusion. We will also present preliminary particleincell simulations of a beamdriven plasma wakefield accelerator used to test the domain of validity for the assumptions made in this model. * V. Lebedev, A. Burov, and S. Nagaitsev, "Efficiency versus instability in plasma accelerators", Phys. Rev. Acc. Beams 20, 121301 (2017). 

Slides WEPLG03 [2.234 MB]  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWICAP2018WEPLG03  
About •  paper received ※ 01 November 2018 paper accepted ※ 28 January 2019 issue date ※ 26 January 2019  
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WEPLG05  Review of Spectral Maxwell Solvers for Electromagnetic ParticleinCell: Algorithms and Advantages  345 


Electromagnetic ParticleInCell codes have been used to simulate both radiofrequency accelerators and plasmabased accelerators. In this context, the ParticleInCell algorithm often uses the finitedifference method in order to solve the Maxwell equations. However, while this method is simple to implement and scales well to multiple processors, it is liable to a number of numerical artifacts that can be particularly serious for simulations of accelerators. An alternative to the finitedifference method is the use of spectral solvers, which are typically less prone to numerical artifacts. In this talk, I will review recent progress in the use of spectral solvers for simulations of plasmabased accelerators. This includes techniques to scale those solvers to large number of processors, extensions to cylindrical geometry, and adaptations to specific problems such as boostedframe simulations.  
Slides WEPLG05 [2.861 MB]  
DOI •  reference for this paper ※ https://doi.org/10.18429/JACoWICAP2018WEPLG05  
About •  paper received ※ 06 November 2018 paper accepted ※ 28 January 2019 issue date ※ 26 January 2019  
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