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arXiv 1405.4153physics.acc-phphysics.comp-ph
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强流束三维粒子模拟中的网格诱导噪声和熵增长

Grid induced noise and entropy growth in 3d particle-in-cell simulation of high intensity beams

Ingo Hofmann, Oliver Boine-Frankenheim

摘要:

用TRACEWIN程序研究了三维高强度聚束粒子模拟中固有的数值噪声,并与Struckmeier的解析模型进行了比较。后者假设熵增长与温度各向异性引起的马尔可夫型随机过程以及使用宏观粒子和计算空间电荷效应引起的人工碰撞有关。由此产生的噪声可以导致六维均方根发射度和适当定义的均方根熵的增长。我们证实了这种增长的依赖性上的束温度各向异性预测的Struckmeier。然而,我们也发现了一个明显的修改机制的噪声产生的非Liouville效应的泊松求解网格,它存在于周期性的聚焦系统,即使在完全没有温度各向异性。我们的研究结果是特别适用于高电流直线加速器模拟,在那里他们可以提供指导,以估计噪声的影响,并帮助找到一个有效的平衡之间的模拟粒子的数量和网格分辨率。

英文摘要:

The numerical noise inherent to particle-in-cell simulation of 3D high intensity bunched beams is studied with the TRACEWIN code and compared with an analytical model by Struckmeier. The latter assumes that entropy growth can be related to Markov type stochastic processes due to temperature anisotropy and the artificial collisions caused by using macro-particles and calculating the space charge effect. The resulting noise can lead to growth of the six-dimensional rms emittance and a suitably defined rms entropy. We confirm the dependence of this growth on the bunch temperature anisotropy as predicted by Struckmeier. However,we also find an apparently modified mechanism of noise generation by the non-Liouvillean effect of the Poisson solver grid, which exists in periodic focusing systems even in the complete absence of temperature anisotropy. Our findings are applicable in particular to high current linac simulation, where they can provide guidance to estimate noise effects and help finding an effective balance between the number of simulation particles and the grid resolution.

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