Decomposition of plasma kinetic entropy into position and velocity space and the use of kinetic entropy in particle-in-cell simulations
Decomposition of plasma kinetic entropy into position and velocity space and the use of kinetic entropy in particle-in-cell simulations
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DOI：
10.1063/1.5098888
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发表时间：
2019-08-01
影响因子：
2.2
通讯作者：
Delzanno, Gian Luca
中科院分区：
文献类型：
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作者：
Liang, Haoming;Cassak, Paul A.;Delzanno, Gian Luca
我们描述了动力学熵的系统发展，作为全动力学粒子在细胞（PIC）模拟中的诊断，并用它来解释日光层、行星和天体物理系统中的等离子体物理过程。首先，我们以两种形式计算动熵：与每个宏观状态的微观状态数量的对数相关的组合形式和与 flnf 相关的连续形式，其中 f 是粒子分布函数。我们讨论每种方法的优点和缺点，并讨论在 PIC 代码中实现它们的微妙之处。使用位置空间二维和速度空间三维的无碰撞 PIC 模拟，我们验证了动熵诊断的实现，并讨论了如何优化数值参数以确保结果准确。我们证明，在反平行磁重联的优化模拟中，总动熵守恒为百分之三。动熵可以分解为位置空间熵和速度空间熵之和，我们用它来研究无碰撞重连接过程中动熵输运的性质。我们发现，由于磁重联期间的等离子体加热，电子和离子的速度空间熵随时间增加，而位置空间熵由于等离子体压缩而减少。该项目使用无碰撞模拟，因此无法解决物理耗散机制；尽管如此，这里开发的基础设施应该对碰撞或弱碰撞日光层、行星和天体物理系统的研究有用。除了重连之外，该诊断预计还适用于等离子体湍流和无碰撞冲击。
We describe a systematic development of kinetic entropy as a diagnostic in fully kinetic particle-in-cell (PIC) simulations and use it to interpret plasma physics processes in heliospheric, planetary, and astrophysical systems. First, we calculate kinetic entropy in two forms-the combinatorial form related to the logarithm of the number of microstates per macrostate and the continuous form related to flnf, where f is the particle distribution function. We discuss the advantages and disadvantages of each and discuss subtleties about implementing them in PIC codes. Using collisionless PIC simulations that are two-dimensional in position space and three-dimensional in velocity space, we verify the implementation of the kinetic entropy diagnostics and discuss how to optimize numerical parameters to ensure accurate results. We show the total kinetic entropy is conserved to three percent in an optimized simulation of antiparallel magnetic reconnection. Kinetic entropy can be decomposed into a sum of a position space entropy and a velocity space entropy, and we use this to investigate the nature of kinetic entropy transport during collisionless reconnection. We find the velocity space entropy of both electrons and ions increases in time due to plasma heating during magnetic reconnection, while the position space entropy decreases due to plasma compression. This project uses collisionless simulations, so it cannot address physical dissipation mechanisms; nonetheless, the infrastructure developed here should be useful for studies of collisional or weakly collisional heliospheric, planetary, and astrophysical systems. Beyond reconnection, the diagnostic is expected to be applicable to plasma turbulence and collisionless shocks.