粒子-in-单元（PIC）模拟中，不同权重的宏粒子弛豫至不同温度
Macroparticles with different weights relax to different temperatures in Particle-In-Cell simulations
AI总结：
该研究针对PIC模拟中宏粒子权重差异导致的非物理温度失衡问题，推导了温度演化方程并提出延迟该失衡的策略。
AI中文摘要：
粒子-in-单元（Particle-In-Cell, PIC）模拟的一个显著特征是使用宏粒子，每个宏粒子代表大量物理粒子。相关的宏粒子权重可在不同粒子种类之间，甚至在不同宏粒子之间存在差异。本文表明，当宏粒子具有不同权重时，等离子体将演化至非物理的热平衡状态，其中温度并不相等：权重较低的宏粒子达到比物理预期更高的温度，而权重较高的宏粒子则达到更低的温度。研究显示，这种非物理平衡状态难以避免，但达到该状态需要一定时间，且相关的时间尺度取决于PIC算法的有效碰撞性。本文推导了一个可预测温度完整时间演化的方程，该方程与PIC模拟结果吻合良好，并利用该方程确定了延迟这种非物理热平衡形成的策略。
英文摘要：
A distinctive feature of Particle-In-Cell (PIC) simulations is the use of macroparticles, each representing many physical particles. The associated macroparticle weight can vary from one species to another, or even from one macroparticle to another. Here we show that, when macroparticles have different weights, the plasma evolves toward an unphysical thermal equilibrium in which the temperatures are unequal: lower-weight macroparticles reach a higher temperature than physically expected, and higher-weight macroparticles a lower one. We show that this unphysical equilibrium is difficult to avoid, but that reaching it takes time, and that the associated timescale depends on the effective collisionality of the PIC algorithm. We derive an equation that predicts the full time evolution of the temperatures, in good agreement with PIC simulations, and use it to identify strategies to delay the establishment of this unphysical thermal equilibrium.
