近日，英国牛津大学Robert J. Ewart团队研究了无碰撞等离子体中普遍非麦克斯韦平衡的弛豫。这一研究成果发表在2025年4月22日出版的《美国科学院院刊》杂志上。

研究组通过熵最大化过程推导了湍流弛豫等离子体的通用平衡，该过程解释了某些无碰撞不变量的短时守恒。这些无碰撞不变量的守恒赋予了系统对其先前条件的部分“记忆”，但由于湍流级联向小尺度的发展，在长时间尺度上是不完美的，这打破了相体积的精确守恒，使这种记忆变得不精确。

平衡仍然由短时无碰撞不变量决定，但不变量本身被湍流的性质驱动到一种普遍的形式。这在一维静电等离子体中的束不稳定性的情况下得到了数值证实，在这种情况下，足够强的湍流似乎会导致粒子能量的分布函数形成指数为-2的普遍幂律尾。

附：英文原文

Title: Relaxation to universal non-Maxwellian equilibria in a collisionless plasma

Author: Ewart, Robert J., Nastac, Michael L., Bilbao, Pablo J., Silva, Thales, Silva, Luís O., Schekochihin, Alexander A.

Issue&Volume: 2025-4-22

Abstract: Generic equilibria are derived for turbulent relaxing plasmas via an entropy-maximization procedure that accounts for the short-time conservation of certain collisionless invariants. The conservation of these collisionless invariants endows the system with a partial “memory” of its prior conditions but is imperfect on long time scales due to the development of a turbulent cascade to small scales, which breaks the precise conservation of phase volume, making this memory imprecise. The equilibria are still determined by the short-time collisionless invariants, but the invariants themselves are driven to a universal form by the nature of the turbulence. This is numerically confirmed for the case of beam instabilities in one-dimensional electrostatic plasmas, where sufficiently strong turbulence appears to cause the distribution function of particle energies to develop a universal power-law tail, with exponent 2.

DOI: 10.1073/pnas.2417813122

Source: https://www.pnas.org/doi/abs/10.1073/pnas.2417813122