Fujii Keisuke
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6169, USA.
Phys Rev E. 2023 Aug;108(2-2):025204. doi: 10.1103/PhysRevE.108.025204.
Nonthermal velocity distributions with much greater tails than a Maxwellian have been observed for radical atoms in plasmas for a long time. Historically, such velocity distributions have been modeled by a two-temperature Maxwell distribution. In this paper, I propose a model based on collisional energy cascade, which has been studied in the field of granular materials. In the collisional energy cascade, a particle ensemble undergoes energy input at the high-energy region, entropy production by elastic collisions among particles, and energy dissipation. For radical atoms, energy input may be caused by the Franck-Condon energy of molecular dissociation or charge-exchange collision with hot ions, and the input energy is eventually dissipated by collisions with the walls. I show that the steady-state velocity distribution in the collisional energy cascade is approximated by the generalized Mittag-Leffler distribution, which is a one-parameter extension of the Maxwell distribution. This parameter indicates the degree of the nonthermality and is related to the relative importance of energy dissipation over entropy production. This model is compared with a direct molecular dynamics simulation for simplified gaseous systems with energy input, as well as some experimentally observed velocity distributions of light radicals in plasmas.
长期以来，人们在等离子体中观察到自由基原子的非热速度分布，其尾部比麦克斯韦分布要大得多。从历史上看，这种速度分布是由双温度麦克斯韦分布来建模的。在本文中，我提出了一个基于碰撞能量级联的模型，该模型在颗粒材料领域已经得到了研究。在碰撞能量级联中，一个粒子系综在高能区域经历能量输入、粒子间弹性碰撞产生熵以及能量耗散。对于自由基原子，能量输入可能是由分子解离的弗兰克 - 康登能量或与热离子的电荷交换碰撞引起的，而输入的能量最终通过与壁面的碰撞而耗散。我表明，碰撞能量级联中的稳态速度分布可以用广义米塔格 - 莱夫勒分布来近似，它是麦克斯韦分布的单参数扩展。这个参数表示非热程度，并且与能量耗散相对于熵产生的相对重要性有关。该模型与具有能量输入的简化气体系统的直接分子动力学模拟以及等离子体中一些实验观察到的轻自由基速度分布进行了比较。
