摘要:
通过Fokker-Planck模拟,研究了等离子体在任意强度的直流电场中产生电流的过程以及电子分布函数的演变过程.研究发现,不同强度的电场中等离子体的行为存在着明显的差别.在弱电场中,电流与电场满足Spitzer公式,且电流产生的响应时间约等于撤销电场后电流衰减的弛豫时间;在中等强度的电场中,电子分布函数呈现为静止Maxwell分布和漂移Maxwell分布之和,而且在中等强度或者强直流电场中弛豫时间也将远远大于响应时间.根据电子分布函数的演变规律,推导了一组类似于流体力学方程的公式,这组方程像Spitzer公式一样简便地描述了等离子体中电流与电场的关系,并且对电场强度没有限制.数值模拟显示这组方程比Spitzer公式更适用于等离子体的混合粒子模拟中.
Abstract:
The generation of plasma current and the evolution of electron distribution under arbitrarily strong direct current electric fields have been studied by Fokker-Planck simulation. It is found that the behaviors of plasma are different under different fields: in the weak field, Spitzer' s law is suitable for describing the relationship between plasma current and electric field, and the response time to generate current is approximately equal to the relaxation time to reduce current after switching off the electric field; in the moderate field, the electron distribution is well represented by the sum of a stationary and drifting Maxwellian, and the relaxation time is much longer than the response time. According to the detailed knowledge of electron distributions, a set of hydrodynamic-like equations, similar to Spitzer's but without the weak-field limit, is given for calculating the current. It is more suitable for application in hybrid particle-in-cell simulations.
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