Ly Minh Nhat, Sano Takayoshi, Sakawa Youichi, Sentoku Yasuhiko
Institute of Laser Engineering, Osaka University, Suita, Osaka 565-0871, Japan.
Phys Rev E. 2023 Aug;108(2-2):025208. doi: 10.1103/PhysRevE.108.025208.
Collisionless shock acceleration, which transfers localized particle energies to nonthermal energetic particles via electromagnetic potential, is ubiquitous in space plasma. We investigate dynamics of collisionless electrostatic shocks that appear at the interface of two plasma slabs with different pressures using one-dimensional particle-in-cell (PIC) simulations and find that the shock structure transforms to a double-layer structure at the high density gradient. The threshold condition of the structure transformation is identified as density ratio of the two plasma slabs Γ ∼40 regardless of the temperature ratio between them. We then update the collisionless shock model that takes into account density expansion effects caused by a rarefaction wave to improve the prediction of the critical Mach numbers. These critical Mach numbers are benchmarked by PIC simulations for a wide range of Γ. Furthermore, we introduce a semianalytical approach to forecast the shock velocity just from the initial conditions based on a concept of the accelerated fraction α.
无碰撞激波加速通过电磁势将局部粒子能量转移到非热能粒子，在空间等离子体中普遍存在。我们使用一维粒子模拟单元（PIC）模拟研究了出现在具有不同压力的两个等离子体平板界面处的无碰撞静电激波的动力学，发现激波结构在高密度梯度下转变为双层结构。结构转变的阈值条件被确定为两个等离子体平板的密度比Γ ∼ 40，而与它们之间的温度比无关。然后，我们更新了无碰撞激波模型，该模型考虑了由稀疏波引起的密度膨胀效应，以改进对临界马赫数的预测。这些临界马赫数通过针对广泛的Γ范围的PIC模拟进行了基准测试。此外，我们基于加速分数α的概念引入了一种半解析方法，仅根据初始条件预测激波速度。
