Plasma transport simulations of Rayleigh–Taylor instability in near-ICF deceleration regimes
Plasma transport simulations of Rayleigh–Taylor instability in near-ICF deceleration regimes
复制标题
近 ICF 减速状态下瑞利-泰勒不稳定性的等离子体输运模拟
作者：
E. Vold;L. Yin;B. Albright
在动力学实验和近惯性约束聚变(ICF)中考察了等离子体组分之间的瑞利-泰勒(R-T)不稳定性。等离子体组分动力学的输运近似被用来表示流体力学程序(XRage)内的粘度和组分质量输运。在动力学试验条件下，将R-T模拟结果与完全动力学的格内质点方法[VPIC]以及R-T不稳定性增长率的解析模型进行了比较。在包括最大生长速率波数在内的初始波长范围内，由两个程序和解析模型得到的单模生长速率是合理一致的。这两个代码显示出相似的扩散混合前锋。代码与代码之间的微小差异来自动力学，而模拟-解析模型的差异来自于建立静力平衡初始条件的梯度的选择所主导的几个来源。在证明了VPIC程序在动力学测试区域内的代码一致性之后，将xRage程序与流体等离子体输运近似应用于更接近ICF内爆的减速条件下的单模R-T不稳定性，近似于碳(C)壳在重氢(D)燃料上的内爆。最大不稳定性的解析波长受动力学的限制，主要是在粘性中，当C-D界面离子温度接近1≈ μ时，最不稳定波长为10 m，最不稳定波长随着温度的升高而增大。只要在足够短的持续时间内分析模拟结果(在这种情况下为⪅0.2 ns)，解析粘性模型在初始扰动波长范围内与模拟结果一致。在无粘欧拉方程和算例之间比较了R-T减速过程中流体结构演化的细节，其中包括初始波长和初始扰动幅度范围内的等离子体传输。无粘性欧拉解显示了在R-T不稳定性中常见的依赖于网格的小尺度结构的级联，而在这种减速区域中的等离子体传输的模拟发展了单涡卷起，因为等离子体传输平滑了所有小于几微米的流体动力学结构。当模拟中考虑了动力学效应时，得到了R-T不稳定性的一个网格收敛的瞬态解，从而对ICF相关条件下R-T不稳定混合过程中的热离子进行了直接的数值模拟。
Rayleigh–Taylor (R–T) instability between plasma species is examined in a kinetic test and near-inertial confinement fusion (ICF) regimes. A transport approximation to the plasma species kinetics is used to represent viscosity and species mass transport within a hydrodynamic fluid code (xRage). R–T simulation results are compared in a kinetic test regime with a fully kinetic particle-in-cell approach [vectorized particle-in-cell (VPIC)] and with an analytic model for the growth rate of R–T instability. Single-mode growth rates from both codes and the analytic model are in reasonable agreement over a range of initial wavelengths including the wavenumber of maximum growth rate. Both codes exhibit similar diffusive mixing fronts. Small code-to-code differences arise from the kinetics, while simulation-analytic model differences arise from several sources dominated by the choice of gradients establishing the hydrostatic equilibrium initial conditions. After demonstrating code agreement in the kinetic test regime, which is practically accessible to the VPIC code, then the xRage code, with the fluid plasma transport approximation, is applied to single mode R–T instability under deceleration conditions closer to an ICF implosion, approximated with a carbon (C) shell imploding on a deuterium (D) fuel. The analytic wavelength of maximum instability is limited by the kinetics, primarily in the viscosity, and is found to be ≈10 μm for an ion temperature near 1 keV at this C–D interface, with the most unstable wavelength increasing as temperature increases. The analytic viscous model agrees with simulation results over a range of initial perturbation wavelengths, provided the simulation results are analyzed over a sufficiently short duration ( ⪅0.2 ns in this case). Details of the fluid structure evolution during this R–T deceleration are compared between the inviscid Euler equations and cases, which include plasma transport over a range in initial wavelengths and initial perturbation amplitudes. The inviscid Euler solutions show a grid-dependent cascade to smaller scale structures often seen in the R–T instability, while simulations with plasma transport in this deceleration regime develop a single vortex roll-up, as the plasma transport smoothes all hydrodynamic fluid structures smaller than several micrometers. This leads to a grid-converged transient solution for the R–T instability when kinetic effects are included in the simulations, and thus represents a direct numerical simulation of the thermal ions during R–T unstable mixing in ICF relevant conditions.