AHKASH：用于模拟天体物理无碰撞等离子体的一种新型混合粒子模拟（Hybrid particle-in-cell）代码
AHKASH: a new Hybrid particle-in-cell code for simulations of astrophysical collisionless plasma
AI总结：
本文介绍了在FLASH框架内开发的新型混合PIC代码AHKASH，采用Boris积分器和约束传输方法推进混合动力学方程，通过多种标准物理问题及湍流发电机测试验证了其准确性和并行扩展性，并引入了等温冷却和混合精度方法以降低计算成本。
摘要：
我们介绍了基于FLASH代码的天体物理混合动力学模拟代码（AHKASH）——这是一种在多物理代码FLASH框架内开发的新型混合粒子模拟（PIC）代码。该新代码使用二阶精度的Boris积分器和预测-预测-校正算法来推进混合动力学方程，并使用约束传输方法确保磁场无散度。该代码支持粒子和网格单元之间的各种插值方案，并通过插值后平滑来减少有限粒子噪声。我们进一步实现了δf方法，用于研究弱碰撞等离子体中的不稳定性。新代码在标准物理问题上进行了测试，例如带电粒子在均匀和空间变化磁场中的运动、Alfvén波和哨声波的传播，以及离子声波的Landau阻尼。我们测试了不同的插值核并证明了进行插值后平滑的必要性。我们将TurbGen湍流驱动模块耦合到新的混合PIC代码中，使我们能够在湍流发电机这一高度复杂的物理问题上测试代码。为了研究具有固定声速马赫数的稳态湍流，保持等温等离子体条件非常重要。因此，我们为混合PIC代码引入了一种新颖的冷却方法，并提供了该方法的测试和校准，以保持等离子体等温。我们描述并测试了“混合精度”方法，该方法显著降低了（约1.5倍）计算成本，同时不牺牲数值解的精度。最后，我们测试了新代码的并行可扩展性，结果显示在高达10,000个核心上具有出色的扩展性。
英文摘要：
We introduce $\texttt{A}$strophysical $\texttt{H}$ybrid-$\texttt{K}$inetic simulations with the $\texttt{flASH}$ code ($\texttt{AHKASH}$) -- a new Hybrid particle-in-cell (PIC) code developed within the framework of the multi-physics code $\texttt{FLASH}$. The new code uses a second-order accurate Boris integrator and a predictor-predictor-corrector algorithm for advancing the Hybrid-kinetic equations, using the constraint transport method to ensure that magnetic fields are divergence-free. The code supports various interpolation schemes between the particles and grid cells, with post-interpolation smoothing to reduce finite particle noise. We further implement a $δf$ method to study instabilities in weakly collisional plasmas. The new code is tested on standard physical problems such as the motion of charged particles in uniform and spatially varying magnetic fields, the propagation of Alfvén and whistler waves, and Landau damping of ion acoustic waves. We test different interpolation kernels and demonstrate the necessity of performing post-interpolation smoothing. We couple the $\texttt{TurbGen}$ turbulence driving module to the new Hybrid PIC code, allowing us to test the code on the highly complex physical problem of the turbulent dynamo. To investigate steady-state turbulence with a fixed sonic Mach number, it is important to maintain isothermal plasma conditions. Therefore, we introduce a novel cooling method for Hybrid PIC codes and provide tests and calibrations of this method to keep the plasma isothermal. We describe and test the `hybrid precision' method, which significantly reduces (by a factor $\sim1.5$) the computational cost, without compromising the accuracy of the numerical solutions. Finally, we test the parallel scalability of the new code, showing excellent scaling up to 10,000~cores.
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