Simulations of Laser Experiments to Study the Origin of Cosmic Magnetic Fields

磁场在宇宙中无处不在。然而，它们的起源还不完全清楚。虽然宇宙学家和天体物理学家提出了各种方法来产生小的种子磁场，但我们观察到的宇宙磁场的显著较大值被认为是等离子体湍流放大这些种子场的结果，即所谓的湍流发电机机制。这种机制尚未在受控的实验室环境中得到证实。目前正在计划利用劳伦斯利弗莫尔国家实验室的国家点火装置（NIF）和法国的激光兆焦耳（LMJ）装置——世界上最大的两个激光装置——的高强度激光器，在实验室中演示和研究湍流发电机的实验。该奖项支持的工作包括设计和建模这些高要求的实验，通过使用高性能代码运行大规模3D模拟的模拟活动。模拟是确保实验达到紊流发电机机制运行所需条件的关键，也是解释实验结果的关键。这项工作将进一步转变学术界在大型激光设备上设计和分析高能量密度物理实验的能力，并将培训初级科学家使用经过验证的模拟来设计和解释此类实验——这是国家的关键需求。模拟的实验建立在探路者实验的基础上，探路者实验是在英国卢瑟福-阿普尔顿实验室的Vulcan激光器和罗切斯特大学激光能量学实验室的Omega激光器上进行的。他们还利用FLASH代码（该小组开发的一种功能强大的辐射mhd代码）进行的经过验证的模拟，在设计和解释这些实验方面获得了经验。用于NIF和LMJ实验的配置基于Omega激光器部署的平台，适用于使用FLASH模拟的大型激光设施。NIF实验将探索大磁普朗特数下的发电机，即磁与流体雷诺数之比（Pm = Rm / Re 1），而LMJ实验将重点研究Pm 1下的磁场放大。发电机机制预计在每个状态下运行不同，FLASH模拟将确保实验达到所需的等离子体状态。这些实验有望描述湍流能量在速度、磁场和密度波动中的分布，为磁化湍流等离子体中的能量级联提供全面的图像。由于NIF每天的激光射击次数只有两次，LMJ只有三次，因此没有误差的余地：为了实现展示和表征湍流发电机机制的科学目标，设计和分析实验的数值建模是必不可少的。

Magnetic fields are ubiquitous in the universe. However, their origin is not fully understood. While cosmologists and astrophysicists have proposed a variety of ways in which small seed magnetic fields could be created, the significantly larger values of cosmic magnetic fields we observe are believed to be the result of the amplification of these seed fields by a turbulent flow of plasma, the so-called turbulent dynamo mechanism. This mechanism has not yet been demonstrated in a controlled laboratory environment. Experiments to demonstrate and study turbulent dynamo in the laboratory are now being planned using the high-intensity lasers at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory and the Laser Megajoule (LMJ) facility in France - the two largest laser facilities in the world. The effort supported by this award consists of designing and modeling these highly demanding experiments through simulation campaigns using a highly capable code to run large-scale 3D simulations. The simulations are vital to ensuring the experiments achieve the conditions required for the turbulent dynamo mechanism to operate and are crucial to interpreting the results of the experiments. This work will further the transformation of the academic community's ability to design and analyze High Energy Density Physics experiments at large laser facilities and will train junior scientists to design and interpret such experiments using validated simulations - a critical national need.The experiments to be modeled build on pathfinder experiments that have been conducted on the Vulcan laser at the Rutherford-Appleton Laboratory in the UK and the Omega laser at Laboratory for Laser Energetics at the University of Rochester. They also build on the experience that has been gained designing and interpreting these experiments using validated simulations done with the FLASH code, a highly capable radiation-MHD code developed by the group. The configurations to be used for the NIF and LMJ experiments are based on the platform deployed for the Omega laser, adapted for the large laser facilities using FLASH simulations. The NIF experiment will probe dynamo in the regime of large magnetic Prandtl numbers, the ratio of magnetic-to-fluid Reynolds numbers (Pm = Rm / Re 1), while the LMJ experiment will focus on magnetic field amplification for Pm 1. The dynamo mechanism is expected to operate differently in each regime and FLASH simulations will ensure the experiments reach the required plasma states. These experiments promise to characterize the distribution of turbulent energy among the velocity, magnetic field, and density fluctuations, providing a comprehensive picture of the energy cascade in a magnetized, turbulent plasma. Since the number of laser shots per shot day is only two at NIF and three at LMJ, there is no room for error: numerical modeling to design and analyze the experiments is imperative to accomplish the scientific goals of demonstrating and characterizing the turbulent dynamo mechanism.