Bott Archie F A, Tzeferacos Petros, Chen Laura, Palmer Charlotte A J, Rigby Alexandra, Bell Anthony R, Bingham Robert, Birkel Andrew, Graziani Carlo, Froula Dustin H, Katz Joseph, Koenig Michel, Kunz Matthew W, Li Chikang, Meinecke Jena, Miniati Francesco, Petrasso Richard, Park Hye-Sook, Remington Bruce A, Reville Brian, Ross J Steven, Ryu Dongsu, Ryutov Dmitri, Séguin Fredrick H, White Thomas G, Schekochihin Alexander A, Lamb Donald Q, Gregori Gianluca
Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom;
Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544.
Proc Natl Acad Sci U S A. 2021 Mar 16;118(11). doi: 10.1073/pnas.2015729118.
Understanding magnetic-field generation and amplification in turbulent plasma is essential to account for observations of magnetic fields in the universe. A theoretical framework attributing the origin and sustainment of these fields to the so-called fluctuation dynamo was recently validated by experiments on laser facilities in low-magnetic-Prandtl-number plasmas ([Formula: see text]). However, the same framework proposes that the fluctuation dynamo should operate differently when [Formula: see text], the regime relevant to many astrophysical environments such as the intracluster medium of galaxy clusters. This paper reports an experiment that creates a laboratory [Formula: see text] plasma dynamo. We provide a time-resolved characterization of the plasma's evolution, measuring temperatures, densities, flow velocities, and magnetic fields, which allows us to explore various stages of the fluctuation dynamo's operation on seed magnetic fields generated by the action of the Biermann-battery mechanism during the initial drive-laser target interaction. The magnetic energy in structures with characteristic scales close to the driving scale of the stochastic motions is found to increase by almost three orders of magnitude and saturate dynamically. It is shown that the initial growth of these fields occurs at a much greater rate than the turnover rate of the driving-scale stochastic motions. Our results point to the possibility that plasma turbulence produced by strong shear can generate fields more efficiently at the driving scale than anticipated by idealized magnetohydrodynamics (MHD) simulations of the nonhelical fluctuation dynamo; this finding could help explain the large-scale fields inferred from observations of astrophysical systems.
理解湍流等离子体中磁场的产生和放大对于解释宇宙中磁场的观测现象至关重要。一个将这些磁场的起源和维持归因于所谓波动发电机的理论框架最近在低磁普朗特数等离子体的激光设施实验中得到了验证（[公式：见原文]）。然而，同一框架提出，当[公式：见原文]时，波动发电机的运行方式应该不同，[公式：见原文]是与许多天体物理环境相关的状态，如星系团的星系团内介质。本文报道了一个创建实验室[公式：见原文]等离子体发电机的实验。我们提供了等离子体演化的时间分辨特征，测量了温度、密度、流速和磁场，这使我们能够探索波动发电机在初始驱动激光与靶相互作用期间由比尔曼电池机制作用产生的种子磁场上运行的各个阶段。发现特征尺度接近随机运动驱动尺度的结构中的磁能增加了近三个数量级并动态饱和。结果表明，这些磁场的初始增长速度比驱动尺度随机运动的周转速度快得多。我们的结果表明，强剪切产生的等离子体湍流在驱动尺度上产生磁场的效率可能比非螺旋波动发电机的理想磁流体动力学（MHD）模拟预期的更高；这一发现有助于解释从天体物理系统观测中推断出的大规模磁场。
