Time-resolved turbulent dynamo in a laser plasma

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 (Pm<1). However, the same framework proposes that the fluctuation dynamo should operate differently when Pm≳1, the regime relevant to many astrophysical environments such as the intracluster medium of galaxy clusters. This paper reports an experiment that creates a laboratory Pm≳1 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的磁场产生与放大机制，是阐释宇宙中磁场观测现象的核心前提。近期，一项将此类磁场的起源与维持归因于所谓涨落发电机（fluctuation dynamo）的理论框架，在低磁普朗特数（magnetic Prandtl number）等离子体（Pm<1）的激光装置实验中得到了验证。然而，该理论框架同时提出，当Pm≳1时，涨落发电机的运行机制会产生显著差异——这一参数区域与诸多天体物理环境高度相关，例如星系团的团内介质（intracluster medium）。本研究报道了一项在实验室中实现Pm≳1等离子体发电机的实验。我们对等离子体演化进行了时间分辨表征，测量了其温度、密度、流动速度与磁场，借此得以探究涨落发电机在初始种子磁场作用下的各运行阶段：该初始种子磁场由驱动激光与靶材相互作用过程中的比尔德曼电池机制（Biermann-battery mechanism）所产生。研究发现，特征尺度与随机运动驱动尺度相近的结构内，磁能量增长了近三个数量级，并最终达到动力学饱和。实验表明，此类磁场的初始增长速率远高于驱动尺度随机运动的周转速率。我们的研究结果表明，强剪切产生的等离子体湍流，可在驱动尺度下比非螺旋涨落发电机的理想化磁流体动力学（magnetohydrodynamics，MHD）模拟所预期的效率更高地生成磁场；这一发现或有助于阐释天体物理系统观测中所推断出的大尺度磁场起源。