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

University Of Chicago, Chicago IL

Investigators

Abstract

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.