Here we report the Project Outcomes of the NSF/DOE Partnership in Basic Plasma Science and Engineering project "Numerical Modeling of Laser-Driven Experiments to Study Astrophysical Processes in Magnetized Turbulence" (PI: P. Tzeferacos, PHY-2033925).

Introduction. Astrophysicists frame the understanding of cosmic magnetic fields in a two-part process: the generation of tiny seed magnetic fields and their subsequent amplification by some form of turbulent dynamo. The latter is the result of stochastic motions in the matter that makes up the interstellar and intra-cluster mediums, which lead to an exponential amplification of magnetic energy density, reaching the values we observe today. The resulting magnetic fields can then be salient agents in a myriad of astrophysical processes, regulating how heat flows and how cosmic rays are accelerated and propagate through the cosmos. Even though conditions favorable for dynamos are common in astrophysics, they are extremely difficult to realize in laboratory experiments. The turbulent wrapping of field-lines operates against magnetic diffusivity, an interplay characterized by the system's "magnetic Reynolds number, Rm," the ratio of the magnetic diffusion over the wrapping (advection) timescale. Theory predicts that turbulent dynamo operates when magnetic Reynolds number surpasses values of ~100-200, which are hard to achieve in terrestrial laboratories. As a result, demonstrating experimentally turbulent dynamo has been considered a "holy grail" for laboratory astrophysics. The TDYNO (turbulent dynamo) collaboration, co-led by the University of Rochester and the University of Oxford, was able to achieve this goal through a concerted campaign of laser-driven experiments (Tzeferacos et al. Nat. Comm. 2018), carried out at the Omega Laser Facility at the Laboratory for Laser Energetics of the University of Rochester. The breakthrough established laboratory experiments as a component in the study of turbulent magnetized plasmas and opened a new path to laboratory investigations of other astrophysical processes (APS-DPP John Dawson Award for Excellence in Plasma Physics Research 2019).

Intellectual merit. The goals of this project are the design, execution, and interpretation of experimental campaigns of the TDYNO collaboration at the Omega Laser Facility, the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, and the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, to study astrophysical processes in magnetized turbulence. The experiments aim to demonstrate and study (1) the turbulent dynamo mechanism in the radiative, compressible regime, and (2) the acceleration of charged particles in magnetized turbulence. We designed these experiments through simulation campaigns using FLASH (Fryxell et al. ApJS 2000), a highly capable radiation-MHD code (Tzeferacos et al. HEDP 2015) the Flash Center for Computational Science develops, and large-scale 3D simulations. The simulations guide the experimental platform design for each facility to ensure that the experiments achieve the plasma conditions and the large Rm values required for the turbulent dynamo mechanism to operate; inform the timing and configuration of the experimental diagnostics; and enable the interpretation of the results of the experiments. The current project enabled (1) the first time-resolved characterization of turbulent dynamo in the laboratory (Bott et al. PNAS 2021); (2) the creation of an experimental analogue of ultra-high-energy cosmic ray transport in turbulent magnetized plasmas at Omega (Chen et al. ApJ 2020); (3) the first experimental demonstration of ion acceleration in a stochastic magnetic field at GSI (Campbell et al. 64th APS-DPP 2022); (4) the first realization of magnetized and supersonic, high-Rm plasma turbulence at the Laser Megajoule Facility (LMJ) in France (Bott et al. Phys. Rev. Lett. 2021); (5) the experimental demonstration at Omega of the insensitivity of dynamo-amplified magnetic fields to the strength of the seed fields from which they originate (Bott et al. MRE 2022); and (6) the demonstration of strong suppression of heat transport in a laboratory replica of galaxy-cluster turbulent plasmas (Meinecke et al. Sci. Adv. 2022), which may explain hot galaxy cluster cores and the observed large temperature variations in small spatial scales (see also accompanying figure).

Broader impacts. The project supported the training of early-career scientists in the design, execution, and interpretation of High Energy Density Physics (HEDP) experiments using validated FLASH simulations. More specifically, the project engaged at the Flash Center five graduate students, three postdocs, and five undergraduate students, who were involved in activities that have significantly enhanced their research experience and advanced their scientific skills. Throughout the course of the project, we enhanced, verified, and validated the capabilities of the FLASH code to enable higher fidelity simulations of laser-driven experiments. These capabilities were made widely available in the public releases of the FLASH code. The project furthered the Flash Center's efforts to transform the academic community's ability to use numerical simulations to design and analyze HEDP experiments at large laser facilities. This was made possible by the successful application and validation of the code in simulating highly demanding experimental campaigns, and with the availability of FLASH and its adoption by the academic HEDP community.

Last Modified: 04/23/2023
Modified by: Petros Tzeferacos