A close-up of the experimental target, consisting of two foils and a pair of grids, held together by cylindrical shields. Each target is about the size of a penny. They were carefully designed and machined to produce a turbulent plasma at conditions never reached before.
An international collaboration, co-led by the University of Oxford, the University of Rochester and the University of Chicago, has conducted experiments that captured for the first time in the laboratory the time history of the growth of magnetic fields by the turbulent dynamo, a physical mechanism thought to be responsible for generating and sustaining astrophysical magnetic fields.
The experiments accessed conditions relevant to most plasmas in the universe and quantified the rate at which the turbulent dynamo amplifies magnetic fields, a property previously only derived from theoretical predictions and numerical simulations. The rapid amplification they found exceeds theoretical expectations and could help explain the origin of the present-day large-scale fields that are observed in galaxy clusters. They reported their results in an article that appeared in the Proceedings of the National Academy of Sciences on 8 March 2021.
Ground-breaking experiements
In their article, the Turbulent Dynamo (TDYNO) team describes the laser-driven experiments they carried out at the Omega Laser Facility of the Laboratory for Laser Energetics (LLE) at the University of Rochester, where they previously demonstrated experimentally the existence of the turbulent dynamo mechanism. That breakthrough earned the team the 2019 John Dawson Award for Excellence in Plasma Physics Research from the American Physical Society. In their more recent TDYNO experiments at Omega, they were able to achieve conditions relevant to the hot, diffuse plasma of the intracluster medium in which the turbulent dynamo mechanism is thought to operate. This feat was accomplished by leveraging laser beams whose total power is equivalent to that of 10,000 nuclear reactors to access plasma regimes that previous liquid-metal and laser-driven experiments could not. Using an extensive suite of plasma diagnostics, the scientists were able to clearly measure as a function of time the magnetic field amplification produced by this mechanism.
‘Understanding how and at what rates magnetic fields are amplified at macroscopic scales in astrophysical turbulence is key for explaining the magnetic fields seen in galaxy clusters, the largest structures in the Universe,’ said Archie Bott, Postdoctoral Research Associate in the Department of Astrophysical Sciences at Princeton and lead author of the study. ‘While numerical models and theory predict fast turbulent dynamo amplification at very small scales compared to turbulent motions, it had remained uncertain as to whether the mechanism operates rapidly enough to account for dynamically significantly fields on the largest scales.’
Large-scale magnetic fields
At the core of the astrophysical dynamo mechanism is turbulence. Primordial magnetic fields are generated with strengths that are considerably smaller than those seen today in galaxy clusters. Stochastic plasma motions, however, can pick up these weak “seed” fields and amplify their strengths to significantly larger values via stretching, twisting and folding of the field. The rate at which this amplification happens, the “growth rate,” differs for the different spatial scales of the turbulent plasma motions: theory and simulations predict that the growth rate is large at the smallest length scales but far smaller at length scales comparable to those of the largest turbulent motions. The TDYNO experiments demonstrated that this may not be the case: turbulent dynamo – when operating in a realistic plasma – can generate large-scale magnetic fields much more rapidly than currently expected by theorists.
‘Our theoretical understanding of the workings of turbulent dynamo has grown continuously for over half a century,’ said Gianluca Gregori, Professor of Physics at the Department of Physics of the University of Oxford and the experimental lead of the project. ‘Our recent TDYNO laser-driven experiments were able to address for the first time how turbulent dynamo evolves in time, enabling us to experimentally measure its actual growth rate.’
Answering key questions
These experiments are part of a concerted effort by the TDYNO team to answer key questions that are debated in the turbulent dynamo literature, establishing laboratory experiments as a component in the study of turbulent magnetised plasmas. The collaboration has built an innovative experimental platform that, coupled with the power of the OMEGA laser, enables the team to probe the different plasma regimes relevant to various astrophysical systems. The experiments are designed using numerical simulations performed with the FLASH code, a publicly available simulation code that can accurately model laser-driven experiments of laboratory plasmas. FLASH is developed by the Flash Center for Computational Science, which recently moved from the University of Chicago to the University of Rochester.
‘The ability to do high-fidelity, predictive modeling with FLASH, and the state-of-the art diagnostic capabilities of the Omega Laser Facility at the LLE, have put our team in a unique position to decisively advance our understanding of how cosmic magnetic fields come to be,’ said Petros Tzeferacos, Director of the Flash Center, Associate Professor at the Department of Physics and Astronomy, and Senior Scientist at the LLE at the University of Rochester – the simulation lead of the project.
Blazing a path
‘This work blazes a path to laboratory investigations of a variety of astrophysical processes mediated by magnetised turbulence,’ added Don Lamb, the Robert A. Millikan Distinguished Service Professor Emeritus in Astronomy and Astrophysics at the University of Chicago and Principal Investigator of the TDYNO National Laser User’s Facility (NLUF) project. ‘It’s truly exciting to see the scientific results that the ingenuity of this team is making possible.’
The project was carried under the auspices of the U.S. Department of Energy (DOE), the National Science Foundation (NSF), the European Research Council (ERC), and the Engineering and Physical Sciences Research Council (EPSRC). The NLUF program of DOE’s National Nuclear Security Administration provided the collaboration with access to the Omega Laser Facility and critical funding for the design and execution of the experiments. The FLASH numerical simulations were conducted at the Argonne Leadership Computing Facility with support from the ASCR Leadership Computing Challenge (ALCC) programs of the DOE Office of Science.
Time-resolved turbulent dynamo in a laser plasma, AFA Bott et al, Proceedings of the National Academy of Sciences, March 2021
Image caption: a close-up of the experimental target, consisting of two foils and a pair of grids, held together by cylindrical shields. Each target is about the size of a penny. They were carefully designed and machined to produce a turbulent plasma at conditions never reached before. Image © Eugene Kowaluk, LLE.