An international team led by the Department of Physics at Oxford, University of Rochester, and University of Chicago has been able to unravel the inner workings of heat conduction in the largest building blocks of our Universe – clusters of thousands of galaxies bound together by gravity.
Most matter in galaxy clusters is in the form of tenuous ionised gas called plasma that is threaded by magnetic fields and is in a turbulent state; in observing many of these clusters of galaxies, astronomers have been facing a difficult conundrum: they all appear much hotter than expected.
Answering fundamental questions
The team, led by Dr Jena Meinecke, former DPhil student at Oxford and Junior Research Fellow at Christ Church, used the largest laser system in the world – the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) in California – to create a replica of the plasma conditions expected to occur in the clusters of galaxies.
Professor Alexander Schekochihin, astrophysicist at the University of Oxford, who was involved in this work says: ‘How energy is injected into the plasma that fills galaxy clusters by the violently active galaxies at their centres, how it is then spread around and heats up the entire enormous system, producing the X-ray glow that observatories like Chandra pick up — these are fundamental questions about the largest building blocks of our Universe. Both observations and the logic of our theoretical models suggest that heat conduction in these plasmas is strongly suppressed compared to naïve expectations. Several schemes for such suppression have been theorised about and simulated numerically, but very tentatively. Here suddenly we have it in a real laboratory plasma — and so experiment now has a chance to leapfrog theory in helping sort out the basic properties of an astrophysical plasma, an exciting prospect.’
Stepping into the unknown
Dr Meinecke continues: ‘The experiments conducted at the NIF are literally out of this world. Capable of bringing the powerful dynamics of the Universe to the laboratory, the NIF truly provides opportunities to step into the unknown.’
In the experiments, laser beams were used to vaporise plastic foils and generate a turbulent and magnetised plasma, in similar fashion to the laser-driven experiments the team performed at the Laboratory for Laser Energetics to demonstrate the turbulent dynamo mechanism for the first time.
‘What is unique in these NIF experiments is that electrons in the plasma collide sufficiently infrequently with each other that they end up following the tangled magnetic field lines,’ says Dr Archie Bott, researcher at Princeton University and also a former DPhil student at Oxford. ‘This phenomenon, which is precisely what is believed to occur in clusters of galaxies, gives rise to suppressed heat conduction.’
This effect is clearly seen in the laboratory data: the measurements show pockets of hot plasma that persist in time and heat cannot escape. Professor Gianluca Gregori, who was the principal investigator in these experiments says: ‘This work is an important stepping stone in understanding the microscopic processes that occur in plasmas that are both magnetised and turbulent. The experimental findings are somewhat surprising as they demonstrate that energy is transported in ways that is very different from what we would have expected from simple theories.’
An astonishing result
‘This is indeed an astonishing result,’ confirms Professor Petros Tzeferacos, director of the Flash Center of Computational Science, which recently moved from the University of Chicago to the University of Rochester and led the simulation efforts to design and help interpret the NIF experimental campaign. ‘To model the NIF experiments, we brought to bear the full array of physics capabilities of FLASH, the multi-physics simulation code we develop. The FLASH simulations were key for untangling the physics at play in the turbulent, magnetised plasma, but the level of thermal transport suppression was beyond what we expected.’
While the simulations reproduce the experimental results by controlling the electron heat transport, they cannot to tell us what is the microscopic mechanism that is ultimately responsible for its observed suppression. Further work on the NIF laser is being prepared to look at the details of these interactions. ‘These experiments provide insight into complex physics processes and also raise additional questions that we hope to answer in upcoming NIF Discovery Science experiments with an optimised target design and diagnostic configuration,’ says Dr James Steven Ross, the liaison scientist of the project at LLNL.
Experiments complementing observation
These experiments demonstrate how laboratory explorations can help the understanding of astrophysical systems in a way that is complementary to observations. Professor Don Lamb of the University of Chicago, also involved in the project, says ‘The Discovery Science programme at the National Ignition Facility – the most energetic laser in the world – made it possible for us to study fundamental physical processes that are essential to understanding the hot, tenuous, magnetised plasma in the cores of galaxy clusters.’
The research was carried out under the auspices of the US Department of Energy (DOE) National Nuclear Security Administration (NNSA), the US DOE Office of Science - Fusion Energy Sciences (FES), the National Science Foundation (NSF), the European Research Council (ERC), and the Engineering and Physical Sciences Research Council (EPSRC). The Discovery Science programme of the US DOE NNSA provided the collaboration with access to the NIF. Compute time for the FLASH numerical simulations was provided by the US DOE ALCC and ERCAP programs, and the LLE High Performance Computing group.
Strong suppression of heat conduction in a laboratory replica of galaxy-cluster turbulent plasmas, by J Meinecke et al, Science Advances, vol 8, issue 10, 9 March 2022