Laboratory Simulation of Magnetized Plasma Turbulence in the Intergalactic Medium

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

We propose an experimental programme to probe one of the greatest puzzles of modern astrophysics: the generation and amplification of magnetic fields ubiquitously found in the Universe. The aim is to demonstrate amplification of magnetic fields by turbulent dynamo - a great challenge of modern experimental plasma physics. We will also study the distribution of turbulent energy between velocity, magnetic and density fluctuations, providing a comprehensive experimental characterisation of the energy cascade in a turbulent plasma.

Magnetic fields are ubiquitously observed in the Universe. Their energy density is comparable to the energy density of the mean plasma flows, so the magnetic fields are essential players in the dynamics of the luminous matter. The total magnetic energy represents a sizable fraction of the cosmic energy budget. What is the origin of these fields? The fact that they are ubiquitous, stochastic and dynamically strong suggests that a universal physical mechanism is at play. The most popular scenario of the cosmic magnetogenesis is that the field grows via some form of turbulent dynamo - fast (exponential) amplification of stochastic field by turbulent motions into which it is embedded, starting from an initial small seed. Understanding magnetogenesis is part of the broader challenge of understanding cosmic turbulence, and the way different form of energies (thermal, turbulent, magnetic) are partitioned on various scales.

With the advent of high-power lasers, a new field of research has opened where, using simple scaling relations, astrophysical environments can be reproduced in the laboratory. The similarity is sufficiently close to make such experiments of high interest. Here we propose to establish an experimental platform using laser-produced plasmas where magnetic fields are produced and amplified by turbulence. In the turbulent plasma, small magnetic fields are initially generated by electrical currents resulting from mis-aligned density and temperature gradients - the so-called Biermann battery effect. By then characterizing the properties of such plasmas and the embedded magnetic fields, we intend to show that those tiny fields can be amplified to much larger values, and up to equipartition with the kinetic energy of the turbulent motions. We will use these experiments to measure the magnetic-energy, density and velocity spectra in the turbulent plasma, thus addressing the details of the energy cascade. Thus, our work would establish, for the first time experimentally, the soundness of the theoretical expectation that tiny seeds produced at protogalactic structures (~10^-21 G) can be amplified to observed dynamically significant values (~10^-6 G) in cosmologically short times.

Planned Impact

With the need of reduction in carbon footprint, fusion energy may represent a long-term goal for a sustainable and clean form of energy. The knowledge basis that our project produces is at the root of fusion energy, and applicable to either inertial or magnetic confinement schemes. The expertise gained in conducting the research work has broad relevance to the development of advanced (and clean) energy sources, and it will lead to enabling critical sciences and technologies in the short term. These will include the capability of performing experiments at fusion-scale laser facilities and the development of advanced diagnostics involved in inertial confinement fusion research. We are training the next generation of scientists that will eventually lead the technical realization of a fusion power plant. In this sense, it is indicative the National Ignition Facility laser, where we have proposed our flagship experiments, has, as its core mission, the technical demonstration of inertial fusion energy. In this context, we will introduce the PDRA to the wider energy network in Oxford (http://www.energy.ox.ac.uk), by participating in open discussions on public and regulatory acceptance of fusion energy - which is carried through diverse disciplines ranging from politics, law, science and engineering.

Laser-plasma experiments, shocks and turbulence are relevant to the UK nuclear defense programmes, both because they provide knowledge on related physics, but also (and more importantly so) because they increase the user base for the AWE Orion laser facility with a larger pool of scientists trained in high energy density science. The fact that one of our collaborators (Dr Foster) is affiliated with AWE confirms the strategic interest in the research topics of this proposal. The PDRA trained on this grant will gain relevant skills enabling him/her to either continue with academic jobs or to gain employment in (defense) national laboratories. Indeed, our former students and PDRAs have moved to positions in academia, national laboratories, and industry - particularly an Oxford-based spin-off company (Oxyntix) focused on the exploitation of fusion energy for commercial purposes.

In addition to publishing our results in high profile journals, we expect that our work will be of large interest for the public understanding of science. Our previous experimental work has received considerable impact on the general public media, with news coverage on Discovery News, CERN Courier and the Daily Mail. We expect the same will occur with the work proposed here. Proposed activities will include visit to local schools and a dedicated web page. The physics department at Oxford University has already a dedicated staff member (an outreach officer) that would help us in coordinating our outreach activities through the network of schools that are part of the Oxford University outreach program. We envision that our PDRA will engage with teachers and schools, about once a year. Our outreach web page will also be linked to the university's resources such as http://www.oxfordsparks.net/subjects which is specifically catered to public engagement.

Publications

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Description We have conducted a series of experiments at large laser facilities which show significant amplification of magnetic field in a turbulent plasma. This agrees with the expectation that turbulent dynamo is operative. Such conditions are believed to occur in the plasma found in cluster of galaxies. Our results thus provide an experimental confirmation of theoretical model of magnetogenesis during the formation of galaxies. Our results have been recently published in Nature Communications. In addition, we have developed advanced analytical techniques for quantitative proton radiography analysis. These techniques are important to the whole plasma/laser physics community.

In the last part of the project, we have extended our experimental platform to study the amplification of magnetic fields in plasmas where compressibility effects become important.
We have, first, worked on understanding the properties of compressible turbulence - results have appeared in Nature Communications - and then looked at how compressibility changes the onset of dynamo. We are preparing our work for submission to a high-impact journal.
Exploitation Route Experimental verification of turbulent dynamo has impacts in astrophysics, plasma physics and turbulence research. While this process has been predicted in numerical simulations, due to the limitations in computer resources, the quantitative details on how dynamo evolves are still unclear. Our work can thus provide a direct benchmark of the theory.
Sectors Education,Energy

 
Description Our results on magnetic field amplification by turbulence have been advertised to the general public. The most prominent activity we have undertaken was the Royal Society Summer of Science 2017 exhibit where some of our work has been presented at a level suitable for primary school students.
First Year Of Impact 2007
Sector Education,Energy
Impact Types Cultural

 
Description Don Lamb 
Organisation University of Chicago
Department Department of Astronomy and Astrophysics
Country United States 
Sector Academic/University 
PI Contribution We provide the team in Chicago our expertise in the experimental diagnostics.
Collaborator Contribution Don Lamb and his team provides us access to the FLASH code. Also, because of this collaboration, we can submit applications for laser time on the Omega laser facility
Impact Astronomy, Plasma Physics, Lasers
Start Year 2010