Particle acceleration in magnetised shocks produced by laser and pulsed power facilities

Lead Research Organisation: Imperial College London
Department Name: Dept of Physics

Abstract

We propose an ambitious multi-institution experimental programme to investigate one of the greatest mysteries in astrophysics: the acceleration mechanism that leads to generation of high energy cosmic rays. The presence of energetic particles in the Universe is a well established fact, with measurements of the cosmic ray (CR) spectrum extending up to astonishing 1e20 eV. In spite of this, the exact mechanism that leads to such high energy particles still remains controversial. The central theme of this proposal is to conduct a programme of linked earth-based experimental and theoretical investigations into CR acceleration mechanisms to address this long running problem. Although many different processes may result in CR acceleration, the present day understanding is that shock waves and turbulence play an essential role in energizing both the electrons and ions present in the interstellar medium.
We will perform linked experimental and numerical studies of the acceleration of electrons in strong shocks formed in magnetised plasmas. The shocks will be formed by supersonic plasma flows created by high intensity lasers and Mega-Ampere-level pulsed currents. The first set of experiments will investigate the initial acceleration of electrons, which should allow the formation of electron population with energies significantly exceeding their initial thermal energy. This is expected to occur due to plasma wave turbulence which is excited in the pre-shock plasma by the ions reflected from the shock front, but this mechanism has never been tested by experiment. We will characterise the development of the turbulence and measure the parameters of the accelerated electrons using state-of-the-art diagnostic techniques previously developed by us. In the second set of experiments, we will investigate the so-called diffusive shock acceleration mechanism, which is considered as the most plausible mechanism of cosmic ray acceleration. This will be achieved by injecting sufficiently energetic electrons into the shock, in such a way that these electrons will then sample both the pre- and post-shock regions, performing multiple passages through the shock front as required for this mechanism to operate efficiently. Use of a magnetic spectrometer will allow direct measurements of the energy of the accelerated electrons which will be compared with theoretical predictions. As part of this project we will also perform numerical simulations using state of the art hybrid-MHD and PIC codes and cross-compare the results with our experimental data. The computational and theoretical components of the project will allow us to forge a strong connection between experiment, astrophysical models and observations.
The proposed research lies at the border between Plasma Physics and Astrophysics, and will advance the development of the novel research area of Laboratory Astrophysics, which seeks to enhance the understanding of the physics governing the behaviour of astrophysical objects directly via scaled laboratory experiments, combined with computer modelling. Creating the extreme plasma conditions required for scaled reconstruction of astrophysical environments in the laboratory, became possible only recently thanks to the advent of high energy lasers and fast rise-time high-current pulsed power facilities. The similarity between the lab and nature in terms of key dimensionless parameters (e.g. Mach number) is sufficiently close to make such experiments highly relevant. The timeliness of this proposal is also underlined by the growing interest in this field internationally with major efforts in USA (Rochester, Livermore - NIF) and Europe (Bordeaux - LaserMegajoule). The combined expertise of the authors of this proposal and the involvement of international collaborators from Astrophysics community will allow us to create and exploit an unprecedented capability for the Laboratory Astrophysics research and provide both breadth and depth to the programme.

Planned Impact

With the clear need to reduce carbon footprint, fusion energy may represent a long-term goal of the highest importance 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 planning, conducting and interpreting 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 essential for driving forward inertial confinement fusion research. We are training the next generation of young scientists that will eventually lead the technical realization of a fusion power plant. In this sense, it is indicative the National Ignition Facility laser (a >$4 billion multi-decade project), where we have proposed to field our flagship experiments, has, as its core mission, the technical demonstration of inertial fusion energy. In this context, we will introduce our PDRAs and PhD students to the wider energy network in Oxford (http://www.energy.ox.ac.uk) and at Imperial College, 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 defence 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 PDRAs trained on this grant will gain highly relevant skills enabling him/her to either continue with academic jobs or to gain employment in (defence) national laboratories. Indeed, our former students and PDRAs have moved to positions in academia, national and international laboratories, and industry.

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 broad impact will occur with the work proposed here. Proposed activities will include visit to local schools and a dedicated web page. The physics departments at both Oxford University and Imperial College already have dedicated staff members (outreach officers) that would help us in coordinating our outreach activities through the network of schools that we are regularly engaged with. We envision that our PDRAs 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

10 25 50
 
Description The key goal of the project is to develop an experimental platform for investigations of the acceleration of electrons in magnetized shocks and by plasma turbulence developing at these shocks. Combining of experimental studies executed under well controlled conditions with numerical modelling allows bridging the gap between astrophysical observations, theoretical understanding and numerical modelling. Experimental work is being performed at the MAGPIE pulsed power facility at Imperial College and at the LULI, Vulcan and ORION laser facilities. The project leveraged significant funds (US DOE via Cornell University) for complementary HEDP experiments at the MAGPIE facility. The project is progressing very well and has led to a number of very significant advances.
Experiments at the MAGPIE pulsed power facility concentrated on studies of magnetized shock waves formed in the interaction of supersonic plasma flows with conductive obstacles, and on the development of magnetic reconnection configuration via collisions of counter-streaming supersonic magnetised plasmas. Both of these configurations provide appropriate and complementary conditions for studies of particle acceleration.
In experiments with magnetised shocks formed at various obstacles we demonstrated that the shape and the strength of the bow shocks strongly depend on the orientation of magnetic field in respect to the obstacle, which controls the level of magnetic field pile-up at the obstacle. The stand-off distance between the shock and the obstacle is strongly affected by the two-fluid plasma effects. This creates conditions for generation of whistler and lower-hybrid plasma waves, with parameters suitable for electron acceleration at these shocks.
In experiments with counter-streaming magnetized plasma flows we have demonstrated that a well-defined and long living magnetic reconnection layer can be created using the MAGPIE pulsed power experimental platform. Detailed investigation of the formed magnetic reconnection layer were performed using combination of advanced plasma diagnostics, such as Thomson scattering, Faraday rotation and interferometry. The measurements demonstrated an anomalously strong ion heating in the reconnection layer, with the ion temperature significantly, by a factor of ~5, exceeding the electron temperature. The observed relation between the two temperatures with Ti = ZTe is a clear indication of the presence of kinetic plasma turbulence in the reconnection layer. We have obtained first measurements of the energetic electrons, ejected from the magnetic reconnection layer. This was done using X-ray imaging spectroscopy with spherically bend crystals, and the measurements show the presence of K-alfa spectral lines, requiring the presence of electrons accelerated to energies exceeding 3-5 KeV. We have also performed measurements of the spatial distribution of the accelerated electrons and also temporal variability of their generation. We found that there is a correlation of the enhanced generation of fast electrons with development of plasmoid instability in the reconnection layer. Detailed characterization of the temporal and spatial characteristic of these accelerated electrons and comparison with theoretical prediction will be continued.
We also performed computer simulations of experiments using resistive MHD code GORGON developed for laboratory plasma physics. The additional benchmarking of the code via comparison with obtained experimental data will allow it to be more reliably used in the areas beyond laboratory applications, e.g. in simulations of space physics problems.
We have performed experiments aimed at measuring the acceleration of electrons and protons in a turbulent plasma. These experiments have been performed at the LULI and OMEGA laser facilities. At LULI, we have found evidence of electron acceleration by lower-hybrid waves. This result indicates that wave-plasma turbulence can be important in the pre-acceleration mechanism and the result obtained here can be used to explain excess x-ray emission seen, for example, around comets entering the solar system. This work has recently been accepted for publication in Nature Physics. On the OMEGA laser, instead, we have looked at the effect of turbulence in the propagation of charged particles (protons). The results of the experiment are still being analysed, but they clearly show that as the magnetized turbulence is increased, there is a marked enhancement of the proton diffusion. These results are also important for the understanding of propagation of cosmic rays throughout the interstellar and intergalactic medium.
Exploitation Route Publication of results in research journals, presentations at international conferences. The data provided by these experiments provides important benchmarks for the understanding of processes related to cosmic ray physics. This has impact in astronomy and astrophysics as well as plasma science.
Sectors Aerospace, Defence and Marine,Energy

 
Description Post-doctoral researcher who was one of the key people in developing technique to generate supersonic plasma flows has moved to an industrial company in the UK (First Light Fusion). His expertise in the use of pulsed power technology will benefit development of technology used by this company. PhD student who was involved in research on this project has moved to an industrial company in the UK (First Light Fusion) as a staff scientists. Post-doctoral researcher is moving to research organization in Germany to work on an International fusion project ITER.
First Year Of Impact 2018
Sector Aerospace, Defence and Marine,Energy
Impact Types Societal,Economic

 
Description First Light Fusion, Collaborative research agreement.
Amount £1,000,000 (GBP)
Organisation First Light Fusion Ltd 
Sector Private
Country United Kingdom
Start 10/2016 
End 10/2021
 
Description US AF OFFICE OF SCIENTIFIC RESEARCH
Amount $374,922 (USD)
Funding ID FA9550-17-1-0036 
Organisation United States Air Force 
Sector Public
Country United States
Start 12/2016 
End 12/2019
 
Description US DOE SSAA via Cornell University
Amount $6,000,000 (USD)
Funding ID DE-FOA-0001634 
Organisation U.S. Department of Energy 
Sector Public
Country United States
Start 10/2017 
End 09/2022
 
Description Dr Frederico Fiuza 
Organisation Stanford University
Department SLAC National Accelerator Laboratory
Country United States 
Sector Public 
PI Contribution We provide experimental data on electron acceleration obtained in our experiments for comparison with theoretical and computational models developed by the collaborators.
Collaborator Contribution Dr Fiuza and his team use their numerical simulation codes to help in interpretation of experimental results and to plan future experiments.
Impact Publication of results is in preparation
Start Year 2018
 
Description First Light Fusion 
Organisation First Light Fusion Ltd
Country United Kingdom 
Sector Private 
PI Contribution We run a number of joint PhD projects either fully funded or co-funded by First Light Fusion, investigating the production and interrogation of high energy density plasmas created in ultra-high velocity impacts. We provide access to state of the art plasma and shocked material diagnostics, including collaborative experiments on advanced x-ray and laser light sources. We provide access to advanced laser and pulsed power systems and expertise.
Collaborator Contribution ~£200k PA for 5 years to run a portfolio of PhD studentships and related research activities. Collaborative experiments including access to high velocity impact systems.
Impact Multi disciplinary
Start Year 2016
 
Description MIT, Prof Loureiro 
Organisation Massachusetts Institute of Technology
Department Plasma Science and Fusion Center
Country United States 
Sector Academic/University 
PI Contribution Our team provided experimental data for comparison with results of theoretical work of Prof Loureiro
Collaborator Contribution Prof Loureiro provided theoretical support to our experimental work on magnetic reconnection and magnetized shocks
Impact Joint publications of several papers in Phys Rev Letters 2016, 2017, and in Physics of Plasmas in 2017
Start Year 2016
 
Description Royal Society Summer Exhibition 2017 - How to Make a Supernova 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Royal Society Summer Exhibition - How to Make a Supernova - Centre for Inertial Fusion Studies participated in an exhibit. Visited by several thousands members of general public over one week.
Year(s) Of Engagement Activity 2017
URL https://royalsociety.org/science-events-and-lectures/2017/summer-science-exhibition/exhibits/how-to-...