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

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

Publications

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Description We have performed initial 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 been published 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 clearly show that as the magnetized turbulence is increased, there is marked enhancement of the proton diffusion. These results are also important for the understanding of the propagation of cosmic rays throughout the interstellar and intergalactic medium. Our work has been accepted in the Astrophysical Journal.
In the last part of the project, we have also started to look at the energy change of these protons, a process known as Fermi acceleration. While we have used a simplified model of the turbulence, our calculations indicate that the energy gain is large enough to possibly become measurable on facilities like NIF and LMJ. This work could lie the basis for a future experimental proposal to those lasers.
Exploitation Route 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 Education,Energy

 
Description The results of our work have appeared in Nature Physics. We have done a press release and expect interest in the scientific community and the general public to raise. We have also presented the work on proton diffusion at several international meetings and conferences in plasma astrophysics.
First Year Of Impact 2018
Sector Education,Other
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