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Electron acceleration by wave turbulence in a magnetized plasma

Abstract

Astrophysical shocks are commonly revealed by the non-thermal emission of energetic electrons accelerated in situ1,2,3. Strong shocks are expected to accelerate particles to very high energies4,5,6; however, they require a source of particles with velocities fast enough to permit multiple shock crossings. While the resulting diffusive shock acceleration4 process can account for observations, the kinetic physics regulating the continuous injection of non-thermal particles is not well understood. Indeed, this injection problem is particularly acute for electrons, which rely on high-frequency plasma fluctuations to raise them above the thermal pool7,8. Here we show, using laboratory laser-produced shock experiments, that, in the presence of a strong magnetic field, significant electron pre-heating is achieved. We demonstrate that the key mechanism in producing these energetic electrons is through the generation of lower-hybrid turbulence via shock-reflected ions. Our experimental results are analogous to many astrophysical systems, including the interaction of a comet with the solar wind9, a setting where electron acceleration via lower-hybrid waves is possible.

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Fig. 1: Illustration of a magnetized plasma–sphere interaction.
Fig. 2: Optical data and radiation-hydrodynamic simulations.
Fig. 3: X-ray data.
Fig. 4: OSIRIS PIC simulations.
Fig. 5: Atomic transition simulations.

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Acknowledgements

We thank all the LULI technical staff at École Polytechnique for their support during the experiment. The research leading to these results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreements no. 256973 and 247039, AWE plc, the Engineering and Physical Sciences Research Council (grant numbers EP/M022331/1, EP/N014472/1, EP/N013379/1 and EP/N002644/1) and the Science and Technology Facilities Council of the United Kingdom. F.C. and L.O.S. acknowledge support from the European Research Council (InPairs ERC-2015-AdG 695088), FCT Portugal (grant no. PD/BD/114307/2016) the Calouste Gulbenkian Foundation and PRACE for awarding access to resource MareNostrum, based in Spain at the Barcelona Supercomputing Center. The PIC simulations were performed at the IST cluster (Lisbon, Portugal), and MareNostrum (Spain). This work was supported in part at the University of Chicago by the US DOE NNSA ASC through the Argonne Institute for Computing in Science under FWP 57789 and the US DOE Office of Science through grant no. DE- SC0016566. The software used in this work was developed in part by the DOE NNSA ASC- and DOE Office of Science ASCR-supported Flash Center for Computational Science at the University of Chicago.

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G.G., B.R. A.R.B., F.F., S.L., F.M., S.S. and R.Bi. conceived this project, which was designed by G.G., S.L. and M.K. The LULI experiment was carried out by A.R., B.A., J.E.C., Y.H., P.M.K., Y.K., J.R.M., T.M., M.O., Y.S. and M.K. The paper was written by A.R., F.C., B.R. and G.G. The data were analysed by A.R. Numerical simulations were performed by F.C. and P.T. Further experimental and theoretical support was provided by R.Ba., P.G., D.Q.L., C.S., R.T. and L.O.S.

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Correspondence to A. Rigby.

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Supplementary figures 1,2, Supplementary Table 1, Supplementary notes and supplementary references

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Rigby, A., Cruz, F., Albertazzi, B. et al. Electron acceleration by wave turbulence in a magnetized plasma. Nature Phys 14, 475–479 (2018). https://doi.org/10.1038/s41567-018-0059-2

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