Magnetic reconnection rapidly converts magnetic energy into some combination of plasma flow energy, thermal energy and non-thermal energetic particles. Various reconnection acceleration mechanisms have been theoretically proposed and numerically studied in different collisionless and low-β environments, where β refers to the plasma-to-magnetic pressure ratio. These mechanisms include Fermi acceleration, betatron acceleration, parallel electric field acceleration along magnetic fields and direct acceleration by the reconnection electric field. However, none of them have been experimentally confirmed, as the direct observation of non-thermal particle acceleration in laboratory experiments has been difficult due to short Debye lengths for in situ measurements and short mean free paths for ex situ measurements. Here we report the direct measurement of accelerated non-thermal electrons from magnetically driven reconnection at low β in experiments using a laser-powered capacitor coil platform. We use kilojoule lasers to drive parallel currents to reconnect megagauss-level magnetic fields in a quasi-axisymmetric geometry. The angular dependence of the measured electron energy spectrum and the resulting accelerated energies, supported by particle-in-cell simulations, indicate that the mechanism of direct electric field acceleration by the out-of-plane reconnection electric field is at work. Scaled energies using this mechanism show direct relevance to astrophysical observations.
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Data are available from the corresponding authors upon reasonable request.
Information about the VPIC code is available via GitHub at https://github.com/lanl/vpic. Data analysis code is available from the corresponding authors upon reasonable request.
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This work was supported by the Department of Energy (DOE) Office of Science, Fusion Energy Sciences (FES), under the LaserNetUS initiative at the Jupiter Laser Facility and OMEGA Laser Facility. This work was mainly supported by the High Energy Density Laboratory Plasma Science program by Office of Science and NNSA under grant no. DE-SC0020103 (H.J., A.C., L.G., S.Z. and E.G.B.), and also by DOE grants GR523126 and NSF grant PHY-2020249 (E.G.B.). We express our gratitude to General Atomics, the University of Michigan and the Laboratory for Laser Energetics for target fabrication, and to the Jupiter Laser Facility, OMEGA and OMEGA EP crews for experimental and technical support. H.J. thanks M. Yamada, J.-Y. Zhong, S. Prager, C. Ren, K. Huang and Q.-M. Lu for their contributions during initial development of this project.
The authors declare no completing interests.
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a, The number of positrons near the laser spot are plotted as a function of OU-ESM channel, coil configuration, and coil current. In all coil configurations, positrons are deflected near the laser spot only for small finite coil currents 0 < I ≲ 3kA, as larger coil currents deflect the positrons below the bottom of the back plate. Further, across all channels, left and right coil configurations consistently exhibit larger positron impacts than the double coil configuration. This exercise implies that LPI-generated electrons are unlikely to be preferentially accelerated to the OU-ESM by double coil magnetic fields, as compared to single coil magnetic fields. b, A simulated five-channel energy spectrum is generated by transforming the experimental no-coil spectrum via positron raytracing at energies of 20 − 100keV, in 10keV increments. The applied magnetic field is the two-coil configuration, with a constant coil current of I = 1kA. Across all channels, simulated spectral dips are observed at 30 − 50keV, in contrast with the experimental spectral bumps (Fig. 2a,b,c) at 40 − 70keV. c, Application of the same raytracing technique to the left-coil configuration can demonstrate the selection mechanism at low energies in Channels 4 and 5, as described in the Fig. 2e caption. In the raytracing spectrum, dips are observed at low energy in Channels 4 and 5, as the lower-energy positrons are preferentially deflected to large θ − z angle by the coil magnetic field.
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Chien, A., Gao, L., Zhang, S. et al. Non-thermal electron acceleration from magnetically driven reconnection in a laboratory plasma. Nat. Phys. 19, 254–262 (2023). https://doi.org/10.1038/s41567-022-01839-x
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