Particle acceleration due to magnetic reconnection using laser-powered capacitor coils

Abstract

Magnetic reconnection is a ubiquitous astrophysical process that rapidly converts magnetic energy into some combination of plasma flow energy, thermal energy, and non-thermal energetic particles, including energetic electrons. Various reconnection acceleration mechanisms in different low-beta and collisionless environments have been proposed theoretically and studied numerically, including 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 heretofore confirmed experimentally in the laboratory, 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. Recently, laser-powered capacitor coils in high-energy-density (HED) plasmas have emerged as a new source for generating strong MegaGauss-level magnetic fields. These targets are comprised of two parallel copper plates connected by a coil. As high-power lasers irradiate the back plate, an electric potential is built, driving a strong current in the coil. Evolution of the coil current and its associated magnetic field is characterized as a function of laser parameters using ultrafast proton radiography and lumped-circuit modeling. We proceed to extend the platform to study magnetic reconnection by adding a parallel second coil, forming a reconnection geometry between the coils. We quantify electromagnetic field evolution in the reconnection region using ultrafast proton radiography and identify proton features corresponding to reconnection fields. We measure reconnection plasma parameters using Thomson scattering. Thus, we confirm low-beta, magnetically driven reconnection is achieved in a quasi-axisymmetric geometry. Electron energy spectra, measured with a magnetic particle spectrometer, indicate accelerated non-thermal electrons from the reconnection process. These are manifested as spectral bumps with energies of 50 - 70 keV. The non-thermal electron spectra exhibit strong angular dependence, with a peaked signal aligned anti-parallel to the reconnection electric field direction and weakening with increasing pitch angle. 2-D, cylindrical particle-in-cell (PIC) reconnection simulations using the VPIC code demonstrate a formation of a non-thermal electron tail, with a similar angular dependence. These together 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 where reconnection is hypothesized to play a role in accelerating electrons. Our results therefore validate one of the proposed acceleration mechanisms by reconnection, and establish a new approach to study reconnection particle acceleration with laboratory experiments in relevant regimes.