Pair-loaded electron-only magnetic reconnection using laser-driven capacitor coils

Abstract

We propose and simulate a laboratory platform to study the effects of positrons in magnetic reconnection using laser-driven capacitor coils. Using particle-in-cell simulations, we show that externally injected MeV electron-positron pairs are trapped in the coil current sheet, significantly modifying the reconnection dynamics and particle acceleration. These pairs increase the reconnection rate by a factor of approximately 8, which Ohm's law decomposition reveals to be driven by the divergence of the generalized pressure tensor. Based on their high energy and magnetization, the pairs also substantially broaden the diffusion region. Particle tracking simulations in realistic coil magnetic fields further demonstrate that injected pairs can remain confined for several picoseconds, providing conditions for sustained interaction with the reconnection region. These results establish a near-term pathway to laboratory studies of positron-influenced reconnection, bridging high-energy-density experiments with pair-dominated astrophysical environments.

Figures (3)

• Figure 1: Proposed experimental concept where pairs from a laser-irradiated Au foil are injected into the coils of a laser-driven capacitor coil. Magnetic fields produced around the legs of the coil create a reconnection configuration. The sub-millimeter separation between the legs allows for electron-only magnetic reconnection with pair loading. Injected pairs are represented by green and blue spheres.
• Figure 2: Two-dimensional VPIC simulation of reconnection with pair injection with a density $\sim$30% of the background electron-ion plasma in the current sheet. The evolution of the out-of-plane current density $J_y$, electron density and out-of-plane magnetic field $B_y$ are shown in (a) where 1 MeV pairs are injected from 50-60 ps. In row 3, arrows show the direction of the in-plane current. Plots (b) and (c) demonstrate the decomposition of the reconnection electric field into the terms given by Ohm's law at $t = 57.96$ ps for a simulation without and with the injection of pairs respectively. The electric field has been normalized by the electron Alfvén speed $V_{Ae}$ multiplied the magnetic field $B_0$ calculated at $z= 1d_e$. This region has been marked in gray in (b) and (c). See Appendix A for the definition of the Ohm's law terms that are labeled in (b) and (c) with colors matching the corresponding lines.
• Figure 3: Trapping of positrons between the coils using the magnetic fields 50 ps into the 100 ps current rise time. Plots (a) and (b) show the integrated time spent by pairs in the x-y and x-z planes respectively. Trapped trajectories of positrons are shown with overlays for the coil positions in blue. The fraction of particles within the red rectangle in (a) and (b) are plotted in (c) showing trapping over several ps.