Nonthermal electron and ion acceleration by magnetic reconnection in large laser-driven plasmas

Abstract

Magnetic reconnection is a fundamental plasma process that is thought to play a key role in the production of nonthermal particles associated with explosive phenomena in space physics and astrophysics. Experiments at high-energy-density facilities are starting to probe the microphysics of reconnection at high Lundquist numbers and large system sizes. We have performed particle-in-cell (PIC) simulations to explore particle acceleration for parameters relevant to laser-driven reconnection experiments. We study particle acceleration in large system sizes that may be produced soon with the most energetic laser drivers available, such as at the National Ignition Facility. In these conditions, we show the possibility of reaching the multi-plasmoid regime, where plasmoid acceleration becomes dominant. Our results show the transition from X point to plasmoid-dominated acceleration associated with the merging and contraction of plasmoids that further extend the maximum energy of the power-law tail of the particle distribution for electrons. We also find for the first time a system-sizedependent emergence of nonthermal ion acceleration in driven reconnection, where the magnetization of ions at sufficiently large sizes allows them to be contained by the magnetic field and energized by direct X point acceleration. For feasible experimental conditions, electrons and ions can attain energies of max,e /k B T e > 100 and max,i /k B T i > 1000. Using PIC simulations with binary Monte Carlo Coulomb collisions we study the impact of collisionality on plasmoid formation and particle acceleration. The implications of these results for understanding the role reconnection plays in accelerating particles in space physics and astrophysics are discussed.

A nuclear-reaction-based method for probing the nonthermal ion energy spectrum in high energy density laboratory plasmas

“…Similar to those previously presented in [36,37], A series of 2D PIC simulations with a newly-developed nuclear reaction calculation module are carried out. Our simulations adopt an initial condition corresponding to a time about halfway through the experiments, after the bubbles are created, have expanded, and have generated their magnetic ribbons, but before the pair of bubbles begin to interact, as shown in figure 2(b).…”

“…Firstly, the nuclear reaction yields increase in a stepwise manner, and the total yield can reach the order of 10 6 , which is sufficient to be measured experimentally. Secondly, according to the inference of the maximum energy after the acceleration of electrons and ions by the reconnection electric field mentioned in the literature [37], E max ∝ M s S/β can be obtained. Therefore, compared with the reference case, the β of case A is 16, and the magnetic field gradient caused by the accumulation of plasma is smaller, so the weakened reconnection electric field accelerates and heats protons to lower energy, and finally, the total yield can only reach 10 5 .…”