Formation of electron energy spectra during magnetic reconnection in laser-produced plasma

The energetic electron spectra formed during magnetic reconnection between two laser-produced plasma bubbles are investigated by use of two-dimensional particle-in-cell simulations. It is found that the evolution of such interaction between the two plasma bubbles can be separated into two distinct stages: the squeezing and reconnection stages. In the squeezing stage, when the two plasma bubbles expand quickly and collide with each other, the magnetic field in the inflow region is greatly enhanced. In the second stage, a thin current sheet is formed between the two plasma bubbles, and then magnetic reconnection occurs therein. During the squeezing stage, electrons are heated in the perpendicular direction by betatron acceleration due to the enhancement of the magnetic field around the plasma bubbles. Meanwhile, non-thermal electrons are generated by the Fermi mechanism when these electrons bounce between the two plasma bubbles approaching quickly and get accelerated mainly by the convective electric field associated with the plasma bubbles. During the reconnection stage, electrons get further accelerated mainly by the reconnection electric field in the vicinity of the X line. When the expanding speed of the plasma bubbles is sufficiently large, the formed electron energy spectra have a kappa distribution, where the lower energy part satisfies a Maxwellian function and the higher energy part is a power-law distribution. Moreover, the increase of the expanding speed will result in the hardening of formed power-law spectra in both the squeezing and reconnection stages.