Role of 3D plasmoid dynamics on the transition from collisional to kinetic reconnection

Laboratory and solar reconnection events are often simulated with a resistive-MHD collisional fluid model, or a collisionless particle in cell (PIC) model. However, for these applications, the current sheet collisionality can vary with time and lead to a transition between these two regimes. Here we study this transition process by 3D PIC simulations with Fokker-Planck collision operator in both Cartesian slab geometry [1] and a geometry relevant to the upcoming FLARE experiment [2]. This approach is able to self-consistently model the plasma heating and transport during this transition from collisional to kinetic reconnection.



The current layer forms in a collisional regime and rapid Joule heating causes: 1) The current-layer thickness (set by the Sweet-Parker scaling) to rapidly decrease. 2) Strong temperature gradients on the sheet edge that can stabilize the onset of the oblique plasmoid instability at larger angles. The plasmoid instability gives rise to strong super-Dreicer electric fields that cause the current sheet to rapidly thin down to electron scales.



[1] Stanier, A., et al., (2019). Phys. Plasmas, 26(7), 072121.

[2] Ji, H., et al. (2022). Nat. Rev. Phys., 1-20.