Thin current sheet evolution in 3D via magnetic reconnection and other processes*

We perform a large ensemble of 3D particle-in-cell (PIC) simulations of thin current sheets (CSs) in relativistically-hot electron-positron plasma, of interest for theoretical simplicity, computational tractability, and astrophysical relevance. Sandwiched in strongly-sheared magnetic fields, thin CSs facilitate fast conversion of magnetic energy into plasma energy. Previous 2D simulations of CSs show a fractal hierarchy of secondary CSs and plasmoids that explains fast energy conversion via classic 2D magnetic reconnection---i.e., nonlinear interaction of tearing and plasmoid coalescence instabilities. However, our 3D PIC simulations, covering a wide range of CSs, shed new light on the interplay of multiple linear and nonlinear instabilities, including fast magnetic energy release via the nonlinear relativistic drift-kink instability (RDKI). CS conditions can affect the relative prominence of instabilities (e.g., guide magnetic field and high magnetization favor 2D reconnection over RDKI), but some initially-similar CSs take chaotically-diverging paths, triggering different mechanisms with varying rates of energy release. Remarkably, the accelerated particle spectrum depends robustly on ambient plasma conditions, despite various underlying processes.