The Birth of Plasmoids: Weibel-Driven Secondary Tearing and its Suppression in Relativistic Reconnection
Plasmoids, or magnetic islands, dominate relativistic reconnection layers in kinetic simulations, yet the instability that creates them remains poorly understood. By tracking newborn islands in 2D particle-in-cell simulations of pair-plasma reconnection with magnetizations $0.1\leσ_0\le1000$, we show that plasmoids are seeded by a current-filamentation/Weibel instability driven by counter-streaming out-of-plane current carriers near primary x-lines. Measured growth rates are $\sim0.1$ times the background plasma frequency and agree with linear theory evaluated on the simulated distributions. As inflowing plasma loads the sheet away from the x-line, the current is carried by more particles and their out-of-plane drift slows. The filamentation instability shuts off once this drift becomes comparable to the momentum spread in the outflow direction, which occurs at a distance $\ell_{\rm iso}\proptoσ_0$ from the x-line. Because the unstable region extends farther at higher $σ_0$, our results account for the onset of plasmoid-dominated reconnection in the relativistic regime but predict that plasmoid formation is suppressed beyond $\sim\ell_{\rm iso}$ from primary x-lines, even in ultra-relativistic reconnection. Particle acceleration, variability, and coherent radio emission attributed to plasmoid-dominated reconnection may therefore be confined to smaller regions than previously assumed.