  Understanding plasma dynamics and nonthermal particle acceleration in 3D
magnetic reconnection has been a long-standing challenge. In this paper, we
explore these problems by performing large-scale fully kinetic simulations of
multi-xline plasmoid reconnection with various parameters in both the weak and
strong guide field regimes. In each regime, we have identified its unique 3D
dynamics that leads to field-line chaos and efficient acceleration, and we have
achieved nonthermal acceleration of both electrons and protons into power-law
spectra. The spectral indices agree well with a simple Fermi acceleration
theory that includes guide field dependence. In the low-guide-field regime, the
flux-rope kink instability governs the 3D dynamics for efficient acceleration.
The weak dependence of the spectra on the ion-to-electron mass ratio and
$\beta$ ($\ll1$) implies that the particles are sufficiently magnetized for
Fermi acceleration in our simulations. While both electrons and protons are
injected at reconnection exhausts, protons are primarily injected by
perpendicular electric fields through Fermi reflections and electrons are
injected by a combination of perpendicular and parallel electric fields. The
magnetic power spectra agree with in-situ magnetotail observations, and the
spectral index may reflect a reconnection-driven size distribution of plasmoids
instead of Goldreich-Sridhar vortex cascade. As the guide field becomes
stronger, the oblique flux ropes of large sizes capture the main 3D dynamics
for efficient acceleration. Intriguingly, the oblique flux ropes can also run
into flux-rope kink instability to drive extra 3D dynamics. This work has broad
implications for 3D reconnection dynamics and particle acceleration in
heliophysics and astrophysics.
