  We investigate guide-field magnetic reconnection and particle acceleration in
relativistic pair plasmas with three-dimensional particle-in-cell (PIC)
simulations of a kinetic-scale current sheet in a periodic geometry at low
magnetizations. The tearing instability is the dominant mode in the current
sheet for all guide field strengths, while the linear kink mode is less
important even without guide field. Oblique modes seem to be suppressed
entirely. In its nonlinear evolution, the reconnection layer develops a network
of interconnected and interacting magnetic flux ropes. As smaller flux ropes
merge into larger ones, the reconnection layer evolves toward a
three-dimensional, disordered state in which the resulting flux rope segments
contain magnetic substructure on plasma skin depth scales. Embedded in the flux
ropes, we detect spatially and temporally intermittent sites of dissipation
reflected in peaks in the parallel electric field. Magnetic dissipation and
particle acceleration persist until the end of the simulations, with
simulations with higher magnetization and lower guide field strength exhibiting
greater and faster energy conversion and particle energization. At the end of
our largest simulation, the particle energy spectrum attains a tail extending
to high Lorentz factors that is best modeled with a combination of two
additional thermal components. We confirm that the primary energization
mechanism is acceleration by the electric field in the X-line region. We
discuss the implications of our results for macroscopic reconnection sites, and
which of our results may be expected to hold in systems with higher
magnetizations.
