  We present the results of particle-in-cell numerical pair plasma simulations
of relativistic 2D magnetostatic equilibria known as the 'ABC' fields. In
particular, we focus on the lowest-order unstable configuration consisting of
two minima and two maxima of the magnetic vector potential. Breaking of the
initial symmetry leads to exponential growth of the electric energy and to the
formation of two current layers, which is consistent with the picture of
'X-point collapse' first described by Syrovatskii. Magnetic reconnection within
the layers heats a fraction of particles to very high energies. After the
saturation of the linear instability, the current layers are disrupted and the
system evolves chaotically, diffusing the particle energies in a stochastic
second-order Fermi process leading to the formation of power-law energy
distributions. The power-law slopes harden with the increasing mean
magnetization, but they are significantly softer than those produced in
simulations initiated from Harris-type layers. The maximum particle energy is
proportional to the mean magnetization, which is attributed partly to the
increase of the effective electric field and partly to the increase of the
acceleration time scale. We describe in detail the evolving structure of the
dynamical current layers, and report on the conservation of magnetic helicity.
These results can be applied to highly magnetized astrophysical environments,
where ideal plasma instabilities trigger rapid magnetic dissipation with
efficient particle acceleration and flares of high-energy radiation.
