  We discuss the phenomenon of energization of relativistic charged particles
in three-dimensional (3D) incompressible MHD turbulence and the diffusive
properties of the motion of the same particles. We show that the random
electric field induced by turbulent plasma motion leads test particles moving
in a simulated box to be accelerated in a stochastic way, a second-order Fermi
process. A small fraction of these particles happen to be trapped in
large-scale structures, most likely formed due to the interaction of islands in
the turbulence. Such particles get accelerated exponentially, provided their
pitch angle satisfies some conditions. We discuss at length the
characterization of the accelerating structure and the physical processes
responsible for rapid acceleration. We also comment on the applicability of the
results to realistic astrophysical turbulence.
