  The aim of this study is to analyze the interaction of charged particles
(ions and electrons) with randomly formed particle scatterers (e.g.\ large
scale local "magnetic fluctuations" or "coherent magnetic irregularities"),
using the set up proposed initially by \cite{Fermi49}. These scatterers are
formed by the explosive magnetic energy release and propagate with the Alfv\'en
speed along the irregular magnetic fields. They are large scale local
fluctuations ($\delta B/B \approx 1$), randomly distributed inside the unstable
magnetic topology and will here be called {\bf Alfv\'enic Scatterers (AS)}. We
constructed a 3D grid on which a small fraction of randomly chosen grid points
are acting as AS. In particular, we study how a large number of test particles
evolve inside a collection of AS, analyzing the evolution of their energy
distribution and their escape time distribution. We use a well established
method to estimate the transport coefficients directly from the trajectories of
the particles. Using the estimated transport coefficients and solving the
Fokker-Planck (FP) equation numerically, we can recover the energy distribution
of the particles. We have shown that the Stochastic Fermi Energization (SFE) of
mildly relativistic and relativistic plasma can heat and accelerate the tail of
the ambient particle distribution as predicted by \citet{Parker58} and
\citet{Ramaty79}. The temperature of the hot plasma and the tail of the
energetic particles depend on the mean free path ($\lambda_{\rm sc}$) of the
particles between the scatterers inside the energization volume.
