  The spatio-temporal evolution of the energetic particles in the transport
time scale in tokamak plasmas is a key issue of the plasmas confinement,
especially in burning plasmas. In order to include sources and sinks and
collisional slowing down processes, a new solver, ATEP-3D was implemented to
simulate the evolution of the EP distribution in the three-dimensional
constants of motion (CoM) space. The Fokker-Planck collision operator
represented in the CoM space is derived and numerically calculated. The
collision coefficients are averaged over the unperturbed orbits to capture the
fundamental properties of EPs. ATEP-3D is fully embedded in ITER IMAS framework
and combined with the LIGKA/HAGIS codes. The finite volume method and the
implicit Crank-Nicholson scheme are adopted due to their optimal numerical
properties for transport time scale studies. ATEP-3D allows the analysis of the
particle and power balance with the source and sink during the transport
process to evaluate the EP confinement properties.
