  In this work we analyze two classes of Density-Estimation techniques which
can be used to consistently couple different kinetic models of the
plasma-material interface, intended as the region of plasma immediately
interacting with the first surface layers of a material wall. In particular, we
handle the general problem of interfacing a continuum multi-species
Vlasov-Poisson-BGK plasma model to discrete surface erosion models. The
continuum model solves for the energy-angle distributions of the particles
striking the surface, which are then driving the surface response. A
modification to the classical Binary-Collision Approximation (BCA) method is
here utilized as a prototype discrete model of the surface, to provide boundary
conditions and impurity distributions representative of the material behavior
during plasma irradiation. The numerical tests revealed the superior
convergence properties of Kernel Density Estimation methods over Gaussian
Mixture Models, with Epanechnikov-KDEs being up to two orders of magnitude
faster than Gaussian-KDEs. The methodology here presented allows a
self-consistent treatment of the plasma-material interface in magnetic fusion
devices, including both the near-surface plasma (plasma sheath and presheath)
in magnetized conditions, and surface effects such as sputtering,
back-scattering, and ion implantation. The same coupling techniques can also be
utilized for other discrete material models such as Molecular Dynamics.
