For a KV beam the halo is created by test particles, initially out of the core, via nonlinear resonances with the betatron motion. The number of resonances and the related diffusive processes are enhanced by the presence of mismatch oscillations of the core. Numerical (PIC) or physical (collisions) noise modifies the core of a KV beam creating tails on time scales relevant for storage rings. We consider a 2D model defined byNpseudo line‐charges per meter, whose limitN→ ∞, at fixed perveance, is the 2D mean field theory for a coasting beam in a constant focusing channel. Using an algorithm of optimal computational complexity we find that the relaxation time scales linearly withNin agreement with Landau’s kinetic theory we have developed for the 2D model. © 2003 American Institute of Physics