In a high intensity circular accelerator the synchrotron dynamics introduces a slow modulation in the betatronic tune due to the space‐charge tune depression. When the transverse motion is non‐linear due to the presence of multipolar effects, resonance islands move in the phase space and change their amplitude. This effect introduces the trapping and detrapping phenomenon and a slow diffusion in the phase space. We apply the neo‐adiabatic theory to describe this diffusion mechanism that can contribute to halo formation.