Particle diffusion in the presence of trapping

The diffusion of particles in the presence of randomly distributed trapping centres is examined. An analytical approach is developed for three simple models of the trap-release processes. It is shown that the particle motion remains diffusive on the average, but the diffusion coefficient can have large fluctuations. The results of the numerical simulations confirm the main qualitative trends found in the analytical study. Although they are very simple, the models can be useful for the examination of the diffusion in tokamak plasma in the presence of quasi-coherent structures which act as trapping centres. copyright 1997 American Institute of Physics