PARTICLE DIFFUSION IN THE PRESENCE OF TRAPPING

The diffusion of particles in the presence of randomly distributed tra pping centres is examined. An analytical approach is developed for thr ee simple models of the trap-release processes. It is shown that the p article motion remains diffusive on the average, but the diffusion coe fficient can have large fluctuations. The results of the numerical sim ulations confirm the main qualitative trends found in the analytical s tudy. Although they are very simple, the models can be useful for the examination of the diffusion in tokamak plasma in the presence of quas i-coherent structures which act as trapping centres. (C) 1997 American Institute of Physics.