Test particle transport in perturbed magnetic fields in tokamaks

Numerical calculations of magnetic field line trajectories in a tokamak are used to investigate the common hypotheses that (i) field lines in a chaotic field make a Gaussian random walk and (ii) that the poloidal component of the magnetic field is uniform in regions with a chaotic magnetic field. Both hypotheses are found invalid in typical tokamak conditions. A test particle transport model in the so-called "collisionless diffusion" limit is presented, based on the field line excursions in numerical simulations. Decorrelation mechanisms that effectively enhance the transport in a stochastic field are investigated. (C) 1999 American Institute of Physics. [S1070-664X(99)02406-4].