On the Relevance of Magnetic Correlation Functions in Energetic Particle Transport Theory

We explore the transport of energetic particles in different versions of so-called noisy slab turbulence. The latter model was proposed to study the random walk of magnetic field lines and the transport of energetic particles in turbulence with weak transverse complexity. We show that the previously used model leads to long-range correlations which are so extreme that the perpendicular integral scale is not finite. As a consequence, particles stay connected to the original magnetic field line for an extended period of time. Previously developed heuristic arguments as well as systematic analytical theories do not agree well with test-particle simulations in this case. However, if the noisy slab turbulence model is modified so that long-range correlations are eliminated, perpendicular diffusion is restored quickly and heuristic arguments as well as systematic theories become valid. The findings of the current paper can either be used to further generalize and improve existing theoretical tools for particle transport, or they can be used to make the requirement that used turbulence models must not contain long-range correlations.