Statistical properties of the radial transport in the magnetic field with radially bounded stochastic region

Abstract

Statistical properties of the radial heat transport of electrons are clarified in the various types of the radially bounded irregular magnetic field inside an axisymmetric torus plasma, where the collisional stochasticity due to the Coulomb collision and the magnetic stochasticity due to a radially bounded perturbed field coexist. Extensive Monte Carlo numerical analysis are performed in the two dimensional parameter space ( s b / s bc , ν/ ν t ) with a delta function initial distribution in the radial direction. The parameters s b and ν are the strength of a magnetic field perturbation and the collision frequency, respectively. The normalization parameter s bc corresponds to the islands overlapping criterion, and ν t is the characteristic frequency of the passing particle orbits in the corresponding regular magnetic field. It is shown that the type of the transport process is closely connected with the locality of the particle radial displacements. As s b / s bc (⩾1) increases and ν/ ν t decreases, the particle distribution spreads out in a whole stochastic region after the short time ballistic (dynamical) phase reflecting the non-locality of radial displacements. As a consequence the subdiffusive process ( s b / s bc ⩾1, ν/ ν t ⩾0) or the uniform mixing process ( s b / s bc ⪢1, ν/ ν t =0) are observed in the long time limit. The standard diffusion approach is applicable only in the limited parameter region when the fairly frequent collisions inside the radially bounded stochastic region with the mixture of regular and irregular domains can recover the locality of particle displacements.