On diffusion of magnetic field lines

The existence of magnetic flux surfaces is regarded as essential for magnetic confinement of fusion plasmas. The blurring or disintegration of these surfaces is quoted in the literature as cause for the observed anomalously high transport perpendicular to the (ideal) flux surfaces; mass and thermal energy are assumed to spread along magnetic field lines which fill certain volumes ergodically. Thus, the heat conductivity is composed of the radial (perpendicular) spread in space given by the field lines and the motion along these trajectories χr=<(Δr)2>/2τ=Dstv with a mean free path denoted by λ=vτ. The basic quantity in this hypothesis is, therefore, the 'magnetic diffusion' coefficient Dst. This coefficient is usually computed by overlaying a set of perturbations (islands) on an equilibrium field (e.g. solution of the Grad-Schlueter-Shafranov equation); the appearance and approximation size of the stochastic regions is quantified by resonance overlap criteria. Dst is then determined with arguments of unlimited, Gaussian random walk of Markov type. Application of such theory to a more realistic (toroidal) tokamak plasma for a JET equilibrium had shown reasonable agreement between the estimated and the observed confinement. However, the assumptions for the concept of diffusion have never been validated. (author) 6 refs., 4 figs