Chaotic field line diffusion in Tokamaks

We have studied numerically the chaotic field line diffusion in a Tokamak with ergodic magnetic limiters. The model field is analytically obtained by solving a Grad-Shafranov equation in toroidal polar coordinates, and the limiter field is obtained by supposing its action as a sequence of delta-function pulses. The mean square radial deviation of a bunch of field lines in a predominantly chaotic region is analyzed. Our results show an initially superdiffusive behavior, followed by a subdiffusive regime, with subsequent loss of field lines due to collisions with the inner wall.