ANOMALOUS TRANSPORT DUE TO MAGNETIC TURBULENCE

Transport in a toroidal system with broken flux surfaces is considered . Flux surfaces with rational field line winding number can degenerate and form magnetic islands. The experimental evidence for the existenc e of magnetic structures is reviewed. If neighbouring chains of island s overlap, a region of stochastic field is formed. In a stochastic fie ld, a field line fills up a region of space and thus makes significant radial excursions. Particles following a stochastic field line may ex perience rapid radial transport. The problem of computing transport in a stochastic field has to stages: what is the behaviour of field line s in a stochastic region? and what is the implication for particle tra nsport? The much used formula due to Rechester & Rosenbluth for collis ionless transport in a fully ergodic field is treated. It is shown tha t the conditions for the validity of this formula are normally not met in a tokamak.