FINITE ORBIT WIDTH EFFECTS ON STOCHASTIC RIPPLE DIFFUSION

An analytical and numerical investigation is undertaken to study the e ffects of non-standard orbit topology on the stochastic ripple diffusi on of high energy ions in tokamaks. Finite orbit width effects are dem onstrated to lead to large modifications of the threshold for the onse t of stochastic motion. Trapped particles moving along trajectories wi th turning points on the inside of the torus are most severely affecte d. In particular, orbits close to the pinch orbit are very sensitive t o magnetic field ripple, whereas orbits with bounce points close to th e horizontal midplane are less easily perturbed than is suggested by s mall orbit width theory. Applying the results to recent ripple experim ents on JET, it is found that the number of stochastically moving part icles is only moderately affected by these effects. The reason for thi s is the large ripple amplitude and the steepness of the ripple gradie nt over most of the plasma cross-section.