Stochastic diffusion of energetic ions in optimized stellarators

It is shown that the collisionless transformation of locally trapped and passing particle orbits in the optimized stellarators of the Wendelstein line results in stochastic diffusion of energetic ions. This diffusion can lead to the loss of a significant fraction of the energetic ion population from the region where the characteristic diffusion time is small compared to the slowing down time. The loss region and losses can be minimized by shaping the plasma temperature and density profiles so that they satisfy certain requirements. The predictions of the theory developed here are in agreement with the results of numerical modeling of alpha -particle confinement in a Helias reactor, which has been carried out with the use of an orbit following code. (C) 2001 American Institute of Physics.