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The radial transport of circulating particles in a perturbed toroidal magnetic field configuration is calculated using the mapping technique. The specific models are stellarators with external perturbations and tokamaks with tearing modes. Coulomb collisions of the circulating particles are described by a Lorentz scattering model of random changes of the pitch angle. Particle orbits in a perturbed magnetic field lead to a deterministic map whereas the Coulomb collisions introduce an extra stochastic process. First, the diffusion in a single chain of islands is studied by launching a sample of particles on the inner closed magnetic surface and calculating the loss rate. The presence of islands enhances the loss rate. In a second case the enhanced loss in a tokamak with two different islands on different resonant surfaces is investigated, thus simulating tearing modes in tokamaks. It is found that the loss rate across the island region grows rapidly if the islands overlap and the region in between becomes stochastic. In such a case the transport of particles is a mixture of stochastic diffusion of field lines and a random walk induced by Coulomb collisions.

Additional information

Authors: SANDER H, Max-Planck-Institut fur Plasmaphysik, Garching bei Munchen (DE)
Bibliographic Reference: Report: IPP 2/296 DE (1988)
Availability: Available from Max-Planck-Institut für Plasmaphysik, 8046 Garching bei München (DE)
Record Number: 198910241 / Last updated on: 1994-12-01
Original language: de
Available languages: de