Mapping of drift surfaces in toroidal systems with chaotic magnetic fields

Drift orbits of test particles are studied using a recently proposed Hamiltonian theory of guiding-center motion in toroidal systems. A symplectic mapping procedure in symmetric form is developed which allows a fast and accurate characterization of the Poincará plots in poloidal cross sections. It is shown that the stochastic magnetic field acts differently on the onset of chaotic motion for co- and counterpassing particles, respectively. Resonant drift surfaces are shifted inward for the co-passing particles, and are shifted outward for the counterpassing particles, when compared with resonant magnetic surfaces. The overall result is an inward (outward) shift of chaotic zones of co-passing (counterpassing) particles with respect to the magnetic ergodic zone. The influence of a stationary radial electric field is discussed. It shifts the orbits farther inward for the co-passing particles and outward for the counterpassing particles, respectively. The shifts increase with the energies of the particles. A rotation of the magnetic field perturbations and its effect on drift motion is also investigated. Estimates for the local diffusion rates are presented. For applications, parameters of the dynamic ergodic divertor of the Torus Experiment for Technology-Oriented Research are used [Fusion Eng. Design 37, 337 (1997)].