Particle Diffusion in Tokamaks with Partially Destroyed Magnetic Surfaces

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Abstract
A Hamiltonian formalism is developed for the drift-orbit trajectories of particles in toroidal systems in the presence of stochastic fields. The equations of motion are integrated numerically to investigate the modification of neoclassical diffusion in a tokamak due to the onset of stochasticity. A significant increase in the diffusion coefficient is observed below the stochastic threshold for electrons, whereas ions are typically not affected until the magnetic field has become quite stochastic.