Particle Diffusion in Tokamaks with Partially Destroyed Magnetic Surfaces

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.