Contribution of particles and magnetic field lines diffusion toAnomalous transport in tokamak

Abstract

Research into anomalous transport is aimed at two different sources: turbulent fluctuations in either the electric or the magnetic field. In each case a model has been developed describing the influence of fluctuating field on particles transport. The dynamic of particles guiding centers in a configuration of electric and magnetic turbulence is studied in four dimensional equations of motion. An Hamiltonian description shows that the particles trajectories are described by action angle variables system. It has been demonstrated that the island topology appears at positions near resonant surface if there are the small perturbations. Localized chaotic regions appear while increasing the perturbation and spread up to a fully stochastic situation.

The standard definition coefficient D uses the limit of the mean square excursion; an expression of D useful to describe transport in a finite region can be obtained from the exit times τ i of particles. In the present paper, we compute the numerical diffusion coefficient in the case of electric and magnetic turbulence, the comparison with quasi-linear theory is made for the two cases. This study indicates that electric fluctuations might be the dominant source of anomalous transport for the particles at low parallel velocity, whereas magnetic fluctuations effact particles transport at great parallel velocity. The D/D ql ratio converges to one for the case where numerical diffusion coefficient coincides well with the quasi-linear diffusion coefficient.