Fast ion transport in the TORPEX experiment

The simulation of the fast ions dynamics experiment in the TORPEX device is presented. The experiment consists of a fast particles source that injects lithium ions in the TORPEX plasma, characterized by interchange-driven turbulence. The simulations are performed by integrating the particles equations of motion in the turbulent fields provided by the simulations of Dr. Ricci et al. [15]. The accuracy of the integration of the eq. of motion has been established by testing the conservation of energy, and comparing the trajectories computed by the simulation with the predictions of the guiding center model. The study of a single particle motion has revealed chaotic behavior, and especially high sensitivity to the initial conditions. An approximation for the motion of the center of mass of a distribution of a statistically meaningful number of particles has been obtained for specific conditions. Finally the dependence of the spreading of the distribution on the fast particles energy, on the spreading in the initial velocities of the particles and on the spreading in the initial angles of the particles has been studied. It has revealed that the effects of turbulence on the spreading of a distribution are visible in the vertical and parallel directions as long as the spreading in the initial velocities is small (else, the effect of turbulence is masked by the effect of the magnetic field geometry) and in the radial direction for any values of the spreading in the initial velocities.