Fast ion transport in TORPEX: theory and synthetic diagnostic

Full text: The transport of fast ions in interchange-mode turbulence is examined in a basic toroidal plasma scenario, TORPEX, with an open magnetic field analogous to the scrape-off layer in tokamak plasmas. We follow fast ion tracer trajectories using the full Lorentz force, specified by a numerical model of drift-reduced Braginskii equations (GBS) in two dimensions, which was previously validated to describe interchange turbulence on TORPEX. We explore the properties of fast ion transport on timescales and energies accessible to experimental measurements, and also at asymptotic limits. A lithium ion emitter coupled to a double-gridded energy analyzer has been installed in TORPEX, with fast ion energies 100-1000 eV, so that Efast/Te ∼ 20 - 200. At these energies, the ions have kpl∼1 and curvature drift velocities comparable to the E x B drift velocities. Using a synthetic diagnostic, the measured ion dispersion agrees with predictions supported by the GBS+tracer simulations. We find that the rate of dispersion is strongly dependent on the ratio of ion energy to electron temperature. Radial dispersion tends to be superdiffusive for low energies and subdiffusive for higher energies, mainly due to the vertical drift-averaging effect. For low energies, a Levy walk model, based on a flight length diagnostic, is used to describe the random walk process of fast ions in the turbulence. (author)