Fast ion dynamics in the basic plasma physics experiment TORPEX

The transport of fast ions in interchange turbulence is examined in TORPEX, a basic plasma physics experiment with the essential elements of the scrape-off layer (SOL). Extensive modeling in two and three dimensions has shown that TORPEX plasma is well-described by a drift-reduced Braginskii fluid simulation (GBS). We follow fast ion trajectories using the full Lorentz force, specified by the GBS fields. A large ensemble of trajectories allows us to create a synthetic diagnostic, giving results that compare favorably with experimental measurements of current density using a fast ion source/detector pair. We also explore the properties of fast ion transport on timescales longer than those accessible to the experiment. We find that the rate of radial dispersion is strongly dependent on the ratio of ion energy to plasma (electron) temperature. The dispersion tends to be subdiffusive for $\rho_i \sim \Lambda$, where $\rho_i$ is the fast ion Larmor radius and $\Lambda$ is the scale length of the turbulence. The effective diffusivity is examined from the perspectives of (1) trajectory dispersion, (2) the flux-gradient Fick's Law relation, (3) velocity correlation, and (4) waiting-time/flight-length distributions.