Reduced simulations of scrape-off-layer turbulence

This thesis looks at models of plasma transport in the Scrape-Off Layer (SOL) of magnetic fusion devices, which sets critical heat loads on the components in the divertor. The cross-field plasma transport in the SOL is due to filamentary turbulence, carrying particles and energy radially within the SOL. This thesis builds upon previous work using the STORM model, which concentrated on modelling individual filamentary structures. Here we simulate a saturated turbulent state, which is modelled using a system of drift-fluid equations, fed by localised density and energy sources. A review of the STORM model used is given, implemented in BOUT++, and a description of the upgraded analysis software written. Comparisons are made between 3D simulations and reduced 2D simulations, which parameterize the parallel transport physics. We describe a simulation setup which aims to produce the closest possible comparison between the 2D STORM model and the 3D STORM model,run for plasma parameters representative of the MAST SOL in L-mode. We found that an interplay of issues caused by two reasonable-seeming assumptions in the model setup were the main reason for differences between the 2D and analogous 3D models. In very high collisionality conditions, the assumptions used to close 2D models are not valid, because high enough collisionality suppresses thermal conduction so that convection becomes important. We also found that a source localised above the X-point in the parallel direction for the 3D simulations causes supersonic flows beyond the X-point. Despite these differences, the 2D model successfully replicates the mean density profile and much of the higher-order statistics of the 3D models with matched sources and no parameter tuning. After motivating the coupling of a core region to the SOL models, we adjusted our model by adding a core region inside the SOL region. We find that this causes considerable complications, as the spontaneous formation of large binormal mean flows exhibits strongly non-linear behaviour. We also examined the effect of altering perpendicular diffusion coefficients, and find that both viscosity and density diffusion matter a lot for the behaviour of these flows and for SOL profiles, even when only changed in the core.