3D Simulations of Scrape-Off Layer Filaments

In the Scrape-Off Layer (SOL) of magnetic confinement devices, cross-field transport of particles is dominated by the convection of filamentary plasma structures via self-generated E×B velocity fields. This thesis investigates the dynamics of such filaments using three dimensional simulations to further theoretical understanding of SOL transport. A new 3D SOL simulation code called STORM3D has been developed using the BOUT++ framework to implement an isothermal drift-reduced fluid model in a slab geometry. Verification and validation exercises are documented to demonstrate that the code has been implemented correctly and that the physical model adequately reproduces experimental observations. A comprehensive characterisation of how a filament’s initial geometry affects its subsequent dynamics is provided via a series of 3D simulations of isolated filaments. In particular the size of a filament in the plane perpendicular to the magnetic field, δ⊥, is shown to have a strong influence on its motions, as it determines which currents balance the filament’s pressure-driven diamagnetic currents, which in turn determines its E×B velocity. At small δ⊥, this balance is predominantly provided by polarisation currents and the filament’s radial velocity is observed to increase with δ⊥. In contrast, at large δ⊥, parallel currents closing through the target are found to be dominant, and the radial velocity decreases with δ⊥. Comparisons are made between 3D simulations and 2D simulations using different parallel closures; namely the sheath dissipation closure, which neglects parallel gradients, and the vorticity advection closure, which neglects the influence of parallel currents. The vorticity advection closure is found not to replicate the 3D perpendicular dynamics well and overestimates the initial radial velocity of all filaments studied. A more satisfactory comparison is obtained with the sheath dissipation closure, even in the presence of significant parallel gradients, where the closure is no longer valid. The vorticity advection closure’s poor performance occurs because in the 3D case parallel currents closing through the sheath play an important role in reducing the extent to which polarisation currents are driven. In a conduction-limited or detached SOL regime however, low plasma temperatures and high neutral densities near the divertor will produce significantly higher resistivity values in the region than that used in the aforementioned 3D simulations. Therefore the effect of increasing the normalised plasma resistivity in the last quarter of the domain nearest the targets is examined using 3D simulations. Whilst small δ⊥ filaments are observed to be relatively unaffected by this quantity, large δ⊥ filaments exhibit faster radial velocities at higher resistivity values due to two mechanisms. Firstly, parallel currents are reduced meaning that polarisation currents are necessarily enhanced and secondly, a potential difference forms along the parallel direction so that higher potentials are produced in the region of the filament for the same amount of current to flow into the sheath. This indicates that broader SOL profiles could be produced at higher values of normalised resistivity, and hence at larger reference SOL densities and at colder temperatures.