Theory and simulation of ion drag on dust in magnetised plasmas

Abstract

Accurate modelling of the forces acting on dust is imperative to the simulation of dust transport in magnetic confinement fusion (MCF) plasmas. Conventional models of one of the most significant of these forces, the ion drag force, neglect to consider the direct effect of the strong magnetic fields present in an MCF plasma on the trajectories of ions in the vicinity of a dust particle. This thesis presents novel theoretical models of ion drag which show that magnetic effects are significant and cannot be neglected in relevant parameter regimes. This is supported by numerical results obtained through modifications to the DiMPl Monte Carlo code, which demonstrate clear agreement with the theoretical models.

Conventional modelling of ion drag further assumes that dust is smaller than the Debye length of the plasma in which it is immersed. This is not always the case in tokamaks. This thesis describes how existing numerical models of charging of large dust grains can be applied to modelling the ion drag force on such grains. We extend our study of magnetic effects to this large dust regime, presenting numerical results for both the charging of, and ion drag on, large dust grains under strong magnetic fields.

Finally, aspiring to be of immediate practical use to those involved in dust transport modelling, this thesis presents a set of semi-empirical models that may be straightforwardly employed to capture the effects of magnetic fields on small and large dust grains, and therein significantly improve the accuracy of these codes in modelling the trajectories of dust grains under strong magnetic field conditions.