Dust Grain Charging In Tokamak Plasmas

The presence of solid, micro-impurtities in tokamak plasmas has become a topic of great importance. Dust, originating from the reactor wall surfaces as a result plasma surface interactions, has the potential to cause serious safety hazards and confinement issues and therefore the transport of these particle contaminants in the plasma of an operational tokamak is therefore, a matter of great importance. An important aspect of Dust transport models is the charge acquired by a dust grain immersed in plasma. This study analyses methods at computing the corresponding ’floating potential’. The orbital motion limited (OML) theory is presented in 4 guises as a means for calculating this floating potential in a tokamak plasma. In addition OML is compared to the ABR model, which assumes that the incoming ion motion is radial. It is found that ABR gives a floating potential that is an explicitly function of dust grain radius in contrast to OML which gives the floating potential as a function of the ration of the ion to electron temperature only. Analysis of the plasma dust interaction suggests that, whilst small dust grains possess more or less the vacuum potential, large grains modify the plasma on scales of λD and far beyond to such an extent that the potential structure is very different, and includes potential barriers to ion collection. OML is modified to account for 4 situations, small grain flowless (OML), large grain flowless (MOML), small grain flow (SOML)and large grain flow (SMOML). It is shown that in the context of the dust transport code DTOCKS, in the regime of interest both flow and grain size should be accounted for, and instead of using basic OML, SOML and SMOML should be considered for the charging model.