Parallel SOL transport in MAST and JET: the impact of the mirror force

Interpretative modelling of the SOL plasma in conventional (JET) and tight (MAST) aspect ratio devices has been performed using OSM2/EIRENE. A detailed comparison has been made of the solutions of the fluid equations and one key issue uncovered by this modelling is the significance of the mirror force for the spherical tokamak (ST) SOL. This force is proportional to del(parallel to) B/B, which is typically a factor 10 larger in an ST due to the low aspect ratio. This term leads to changes in the charged particle velocity distributions near regions with large V-parallel to B/B representing an effective, upstream particle and momentum source. The modelling performed in this paper indicates that exclusion of the del(parallel to) B/B term may lead to incorrect conclusions on, for example, the upstream density, especially in STs.