The collisional dynamics of test impurity ions in spherical tokamak plasmas
with strongly-sheared radial electric fields is investigated by means of a test
particle full orbit simulation code. The strength of the shear is such that the
standard drift ordering can no longer be assumed and a full orbit approach is
required. The effect of radial electric field shear on neoclassical particle
transport is quantified for a range of test particle mass and charge numbers
and electric field parameters. It is shown that the effect of a sheared
electric field is to enhance the confinement of impurity species above the
level observed in the absence of such a field. The effect may be explained in
terms of a collisional drag force drift, which is proportional to particle
charge number but independent of particle mass. This drift acts inwards for
negative radial electric fields and outwards for positive fields, implying
strongly enhanced confinement of highly ionized impurity ions in the presence
of a negative radial electric field.Comment: 16 pages, 6 figures, submitted to Nuclear Fusio