  Fully-ionised carbon impurity flow is studied in ion-root, neutral beam
heated plasmas by means of Charge Exchange Recombination Spectroscopy (CXRS) in
the TJ-II stellarator. Perpendicular flows are found to be in reasonable
agreement with neoclassical calculations of the radial electric field. The
parallel flow of the impurity is obtained at two locations of the same flux
surface after subtraction of the calculated Pfirsch-Schl\"uter parallel
velocity. For the medium density plasmas studied, $\bar{n}_{\rm
e}\in(1.2-2.4)\times 10^{19}$ m$^{-3}$, the measured impurity flow is found to
be inconsistent with a total incompressible flow, i.e. $\nabla\cdot{\bf
u}_z\ne0$, thus contradicting the usual assumption of a constant density on
each flux surface. The experimentally observed velocity deviations are compared
with the parallel return flow calculated from a modelled impurity density
redistribution driven by ion-impurity friction. Although the calculated return
flow substantially modifies the incompressible velocity pattern, the
modifications do not explain the in-surface variations of impurity parallel
mass flow at the precise locations of the CXRS measurements. Small
inhomogeneities of the electrostatic potential in a surface are also shown to
affect the impurity redistribution but do not provide a better understanding of
the measurements.
