Compressible impurity flow in the TJ-II stellarator

Fully ionized 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–Schlüter parallel velocity. For the medium density plasmas studied, , the measured impurity flow is found to be inconsistent with a total incompressible flow, i.e. ∇ · u z ≠ 0, thus implying a non-constant impurity density on those flux surfaces. 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, modifications at the locations of the CXRS measurements are generally smaller in magnitude and opposite in sign when compared to the experimentally observed deviations. 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.