Incompressibility of Impurity Flows in Low Density TJ-II Plasmas and Comparison with Neoclassical Theory

Poloidal and toroidal velocities of fully-ionised carbon are measured by means of charge exchange recombination spectroscopy (CXRS) in the TJ-II stellarator. We present a detailed treatment of the 3D geometry and show that flow measurements, performed at different locations of the same flux surface, are compatible with flow incompressibility for the low density plasmas under study (line averaged electron densities $\bar{n}_e\,\le\,10^{19}$ m$^{-3}$). Furthermore, comparison with neoclassical calculations shows quantitative agreement with the measured radial electric field and ion bootstrap parallel flow in the absence of an external momentum input.