Ion flow measurements in the Wendelstein 7-X stellarator towards a validation of neoclassical theory

We present a systematic experimental study of the ion plasma velocity field in the Wendelstein 7-X (W7-X) stellarator, in multiple magnetic configurations and operation conditions, and assess the ability of neoclassical theory to explain the observations. In this assessment, we employ a forward model of the velocity measurement from the charge exchange recombination spectroscopy (CXRS) diagnostic with a rigorous treatment of the ion flow and line of sight geometries, as well as wavelength corrections due to atomic physics and instrumental effects.The minimisation of the absolute difference between the experimentally measured velocity profiles and those predicted by the forward model enables the inversion of radial profiles of the radial electric field and the net parallel impurity ion velocity, which can be directly compared with neoclassical computations. Non-trivial redundancy in the multiple simultaneous velocity measurements allows to self-calibrate instrumental wavelength drifts within the forward model. A recent extension in the model representation has resulted in a substantial quantitative improvement in the agreement between CXRS velocity measurements and neoclassical flow expectations, which are now shown to be compatible in the majority of the cases. ... mehrFurthermore, we show that the flow inversions are statistically robust and consistent when two different emission lines are used. The radial electric field compares well with independent Doppler reflectometry measurements within the radial region where they overlap. By and large, the observations presented support the validity of neoclassical flow calculations in the plasma core and mid-radius region (ρ0.7) in W7-X. However, with the presently accessible experimental accuracy and magnetic configuration variations, a clear configuration dependence of the ion flow could not be determined.