Electric field reconstruction via tomographic impurity rotation tracking in Wendelstein 7-X

The investigation of the ambipolar radial electric field is important for understanding transport and confinement properties in the neoclassically optimized stellarator Wendelstein 7-X (W7-X). Hence, a new experimental method to determine the electric field based on the poloidal rotation dynamics of externally injected impurity ions is presented. The tomographic inversion of the line-integrated soft x-ray data together with singular value decomposition resolves the poloidal motion of highly charged impurities injected via laser blow-off into low-beta W7-X plasmas. The radial electric field is reconstructed from the measured poloidal velocities and the known magnetic field. In contrast to established diagnostics, the method only relies on a few assumptions and does not require wavelength calibration, detailed atomic physics modeling, or the availability of neutral beam injection. The method is applied to two nearly identical plasma scenarios with forward and reversed magnetic field directions. In both cases, the inferred radial electric fields are negative in the outer plasma region and exhibit comparable magnitudes, independent of field direction. The experimentally determined values show good agreement with neoclassical predictions obtained from calculations, within estimated uncertainties. These results demonstrate the capability of the presented method as an independent diagnostic for the radial electric field in W7-X and provide a complementary approach that avoids key limitations of established techniques.