Core radial electric field and transport in Wendelstein 7-X plasmas

The results from the investigation of neoclassical core transport and the role of the radial electric field profile (E-r) in the first operational phase of the Wendelstein 7-X (W7-X) stellarator are presented. In stellarator plasmas, the details of the E-r profile are expected to have a strong effect on both the particle and heat fluxes. Investigation of the radial electric field is important in understanding neoclassical transport and in validation of neoclassical calculations. The radial electric field is closely related to the perpendicular plasma flow (u(perpendicular to)) through the force balance equation. This allows the radial electric field to be inferred from measurements of the perpendicular flow velocity, which can be measured using the x-ray imaging crystal spectrometer and correlation reflectometry diagnostics. Large changes in the perpendicular rotation, on the order of Delta u(perpendicular to) similar to 5 km/s (Delta E-r similar to 12 kV/m), have been observed within a set of experiments where the heating power was stepped down from 2MW to 0.6MW. These experiments are examined in detail to explore the relationship between heating power temperature, and density profiles and the radial electric field. Finally, the inferred E-r profiles are compared to initial neoclassical calculations based on measured plasma profiles. The results from several neoclassical codes, SFINCS, FORTEC-3D, and DKES, are compared both with each other and the measurements. These comparisons show good agreement, giving confidence in the applicability of the neoclassical calculations to the W7-X configuration. Published by AIP Publishing.