Measurements of the parameter dependencies of the bootstrap current in the W7-X stellarator

Abstract Intrinsic, diffusion-driven toroidal (bootstrap) currents between ?7 and 17 kA were measured in the optimised stellarator Wendelstein 7-X (W7-X) for several magnetic configurations at line-integrated plasma densities 2 × 10 19 1.6 20 m ?2 heating powers 0.5 6 MW. The sign of bootstrap current changes with field reversal, its magnitude decreases mirror term rotational transform configuration as well density, increases power. Both absolute values dependencies agree neoclassical calculations within their respective uncertainties, confirming optimisation W7-X small currents.