Equilibrium reconstructions for Wendelstein 7-X with V3FIT

The present status of the V3FIT code for the free-boundary reconstruction of the magneto-hydrodynamic (MHD) equilibrium of W7-X plasmas with finite plasma pressure and toroidal current are presented. The reconstruction of the equilibrium is a vital tool for fusion experiments to model and interpret diagnostic signals. The iterative procedure involves solving the MHD equilibrium, calculating synthetic diagnostic signals and comparing the signals to those from the experiment. The parameters that describe the plasma position, boundary location and internal pressure and current profiles are adjusted until a 'best-match' is found between the synthetic signals and measured signals. Profile and parameter uncertainties are based on a best-linear estimate. The reconstruction output includes the shape and location of the plasma boundary, and profile information of the plasma parameters. This set of information is then used to interpret diagnostic signals and can be used for further analysis of the equilibrium state. Typically, the MHD solution is provided by VMEC, which only permits nested closed flux surface. The SIESTA code, which is already coupled with V3FIT, permits islands to be present in the free-boundary MHD solution.