V3FIT: a code for three-dimensional equilibrium reconstruction

The V3FIT code for performing equilibrium reconstruction in three-dimensional plasmas is described. It is a modular code that has the potential to be coupled with a variety of equilibrium solvers to compute the externally measured response to an arbitrary internal state of the plasma. Singular-value decomposition is used to identify the dominant components of the plasma state that can be accurately determined by the reconstruction process and to guide the minimization of the chi(2) variance-normalized mismatch between the measured and computed signals. Comparison of a tokamak plasma equilibrium computed by V3FIT and by the axisymmetric equilibrium reconstruction code EFIT is presented. V3FIT is used to reconstruct an axisymmetric DIII-D equilibrium using experimentally observed magnetic diagnostic signals. Three-dimensional reconstructions of stellarator plasma equilibria in the CTH device show the code behaves as expected in the presence of experimental noise, appropriately ignores near-singular directions in parameter space and robustly reconstructs equilibria starting from substantially different initial parameter values.