Helical equilibrium reconstruction with V3FIT in the RFX-mod Reversed Field Pinch

Abstract

Helical states are routinely found in all reversed field pinch experiments. The inherently three-dimensional feature of these states requires an adequate equilibrium reconstruction. In this paper we present the use of the V3FIT code for the RFX-mod experiment. V3FIT uses VMEC as an equilibrium solver and varies reconstruction parameters to minimize the difference between experimental and modelled signals. Both magnetic (local field, saddle coils and flux loops) and kinetic diagnostics (Thomson scattering, interferometric and soft x-ray emissivity data) are used in order to properly deal with the problem of equilibrium degeneracy. From this point of view a sensitivity study of external magnetic measurements on the internal topological structure is presented.As the work deals with fixed-boundary equilibria, a solution to the issue of vacuum fields computation is also presented.A comparison with previous procedures for obtaining both axisymmetric and helical equilibria is presented, with particular attention to the role of pressure and thermal measurements—information which is missing in previous analyses.The results provide a good match with experimental data showing that indeed a consistent inclusion of pressure with temperature and density measurements is necessary. The equilibria obtained are suitable for both transport and stability analysis.