Helical equilibrium reconstruction with diagnostics in the RFX-mod reversed field pinch

Helical states are routinely observed in the RFX-mod reversed field pinch [1] and their study and interpretation needs adequate equilibrium reconstruction. This has to be done constraining the reconstruction with experimental measurements. Give n the 3D nature of such states, on the one hand for solving the direct equilibrium problem the VMEC code [2] has been adapted and used also for the reversed field pinch configuration. On the other hand, to solve the inverse problem (i.e. equilibrium reconstruction from data) the V3FIT code [3] is being used in order to exploit the large number of measurements present in the RFX-mod experiment. The link between the q profile and topology is addressed showing the role of shear reversal to reach a helical core [4]. Also we present the limitation in using only extern al measurements and the role of pressure in the determination of a helical equilibrium: significant information is obtained using temperature and density measurements as well as SXR emissivity signals. References [1] R. Lorenzini et al., Nature Physics 5 (2009) 570-574 [2] S.P. Hirshman and J.C. Whitson, Phys. Fluids 26 (1983) 3554 [3] J.D. Hanson et al., Nucl. Fusion 49 (2009) 075031 [4] D. Terranova et al., 38th EPS Conference on Controlled Fusion and Plasma Physics, Strasbourg (France), 27 June-1 July 2011, P5.084