Quasi-single-helicity (QSH) states, characterized by a magnetic spectrum dominated by the innermost resonant tearing mode, are common to all the reversed field pinch (RFP) experiments. The internal magnetic field structure produced by the dominant mode is investigated for the QSH observed in the Madison Symmetric Torus (MST) RFP in discharges with zero toroidal magnetic field at the plasma boundary. The reconstruction is based on an MHD model coupled to edge measurements of the magnetic field. The model discards pressure, which has little effect on the equilibrium magnetic profile of present RFP plasmas, but adopts a realistic toroidal geometry. The technique is the adaptation to the MST configuration of a procedure already applied in RFX-mod, but a more general radial profile for the current density is needed for an adequate reconstruction of the MST case. The emerging features are similar to those found in RFX-mod. The helical flux surfaces of the dominant mode provide, with a good degree of reliability, a basis for mapping kinetic quantities such as electron density and soft-x-ray emissivity.
Dynamics of ultralow-q plasmas in the RFX-mod device