Vanishing Magnetic Shear And Electron Transport Barriers In The RFX-Mod Reversed Field Pinch

M. Gobbin, D. Bonfiglio, D. F. Escande, A. Fassina, L. Marrelli, A. Alfier, E. Martines, B. Momo, and D. Terranova

Phys. Rev. Lett. 106, 025001 – Published 13 January 2011

Abstract

We define the safety factor $q$ for the helical plasmas of the experiment RFX-mod by accounting for the actual three-dimensional nature of the magnetic flux surfaces. Such a profile is not monotonic but goes through a maximum located in the vicinity of the electron transport barriers measured by a high resolution Thomson scattering diagnostic. Helical states with a single axis obtained in viscoresistive magnetohydrodynamic numerical simulations exhibit similar nonmonotonic $q$ profiles provided that the final states are preceded by a magnetic island phase, like in the experiment.