Self-Organized Helical Equilibria in the RFX-Mod Reversed Field Pinch

With operation at high plasma current (Ip similar to 1.5 MA), the plasma in the RFX-mod reversed field pinch self-organises in a 3D helical state with almost conserved flux surfaces featuring strong electron temperature barriers. Up to now the equilibrium of such states was obtained by a perturbative solution of the Newcomb equation in toroidal geometry. This allowed for the mapping of the electron temperature, density and SXR emissivity profiles on helical flux surfaces thus proving the correlation between kinetic plasma quantities and the underlying helical magnetic topology. In order to obtain a full 3D equilibrium reconstruction, the magnetic topology of a SHAx state has been also reconstructed with the VMEC code adapted for the RFP using experimental profiles as input data. Previous results for the axisymmetric case were used as benchmark. It is found that helical equilibria are characterised by a particular q profile with a region of zero magnetic shear as this has been found to be a key ingredient to trigger internal transport barriers. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim