Spectral properties of helical RFP equilibria

The helical states commonly occurring in high current RFX-mod plasmas are now routinely reconstructed by means of the equilibrium code VMEC[1], modified to suit the RFP configuration [2]. The existence of such equilibria is the basis on which several stability and transport studies can be established. In this context, one of the important questions to be addressed is the occurrence of resonant tearing instabilities in helical plasmas, which tend to braid the magnetic field and to provide, consequently, one of the most dangerous sources of particle and energy transport. In order to quench them, we have first to understand what they look like when seen by the (inherently toroidal) set of edge magnetic probes. The aim of this work is indeed to clarify some aspects related to the mutual spectral properties of axisymmetric and helical MHD equilibria. In particular we will deal with i) the toroidal/cylindrical mode spectrum produced by helical equilbria; ii) related to this, the problem of the magnetic mode spectrum to be set by the mode-control system to get helical states with zero-amplitude secondary modes; iii) the richness of the (tearing) helical spectrum related to a single Fourier harmonic imposed by the mode-control system; iv) the shape of the related eigenfunctions in helical geometry. A comparison with the perturbative Newcomb's approach in toroidal geometries will be also carried out, so as to understand if the present method can be a valid alternative to the solution of the Newcomb's equation in helical geometry. Furthermore, since the method can be applied to any VMEC equilibrium, similar considerations will be made for the tokamak and stellarator configuration. References [1] S.P. Hirshman and J.C. Whitson, Phys. Fluids 26(1983) 3554 [2] D. Terranova et al., Plasma Phys. Control. Fusion 52, 124023 (2010)