Thermal gradients of QSH states in RFX-mod

The Reversed Field Pinch (RFP) is a fusion-aimed, toroidal magnetic configuration characterized by the presence of several resonant tearing modes that contribute to the sustainment of the configuration itself. However, such modes make the magnetic field stochastic through the overlapping of their magnetic islands. The field stochasticity, and the entailed strong transport typically found in Multiple Helicity (MH) states, is mitigated when the plasma accesses the Quasi-Single Helical state (QSH) [1], where the innermost resonant mode dominates the spectrum of the other (secondary) instabilities. In high current (I = 1.5 MA) experiments of RFX-mod [2] the plasma stays preferentially in the QSH state, although transiently it comes back to MH [3]. QSH states feature steep electron transport barriers, interpreted as Internal Transport Barriers that enclose thermal structures, whose extent can vary considerably. Narrow, off-axis thermal structures are typically located inside the magnetic island of the dominant mode; this first stage of QSH state features two magnetic axes and it is named Double Axis state (DAx) [4]. As the dominant mode grows further, a topological change of magnetic field takes place; the X-point of the dominant mode island anneals with the main magnetic axis and the island Opoint becomes the only magnetic axis [5]. A recent paper [6] shows that the extent of the thermal structure, despite the dramatic change of the magnetic field topology, remains relatively unchanged. This second phase of QSH is named Single Helical Axis state (SHAxn) where the subscribe 'n' indicates the narrow temperature profile. When the dominant mode grows above 4% of the edge magnetic field, a flat, high-temperature region develops in the plasma core, which is named SHAxw where the subscribe 'w' indicates the wide temperature profile comparing with SHAxn; this is the third stage of a QSH state. In this paper we study the thermal gradients of ITBs in the 3 QSH sub-states using the same database obtained from RFX-mod experiments of Ref[6]. Specifically, a routine has been developed to automatically find the top and the foot location of the temperature gradient in the Thomson scattering profile in both machine and helical flux coordinates, the latter calculated by means of the Sheq code [7]. The gradients are investigated studying their steepness, extent and position, which are correlated to the behavior of the dominant mode and secondary instabilities. The confinement time will be also calculated and commented in the light of the results of gradient analyses.