ITG turbulence in finite-beta helical and axisymmetric reversed field pinch plasmas

In the reversed field pinch (RFP) device RFX-mod, good confinement properties are achieved when the magnetic surfaces become helical. This condition causes on overall reduction of the magnetic chaos, but, at the same time, creates the physical conditions mostly favoring the onset of electrostatic/ electromagnetic turbulence, e.g., the occurrence of large pressure gradients. In particular, a helical core imposes an unfavorable e?ect in terms of iontemperature- gradient (ITG) stability and related turbulent transport1. ITG modes turn out to be localized where the magnetic surface compression is higher: here the local temperature gradients become larger, with a consequent growing instability and higher ion heat transport. Moreover, while an axisymmetric RFP presents a very high zonal flow residual level, the helical states of the plasma exhibit a lower zonal flow residual and a stellarator-like oscillating behaviour of the zonal potential. In this contribution we revisit these findings, obtained in simplified conditions of the plasma. Due to the importance of electromagnetic effects in the RFP, we focus on the role of a finite plasma beta on ITG stability, turbulence and zonal flow response. The occurrence of microtearing modes in high-beta plasmas is also evaluated. 1 - I. Predebon and P. Xanthopoulos, Phys. Plasmas 22 , 052308 (2015)