Characterization of Microtearing Modes in the RFX-mod Reversed- Field Pinch Plasmas

In the reversed-field pinch (RFP) plasmas the Quasi Single Helicity (QSH) scenario features a central plasma volume with good magnetic surfaces and appearance of transport barriers with reduced heat transport. As for the other classes of fusion devices, in such plasmas microturbulence is invoked as responsible for a non-negligible level of transport beyond collisional limits. Recently, a theoretical stability analysis performed by means of the gyrokinetic GS2 code revealed the QSH states of the RFP to be prone to microtearing modes [1]. Despite the body of theoretical work on microturbulence in fusion plasmas, up to now most proofs about the existence of the microturbulence are deduced only from measurements of large-scale quantities (profiles, fluxes, flows, etc...), and the relation between the micro- and the macro-scales needs supplementary hypotheses. We provide here direct observations of the presence of microtearing modes in the plasma core of a fusion device. The measurements are carried out in the RFX-mod device by means of a system of in-vessel probes located at the wall detecting magnetic fluctuations with good time and space resolution (of the order of few ion Larmor radii). Small-scale electromagnetic quasi-coherent instabilities are revealed during the helical states of the plasma. Their amplitude is correlated to the electron temperature gradient strength in the core. The comparison of experimental data with dedicated linear gyrokinetic simulations permits to identify such instabilities as microtearing modes. The relation between the observed time evolution of the spectral properties and the magnetic equilibrium is analyzed mainly in terms of the helical q profile. The possible effects on heat transport in RFP plasmas are also discussed. References [1] I. Predebon, F. Sattin, M. Veranda, D. Bonfiglio, and S. Cappello Phys. Rev. Lett. 105, 195001 (2010)