Experimental Observation and Characterization of Microtearing Modes in a Toroidal Fusion Plasma

The Reversed Field Pinch (RFP) device has nowadays achieved the Quasi Single Helicity (QSH) scenario as its standard way of operation. This magnetic configuration features a central plasma volume with good magnetic surfaces and appearance of transport barriers with reduced heat transport. In such plasmas microturbulence is invoked as responsible for a non-negligible level of transport beyond collisional limits, as for the other classes of toroidal fusion devices. Recently, a theoretical stability analysis of microtearing modes in RFP, performed by means of the GS2 code, indeed revealed the QSH states to be prone to microtearing modes. In striking contrast with the body of theoretical work on microturbulence in fusion plasmas, the number of experimental investigations is fairly limited; most proofs about the existence of the microturbulence are indeed deduced from measurements of largescale quantities (profiles, fluxes, flows, etc...), and a number of supplementary hypotheses to link the micro- and the macro-scales are needed. We provide here the first direct observations in laboratory plasma of the presence of microtearing modes in the plasma core. The measurements are carried out in the RFX-mod device by means of a system of in-vessel probes located at the wall, capable of detecting magnetic fluctuations with high time and space resolution. Small-scale electromagnetic modes are revealed during the helical states of the plasma; their amplitude is well correlated to the electron temperature gradient strength in the core. The identification of such instabilities in terms of microtearing modes derives from the comparison of experimental data with dedicated linear gyrokinetic simulations. The possible effect on heat transport in RFP plasma is discussed.