Advances in understanding RFX-mod helical plasmas.

High current (I_p = 1-1.7 MA) operations in the ohmic reversed field pinch RFX-mod spontaneously access the quasi-single helicity (QSH) regime, in which the magnetic dynamics is dominated by the innermost resonant mode (m = 1, n = -7), the magnetic chaos level is reduced and the internal magnetic field configuration is close to a pure helix. During the QSH state strong electron temperature (T_e) gradients can show up, identifying an electron transport barrier (Puiatti 2011 Nucl. Fusion 51 073038). Results and advancements obtained in recent RFX-mod high current campaigns are reported in this paper. The best plasma performances at high I_p (i.e. high power) in QSH regimes have been reached with marginally reversed values of the safety factor q at the edge (-0.01 < q_a < 0) when the lowest amplitudes of secondary m = 0, 1 modes are obtained. The QSH magnetic states are not stationary and show back transitions to the multiple helicity regime; however, the total persistency of the QSH is found to increase with the plasma current and at I_p > 1.5MA it exceeds 90% of the plasma current flat-top. The high electron temperature gradients (>2 keVm^-1) do not persist throughout the magnetic QSH cycle, as documented by a new high time resolution SXR double-filter multichord system, however, they last 5 to 10 ms, more than the energy confinement time. In the barrier region the profile of the electron heat diffusivity Χ_e falls around 2-20m² s^-1, well below ~40-100m^-2 s^-1 typical of the outer regions. The experimental evaluation of Χ_e across the thermal barrier is consistent with a microtearing mode driven heat transport. [ABSTRACT FROM AUTHOR]