Advances in understanding RFX-mod helical plasmas

Abstract

High current (Ip = 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 (Te) 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 Ip (i.e. high power) in QSH regimes have been reached with marginally reversed values of the safety factor q at the edge (−0.01 < qa < 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 Ip > 1.5 MA it exceeds 90% of the plasma current flat-top. The high electron temperature gradients (>2 keV m−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 χe falls around 2–20 m2 s−1, well below ∼40–100 m−2 s−1 typical of the outer regions. The experimental evaluation of χe across the thermal barrier is consistent with a microtearing mode driven heat transport.