2D characterization of core thermal topology changes in controlled RFX-mod QSH states

Abstract

In the reversed field pinch RFX-mod device, the achievement of high plasma currents (Ip ⩾ 1.3 MA) has allowed the appearance of a new kind of large thermal structures emerging in the plasma core during the so-called quasi-single helicity (QSH) regimes. These structures correspond to a helical equilibrium established in the plasma, which has been dubbed as the single helical axis (SHAx) state. The topological features of this new type of thermal structures covering most of the plasma core have been experimentally investigated here. Analyses have been performed by means of three diagnostics, simultaneously and routinely running on RFX-mod: the soft x-ray (SXR) camera, the Thomson scattering diagnostic for the electron temperature (Te) profile estimation and the SXR tomography. In particular, a 2D map reconstruction of Te has been performed: the core electron temperature asymmetries mostly account for the ones in the reconstructed SXR emissivity. The magnetic topology of these QSH thermal structures has also been analysed and numerically investigated by the Hamiltonian guiding centre code ORBIT: magnetic and thermal structures have been identified, in position and topological features. Also the Te profile temporal evolution, provided by the tomographic diagnostic and the SXR camera, has been investigated, showing that the transition to SHAx does not develop in a unique way.