Thermal properties of three sub-states of quasi-single helicity states on RFX-mod

Abstract

Quasi-single helicity (QSH) state has been widely observed in reversed field pinch devices to sustain thermal structures, enclosed by steep temperature gradients interpreted as electron internal transport barriers (eITB). Within the evolution of a QSH cycle, three sub-states can be identified in the RFX-mod experiment, named DAx, SHAxn and SHAxw, where the subscript n and w indicate narrow and wide thermal structures, respectively. Such sub-states feature different magnetic topologies as well as different width of the thermal structures. This paper presents a characterization of the three QSH sub-states mainly devoted to describe the properties of the eITB temperature gradients. The analyses show a decreasing trend of the absolute value of the thermal gradient as the dominant mode increases. Further study on the thermal diffusivity confirms that these behaviors are essentially due to the local change of the heat transport. Besides, the thermal gradient in DAx and SHAxn groups shows an increasing trend with the decrease of secondary modes. Such behavior has been studied and it can be explained by stochastic transport in agreement with what found for SHAXw (Lorenzini et al 2012 Nucl. Fusion 52 062004). As for SHAxw group, the thermal gradient suddenly decreases with respect to DAx and SHAxn values. This behavior can still be explained in the framework of stochastic transport considering the structure of magnetic perturbations at the location of thermal gradients (Lorenzini et al 2016 Phys. Rev. Lett. 116 185002). The total energy confinement time has also been evaluated, adopting a more appropriate method. The result shows that the energy confinement time increases up to 40% from DAx to SHAxw.