Flow measurements and modelling in helical RFX-mod equilibria

Abstract

In the reversed field pinch experiment RFX-mod, plasma currents larger than 1 MA show a spontaneous transition from an axisymmetric to a helical configuration, where electron internal transport barriers (eITB) are observed in the proximity of the maximum of the q safety factor. 3D MHD simulations predict a helical flow pattern associated with the helical deformation of the magnetic flux surfaces. This paper reports the recent experimental findings on plasma flow in these helical regimes. Passive spectroscopy measurements of carbon and boron line Doppler shift allow the reconstruction of the m = 1 component of the velocity pattern associated with the helical equilibrium in a poloidal cross-section. The helical plasma flow is observed to form a m = 1 convective cell creating a localized sheared flow outside the region of strong electron temperature gradients characterizing the eITB. The experimental pattern is compared with the results of 3D MHD simulations (by means of the SpeCyl code). While the code predicts a flow shear peaking in the proximity of the null of the magnetic shear, where the safety factor is maximum, experimentally the maximum flow shear is found to be more external, leaving as a still open question the role played by the flow shear for the thermal barrier formation in RFX-mod helical plasmas.