Behaviour of electron density profiles and particle transport analysis in the RFX reversed field pinch

Abstract

Density profiles measured in RFX by a 13 chord mid-infrared (MIR) interferometer are found to be flat or hollow. Density gradients are large only in a region typically of the order of 2 to 4 cm from the edge. A detailed investigation of the form of the electron density profile during the current flat-top phase in different experimental conditions has been performed. The shape of the profile seems to depend only on the I/N (plasma current/line density) parameter, varying from flat or slightly peaked at high I/N to clearly hollow at low I/N. The systematic existence of stationary hollow density profiles implies the presence of an outward directed fluid velocity. A one dimensional (1-D) particle transport code has been used to simulate the profile behaviour in order to obtain quantitative information on the form of the diffusion coefficient and of the `anti-pinch' velocity. The results are compatible with a transport mechanism based on parallel transport along stochastic magnetic field lines in a large part of the plasma core region. In this framework, the apparent anti-pinch term would be driven by the temperature gradient.