The density limit in fusion plasmas: the role of edge instabilities

This Thesis deals with the study of density limit in magnetically confined fusion plasmas. The density limit causes the termination of the plasma discharge when a threshold value for the electron density is overcome, the so-called Greenwald density (nG), and thus represents an important operative limit for fusion devices. The problem is studied following a multiple point of view approach involving the numerical solution of a single-fluid model for the plasma, the study of plasma transport theory and the analysis of data coming from the reversed-field pinch experiment RFX-mod in Padua, Italy. Analysis of the results from numerical simulations of the reversed-field pinch configuration allows confirming the role of a dimensionless parameter of the model (the Hartmann number H, related to plasma resistivity and viscosity) in describing the transition observed in the dynamics of the plasma and in ruling the behaviour of edge-magnetic field. Analysis of the plasma transport theory allows writing the Hartmann number in terms of plasma measurable quantities. Analysis of a wide set of data coming from the RFX-mod experiment allows linking the trend in edge-magnetic field observed at the onset of the density limit to the (perpendicular) Hartmann number: this supports abandoning the phenomenological nG parameter in favor of H. Furthermore, H describes with a good level of correlation the behaviour of the measured plasma density, temperature and current. The work in this Thesis provides an important confirmation to the use of the single-fluid model in modeling reversed-field pinch plasmas and opens the way towards exploring the possibility that H could be the order parameter also in the tokamak density limit.