The reversed field pinch (RFP) is a magnetic configuration germane to the tokamak, that produces most of its magnetic field by the currents flowing inside the plasma; external coils provide only a small edge toroidal field whose sign is reversed with respect to the central one. Because of the presence of magnetic turbulence and chaos, the RFP had been considered for a long period as a terrible confinement configuration. Then strong enhancements were triggered in the RFX-mod RFP in Padua (Italy): a self-organized helical state with an internal transport barrier develops, and a broad zone of the plasma becomes hot (above 1 keV for a central magnetic field above 0.8 T). The possibility of this helical state and of the corresponding improvement in confinement had been theoretically predicted by three-dimensional nonlinear visco-resistive magnetohydrodynamic (MHD) simulations. This course summarizes the present experimental and theoretical knowledge about this helical state, in particular with the following issues: Lawson criterion, dynamo, MHD and magnetic field bifurcations, analytical description, analogy with the nonlinear tearing mode, attractiveness of the RFP configuration for a reactor, usefulness for fusion science and dynamo physics. The new paradigm for the RFP supports its reappraisal as a low-external field, non-disruptive, ohmically heated approach to magnetic fusion, exploiting both self-organization and technological simplicity.
