Bifurcation in the MHD behaviour of a self-organizing system: the reversed field pinch (RFP)

Within the framework of MHD modelling the RFP is shown to develop turbulent or laminar regimes switching from the former to the latter in a continuous way depending on the strength of dissipative forces (the higher they are the more laminar is the corresponding regime). In either of these cases interesting features can be observed such as the occurrence of quasi-periodic relaxation events involving reconnection processes, or the formation of stationary helical symmetric configurations. The first case corresponds to the conventional turbulent dynamo in the RFP where perturbations with multiple helical harmonic content are present. The second case corresponds to a global single helical deformation of the current channel. This simpler configuration is associated with a laminar electrostatic dynamo field and may also be found as a solution of a helical Ohmic equilibrium problem where a finite beta is necessary. The continuity of the transition between the two regimes suggests that the simple helical symmetric solution can provide a fruitful intuitive description of the RFP dynamo in general. Many of the MHD predictions are in good agreement with experimental findings and suggest possible improvements for the confinement properties of the RFP configuration.