Electrostatic dynamo in reversed field pinch plasmas: simple common fundamental nature of laminar and turbulent regimes

Within the framework of magnetohydrodynamic (MHD) numerical modelling, the ReversedField Pinch (RFP) has been found to develop turbulent or laminar regimes switching from theformer to the latter in a continuous way depending on the strength of dissipative forces. The laminarsolution corresponds to a simple global helical deformation of the current channel. A helicallymodulatedelectrostatic field arises in order to account for the helical modulation of the currentdensity along magnetic field lines. The associated electrostatic drift yields the main component ofthe dynamo velocity field. The continuity of the transition between the two regimes suggests thatthe simple laminar helical solution can provide a fruitful intuitive description of the RFP dynamo ingeneral. In fact, the electrostatic drift remains the main component of the dynamo velocity field inthe non-stationary turbulent regime for a sustained RFP.We show that the same dynamo action, dueto the electrostatic drift, is provided either by one single mode and its harmonics, as in the laminarregime, or by a rich spectrum of modes with the action of full nonlinear coupling, as in the turbulentone. Here, we review our previous work and present new elements to clarify the physics of the RFPdynamo. Many of the MHD predictions are in good agreement with experimental findings.