MHD nonlinear behaviour and dynamo in numerical simulations of the reversed field pinch with external drive of toroidal magnetic flux (PPCD)

A so-called "dynamo" mechanism, due to co-operation of fluctuating velocity and magnetic fields, is usually seen as essential to describe the sustainment and transport of the Reversed Field Pinch (RFP) confinement configuration. 3D MHD nonlinear numerical simulations (SpeCyl code) are discussed in this work where an external drive is applied, similarly to the action of an experimental Pulsed Poloidal Current Drive (PPCD)[1,2]. The PPCD technique has been experimentally proven to provide substantial reduction of heat transport. In this work we show that, during the external action, the MHD (dynamo) perturbations decrease in amplitude and that the current density profiles become steeper and in particular increase in the central region, as observed in experiments [1,2]. Moreover we stress the importance of the pinch velocity term which yields the most important contribution in balancing the resistive diffusion when the turbulent dynamo becomes negligible in Ohm's law. [1] Puiatti et al. , to be submitted to 19th IAEA 2002 conference [2] D.L.Brower et al. , Phys.Rev. Lett., 88, 185005 (2002)