Linear properties and nonlinear behaviour: MHD study of pulsed poloidal current drive (PPCD) in the reversed field pinch (RFP)

The Reversed Field Pinch (RFP) is usually seen as a system in which a "dynamo" mechanism, due to co-operation of fluctuating velocity and magnetic fields, is essential to describe configuration maintenance and transport. The so-called Pulsed Poloidal Current Drive (PPCD) is an experimental technique which consists in pulsing the edge toroidal magnetic field toward more reversed values. This technique has been shown to provide substantial reduction of transport and of fluctuating magnetic fields [1]. In this work we focus on the link between the nonlinear dynamics and the linear stability properties of the evolving configuration. 3D MHD nonlinear numerical simulations (SpeCyl code) are discussed where an external drive, through a time dependent boundary condition on the mean toroidal magnetic field is applied similarly to the action of an experimental PPCD. This kind of modelling describes the main aspects of the experimental behaviour [2]. Indeed we nonlinearly obtain a reduction of the (entire) magnetic spectrum amplitude (quench of dynamo modes) and a concentration and peaking of current density mean profiles. The study of the linear properties of the configuration, using a spectral eigenvalue resistive code, underlines the importance of the m=1 ideal modes (in a resistive plasma: resistive kink modes [3,4]) and of the m=0 resistive modes. In particular during PPCD action the m=1 near axis ideal modes are suppressed, while the resistive part of the spectrum shows only moderate modifications. However when the external drive lasts enough to push the plasma at deep reversal, several resistive m=0 modes are destabilized and consistently the m=0 mode total energy is observed to increase in nonlinear simulations. Interesting implications can be drawn also from the sensitivity of the linear stability properties of the system on apparently relatively small rearrangements of the equilibrium magnetic fields. Indeed, in most of the cases, experimental profiles reconstruction does not allow comparable precision. [1] B. E. Chapman, et. al. PRL 87 (2001) 205001. [2] Puiatti M.E.,Cappello S. et al., 19th IAEA conference (Lyon), paper EX/P5-05 and extended paper submitted to Nucl. Fus. (2002) [3] Kusano K., Sato T., Nucl. Fusion 30 (1990) 2575. [4] Cappello S., Biskamp D., Nuclear Fusion 36 (1996) 571