Stabilization of ideal pressure gradient driven edge modes during pulsed parallel current drive in reversed field pinch

Abstract

Significant improvement of plasma confinement in the Madison Symmetric Torus reversed field pinch (RFP) has been routinely achieved by applying an inductive electric field at the plasma boundary in the direction parallel to the equilibrium magnetic field at the plasma edge. An auxiliary edge current is driven by this electric field with the goal of replacing the dynamo-driven current and modifying the parallel current profile to reduce current-driven instabilities. This current-drive technique is called pulsed parallel current drive (PPCD) in RFP. During PPCD plasma fluctuations are reduced everywhere resulting in tokamak-like confinement parameters, while the edge density profile steepens significantly and plasma beta increases. A steep edge plasma pressure profile, a relatively high plasma beta and a strong unfavourable curvature of equilibrium magnetic field near the edge in RFP could excite pressure-driven fluid turbulence near the edge and worsen plasma confinement, opposite to the experimental observations. In this study stability analysis of edge pressure gradient driven ideal modes in standard-like and in PPCD-like plasma equilibria is performed. An ideal magnetohydrodynamic plasma model in cylindrical RFP equilibrium with a step function plasma pressure profile and a vacuum layer between the plasma boundary and the conducting shell is used. Standard-like and PPCD-like plasma equilibria in the model are defined by the direction of the surface current at the plasma–vacuum interface. The results show that while in standard-like equilibrium the edge pressure gradient driven modes are highly unstable in this model, the transition to PPCD-like equilibrium completely stabilizes these modes. The modes stabilization is primarily due to strengthening of magnetic shear at the location of the pressure gradient during the drive and due to the proximity of this location to the conducting wall. This stabilization mechanism is not specific to RFPs, making PPCD a general method of stabilization of the edge pressure gradient driven instabilities which could be applied in other magnetic confinement systems. Application of PPCD to stabilize the edge localized modes in tokamaks is proposed.