|icc06.pdf||2006-02-22 16:06:15||Vladimir Svidzinski|
On the physics of improved confinement during pulsed poloidal current drive in MST reversed-field pinch.
Author: Vladimir Svidzinski
Submitted: 2005-12-21 18:16:21
Co-authors: S. C. Prager
University of Wisconsin-Madison
1150 University Ave
Madison, Wisconsin 53706
Reduction of core-resonant magnetic fluctuations and improved confinement in the Madison Symmetric Torus reversed-field pinch have been routinely achieved by applying the surface poloidal electric field. The created inductive poloidal electric field drives current in plasma which leads to the improved confinement. 3-D resistive MHD modeling has been used by several groups to study the effect. Due to limitations in computing power the 3-D models are studied with smaller Lundquist numbers and usually the plasma density is not evolved. With these limitations and the ambiguity of the results the exact mechanism of fluctuation reduction is not evident from such modeling. We develop a relatively simple 1-D model in cylindrical geometry which assumes poloidal and axial symmetry during the drive. We use resistive MHD model with realistic plasma parameters and assume that there is a vacuum gap between plasma boundary and conducting wall of the vessel. Evolution of plasma density is taken into account and plasma boundary moves self-consistently with momentum equation. During the drive there is a repulsive force between the image currents in plasma and currents driven in the conducting shell by the applied voltage. The force pushes plasma inwards and drags with it the equilibrium magnetic field embedded in the plasma. This leads to compression (pinching) of magnetic field, plasma current and density toward the core region. We start from an initial unstable equilibrium and examine stability at intermidiate moments of time during the drive. For this we calculate the growth rates of unstable eigenmodes in the plasma. Our results show that the modifications to the plasma current profile during the drive are stabilizing. The initial stabilization is due to the direct modification of the current profile near the edge and it is enhances later in time due to the flattening of lambda profile in the core region which is due to the pinching effect.