On the Physics of Improved Confinement During Pulsed Poloidal Current Drive in MST Reversed-Field Pinch

Abstract

Reduction of core-resonant magnetic fluctuations and improved confinement in the Madison Symmetric Torus reversed-field pinch (MST RFP) 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. To study the effect we developed a relatively simple 1-D model in cylindrical geometry which assumes poloidal and axial symmetry during the drive. We use resistive magnetohydrodynamics 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 selfconsistently with momentum equation. We start from an initial unstable equilibrium and examine stability of plasma configuration at intermediate 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. It enhances later in time due to the flattening of k profile in the core region as plasma and magnetic field compress inward during the drive.

Nonlinear magnetohydrodynamics of pulsed parallel current drive in reversed-field pinches

“…While this is similar to the conclusions of Ref. 19, we show that it is also consistent with pinching due to loss of dynamo, which can dominate the subsequent profile evolution, as discussed in Ref. 17.…”

“…The subsequent stabilization is similar to the effect of edge current noted in Ref. 19. However, slightly later in our nonlinear simulation, non- local profile changes due to loss of dynamo begin to occur, and the gradient in the -profile near the rational surface builds.…”

“…The computations are similar to the profile evolution considered in Ref. 19, except that we use lower S-value, and an effective small flux of mass prevents a vacuum region from opening. The computations are started from an Ohmic equilibrium with pinch and reversal parameters of ⌰ = 1.56 and F = 0.01, respectively, where ⌰ ϵ 0 aI z / 2⌽ z , F ϵ a 2 ͗B z ͑a͒͘ / ⌽ z , I z is the axial current, and ⌽ z is the axial flux.…”

“…The modification in edge current by the applied poloidal electric field is found to alter the equilibrium to a sufficient degree to make resonant tearing modes stable. 19 In Ref. 17, it is noted that an edge current is absent in three-dimensional simulations that show nonlocal pulse penetration due to dynamo loss.…”

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

“…This study is an addition to the previous modelling of stabilization of core resonant (resonance surface is within the reversal surface) current-driven modes in RFP during PPCD [14,15] which explained their stabilization during the drive. The previous and present studies demonstrated stabilization of current-and pressure-driven modes during PPCD, thus explaining the significant improvement in plasma confinement.…”