Evolution of Plasma Flow Shear and Stability in the ZaP Flow Z-Pinch

Abstract

Summary form only given. The ZaP flow Z-pinch experiment at the University of Washington investigates the concept of using sheared flows to stabilize an otherwise unstable plasma configuration. The stabilizing effect of a sheared axial flow on the m=1 kink instability in Z-pinches has been studied using linearized, ideal MHD theory to reveal that a sheared axial flow stabilizes the kink mode when the shear exceeds a threshold. The ZaP experiment generates a Z-pinch plasma that is 1 m long with a 1 cm radius with an embedded axial flow. Time-resolved Doppler shifts of plasma impurity lines are measured along 20 chords to determine the plasma axial velocity profiles. An azimuthal array of magnetic probes measures the fluctuation levels of the azimuthal modes m=1, 2, and 3. After pinch assembly a quiescent period is found where the mode activity is significantly reduced. The plasma axial velocity evolves from a uniform to a sheared and back to a uniform flow profile. The sheared flow profile is coincident with a plasma quiescent period where magnetic mode fluctuations are low. The value of the velocity shear satisfies the theoretical threshold for stability during the quiescent period and does not satisfy the threshold during high mode activity. Evolution of the plasma parameters including ion temperature and axial velocity will be presented