Equilibrium Evolution in the ZaP Flow Z-Pinch

The ZaP Flow Z-pinch experiment at the University of Washington investigates the innovative plasma confinement concept of using sheared flows to stabilize an otherwise unstable 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 an axially flowing Z-pinch that is 1 m long with a 1 cm radius with a coaxial accelerator coupled to a pinch assembly chamber. Magnetic probes measure the fluctuation levels of the azimuthal modes m = 1, 2, and 3. After assembly the plasma is magnetically confined for an extended quiescent period where the mode activity is significantly reduced. Time-resolved Doppler shifts of plasma impurity lines are measured along 20 chords to determine the plasma axial velocity profiles showing a large, but sub-Alfvenic, sheared flow during the quiescent period and low shear profiles during periods of high mode activity. The plasma has a sheared axial flow that exceeds the theoretical threshold for stability during the quiescent period and is lower than the threshold during periods of high mode activity. 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. Multichord and holographic interferometers measure a Z-pinch plasma with a peaked radial profile during the quiescent period. Internal magnetic fields have been recently determined by measuring the Zeeman splitting of impurity carbon emission. The measurements are consistent with a well-confined pinch plasma.