Equilibrium, flow shear and stability measurements in the Z-pinch

Abstract

The stabilizing effect of a sheared axial flow is investigated in the ZaP flow Z-pinch experiment at the University of Washington. Long-lived, hydrogen Z-pinch plasmas are generated that are 1 m long with an approximately 10 mm radius and exhibit gross stability for many Alfvén transit times. Large magnetic fluctuations occur during pinch assembly, after which the amplitude and frequency of the fluctuations diminish. This stable behaviour continues for an extended quiescent period. At the end of the quiescent period, fluctuation levels increase in magnitude and frequency. Axial flow profiles are determined by measuring the Doppler shift of plasma impurity lines using a 20-chord spectrometer. Experimental measurements show a sheared flow that is coincident with low magnetic fluctuations during the quiescent period. The experimental flow shear exceeds the theoretical threshold during the quiescent period, and the flow shear is lower than the theoretical threshold at other times. The observed plasma behaviour and correlation between the sheared flow and stability persists as the amount of injected neutral gas and experimental geometry are varied. Computer simulations using experimentally observed plasma profiles show a consistent sheared flow stabilization effect. Plasma pinch parameters are measured independently to demonstrate an equilibrium consistent with radial force balance.

High energy density Z-pinch plasmas using flow stabilization

“…The quiescent period is coincident with the establishing of a sheared flow state. [1] The quiescent period lasts 20-100 µs, which is 1000's of theoretical growth times (20 ns) for a static plasma with the same parameters. The quiescent period also corresponds to other experimental measurements indicating stable plasma behavior, such as peaked density profiles and increasing plasma temperatures.…”

“…The details of the formation process and experimental measurements are provided in Refs. [1] and [11], and references therein.…”

“…The ZaP Flow Z-Pinch research project [1] investigates using sheared flows to stabilize these classical instabilities without requiring close-fitting walls or axial magnetic fields. Theoretical analysis suggests that a sufficiently radially-sheared axial flow can stabilize the modes in a pure Z-pinch provided dv z / dr ≥ 0.1kV A .…”

Controlling Rayleigh-Taylor Instabilities in Magnetically Driven Solid Metal Shells by Means of a Dynamic Screw Pinch

“…However, stabilizing mass ablation does not occur in magnetically driven liner implosions. In diffuse z-pinches, MRTI is controlled by mass accretion [43] or by tailoring the initial density [44,45] or velocity shear [46,47] profiles. Yet, these methods are not easily applicable to the relatively slow implosions of dense metal shells.…”

Calculation of the Equilibrium Evolution of the ZaP Flow $Z$ -Pinch Using a Four-Chord Interferometer

“…This paper describes a method in which two of the temporally evolving equilibrium properties of the ZaP flow Z -pinch [11]- [14], magnetic field and temperature, are calculated using electron density measurements from a four-chord interferometry system to completely describe the Z -pinch MHD equilibrium. Interferometry and magnetic probe measurements are coupled with the equations of radial force balance and radial heat conduction.…”