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Formation and Sustainment of a Sheared Flow Z- Pinch
| Author: | Golingo R.P. |
| Coauthor: | . Shumlak, B.A. Nelson, S.L. Jackson, D.J. Den Hartog, and the ZaP Team |
| Institution : | University of Washington |
| Abstract text: | The ZaP Flow Z-Pinch project is experimentally studying the effect of sheared flows on Z-pinch stability. It has been shown numerically that when dVz/dr exceeds 0.1kVA the kink (m=1) mode is stabilized [U. Shumlak, C.W. Hartman, Phys. Rev. Lett. 75 (18), 3285 (1995)]. A coaxial accelerator coupled with a 1 meter assembly region is used to form Z-pinches with an embedded axial flow. Operating parameters have been identified which generate long-lived, quiescent Z-pinches throughout the first half cycle of the current.
During the initial plasma acceleration phase, the motion of the current sheet is consistent with snowplow models previously developed for other coaxial accelerators. Magnetic probes in the assembly region measure the azimuthal modes of the magnetic field. The m=1 mode is proportional to the radial displacement of the Z-pinch away from the axis. The magnetic mode levels show a quiescent period which is over 1,000 times the growth time of a static Z-pinch. The axial velocity profile is measured with a 20-chord ICCD spectrometer. Using data from multiple pulses, the time evolution of the velocity profile has been measured during formation, throughout the quiescent period, and into the transition to instability. The evolution shows that a sheared plasma flow develops as the Z-pinch forms. Throughout the quiescent period, the flow shear is greater than the theoretically required threshold for stability. As the flow shear decreases, the magnetic mode activity increases. The coaxial accelerator provides axially moving plasma throughout the quiescent period and may explain the evolution of the velocity profile and the sustainment of the flow Z-pinch. The present understanding of the project may lead to steady state Z-pinches.
An overview of the experimental program, recent results, and future work will be presented.
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