Dynamics of electron plasma vortices subject to time-dependent external flows*

The behavior of fluid vortices in response to time-dependent externally imposed flows is studied using pure electron plasmas in the laboratory. These plasmas obey guiding-center ExB drift dynamics in the plane perpendicular to the magnetic field which are isomorphic to the dynamics of a two-dimensional (2D) inviscid, incompressible (ideal) fluid, where electron density plays the role of fluid vorticity [1]. External flows are applied by adjusting the boundary conditions of the cylindrical container. Previous work has focused primarily on the dynamics of electron vortices under constant strain flow [2], whereas here the flow strength is varied in time. Transient strain pulses are used to generate filaments of vorticity, and the resulting shear (e.g., Kelvin-Helmholtz) instability is studied. When the strain is ramped slowly, adiabatic behavior is observed. When the strain is varied periodically, under certain conditions chaotic and turbulent behavior are observed. These results contribute to a comprehensive picture of the behavior of vortices and shear layers in the presence of external influences. The experimental results are compared to theoretical models [3,4] and to corresponding vortex-in-cell simulations. These studies may be relevant to a variety of other quasi-2D fluid systems in which vortex structures commonly arise, including the dynamics of geophysical fluids, astrophysical disks, and magnetically confined plasmas for fusion research.

* In collaboration with J. R. Danielson, D. H. E. Dubin, and C. M. Surko.

[1] C. F. Driscoll and K. S. Fine, Phys. Fluids B 2, 1359 (1990)

[2] N. C. Hurst, J. R. Danielson, D. H. E. Dubin, and C. M. Surko, J. Fluid Mech. 848, 256-287 (2018)

[3] S. Kida, J. Phys. Soc. Japan 50, 3517 (1981)

[4] D. G. Dritschel, P. H. Haynes, M. N. Juckes, and T. G. Shepherd, J. Fluid Mech. 230, 647-665 (1991)