Dynamics and precise control of fluid V-states using an electron plasma

Abstract

An electron plasma can be confined for a theoretically infinite time in a Penning-Malmberg trap, a linear, azimuthally-symmetric magneto-electrostatic device where upon suitable conditions (high magnetization) the transverse dynamics of the plasma column is isomorphic to the one displayed by a two-dimensional ideal fluid. Fluid dynamics can thus be reproduced in these systems with a very high degree of control on the system's parameters and active excitation of fluid perturbations is made possible by the use of static or time-dependent electric fields (i.e., fluid strains) imparted by electric potentials applied to the azimuthal patches of a sectored electrode of the trap. An example is represented by azimuthal velocity shear phenomena and the insurgence of Kelvin-Helmholtz (KH) instabilities in fluid vortices. We present a study where we exploit multipolar rotating electric fields to generate V-states and observe their dynamics and stability properties. A V-state is the generalization of the 2D Kirchhoff (elliptical) fluid vortex to a generic KH mode, in the nonlinear regime. In particular, we discuss first how we can exploit a combination of techniques (plasma evaporation and tilt-induced transport) to tune the radial vorticity profile, which may have an effect on the dynamics of the growth and decay of the selected KH wave. We also investigate autoresonant (swept-frequency, self-locking) excitation - useful, e.g., for the precise control of the KH mode growth - and discuss the features of autoresonance applied to higher-order KH waves.