Speaker
Description
A single-species (e.g., electron) plasma can have a theoretically indefinite lifetime in a magneto-electrostatic device such as a Penning-Malmberg trap, a linear, azimuthally-symmetric electrostatic confinement environment immersed in an intense axial magnetic field. Here the transverse dynamics of the sample is isomorphic to the one exhibited by a two-dimensional ideal fluid – with significant experimental advantages lying in the high degree of control on the system's parameters, such as initial conditions and active fluid strain perturbations, as well as the effective diagnostic opportunities [1,2].
An example of such 2D fluid dynamics is the evolution of l-fold symmetric fluid vortices. These structures may be observed in a range of natural environments, such as geophysical and astrophysical flows. Such vortices can be generated from an isolated, cylindrically symmetric vorticity patch and brought to the nonlinear deformation regime (V-state) by means of a resonant excitation of a single Kelvin-Helmholtz (KH) mode. In the plasma analogue, this translates into perturbing an electron column of circular cross-section by means of oscillating multipolar electric fields [3]. The stability of V-states has been subject of investigation ever since Kirchhoff’s prediction in the case of a quadrupolar deformation and the generalisation by Deem and Zabusky for arbitrary deformation order [4].
We present here a review of our experimental and numerical investigations on this subject. We show first how we can exploit a combination of techniques to tune the initial radial vorticity profile. We analyse then some characteristic features of V-state insurgence and evolution, as the locking and the strength of the coupling between the perturbation and the external strain field. In particular, we observe the influence of the initial vorticity profile and the structure of the strain field on the vortex-forcing interaction. Following the forced and free relaxation of the V-state after its saturation, we observe that the decay to a “natural” axisymmetric equilibrium may be interrupted, in favour of permanently or intermittently deformed structures. We also implement an autoresonant (swept-frequency, self-locking) excitation scheme - useful, e.g., for the precise control of the KH mode growth – which shows again some peculiar features.
References:
[1] C. F. Driscoll and K. S. Fine, Phys. Fluids B 2, 1359 (1990).
[2] P. Wongwaitayakornkul, J. R. Danielson, N. C. Hurst, D. H. E. Dubin and C. M. Surko, Phys. Plasmas 29, 052107 (2022).
[3] G. Maero, N. Panzeri, L. Patricelli and M. Romé, J. Plasma Phys. 89, 935890601 (2023).
[4] G. S. Deem and N. J. Zabusky, Phys. Rev. Lett. 40, 859 (1978).
