Evolution of ExB electron plasma vortex under strain pulse: Competition Between Adiabaticity and Instability.

Strongly magnetized pure electron plasmas provide a clean laboratory analog for two dimensional (2D) vortex dynamics relevant to geophysical and astrophysical flows, high-intensity beam transport, and drift-wave turbulence. The current study reports systematic experiments and numerical modeling of the response of an initially axisymmetric vortex to a half-sinusoidal strain pulse (HSP)—a finite waveform that probes both adiabatic invariant breaking [1] and the onset of flow-driven instabilities [2]. Here, the adiabatic invariant (I) is the area enclosed by the vortex orbit in a phase space parameterized by its aspect ratio (λ) and orientation (ξ). The change, △ I > 0, due to HSP quantifies the breaking of adiabaticity. Scanning the peak strain amplitude ε_m and pulse duration T reveals two distinct regimes of response. At lower ε_m, we observe clear periodic increase of △ I as well as a robust T^-2 scaling with pulse duration. This regime shows excellent agreement between experiment and a constant vorticity elliptical model [3]. In contrast, at higher ε_m, the vortex transiently crosses the separatrix in phase space, and the system departs significantly from the simple power-law behavior. Further, when the vortex aspect ratio surpasses λ > 3, the experimental observation deviate notably from the elliptical model indicating the onset of instability-driven dynamics [4]. Using the numerical model, a 2D (ε_m , T ) map of △ I shows chains of periodic islands. By examining individual vortex trajectories, we find that the phase at which the vortex reaches its maximum aspect ratio strongly influences the structure and amplitude of the contours. Future work will investigate how variations in vortex profile shape and multi‑pulse forcing protocols can drive transitions to complex or turbulent flow regimes.

We acknowledge support from the DOE grant DE-SC0016532 and UCSD foundation.

[1] N. C. Hurst et al.,Phys. Rev. Fluids 6, 054703 (2021).

[2] N. C. Hurst et al.,Phys. Plasmas 27, 042101 (2020).

[3] S. Kida, J. Phys. Soc. Japan 50, 3517–3520 (1981).

[4] D. W. Moore, and P. G. Saffman. Aircraft Wave Turbulence and its Detection, Plenum Press, New York, pp. 339-354 (1971).