Role of interchange and mixed modes in magnetic Rayleigh-Taylor instability dynamics --- a 2D vs 3D study

Abstract

The magnetic Rayleigh-Taylor instability (MRTI) governs plasma mixing and transport in diverse systems. Owing to computational constraints, two-dimensional (2D) MRTI simulations are common. It is intuitive that 2D and three-dimensional (3D) counterparts differ significantly. One of the crucial differences is the types of wave modes admissible in the two systems. Perturbations in 3D MRTI facilitate the coexistence of interchange, undular, and mixed modes. Such rich physics is not possible in 2D, where the perturbations can only have undular modes. This raises the question, how do the interchange and mixed modes change the MRTI dynamics? To address this aspect, we simulate MRTI in 2D and 3D, across different magnetic field strengths. Consequently, this paper provides, currently lacking, a rigorous qualitative and quantitative evaluation of the similarities and differences between 2D and 3D MRTI. The 2D MRTI is characterized by large-scale plumes with inefficient mixing and significant fluid interpenetration, while the 3D is characterized by significant mixing and small-scale plumes. Consequently, higher gravitational potential energy is extracted in 2D. The small-scale vortices and large current sheets lead to higher energy dissipation in 3D. Extending to fundamental nature of evolution, 3D simulations tend towards a self-similar state (due to interchange and mixed modes). But such behaviour was not observed in our 2D simulations, due to the persistent influence of magnetic field. Surprisingly, the reconnection driven magnetic-to-kinetic energy conversion is similar in 2D and 3D. Overall, the work concludes that interchange and mixed modes influence the dynamics significantly leading to stark differences between 2D and 3D MRTI. The work thus guides the extent to which one can extrapolate the 2D simulations results to understand MRTI in astrophysical and fusion energy systems.

arXiv comment21 pages, 22 figures, 1 Table