The effect of external magnetic field on the linear stage evolution of Kelvin–Helmholtz instability in laser driven plasma

Abstract

A Kelvin–Helmholtz instability (KHI) is a fundamental physical process of fluids and magnetized plasma. We report the experimental formation of a KHI produced by intense laser-driven thin plastic foils. For an external magnetic field in different directions, the KHI shows different evolutionary features. A theoretical derivation shows that an external magnetic field has a stronger effect on KHI growth when its component is perpendicular to the plasma flow. A linear evolutionary stage of the KHI is captured by optimizing the target design, which is reproduced by FLASH simulation.

Introduction

A Kelvin–Helmholtz instability (KHI) occurs when two parallel fluids of different velocities and densities flow alongside each other, with a shear exceeding a critical value [1]. In a collisionless plasma, wave–particle interactions play essential roles in transport and dissipation [2], governing the intermixing of fluids in both natural and engineering systems. Although not expected to appear, KHIs are widespread in inertial confinement fusion [3]. The KHI is also essential to many astrophysical and space phenomena such as the dynamic structure of cometary tails [4], relativistic outflows and oscillations in astrophysical jets [5], [6], mergers between neutron stars or energy and momentum transfer between the solar wind and the magnetosphere of the Earth [7]. Recent observations have found that the KHI plays a very essential role in plasma heating as it develops on small scales, thereby enhancing dissipation such as turbulent viscosity at the boundaries of larger-scale coronal mass ejections and solar jets [1], [8], [9], [10], [11]. Wan et al. [12] first observed a KHI evolving from well-controlled dual-mode seed perturbations. Kinetic simulations show that an electron vortex can generate secondary magnetic reconnection, a formation of secondary magnetic islands and flux ropes. This is a nonlinear effect of electron KHI driven by electron shear flows produced during magnetic reconnection, which leads to strong electron energization in the outflow region. However, this has not been verified experimentally. Simulation and observation have also found that a KHI can generate secondary magnetic reconnection [13], [14], [15], [16].

Laboratory studies are an important platform for observing such macroscopic phenomena in astrophysical plasma. A key assumption of all laser-driven high-energy-density (HED) experiments is that the dynamics of the system is repeatable across experiments as long as the defining metrics of the platform remain constant. Repeatability allows users to map the entire evolution of a system. Wan et al. [17] used the OMEGA EP laser facility to observe single-mode KHI behavior in the supersonic regime. Our experiment also establishes a platform that can be iterated to explore compressible HED shear flows in more detail. In addition, Harding et al. used radiographic diagnosis to induce a new and highly pronounced HED KHI [18]. In recent years, there have been many studies on linear and weakly nonlinear behavior of incompressible KHIs. A slight disturbance at the fluid interface grows linearly and weakly nonlinearly, and it develops from a strong nonlinear effect on a turbulent mixture [19], [20], [21]. However, no past study has used an external magnetic field when observing KHI behavior in the supersonic regime.

In this work, we study the linear evolution of a KHI in the laboratory. We use laser-induced plasma to drive KHI formation and observe the effect of an external magnetic field on KHI development and evolution.