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This study presents three-dimensional (3D) resistive Hall-magnetohydrodynamic simulations of the Kelvin–Helmholtz instability (KHI) dynamics at Earth's magnetospheric flanks during northward interplanetary magnetic field periods. By comparing two simulations with and without initial magnetic shear, we analyze the impact of distinct magnetic field orientations on plasma dynamics and magnetic reconnection events taking into account 3D mechanisms, such as KHI high latitude stabilization. The identical nature of the simulations, except for the presence/absence of an initial magnetic shear, enables, for the first time, a complete and coherent comparative analysis of the latitudinal distribution of KH vortices, current sheets, reconnection events, and the evolution of the mixing layer. In one configuration, a uniform magnetic field leads to double mid-latitude reconnection (MLR), while in the other, magnetic shear induces both type I vortex-induced reconnection (VIR) and MLR. Notably, the type I VIR observed in this second scenario results from the combined action of line advection and vortex-induced current sheet pinching (the classic mechanism driving two-dimensional type I VIR). Of particular importance is our quantification of newly closed field lines that experienced double reconnection, ultimately becoming embedded in solar wind plasma at low latitudes while remaining connected to magnetospheric plasma at high latitudes. The varying abundance of such lines in the two simulations holds implications for plasma transport at the magnetopause. [ABSTRACT FROM AUTHOR]

Measured filament velocities in the DIII-D tokamak are compared against theoretical scalings, finding that the latter often represents an upper limit on experimental velocity distributions with most filaments possessing lower velocity. Filament spin from internal E × B drift is experimentally demonstrated to alter filament radial velocity. A critical spin velocity, where filament radial velocity peaks, is observed and corresponds to approximately 5 km/s. This transition is corroborated using a less direct measure of filament spin in the form of a temperature ratio. These techniques are combined to find that the critical spin velocity closely aligns with transport times along and across filaments becoming comparable. The normalized filament size distribution is consistent with the most stable size as dictated by Kelvin–Helmholtz and curvature-driven instabilities. Overall, the findings suggest filament stability and spin alter filamentary transport that may threaten the integrity of first walls in fusion devices. [ABSTRACT FROM AUTHOR]

The Kelvin–Helmholtz instability (KHI) occurs on the interface of gold cones and embedded fuels for fusion schemes with gold cones. The development of KHI on the inner surface of gold cones in the double-cone ignition scheme is investigated with two-dimensional radiation hydrodynamic simulations. It has been found that the colliding high-density fuel plasma between the tips of the two cones is spatiotemporally separated from the mixed gold ions from the inner surface of the gold cones due to the KHI. Furthermore, it is found that fuel layers coated on the inner surface of the cones can effectively mitigate the energy loss in the compression process. These results could provide a reference for fast ignition schemes with gold cones. [ABSTRACT FROM AUTHOR]

A two-field gyrofluid model including ion finite Larmor radius (FLR) corrections, magnetic fluctuations along the ambient field, and electron inertia is used to study two-dimensional reconnection in a low βe collisionless plasma, in a plane perpendicular to the ambient field. Both moderate and large values of the ion-to-electron temperature ratio τ are considered. The linear growth rate of the tearing instability is computed for various values of τ, confirming the convergence to reduced electron magnetohydrodynamics predictions in the large τ limit. Comparisons with analytical estimates in several limit cases are also presented. The nonlinear dynamics leads to a fully developed turbulent regime that appears to be sensitive to the value of the parameter τ. For τ = 100, strong large-scale velocity shears trigger Kelvin–Helmholtz instability, leading to the propagation of the turbulence through the separatrices, together with the formation of eddies of size of the order of the electron skin depth. In the τ = 1 regime, the vortices are significantly smaller and their accurate description requires that electron FLR effects be taken into account. [ABSTRACT FROM AUTHOR]

The Kelvin–Helmholtz instability (KHI) stems from the velocity shear in a single continuous fluid or a velocity difference across the interface between two distinct fluids. The effect of the transverse magnetic field on the KHI of the supersonic mixing layer is investigated by numerical method. An algorithm with corner-transport-upwind and constrained-transport is used to solve the equations of magnetohydrodynamic (MHD). The evolutions of vorticity, pressure, and shock-vortex structure of the supersonic mixing layer with and without the magnetic field are studied qualitatively and quantitively. The suppression mechanism of the transverse magnetic field on the KHI is analyzed from the aspects of magnetic pressure and magnetic tension, respectively. The results show that the transverse magnetic field has a significant influence on the evolution of shock-vortex structure in the supersonic mixing layer. The magnetic pressure makes the vorticity deposition at the shear layer, and the magnetic tension produces an anti-bending torque on the shear layer. Under these two kinds of effects, the instability of the shear layer is suppressed effectively, and a long-standing banded structure of the shear layer is formed and teared later, which results in a "fishhook-like" structure occurred. Moreover, the transverse magnetic field inhibits the formation of the high-convection Mach value (Mc) regions, which effectively inhibits the formation of shocklets. In addition, with the effect of the magnetic field, a premature laminar-turbulent transition is stimulated at the core region of the vortex structure. [ABSTRACT FROM AUTHOR]

On electron kinetic scales, ions and electrons decouple, and electron velocity shear on electron inertial length ∼ d e can trigger electromagnetic (EM) electron Kelvin–Helmholtz instability (EKHI). In this paper, we present an analytic study of EM EKHI in an inviscid collisionless plasma with a step-function electron shear flow. We show that in incompressible collisionless plasma, the ideal electron frozen-in condition E + v e × B / c = 0 must be broken for the EM EKHI to occur. In a step-function electron shear flow, the ideal electron frozen-in condition is replaced by magnetic flux conservation, i.e., ∇ × (E + v e × B / c) = 0 , resulting in a dispersion relation similar to that of the standard ideal and incompressible magnetohydrodynamics KHI. The magnetic field parallel to the electron streaming suppresses the EM EKHI due to magnetic tension. The threshold for the EM mode of the EKHI is (k · Δ U e) 2 > n e 1 + n e 2 n e 1 n e 2 [ n e 1 (v A e 1 · k) 2 + n e 2 (v A e 2 · k) 2 ] , where v A e = B / (4 π m e n e) 1 / 2 , Δ U e , and ne are the electron streaming velocity shear and densities, respectively. The growth rate of the EM mode is γ em ∼ Ω ce , which is the electron gyro-frequency. [ABSTRACT FROM AUTHOR]

Kelvin–Helmholtz instability (KHI) is considered important in transporting energy and mass at the magnetopause of Earth and other planets. However, the ion kinetic effect influences the generation and evolution of KHI, as the spatial length of the magnetopause may be smaller than the Larmor radius of the ion; this influence is not yet fully understood. In this investigation, laboratory experiments were designed to study the excitation of KHI at the ion kinetic scale. The ion kinetic scale was modeled by controlling the ratio of the Larmor radius and the electric scale length ρ i / L E > 1 , and the KHI was excited at the spatial scale of LE by a controllable sheared E × B flow. It was found that the ion kinetic effect on KHI growth manifests as the ion Larmor radius reaches the shear length scale, and the KHI is suppressed as the ion Larmor radius increases. Incorporating a theoretical analysis by substituting our experimental parameters, the suppression of the KHI was attributed to the fact that the KHI linear growth rate decreases with the ratio change of the ion Larmor radius because the relative orientations of the ion diamagnetic drift velocity ( V d ) and the shear flow velocity ( V 0 ) are opposite. Our experimental conditions ( V d / V 0 < 0) are similar to the dusk-side conditions of the magnetospheres of Earth and Mercury under northward interplanetary magnetic fields; therefore, this result can be extended to understand the evolution of KHI in the planetary boundary layer. [ABSTRACT FROM AUTHOR]

The Richtmyer–Meshkov instability (RMI) is caused by an incident planar shock wave impinging on the heavy-gas-density interface. We have numerically investigated the RMI controlled by different transverse magnetic-field strengths based on the ideal compressible magnetohydrodynamics (MHD) equations. The MHD equations are solved by the corner transport upwind + constrained transport algorithm, which guarantees a divergence-free constraint on the magnetic field. We discuss the flow characteristics and shock patterns in both classical hydrodynamic and MHD situations and verify our conclusions by comparing the experimental results with the numerical results. The results show that the magnetic field modifies the pressure-gradient distribution, and the baroclinic vorticity splits and attaches to the MHD shock waves. In addition, the results indicate that the interaction of shock wave and density interface changes the distribution of magnetic-field energy and distorts the magnetic induction line in the region of magnetic-field energy accumulation. The distortion of the magnetic induction lines alters the magnetic field gradient and creates a magnetic tension that produces a torque opposing that generated by the shear force on the vorticity layer, so the Kelvin–Helmholtz instability is effectively suppressed and no Kelvin–Helmholtz vortex appears on the vorticity layer. The result is that the interface instability is suppressed. [ABSTRACT FROM AUTHOR]

Shear flows along a plasma interface will quickly grow unstable due to the Kelvin–Helmholtz instability. If there is a concurrent temperature gradient across the interface, higher modes are stabilized by the thermal diffusion. These ablative effects must be considered in, for example, jet features in inertial confinement fusion hot-spots, or plasma plumes in young supernovae. We show that magnetization of the plasma can greatly affect the instability, even if magnetic pressure is small. This is because electrons are localized by their gyromotion, reducing the heat flux and material ablation. We use a two-dimensional numerical extended-magnetohydrodynamics approach to assess this effect for dense fusion conditions. In comparison with the unmagnetized case, self-generated Biermann fields make only a minor difference to growth rates. However, simulations with a large 50 kT external field found that the growth rate of the least stable mode increased by 40%. This has implications for mix processes in Z-pinches and magnetized inertial confinement fusion concepts. [ABSTRACT FROM AUTHOR]

At the Earth's low-latitude magnetopause, the Kelvin–Helmholtz instability (KHI), driven by the velocity shear between the magnetosheath and magnetosphere, has been frequently observed during northward interplanetary magnetic field (IMF) periods. However, the signatures of the KHI have been much less frequently observed during southward IMF periods, and how the KHI develops under southward IMF has been less explored. Here, we performed a series of realistic 2D and 3D fully kinetic simulations of a KH wave event observed by the Magnetospheric Multiscale (MMS) mission at the dusk-flank magnetopause during southward IMF on September 23, 2017. The simulations demonstrate that the primary KHI bends the magnetopause current layer and excites the Rayleigh–Taylor instability (RTI), leading to penetration of high-density arms into the magnetospheric side. This arm penetration disturbs the structures of the vortex layer and produces intermittent and irregular variations of the surface waves which significantly reduces the observational probability of the periodic KH waves. The simulations further demonstrate that in the non-linear growth phase of the primary KHI, the lower-hybrid drift instability (LHDI) is induced near the edge of the primary vortices and contributes to an efficient plasma mixing across the magnetopause. The signatures of the large-scale surface waves by the KHI/RTI and the small-scale fluctuations by the LHDI are reasonably consistent with the MMS observations. These results indicate that the multi-scale evolution of the magnetopause KH waves and the resulting plasma transport and mixing as seen in the simulations may occur during southward IMF. [ABSTRACT FROM AUTHOR]

In this study, we present the first observations from the Magnetospheric Multiscale (MMS) mission of the Kelvin–Helmholtz instability (KHI) at the dusk-flank magnetopause during southward interplanetary magnetic field conditions on September 23, 2017. The instability criterion for the KHI was fulfilled for the plasma parameters observed throughout the event. An analysis of the boundary normal vectors based on the application of the timing method onto the magnetic field and the electron density data and the minimum directional derivative method onto the magnetic field data shows signatures of surface waves in the plane parallel to the velocity shear. A comparison to 2D fully kinetic simulations demonstrates reasonable consistencies with the formation of surface waves generated by the KH instability, as well as the structures of rolled-up KH waves. The observations further indicated low density faster than sheath plasma as an indicator of rolled-up vortices, which is also consistent with the simulations. All of these results show that the observed waves and vortices are most likely generated by the KH instability. High-time resolution MMS measurements further demonstrate kinetic-scale electric field fluctuations on the low-density side of the edges of surface waves. Detailed comparisons with the simulations suggest that the observed fluctuations are generated by the lower-hybrid drift instability excited by the density gradient at the edges of these surface waves. These secondary effects can lead to a flattening of the edge layers, indicating the connection between kinetic and larger scales within the KH waves and vortices. [ABSTRACT FROM AUTHOR]

Using a 2D Viscoresistive Reduced MagnetoHydroDynamic model, the magnetic island coalescence problem is studied in the presence of in-plane, parallel shear flows. Extending the analytical work of Waelbroeck et al. [Phys. Plasmas 14, 022302 (2007)] and Throumoulopoulos et al., [J. Phys. A 42, 335501 (2009)] in the sub-Alfvénic flow shear regime for Fadeev equilibrium, the super-Alfvénic regime is studied for the first time numerically. A wide range of values of shear flow amplitudes and shear scale lengths have been considered to understand the effect of sub-Alfvénic and super-Alfvénic flows on the coalescence instability and its nonlinear fate. We find that for flow shear length scales greater than the magnetic island size, the maximum reconnection rate decreases monotonically from sub-Alfvénic to super-Alfvénic flow speeds. For scale lengths smaller than the island size, the reconnection rate decreases up to a critical value v 0 c , beyond which the shear flow is found to destabilize the islands. The value of v 0 c decreases with a decrease in the value of shear flow length scale. Interestingly, for our range of parameters, we find suppression of the Kelvin–Helmholtz instability in super-Alfvénic flows even when the shear scale length is smaller than the island width. Observation of velocity streamlines shows that the plasma circulation inside the islands has a stabilizing influence in strong shear flow cases. Plasma circulation is also found to be responsible for the decrease in upstream velocity, causing less pileup of magnetic flux on both sides of the reconnection sheet. [ABSTRACT FROM AUTHOR]

A fourth-order accurate continuum kinetic Vlasov solver and a systematic method for constructing customizable kinetic equilibria are demonstrated to be powerful tools for the study of nonuniform collisionless low-beta plasmas. The noise-free methodology is applied to investigate two gradient-driven instabilities in 4D (x , y , v x , v y) phase space: the Kelvin–Helmholtz instability and the lower hybrid drift instability. Nonuniform two-species configurations where ion gyroradii are comparable to gradient scale lengths are explored. The approach sheds light on the evolution of the pressure tensor in Kelvin–Helmholtz instabilities and demonstrates that the associated stress tensor deviates significantly from the gyroviscous stress tensor. Even at high magnetization, first-order approximations to finite-gyromotion physics are shown to be inadequate for the Kelvin–Helmholtz instability, as shear scales evolve to become on par with gyromotion scales. The methodology facilitates exploring transport and energy partitioning properties associated with lower hybrid drift instabilities in low-beta plasma configurations. Distribution function features are captured in detail, including the formation of local extrema in the vicinity of particle-wave resonances. The approach enables detailed targeted investigations and advances kinetic simulation capability for plasmas in which gyromotion plays an important role. [ABSTRACT FROM AUTHOR]

Studies have shown that in sheared E × B flows in an inhomogeneous ionospheric plasma, the gradient drift (GDI) or the Kelvin–Helmholtz (KHI) instability may grow. This work examines the conditions that cause one of these instabilities to dominate over the other using a novel model to study localized ionospheric instabilities. The effect of collisions with neutral particles plays an important role in the instability development. It is found that the KHI is dominant in low collisionality regimes, the GDI is dominant in high collisionality regimes, and there exists an intermediate region in which both instabilities exist in tandem. For low collisionality cases in which the velocity shear is sufficiently far from the density gradient, the GDI is found to grow as a secondary instability extending from the KHI vortices. The inclusion of a neutral wind-driven electric field in the direction of the velocity shear does not impact the dominance of either instability. Using data from empirical ionospheric models, two altitude limits are found. For altitudes above the higher limit, the KHI is dominant. For altitudes below the lower limit, the GDI is dominant. In the intermediate region, both instabilities grow together. Increasing the velocity shear causes both limits to be lower in altitude. This implies that for ionospheric phenomena whose density and velocity gradients span large altitude ranges, such as subauroral polarization streams, the instabilities observed by space-based and ground-based observation instruments could be significantly different. [ABSTRACT FROM AUTHOR]

Hall-magnetohydrodynamic (Hall-MHD) theory, two-fluid simulations, and kinetic simulations are used to investigate the cross-field transport properties of Kelvin–Helmholtz instabilities in nonuniform low-beta collisionless plasmas. Hall-MHD analysis shows how the linear properties of the instability are modified by density gradients and magnetization. High-order accurate two-fluid and kinetic simulations, with complete dynamics of finite-mass electrons and ions, are applied to a suite of parameter cases to systematically assess the effects of diamagnetic drift, magnetization, charge separation, and finite Larmor motion. Initialization of exact two-species kinetic equilibria facilitates the study of isolated physical effects and enables detailed cross-comparisons between two-fluid and kinetic simulations, including for cases where ion gyroradii are comparable to gradient scale lengths. For nonuniform plasmas with significant space charge, the results of two-fluid and kinetic simulations are found to disagree with Hall-MHD predictions. Kelvin–Helmholtz instability growth rates, per unit shear, are shown to be smaller when ion diamagnetic drift and E × B drift are parallel and larger when the two drifts are antiparallel. The effect is attributed to polarization drift in the shear layer, which leads to redistribution of charge, alters the electric field that drives plasma advection, and consequently modifies growth rates. Instability-induced mass transport for different parameters is characterized in terms of the flux across the shear layer and a simplified diffusion model. Distribution functions from kinetic simulations are shown to deviate substantially from Maxwellian reconstructions, indicating the importance of kinetic physics during the nonlinear phase of the instability. [ABSTRACT FROM AUTHOR]

In the framework of the studies on magnetic reconnection, much interest has been recently devoted to asymmetric magnetic configurations, which can naturally be found in solar and astrophysical environments and in laboratory plasmas. Several aspects of this problem have been investigated, mainly in a two-dimensional geometry and by means of particle-in-cell (PIC) simulations. Still, there are open questions concerning the onset and the effects of secondary instabilities in the nonlinear phase of an asymmetric reconnection process. In this work, we focus on the conditions that lead to the appearance of the Kelvin-Helmholtz instability following an asymmetric reconnection event in a collisionless plasma. This investigation is carried out by means of two-dimensional numerical simulations based on a reduced fluid model assuming a strong guide field. We show that, unlike the symmetric case, in the presence of asymmetry, a Kelvin-Helmholtz-like instability can develop also for a finite equilibrium electron temperature. In particular, simulations indicate the formation of steep velocity gradients, which drive the instability, when the resonant surface of the equilibrium magnetic field is located sufficiently far from the peak of the equilibrium current density. Moreover, a qualitative analysis of the vorticity dynamics shows that the turbulent behavior induced by the secondary instability not only is confined inside the island but can also affect the plasma outside the separatrices. The comparison between simulations carried out with an adiabatic closure and a Landau-fluid closure for the electron fluid indicates that the latter inhibits the secondary instability by smoothing velocity gradients. [ABSTRACT FROM AUTHOR]

In diverse areas of science and technology, including inertial confinement fusion (ICF), astrophysics, geophysics, and engineering processes, turbulent mixing induced by hydrodynamic instabilities is of scientific interest as well as practical significance. Because of the fundamental roles they often play in ICF and other applications, three classes of hydrodynamic instability-induced turbulent flows—those arising from the Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholtz instabilities—have attracted much attention. ICF implosions, supernova explosions, and other applications illustrate that these phases of instability growth do not occur in isolation, but instead are connected so that growth in one phase feeds through to initiate growth in a later phase. Essentially, a description of these flows must encompass both the temporal and spatial evolution of the flows from their inception. Hydrodynamic instability will usually start from potentially infinitesimal spatial perturbations, will eventually transition to a turbulent flow, and then will reach a final state of a true multiscale problem. Indeed, this change in the spatial scales can be vast, with hydrodynamic instability evolving from just a few microns to thousands of kilometers in geophysical or astrophysical problems. These instabilities will evolve through different stages before transitioning to turbulence, experiencing linear, weakly, and highly nonlinear states. The challenges confronted by researchers are enormous. The inherent difficulties include characterizing the initial conditions of such flows and accurately predicting the transitional flows. Of course, fully developed turbulence, a focus of many studies because of its major impact on the mixing process, is a notoriously difficult problem in its own right. In this pedagogical review, we will survey challenges and progress, and also discuss outstanding issues and future directions. [ABSTRACT FROM AUTHOR]

We investigate the effects of the half angle on the intersection and merger of two plasma jets and verify the observation of the density drop and undulations at the merger interface seen in recent experiments. To perform this analysis, we employed a Smooth Particle Hydrodynamic code to model the jets and their merger. We validate the code against well-known test cases, Sod shock tube, Noh-Cylindrical implosion, and Kelvin-Helmholtz instability. These cases stress and quantify the Smoothed Particle Hydrodynamic code's ability to handle the expected physics of two jet merging. The half angle influences the shock region of the jets, but all jets merge and demonstrate density undulations along their centerline. The merged jet has velocity and density profiles consistent with experimental observations. The density drop was observed in the simulation and can be attributed to the sequence of thermal expansion of the free jets prior to the merge and subsequent density jump across the shock, where the centerline jump would be at the lowest density. A potential cause of the undulations along the centerline can be attributed to Kelvin-Helmholtz instability as it was the culprit in the simulation, although perturbations in the experiments or other causes cannot be ruled out. [ABSTRACT FROM AUTHOR]

A three-dimensional turbulence simulation is performed in order to understand the role of spiral structures observed in Kelvin-Helmholtz turbulence. The simulation is performed by introducing a vorticity source to drive the plasma rotation. By scanning the intensity of the vorticity source, a quasi-periodic formation of a spiral structure is obtained above a certain source intensity. The quasi-periodic oscillation can be seen in the fluctuation energy and the background, which exhibits a limit cycle oscillation. We clarify the roles of the spiral formation in the limit cycle oscillation: The dynamical change in the radial variation of the phase of the fluctuations causes a strong coupling of the fluctuations with the background through the transport of particles and momentum. The formation mechanism of the spiral structure is also studied. An instability due to the combination of the cylindrical effect and the flow inhomogeneity is consistent with the fluctuation which drives the spiral structure obtained in the simulation. [ABSTRACT FROM AUTHOR]