Strong Vertical Magnetic Field Research Articles

Investigation of the spreading of a liquid metal droplet under a vertical magnetic field

In the liquid metal divertor of a magnetic confinement fusion device, the spreading characteristics of the liquid metal are crucial for ensuring the stable operation of the divertor. This study has experimentally investigated the spreading characteristics of a GaInSn alloy droplet on a solid substrate under a strong vertical magnetic field, with the magnetic field intensity ranging from 0 to 2.5 T. First, several parameters of the droplet, such as droplet shape, spreading factor, dynamic contact angle, spreading velocity, and rebound behavior after impacting, were studied without a magnetic field. The fitting relationship between maximum spreading factor βmax and Weber number We was obtained and has been compared with the scaling laws from the literature. Furthermore, the effect of the vertical magnetic field on those parameters has been investigated systematically. Quantitative results on βmax and the maximum spreading time tDmax, varied with the Hartmann number (Ha) and the We number, provide a comprehensive understanding of the spreading dynamics. The specific relationship between βmax and We number under different magnetic field intensities (B) shows that a vertical magnetic field has a great inhibiting effect on liquid metal droplet spreading. Finally, the influence of oxidation on droplet spreading characteristics also has been studied. These basic findings are important for the application of liquid metal on a divertor/limiter in a fusion reactor, offering a theoretical reference engineering design.

Scaling relations in quasi-static magnetoconvection with a strong vertical magnetic field

The scaling law for the horizontal length scale $\ell$ relative to the domain height $L$ , originating from the linear theory of quasi-static magnetoconvection, $\ell /L \sim Q^{-1/6}$ , has been verified through two-dimensional (2-D) direct numerical simulation (DNS), particularly at high values of the Chandrasekhar number ( $Q$ ). This relationship remains valid within a specific flow regime characterized by columnar structures aligned with the magnetic field. Expanding upon the $Q$ -dependence of the horizontal length scale, we have derived scaling laws for the Nusselt number ( $Nu$ ) and the Reynolds number ( $Re$ ) as functions of the driving forces (Rayleigh number ( $Ra$ ) and $Q$ ) in quasi-static magnetoconvection influenced by a strong magnetic field. These scaling relations, $Nu \sim Ra/Q$ and $Re \sim Ra Q^{-5/6}$ , have been successfully validated using 2-D DNS data spanning a wide range of five decades in $Q$ , ranging from $10^5$ to $10^9$ . The successful validation of the relations at large $Q$ values, combined with our theoretical analysis of dissipation rates and the incorporation of the horizontal length scale's influence on scaling behaviour, presents a valid approach for deriving scaling laws under various conditions.

Regulation of nanosecond pulse breakdown process by vertical magnetic field

Effects of vertical magnetic field on the breakdown process of the nanosecond pulse discharge in atmospheric air are studied via two-dimensional particle-in-cell/Monte Carlo collision simulations. The numerical model is chosen and defined reasonably, with reference to experimental situations and literature reports. It is shown that when the applied magnetic field is strong enough, the evolutionary characteristics of the ionization channel are greatly affected due to the Lorentz force on charged particles. The impact is manifested macroscopically by the slowing down of the ionization channel evolution speed, the ionization channel shift, and the improvement of the discharge uniformity. At the microscopic level, the impact is mainly reflected in the regulation of the highest-energy electrons and the regulation of the overall electron energy distribution. That is, the adoption of a strong vertical magnetic field is capable of suppressing the generation of energetic electrons. The authors' results explicitly demonstrate the regulation of vertical magnetic field on the breakdown process of the nanosecond pulse discharge, which provides more comprehensive knowledge for the atmospheric air gap nanosecond pulse discharge physics and the theoretical basis for application design.

On the Jet–Ejecta Interaction in 3D GRMHD Simulations of a Binary Neutron Star Merger Aftermath

Short γ-ray burst (sGRB) jets form in the aftermath of a neutron star merger, drill through disk winds and dynamical ejecta, and extend over four to five orders of magnitude in distance before breaking out of the ejecta. We present the first 3D general-relativistic magnetohydrodynamic sGRB simulations to span this enormous scale separation. They feature three possible outcomes: jet+cocoon, cocoon, and neither. Typical sGRB jets break out of the dynamical ejecta if (i) the bound ejecta’s isotropic equivalent mass along the pole at the time of the BH formation is ≲10−4 M ⊙, setting a limit on the delay time between the merger and BH formation, otherwise, the jets perish inside the ejecta and leave the jet-inflated cocoon to power a low-luminosity sGRB; (ii) the postmerger remnant disk contains a strong large-scale vertical magnetic field, ≳1015 G; and (iii) if the jets are weak (≲1050 erg), the ejecta’s isotropic equivalent mass along the pole must be small (≲10−2 M ⊙). Generally, the jet structure is shaped by the early interaction with disk winds rather than the dynamical ejecta. As long as our jets break out of the ejecta, they retain a significant magnetization (≲1), suggesting that magnetic reconnection is a fundamental property of sGRB emission. The angular structure of the outflow isotropic equivalent energy after breakout consistently features a flat core followed by a steep power-law distribution (slope ≳3), similar to hydrodynamic jets. In the cocoon-only outcome, the dynamical ejecta broadens the outflow angular distribution and flattens it (slope ∼1.5).

Columnar dipolar clusters defying gravity.

A striking and highly versatile feature of magnetic (nano)particles is their ability to be manipulated at will at a distance by external fields. In this paper, the influence of gravity on the self-assembly of dipolar particles near a surface in the presence of a strong vertical magnetic field is investigated theoretically. A rich ground-state phase diagram stems from the effects of the number of particles N and gravity. Two distinct regimes are discovered for the gravity-mediated breakup of a standing chain. When N is small, there is a chain fragmentation (with two widely separated repulsive chain fragments) above a critical value for the gravity, whereas for higher chains, ribbonization (with two cohesive chain fragments) sets in. In both scenarios, simple algebraic decays for the transition gravity as a function of N are analytically predicted and accurately corroborate the exact numerical results. Further intricate chain fragmentations and internal ribbon transformations operate upon further increasing the gravity until all N constitutive particles lie on the surface. Our findings shed additional light on various recent experiments and computer simulations on magnetic colloids and granular media.

Non-Invasive Groundwater Velocity Measurements Using a Novel Electromagnetic Flowmeter

Tracing groundwater flow is of vital importance for managing water resources and understanding the natural water cycle. However, it is a challenge to make accurate measurements of groundwater flow due to its extremely low velocity. A new electromagnetic flowmeter has been developed to conduct non-invasive groundwater flow velocity measurements. In the setup, a customized trapezoidal current source was connected to a 1 m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 1 m square coil to generate a strong vertical magnetic field. Two electrodes were used to sense the potential difference caused by water flowing through the magnetic field. A simulation has been developed based on Faraday’s law to evaluate the flow signal under an idealized condition. Two different lab environments have been built for testing the device: an artificial mini-aquifer and a rolling gantry. The measurements show that the flow signal, in practice, generated by the slow-moving water was orders of magnitude smaller than the inductive and capacitive interferences in the system. A linear signal processing model has been developed, allowing the flow signal to be extracted from the measured results. The results show good agreement with simulation and a strong correlation between the flow speed and the processed flow signal, having an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R^{2}$ </tex-math></inline-formula> coefficient of 0.94.

Noncollinear Remanent Textures Induced by Surface Spin Flop in Synthetic Antiferromagnets with Perpendicular Anisotropy

The surface spin flop, observed in synthetic antiferromagnets (SAFs) with uniaxial anisotropy and strong antiferromagnetic (AF) interlayer exchange coupling, can be considered as a laterally homogeneous, vertical AF domain wall pushed into the SAF from either the top or the bottom in the presence of a strong external vertical magnetic field. As a result, the AF domain wall can be described as a one-dimensional entity. In this work, we present a concept to stabilize laterally homogeneous vertical AF domain walls by local variation of the perpendicular magnetic anisotropy in $\mathrm{Co}/\mathrm{Pt}$-based SAFs. Our approach not only allows the stabilization of the vertical AF domain wall in the absence of any external magnetic field, but furthermore enables a deterministic selection among four different remanent states, each one stable within a broad external magnetic field range of almost one tesla. We also demonstrate an extension to our concept by stabilizing two coexisting vertical AF domain walls, thus yielding a system with a total of six different selectable (and reprogrammable) remanent states. The controlled stabilization of noncollinear AF textures in the form of vertical AF domain walls at remanence could be used as an infrastructure for propagating spin waves within the AF domain wall itself, as well as for tuning the dynamic behavior of perpendicular standing spin wave modes existing vertically across the SAF.

Vortex in a strongly coupled dusty plasma embedded in an external magnetic field

Vortex motion of a dust fluid suspended in an argon rf power has been investigated for both the kinetic and hydrodynamic limit of a strongly coupled compressible dusty plasma below crystallization limit. Dust particles are confined by a radial weak electric field under the influence of a strong vertical magnetic field, dust-dust correlation, ion drag, particle attachment, grains drift, and diffusion. Radial distribution of different vortex properties is observed numerically. Modifications of those properties due to the variations in grain size and ion temperature are compared between hydrodynamic and kinetic limits and are found to be more prominent in the hydrodynamic regime. With the decreasing correlation among the grains, the range of radial distribution of dust particles expands and the influence of shear rotation increases. Moreover, vortex flow dissipation is found to be enhanced at a large rate in the case of hydrodynamic limit compared to the case of kinetic one.

Anticorrelation of Variations of the Magnetic Field of a Sunspot and the Brightness of Its Umbra in Long-Period Sunspot Oscillations

Changes in the umbra brightness and in the magnetic field over time should occur on long-period sunspot oscillations in accordance with Birman’s idea of inhibition of circulation (overturning) convection by a strong vertical magnetic field in antiphase mode.When the sunspot as a whole shifts upward, its power flux tube expands with a fixed magnetic flux, the magnetic field decreases, and the umbra brightness should increase. Vice versa, when the magnetic flux tube of the sunspot is lowered, it is compressed, the field grows, and the umbra brightness should decrease. Until now, this oscillatory process, which is described by the theoretical model of a shallow sunspot, has remained unexplored. This paper attempts to prove the Birman effect (Biermann, 1941) on the example of long-period oscillations of the sunspot 2010/12/07-13 (NOAA 11 133), for which there are available data from both the Solar and Heliospheric Observatory (SOHO) and Solar Dynamics Observatory (SDO).

Relationship of the Magnetic-Field Strength and the Brightness of the Sunspot Umbra and the Center of a Facular Knot

Two main features define a sunspot as an object: a strong magnetic field and a low umbra temperature. The same can be said about a facular knot, in the center of which there is a dark micropore, and the magnetic field is several hundred Gauss (which noticeably exceeds the level of equipartition of about 200–250 G), sometimes reaching 1000 or 1200 G. The temperature decrease in these activity elements is explained by the suppression of circulation (overturning) convection by a strong vertical magnetic field. The first to express this idea was Biermann (1941). Within the framework of this conception, a saturation effect should be expected: when the magnetic field completely suppresses convection, its further growth should not lead to a decrease in temperature. There is still no clear answer to the question of the saturation effect. We will try to answer it in the first part of the work according to the Solar Dynamics Observatory. We show that saturation occurs at B = 2300–2400 G. A further increase in the magnetic field of the spot no longer leads to a decrease in the brightness of its umbra. The relationship of the magnetic field and intensity is also manifested in long-period oscillations of the magnetic field of spots. In this case, the brightness of the spot umbra changes in antiphase with its magnetic field (Efremov et al., 2020) in accordance with the theoretical model of a shallow sun spot (Solov’ev and Kirichek, 2014, 2016). A similar effect was established by us in the second part of this work for facular knots: during slow facular oscillations with periods of more than 1 h, its brightness changes in antiphase with a magnetic field. The obtained results convincingly confirm the Birman hypothesis.

Rayleigh--Bénard convection in strong vertical magnetic field: flow structure and verification of numerical method

Direct numerical simulations are performed to study turbulent Rayleigh-Benard convection in a vertical cylindrical cavity with uniform axial magnetic field. Flows at high Hartmann and Rayleigh numbers are considered. The calculations reveal that, similarly to the behavior observed in Rayleigh-Benard convection with strong rotation, flows at strong magnetic field develop a central vortex, while the heat transfer is suppressed.

Conductivity influence on interfacial waves in liquid metal batteries and related two-layer systems

Fluid flows in liquid metal batteries can be generated by a number of effects. We start with a short overview of different driving mechanisms and then address questions specific to the metal pad role instabilities in three-layer systems. We focus on the role of the conductivity distribution in the cell, noting at the same time that interfacial tension should be considered as well for smaller cells. Following this discussion, numerical results on the excitation of interfacial waves in two-layer liquid metal systems with miscibility gaps bearing an interface normal electric current are presented. Confirming recent results from the literature, we find that magnetic damping plays a decisive role for strong vertical magnetic fields. In addition, boundary conditions for the electric field strongly influence critical currents and growth rates.

Effects of the vessel bottom wall on a particle rising in an electrically conducting fluid under a strong vertical magnetic field

The drag of a particle rising along a vertical magnetic field in an electrically conducting fluid is studied when the vessel bottom wall exists. A dynamically similar flow using compressed coordinates is obtained when the Hartmann number is much greater than one, the Reynolds number is much smaller than the Hartmann number, and the magnetic Reynolds number is much smaller than one. The drag influenced by the vessel bottom wall is derived from this similar solution. A correction term with respect to the vessel bottom wall is added to Chester’s drag in free space. The drag of the particle increases when the distance between the particle and vessel bottom wall decreases. The region of influence of the vessel bottom wall spreads along the magnetic flux lines when the Hartmann number increases. Therefore, the correction term of drag affects a large region in the vessel when the magnetic field is very strong.

Quasi-periodic oscillations, trapped inertial waves, and strong toroidal magnetic fields in relativistic accretionwhen th discs

The excitation of trapped inertial waves (r-modes) by warps and eccentricities in the inner regions of a black hole accretion disc may explain the high-frequency quasi-periodic oscillations (HFQPOs) observed in the emission of Galactic X-ray binaries. However, it has been suggested that strong vertical magnetic fields push the oscillations' trapping region toward the innermost stable circular orbit (ISCO), where conditions could be unfavourable for their excitation. This paper explores the effects of large-scale magnetic fields that exhibit \textit{both} toroidal and vertical components, through local and global linear analyses. We find that a strong toroidal magnetic field can reduce the detrimental effects of a vertical field: in fact, the isolation of the trapping region from the ISCO may be restored by toroidal magnetic fields approaching thermal strengths. The toroidal field couples the r-modes to the disc's magneto-acoustic response and inflates the effective pressure within the oscillations. As a consequence, the restoring force associated with the vertical magnetic field's tension is reduced. Given the analytical and numerical evidence that accretion discs threaded by poloidal magnetic field lines develop a strong toroidal component, our result provides further evidence that the detrimental effects of magnetic fields on trapped inertial modes are not as great as previously thought.

Effects of vertical, horizontal and rotational magnetic fields on convection in an electromagnetically levitated droplet

A series of three-dimensional numerical simulations was conducted to investigate the convection inside an electromagnetically levitated silicon droplet under vertical, horizontal and rotational magnetic fields, respectively. The results show that the flow is totally axisymmetric and two counter-circulating vortexes appear in the droplet with an inner-outer distribution under vertical magnetic fields. The inner vortex brings fluids from the top area to bottom area directly, which is potentially responsible for the requirement of quite strong vertical magnetic fields in the experimental measurement of thermal conductivity of molten silicon. With horizontal magnetic fields, the vortexes inside the droplet turn to an up-down distribution, which is more beneficial in the thermal conductivity measurement since it prevents the direct convective heat transfer effectively. However, the horizontal magnetic field fails to suppress the convection effectively and the flow is intense and spatially imbalanced, leading to a less stable flow state. The rotational magnetic field combines the advantages of the formers, suppresses the convection along z-axis apparently and creates an up-down distribution of vortexes simultaneously. It suggests that the rotational magnetic field shows potential interest in the measurement of thermal conductivity of melts. Besides, the rotational magnetic field would evoke forced convection in azimuthal direction, which can be better controlled by varying magnetic intensity and rotating frequency. The azimuthal flow is expected to balance the rotation of melts induced by helicity of coils in experiments and shows potential advantages in melt stirring and solidification from undercooled melts.

Wall modes in magnetoconvection at high Hartmann numbers

Three-dimensional turbulent magnetoconvection at a Rayleigh number of $Ra=10^{7}$ in liquid gallium at a Prandtl number $Pr=0.025$ is studied in a closed square cell for very strong external vertical magnetic fields $B_{0}$ in direct numerical simulations which apply the quasistatic approximation. As $B_{0}$, or equivalently the Hartmann number $Ha$, are increased, the convection flow, which is highly turbulent in the absence of magnetic fields, crosses the Chandrasekhar linear stability limit for which thermal convection ceases in an infinitely extended layer and which can be assigned a critical Hartmann number $Ha_{c}$. Similar to rotating Rayleigh–Bénard convection, our simulations reveal subcritical sidewall modes that maintain a small but finite convective heat transfer for $Ha&gt;Ha_{c}$. We report a detailed analysis of the complex two-layer structure of these wall modes, their extension into the cell interior, and a resulting sidewall boundary layer composition that is found to scale with the Shercliff layer thickness.

Interplay of cell death signaling pathways mediated by alternating magnetic field gradient

The ability to control or manipulate the pathways leading to cell death plays a pivotal role in cancer treatment. We demonstrate magneto-actuation of magnetic nanoparticles (MNPs) to induce different cell death signaling pathways, exemplifying the intricate interplay between apoptosis and necrosis. In vitro cell experiments show the cell viabilities decreases with increasing field strength and is lower in cells treated with low aspect ratio MNPs. In a strong vertical magnetic field gradient, the MNPs were able to apply sufficient force on the cell to trigger the intracellular pathway for cell apoptosis, thus significantly reducing the cell viability. The quantification of apoptotic and necrotic cell populations by fluorescence dual staining attributed the cell death mechanism to be predominantly apoptosis in a magnetic field gradient. In contrast, the MNPs in an alternating magnetic field gradient can effectively rupture the cell membrane leading to higher lactate dehydrogenase leakage and lower cell viability, proving to be an effective induction of cell death via necrosis.

Vertical oscillations of dust particles in a strongly magnetized plasma sheath induced by horizontal laser manipulation

Experimental studies are presented where dust particles are suspended in the lower sheath region of an argon rf discharge at a strong vertical magnetic field from B=1.5 T up to 2.27 T. There the particles arranged in an ordered pattern imposed by the upper mesh electrode. It is observed that the particles jump to a new equilibrium position, where they exhibit self-excited vertical oscillations when illuminated by a horizontal laser beam. The dust motion is weakly damped during an upward jump and strongly damped during the return to the equilibrium after the laser is switched off. A model based on delayed charging is presented that can describe the observed behavior.

Heat and momentum transfer for magnetoconvection in a vertical external magnetic field.

The scaling theory of Grossmann and Lohse [J. Fluid Mech. 407, 27 (2000)JFLSA70022-112010.1017/S0022112099007545] for turbulent heat and momentum transfer is extended to the magnetoconvection case in the presence of a (strong) vertical magnetic field. A comparison with existing laboratory experiments and direct numerical simulations in the quasistatic limit allows us to restrict the parameter space to very low Prandtl and magnetic Prandtl numbers and thus to reduce the number of unknown parameters in the model. Also included is the Chandrasekhar limit, for which the outer magnetic induction field B is large enough such that convective motion is suppressed and heat is transported by diffusion. Our theory identifies four distinct regimes of magnetoconvection that are distinguished by the strength of the outer magnetic field and the level of turbulence in the flow, respectively.

Some new aspects of the transient ionization layer of comet Siding Spring origin in the Martian upper atmosphere

AbstractThe close encounter of comet Siding Spring with Mars resulted in the formation of a dense transient ionization layer in the Martian upper atmosphere at altitudes between 80 and 120 km. Instruments on three spacecraft orbiting Mars detected the presence of this layer, as reported in previous publications. In this study, we reanalyzed the ionograms of the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on Mars Express to get further insight about the recurrence of the layer. For this purpose, data from three periapsis passes of MARSIS that took place 5 h, 12 h, and 19 h after peak dust deposition are used. We found that the transient ionization layer was sustained at least for 19 h on the nightside and 12 h on the dayside. While the peak density of the layer on the nightside gradually decreases from orbit to orbit, it does not change much on the dayside. Some ionograms in all three orbits show two transient ionization layers that are separated by ~60 km in apparent altitude. These double layers occur preferentially in regions of strong vertical magnetic fields. The bottom layer of the double structure is probably an oblique echo due to reflections from ionization bulges (formed in regions of vertical magnetic fields) at altitudes of the transient ionization layer. Horizontal bifurcation of the original layer is considered as another plausible mechanism for explaining the double‐layer structure.