Strong Horizontal Magnetic Field Research Articles

Numerical simulations of a coaxial bubble pair rising in a liquid metal column under horizontal magnetic fields

The motion and coalescence of a coaxial bubble pair rising in a liquid metal column under horizontal magnetic fields were numerically examined using the VOF method in this present paper. The MHD (Magnetohydrodynamics) effects on the characteristics of rise velocity, flow field, and coalescence process of bubble pair by considering various wall confinement ratios (Cr) were analyzed. The results indicate that the effects of magnetic field and wall confinement on the coalescence of the coaxial bubbles are non-monotonic. For smaller Cr, a higher initial rise velocity of the bubbles is generated by strengthening the counter-rotating toroidal vortices around bubbles in the initial stage, but the terminal rise velocity decreases in the stable stage. In the presence of the magnetic field, both the initial rise velocity and terminal rise velocity decrease. A horizontal magnetic field makes the flow field around the bubbles be anisotropic by weakening the toroidal vortices on both sides of the bubbles along the magnetic field direction, which also dampens the wake vortices of the leading bubble and thus reduces the attractive force of wake effect acting on the tailing bubble. On the other hand, the downward Lorentz force induced by the magnetic field on the top of the leading bubble suppresses its upward motion, which makes the tailing bubble collide with the leading bubble earlier. As the competition between the above two mechanisms varies with the magnetic field strength, the coalescence time of the bubble pair also changes accordingly. Particularly, a strong horizontal magnetic field tends to promote the bubbles coalescence under the wall effects and delay that when the wall effects are minor or negligible. For Ha=771, bubbles coalescence at Cr=2 is about 32 % earlier than that at Cr=4.

Wall-attached convection under strong inclined magnetic fields

We employ a linear stability analysis and direct numerical simulations to study the characteristics of wall modes in thermal convection in a rectangular box under strong and inclined magnetic fields. The walls of the convection cell are electrically insulated. The stability analysis assumes periodicity in the spanwise direction perpendicular to the plane of a homogeneous magnetic field. Our study shows that for a fixed vertical magnetic field, the imposition of horizontal magnetic fields results in an increase of the critical Rayleigh number along with a decrease in the wavelength of the wall modes. The wall modes become tilted along the direction of the resulting magnetic fields and therefore extend further into the bulk as the horizontal magnetic field is increased. Once the modes localized on the opposite walls interact, the critical Rayleigh number decreases again and eventually drops below the value for onset with a purely vertical field. We find that for sufficiently strong horizontal magnetic fields, the steady wall modes occupy the entire bulk and therefore convection is no longer restricted to the sidewalls. The aforementioned results are confirmed by direct numerical simulations of the nonlinear evolution of magnetoconvection. The direct numerical simulation results also reveal that at least for large values of horizontal magnetic field, the wall-mode structures and the resulting heat transfer are dependent on the initial conditions.

Non-linear magnetic buoyancy instability and turbulent dynamo

ABSTRACT Stratified discs with strong horizontal magnetic fields, are susceptible to magnetic buoyancy instability (MBI). Modifying the magnetic field and gas distributions, this can play an important role in galactic evolution. The MBI and the Parker instability, in which MBI is exacerbated by cosmic rays, are often studied using an imposed magnetic field. However, in galaxies and accretion discs, the magnetic field is continuously replenished by a large-scale dynamo action. Using non-ideal MHD equations, we model a section of the galactic disc (we neglect rotation and cosmic rays considered elsewhere), in which the large-scale field is generated by an imposed α-effect of variable intensity to explore the interplay between dynamo instability and MBI. The system evolves through three distinct phases: the linear (kinematic) dynamo stage, the onset of linear MBI when the magnetic field becomes sufficiently strong and the non-linear, statistically steady state. Non-linear effects associated with the MBI introduce oscillations which do not occur when the field is produced by the dynamo alone. The MBI initially accelerates the magnetic field amplification but the growth is quenched by the vertical motions produced by MBI. We construct a 1D model, which replicates all significant features of 3D simulations to confirm that magnetic buoyancy alone can quench the dynamo and is responsible for the magnetic field oscillations. Unlike similar results obtained with an imposed magnetic field, the non-linear interactions do not reduce the gas scale height, so the consequences of the magnetic buoyancy depend on how the magnetic field is maintained.

CHIMERA Fusion Technology Facility: Testing and Virtual Qualification

The Combined Heating and Magnetic Research Apparatus (CHIMERA) fusion technology test facility is under construction. The facility will be uniquely capable of semi-integral testing of fusion materials and component modules up to the size of the ITER test blanket module box, under combined conditions of in-vacuum high heat flux, static and pulsed magnetic fields, and high-temperature/high-pressure water cooling. This paper reports the high-level capabilities of the CHIMERA baselined design and the planned program of testing and describes the proposed strategy for use of simulations for virtual testing, qualification, and in-situ monitoring. The first step in testing of a component mock-up is to take data from as-built geometry and other measurements and transmit them to an integrated computational model that can closely mimic the physical asset and form a digital replica. Not only can this digital replica be queried in advance of physical testing in the facility, allowing optimization of the test program, but combined with subsequent test data, it also can deliver much greater insight into experimental results than can be obtained using test data alone. The digital replica is used as the basis for a digital twin, which is live coupled to the running experiment, and is under development as a proposed key facet of fusion reactor surveillance in-service. Physical mock-ups for testing can be subjected to in-vacuum heat flux up to 0.5 MW/m2 over the entire surface while within a strong horizontal magnetic field. The central field can be up to 4 T with a peak in the test region of 5 T. The same component mock-ups can also be subjected to repeated magnetic field pulses with ramp rate 12 T/s, which can simulate loading conditions of a plasma disruption. Facility upgrades are underway to include a liquid metal circulation loop to allow the study of magnetohydrodynamics effects and to add a high-heat-flux system using a very high-power continuous-wave laser to achieve divertor-relevant heat fluxes of 20 MW/m2 over the area of a small-scale mock-up. Four examples are given to illustrate the physical testing program that is currently foreseen.

Development of a Longitudinal Wave EMAT With High Conversion Efficiency

Until now, most of the bulk-wave electromagnetic acoustic transducers (EMATs) are developed for shear wave (S-wave) testing. This article presents the development of new longitudinal wave (L-wave) EMATs with high conversion efficiency by using a new magnetic configuration and a meander-line coil. The new magnetic configuration consists of several rectangular magnets arranged face to face to generate a strong horizontal magnetic field in periodical distribution under the magnets. By using the meander-line coil under the magnets, a strong L-wave can be generated and detected. The new EMATs have been characterized and optimized using numerical simulation methods. Simulation results show that the flux density of the horizontal magnetic field of the new EMATs can be increased by a factor of ~4.7, compared with that of the conventional U-type magnet. So, the conversion efficiency for L-wave generation and detection can be significantly improved. Finally, experimental measurement and comparison with the current L-wave EMATs with the U-type magnet have also been carried out. The experimental results have confirmed that the signal amplitude of the new L-wave EMATs is increased by a factor of ~8.3.

3D unsteady and steady modeling of heat and mass transfer during Cz Si crystal growth with a horizontal magnetic field

A combined LES/RANS approach is applied to modeling Czochralski (Cz) Si growth with a strong horizontal magnetic field stabilizing the melt flow. A model furnace geometry with a cylindrical crucible is considered, which can be suggested for benchmarking Cz Si melt convection of industrial scale. Grid resolution in boundary layers is carefully adjusted and the 4th approximation order for diffusion terms is shown to provide sufficient computation accuracy even with comparatively rough grids. The computed flow structure, temperature distribution on the melt free surface, and RMS temperature fluctuations of the melt are in good agreement with published experimental data and modeling results for industrial furnaces. The effect of the crystal rotation rate on the interface shape and oxygen incorporation into the crystal is analyzed. Due to high thermal conductivity (or low Pr number) of the melt, the unsteady oscillations do not exert a significant effect on the temperature and heat flux distribution in the melt, which allows quite accurate predictions of the interface shape, even neglecting the flow unsteadiness. In contrast, due to a low diffusivity (or high Sc number) of atomic oxygen in the melt, the unsteady oscillations quite significantly affect its transport through the melt and incorporation into the crystal, which results in remarkable overestimation of the oxygen concentration without consideration of unsteady melt flow evolution. The formulation of the LES/RANS approach used in the benchmark problem was verified on experimental data for an industrial furnace for Magnetic Cz (MCz) growth of 300 mm silicon crystals.

Formation of superstrong horizontal magnetic field in delta-type sunspot in radiation magnetohydrodynamic simulations

ABSTRACT We perform a series of radiative magnetohydrodynamic simulations to understand the amplification mechanism of the exceptionally strong horizontal magnetic field in delta-type sunspots. In the simulations, we succeed in reproducing the delta-type sunspot and resulting strong magnetic field exceeding 6000 G in a light bridge between the positive and negative polarities. Our conclusions in this study are summarized as follows: (1) The essential amplification mechanism of the strong horizontal magnetic field is the shear motion caused by the rotation of two spots. (2) The strong horizontal magnetic field remains the force-free state. (3) The peak strength of the magnetic fields does not depend on the spatial resolution, top boundary condition, or Alfvén speed limit. The origin of the rotating motion is rooted in the deep convection zone. Therefore, the magnetic field in the delta-spot light bridge can be amplified to the superequipartition values in the photosphere.

Effects of hall currents with heat and mass transfer on the peristaltic transport of a Casson fluid through a porous medium in a vertical circular cylinder

In the current paper, the peristaltic transport of a non-Newtonian fluid obeying a Casson model with heat and mass transfer inside a vertical circular cylinder is studied. The considered system is affected by a strong horizontal uniform magnetic field together with the heat radiation and the Hall current. The problem is modulated mathematically by a system of PDE that describe the basic behavior of the fluid motion. The boundary value problem is analytically solved with the appropriate boundary conditions in accordance with the special case, in the absence of the Eckert number. The solutions are obtained in terms of the modified Bessel function of the first kind. Again, in the general case, the system is solved by means of the homotopy perturbation and then numerically through the Runge-Kutta Merson with a shooting technique. A comparison is done between these two methods. Therefore, the velocity, temperature and concentration distributions are obtained. A set of diagrams are plotted to illustrate the influence of the various physical parameters in the forgoing distributions. Finally, the trapping phenomenon is also discussed.

Observations of solar small-scale magnetic flux-sheet emergence

Aims. Two types of flux emergence were recently discovered in numerical simulations: magnetic loops and magnetic sheet emergence. While magnetic loop emergence has been documented well in recent years using high-resolution full Stokes data from ground-based telescopes as well as satellites, magnetic sheet emergence is still an understudied process. We report here on the first clear observational evidence of a magnetic sheet emergence and characterise its development. Methods. Full Stokes spectra from the Hinode spectropolarimeter were inverted with the Stokes Inversion based on Response functions (SIR) code to obtain solar atmospheric parameters such as temperature, line-of-sight velocities, and full magnetic field vector information. Results. We analyse a magnetic flux emergence event observed in the quiet-Sun internetwork. After a large-scale appearance of linear polarisation, a magnetic sheet with horizontal magnetic flux density of up to 194 Mx cm−2 hovers in the low photosphere spanning a region of 2–3 arcsec. The magnetic field azimuth obtained through Stokes inversions clearly shows an organised structure of transversal magnetic flux density emerging. The granule below the magnetic flux sheet tears the structure apart leaving the emerged flux to form several magnetic loops at the edges of the granule. Conclusions. A large amount of flux with strong horizontal magnetic fields surfaces through the interplay of buried magnetic flux and convective motions. The magnetic flux emerges within 10 minutes and we find a longitudinal magnetic flux at the foot points of the order of ∼1018 Mx. This is one to two orders of magnitude larger than what has been reported for small-scale magnetic loops. The convective flows feed the newly emerged flux into the pre-existing magnetic population on a granular scale.

The Origin and Early Evolution of a Bipolar Magnetic Region in the Solar Photosphere

Abstract Finding the formation mechanisms for bipolar configurations of a strong local magnetic field under control of the relatively weak global magnetic field of the Sun is a key problem of the physics of solar activity. This study is aimed at discriminating whether the magnetic field or fluid motion plays a primary, active role in this process. The very origin and early development stage of Active Region 12548 are investigated based on Solar Dynamics Observatory/Helioseismic and Magnetic Imager observations of 2016 May 20–25. Full-vector magnetic and velocity fields are analyzed in parallel. The leading and trailing magnetic polarities are found to grow asymmetrically in terms of their amplitude, magnetic flux, and the time variation of these quantities. The leading-polarity magnetic element originates as a compact feature against the background of a distributed trailing-polarity field, with an already existing trailing-polarity magnetic element. No signs of strong horizontal magnetic fields are detected between the two magnetic poles. No predominant upflow between their future locations precedes the origin of this bipolar magnetic region (BMR); instead, upflows and downflows are mixed, with some prevalence of downflows. Any signs of a large-scale horizontal divergent flow from the area where the BMR develops are missing; in contrast, a normal supergranulation and mesogranulation pattern is preserved. This scenario of early BMR evolution is in strong contradiction with the expectations based on the model of a rising Ω-shaped loop of a flux tube of strong magnetic field, and an in situ mechanism of magnetic-field amplification and structuring should operate in this case.

Onset of Photospheric Impacts and Helioseismic Waves in X9.3 Solar Flare of 2017 September 6

Abstract The X9.3 flare of 2017 September 6, was the most powerful flare of Solar Cycle 24. It generated strong white-light emission and multiple helioseismic waves (sunquakes). By using data from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory as well as hard X-ray (HXR) data from the KONUS instrument on board the WIND spacecraft, and Anti-Coincidence System on board the INTERGRAL space observatory, we investigate spatio-temporal dynamics of photospheric emission sources, identify sources of helioseismic waves, and compare the flare photospheric dynamics with the HXR temporal profiles. The results show that the photospheric flare impacts started to develop in compact regions in close vicinity of the magnetic polarity inversion line (PIL) in the preimpulsive phase before detection of the HXR emission. The initial photospheric disturbances were localized in the region of strong horizontal magnetic field of the PIL, and, thus, are likely associated with a compact sheared magnetic structure elongated along the PIL. The acoustic egression power maps revealed two primary sources of generation of sunquakes, which were associated with places of the strongest photospheric impacts in the preimpulsive phase and the early impulsive phase. This can explain the two types of helioseismic waves observed in this flare. Analysis of the high-cadence HMI filtergrams suggests that the flare energy release developed in the form of sequential involvement of compact low-lying magnetic loops that were sheared along the PIL.

Experimental study on rise velocities of single bubbles in liquid metal under the influence of strong horizontal magnetic fields in a flat vessel

The ascent of single argon bubbles with equivalent diameters (deq) between 3.43 and 6.28 mm is investigated at room temperature in a flat, rectangular vessel by means of ultrasound Doppler velocimetry (UDV). GaInSn is used as a working liquid and magnetic flux intensities up to B ≈ 0.918 T are applied. A decelerating effect on the rise velocity is observed at lower, an accelerating effect at medium and a reduction at higher field strengths. Maximum velocities are achieved when the ratio of N/CD ≈ 1, bubble paths are substantially rectilinear at N/CD > 2. The mean ascent velocities are compared with literature and data of this work as well as other publications is provided in tables.

Physical modelling of Czochralski crystal growth in horizontal magnetic field

This study addresses experimentally the heat transfer, the temperature azimuthal non-uniformity and the onset of oscillations in a low temperature physical model of a medium-sized Czochralski crystal growth process with a strong horizontal magnetic field (HMF). It is observed that under certain conditions the integral heat flux may decrease with increasing magnetic field strength at the same time as the flow velocity increases. The azimuthal non-uniformity of the temperature field in the melt near the crystal model rim is only little influenced by its rotation rate outside of a narrow range where the centrifugal force balances the buoyant one. The flow oscillation onset has been observed for two values of the HMF strength. Conditions of this onset are little influenced by the crystal rotation. The critical temperature difference of the oscillation onset considerably exceeds that of the Rayleigh–Bénard (RB) cell in a strong HMF.

Numerical investigation of MHD heat transfer in a vertical round tube affected by transverse magnetic field

Numerical simulations of the flow of a liquid metal in a vertical pipe are performed. The configuration reproduces the test section of the recent experiment [1]. The mean flow is directed downward, a half of the pipe's wall is heated, and a strong horizontal magnetic field perpendicular to the temperature gradient is imposed. The simulations produce results in good agreement with the experiment and lead us to an explanation of the observed phenomenon of anomalous high-amplitude temperature fluctuations. The fluctuations are caused by growth and quasi-periodic breakdown of the pairs of ascending and descending jets related to the elevator-mode thermal convection. Implications for operation of liquid metal blankets with poloidal ducts are discussed.

On Linear Instability and Stability of the Rayleigh–Taylor Problem in Magnetohydrodynamics

We investigate the stabilizing effects of the magnetic fields in the linearized magnetic Rayleigh-Taylor (RT) problem of a nonhomogeneous incompressible viscous magnetohydrodynamic fluid of zero resistivity in the presence of a uniform gravitational field in a three-dimensional bounded domain, in which the velocity of the fluid is non-slip on the boundary. By adapting a modified variational method and careful deriving \emph{a priori} estimates, we establish a criterion for the instability/stability of the linearized problem around a magnetic RT equilibrium state. In the criterion, we find a new phenomenon that a sufficiently strong horizontal magnetic field has the same stabilizing effect as that of the vertical magnetic field on growth of the magnetic RT instability. In addition, we further study the corresponding compressible case, i.e., the Parker (or magnetic buoyancy) problem, for which the strength of a horizontal magnetic field decreases with height, and also show the stabilizing effect of a sufficiently large magnetic field.

Mixed convection in horizontal duct flow with transverse magnetic field and heating of side wall

Computational study of mixed convection in a horizontal duct with very strong (Hartmann numbers Ha up to 800) horizontal transverse magnetic field and electrically insulated walls is conducted. Strong constant-rate heating (Grashof numbers Gr up to 109) is applied to one vertical wall, while the other walls are thermally insulated. It is found that in a wide range of Ha and Gr the flow is two-dimensional (uniform in the streamwise direction) and steady-state. No instability to convection rolls aligned with the magnetic field detected recently in a similar duct and pipe with bottom heating is observed. The velocity field is a combination of convection-modified streamwise flow and convection-induced circulation in the transverse plane. The circulation has strong jets near the heated and horizontal walls as prominent features. It is demonstrated that, at a constant Reynolds number, integral characteristics and spatial structure of the flow can be approximately considered as determined by Gr/Ha2 as a single parameter. We also present results of a systematic comparison study of several commonly used grid clustering schemes in their application to wall-bounded flows with strong magnetic fields.

The Role of Magnetic Fields in Transient Seismic Emission Driven by Atmospheric Heating in Flares

Transient seismic emission in flares remains largely mysterious. Its discoverers proposed that seismic transients are driven by impulsive heating of the flaring chromosphere. Simulations of such heating show strong shocks, but these are damped by heavy radiative losses as they proceed downward. Because compression of the gas the shock enters both heats it and increases its density, the radiative losses increase radically with the strength of the shock, leaving doubt that sufficient energy can penetrate into the solar interior to explain helioseismic signatures. We note that simulations to date have no account for strong, inclined magnetic fields characteristic of transient-seismic-source environments. A strong horizontal magnetic field, for example, greatly increases the compressional modulus of the chromospheric medium, greatly reducing compression of the gas, hence radiative losses. Inclined magnetic fields, then, must be fundamental to the role of impulsive heating in transient seismic emission.

Fabrication of the oriented LiCoO2 sheet using a strong magnetic field

LiCoO2 is a popular positive electrode material for Li-ion secondary batteries. The perpendicular alignment of the interlayer of the LiCoO2 lattice on the electrode contributes to easy access of Li-ions. In this study, we report the preparation of an oriented LiCoO2 sheet using the conventional coating method followed by drying in a maximum 12 T strong magnetic field. We discuss the effect of the particle shape and viscosity of the paste on the degree of orientation. As a result, when the sheets are dried in a strong horizontal magnetic field, the electrochemical active planes of the LiCoO2 are aligned perpendicular to the sheet surface.

Effects of Martian crustal magnetic field on its ionosphere

The effect of the Martian crustal magnetic field is one of the hot topics of the study of Martian ionosphere. The studies on this topic are summarized in this paper. Main data of the Martian ionosphere were resulted from radio occultation experiments. According to the observations, the electron density scale height and the peak electron density of the Martian ionosphere are influenced by its crustal magnetic field. The strong horizontal magnetic field prevents the vertical diffusion of the plasma and makes the electron density scale height in the topside ionosphere close to that in the photo equilibrium region. In the cusp-like regions with strong vertical magnetic field, the enhanced vertical diffusion leads to a larger electron density scale height in the diffusion equilibrium region. The observation of radio occultation experiment onboard Mars Global Surveyor (MGS) showed that the averaged peak electron density observed in the southern hemisphere with strong crustal magnetic field was slightly larger than that in the northern hemisphere with weak crustal magnetic field. The Mars advanced radar for subsurface and ionosphere sounding (MARSIS) onboard Mars Express (MEX) was the first topside sounder to be used to observe Martian ionosphere. The MARSIS results confirmed that the enhancement of the peak electron density occurred in cusp-like regions with open field lines, and the amount of the enhancement was much larger than that observed by the radio occultation experiment. There are two possible mechanisms for the peak electron density enhancement in the cusp-like crustal magnetic field regions: One is the precipitation of the energetic particles and the other is the heating by the waves excited by plasma instabilities. It’s difficult to determine which one is the key mechanism for the peak electron density enhancement. Based on these studies, several interesting problems on the Martian ionosphere and plasma environment are presented.

Magnetic Field Structures in a Facular Region Observed by THEMIS and Hinode

The main objective of this paper is to build and compare vector magnetic maps obtained by two spectral polarimeters, i.e. THEMIS/MTR and Hinode SOT/SP, using two inversion codes (UNNOFIT and MELANIE) based on the Milne-Eddington solar atmosphere model. To this end, we used observations of a facular region within active region NOAA 10996 on 23 May 2008, and found consistent results concerning the field strength, azimuth and inclination distributions. Because SOT/SP is free from the seeing effect and has better spatial resolution, we were able to resolve small magnetic polarities with sizes of 1" to 2", and we could detect strong horizontal magnetic fields, which converge or diverge in negative or positive facular polarities. These findings support models which suggest the existence of small vertical flux tube bundles in faculae. A new method is proposed to get the relative formation heights of the multi-lines observed by MTR assuming the validity of a flux tube model for the faculae. We found that the Fe 1 6302.5 \AA line forms at a greater atmospheric height than the Fe 1 5250.2 \AA line.