Strong Transverse Magnetic Field Research Articles

Understanding asymmetry in electromagnetically induced transparency for 87Rb in strong transverse magnetic field

We present the results of our experimental investigation performed for D2 line of 87Rb. In this work, we have studied the phenomenon of electromagnetically induced transparency of 87Rb in presence of a transverse magnetic field and it is interesting to find the EIT spectrum shows signature of the closely lying hyperfine excited states in particular F′ = 1. We choose two different configurations for our study namely, Λ1 and Λ2 realised by locking probe beam at two different transition i.e. |F = 1〉 → |F′ = 2〉 and |F = 2〉 → |F′ = 2〉 respectively. We observe asymmetric features in both configurations at high magnetic field and for Λ1 configuration we find complete conversion from transmission to absorption. We explain the observations by quantitative assessment of the impurities in the dark states which arises because of the influence of the neighbouring states. We substantiate our experimental findings with density based numerical calculations.

Numerical solution of steady MHD duct flow in a square annulus duct under strong transverse magnetic field

ABSTRACT This paper explains how the two-dimensional magnetohydrodynamics (MHD) flow through a duct with square annulus cross-section with electrically conducting walls is analysed using radial basis function-generated finite difference method. By applying the transverse magnetic field on the walls with some inclination, the solution is obtained in terms of velocity and magnetic field. The coupled equations are decoupled by using non-dimensional quantities and these equations are solved numerically. The simulations are done by changing the width of the annular region and the shape of the inner boundary. The solutions for induced magnetic field and velocity are obtained for different values of the Hartmann number and the angle of inclination of the applied magnetic field on the walls.

Linear global stability of liquid metal mixed convection in a horizontal bottom-heating duct under strong transverse magnetic field

Two-dimensional steady-state solutions of liquid metal mixed convection in a horizontal bottom-heating duct under a strong magnetic field are first computed numerically by the Newton iteration method along with the spatial discretization of the Taylor–Hood finite element. Two branches of steady solutions with symmetrical rolls and a pair of asymmetrical solutions with a single roll are identified and can be regarded as the base flow for linear global stability analysis. The symmetrical steady solution for the first branch has a nearly uniform distribution for the temperature field in the transverse direction, while the second branch occurs at much larger Grashof numbers and the temperature field becomes nonuniform transversely. The linear stability analysis is performed for a fixed Reynolds number and Prandtl number with Re = 5000 and Pr = 0.0321. For the symmetrical rolls of the first branch, with an increase in the Grashof number, two-dimensional stationary instabilities first occur at small Hartmann numbers, while three-dimensional oscillatory instabilities first appear at moderate or large Hartmann numbers. From the further study of the two-dimensional instabilities, it is revealed that the asymmetrical solution is actually bifurcated supercritically from the symmetrical solution at a two-dimensional critical Grashof number. In addition, the critical curve of the Grashof number with respect to the Hartmann number for the three-dimensional oscillatory mode shows that there exists a minimum critical Grashof number, which occurs at a moderate Hartmann number. The critical curves of the one-roll asymmetrical solution are also exhibited and determined by two three-dimensional oscillatory unstable modes. It is revealed that there exists a minimum Hartmann number below which the asymmetrical steady-state can always remain stable for all Grashof numbers (5 × 105–107). The energy analyses at the oscillatory critical thresholds with different Hartmann numbers are performed to exhibit that buoyancy is the dominant destabilizing term, and the magnetic force is always the main stabilization term for both symmetrical and asymmetrical solutions. In addition, both streamwise and cross-sectional shears of the basic flow are important for the determination of the linear stability boundary of the asymmetrical solution.

An alternative way to study magnetic fluid magnetization and viscosity

In the article, magnetization, viscosity and its increment (“magnetoviscous” effect) in a thin near-wall layer of the column of magnetic fluid that oscillates in the tube when applying a strong transverse magnetic field are assessed. Calculations were performed using formulae obtained on the basis of two different theoretical approaches. The described technique is of interest for studying magnetophoresis and the processes of aggregation of magnetic nanoparticles.

Thermal Radiation, Chemical Reaction, Viscous and Joule Dissipation Effects on MHD Flow Embedded in a Porous Medium

An analysis is presented to study the effects of thermal radiation, chemical reaction, viscous and Joule dissipation on MHD free convection flow between a pair of infinite vertical Couette channel walls embedded in a porous medium. The fluid flows by a strong transverse magnetic field imposed perpendicularly to the channel wall on the assumption of a small magnetic Reynolds number. The governing non linear partial differential equations are transformed in to ordinary differential equations and are solved analytically. The effect of various parameters viz., Eckert number, electric conductivity, dynamic viscosity and strength of magnetic field on temperature profile has been discussed and presented graphically.

“Wall Viscosity” of Magnetic Fluid Oscillations in a Strong Magnetic Field

The authors provide an estimation of the “wall viscosity” and its increment (“magneto-viscous” effect) in a thin near-wall layer of a column of magnetic fluid oscillating in the tube when a strong transverse magnetic field is applied. The calculation is performed according to the formula obtained on the basis of two different theoretical approaches and applied to the previously published experimental results of the complex measurement of oscillation frequency and saturation magnetization of the magnetic fluid. These results are commented upon, stemming from the assumed absence of the field dependence of viscosity. The estimates of “wall viscosity” “from above” and “from below” are used to calculate their contribution to the damping coefficient of oscillations.

Viscosity of Magnetic Fluid in Oscillation System in a Strong Magnetic Field

The article gives an assessment of viscosity and its increment (‘magnetoviscous’ effect) in a thin near-wall layer of the column of magnetic fluid that oscillates in the tube when applying a strong transverse magnetic field. Viscosity is calculated according to a formula derived from two different theoretical approaches. For calculation, previously published experimental results, which were discussed earlier on the assumption that there is no field dependence of viscosity, are used in the article. A comparative analysis of the “near-wall viscosity” estimates obtained using dynamic elasticity as well as the magnetization curve and the coefficient of static elasticity is carried out. The significance of the results obtained for ‘magnetoviscous’ effect for performing diagnosis of interparticle aggregation is indicated.

Heat Transfer during Mixed Convection of a Molten Salt in the Presence of Magnetic Fields

Results of an experimental study of the salt melt downflow in a uniformly heated pipe under the influence of a strong transverse magnetic field are presented. The changes of heat transfer coefficients and statistical characteristics of temperature fluctuations under the influence of the magnetic field are investigated. The peculiarities of the transition of the viscous-gravitational flow in the viscous-inertial-gravitational flow at Reynolds numbers (Re = 3000–5000) under the influence of the magnetic field (Ha = 17) were studied.

Decay of turbulence in a liquid metal duct flow with transverse magnetic field

Decay of honeycomb-generated turbulence in a duct with a static transverse magnetic field is studied via direct numerical simulations. The simulations follow the revealing experimental study of Sukoriansky et al. (Exp. Fluids, vol. 4 (1), 1986, pp. 11–16), in particular the paradoxical observation of high-amplitude velocity fluctuations, which exist in the downstream portion of the flow when the strong transverse magnetic field is imposed in the entire duct including the honeycomb exit, but not in other configurations. It is shown that the fluctuations are caused by the large-scale quasi-two-dimensional structures forming in the flow at the initial stages of the decay and surviving the magnetic suppression. Statistical turbulence properties, such as the energy decay curves, two-point correlations and typical length scales are computed. The study demonstrates that turbulence decay in the presence of a magnetic field is a complex phenomenon critically depending on the state of the flow at the moment the field is introduced.

Analytical Solutions to Nonuniform Surface Heat Transfer With Volumetric Sources in Magnetohydrodynamic Duct Flow

A detailed understanding of the flow of a liquid metal in a rectangular duct subject to a strong transverse magnetic field is vital in a number of engineering applications, notably for proposed blanket technologies for fusion reactors. Fusion reactors offer the potential for clean base-load energy and their development is now entering an engineering phase where the practical means by which the energy released can be converted into useful heat must be addressed. To such ends, this article considers the convective heat transfer processes for fully developed laminar magnetohydrodynamic (MHD) flows in rectangular ducts of the kind proposed in some blanket designs. Analytical solutions which incorporate the nonuniformity of peripheral temperature and heat flux and the effect of volumetric heating, are developed as functions of magnetic field strength and duct aspect ratio. A distinct feature of these MHD problems, not yet addressed in the literature, is that unlike the conventional characterization of heat transfer by a Nusselt number, it is necessary to generalize the concept to vectors and matrices of Nusselt coefficients, due to the extreme anisotropy of both the flow and heating. The new analytical results presented here capture more complex heat transfer behavior than non-MHD flows and in particular characterize the importance of aspect ratio. The importance of these new results lie not only in the improved understanding of this complex process but also in the provision of characterizations of convective heat transfer which underpin progress toward systems scale simulations of fusion blanket technology which will be vital for the realization of practical fusion reactors.

Теплообмен при смешанной конвекции расплава соли в присутствии магнитных полей

The results of an experimental study of the salt melt downflow in a uniformly heated pipe under the influence of a strong transverse magnetic field are presented. The changes of heat transfer coefficients and statistical characteristics of temperature fluctuations under the influence of the magnetic field are investigated. The peculiarities of the transition of the viscous-gravitational flow in the viscous-inertial-gravitational flow at Reynolds numbers (Re=3000-5000) under the influence of the magnetic field (Ha=17) were studied.

Convection instability in a downward flow in a vertical duct with strong transverse magnetic field

The downward flow in a vertical duct with one heated and three thermally insulated walls is analyzed numerically using the two-dimensional approximation valid in the asymptotic limit of an imposed strong transverse magnetic field. The work is motivated by the design of liquid metal blankets with poloidal ducts for future nuclear fusion reactors, in which the main component of the very strong magnetic field is perpendicular to the flow direction and very strong heating is applied at the wall facing the reaction chamber. The flow is found to be steady-state or oscillating depending on the strengths of the heating and magnetic field. A parametric study of the instability leading to the oscillations is performed. It is found among other results that the flow is unstable and develops high-amplitude temperature oscillations at the conditions typical for a fusion reactor blanket.

Effect of initial stress and Hall current on a magneto-thermoelastic porous medium with microtemperatures

In this paper, the effect of initial stress on a thermoelastic porous medium with microtemperatures in the presence of magnetic field taking Hall current into account is investigated. The medium is permeated by a strong transverse magnetic field imposed perpendicularly on the displacement plane so the induced electric field is neglected. The normal mode analysis is used to obtain the exact expressions for displacement components, normal conduction current density field, transverse conduction current density field, microtemperature components, heat flux moments, stress components, temperature distribution and change in the volume fraction field. The variations of the considered variables with the horizontal distance are illustrated graphically. The results of comparisons we obtained were with and without initial stress and Hall current effect. Some particular cases of special interest have been deduced from the present investigation.

Effect of vortex promoter shape on heat transfer in MHD duct flow with axial magnetic field

The heat transfer from the side-wall of a duct through which an electrically conducting fluid flows within a strong transverse magnetic field is numerically investigated using high-resolution numerical simulation. Parameter ranges considered are 0≤Ha≤2400 and 100≤Re≤3000 for a constant blockage ratio of 1/4. The gain in the heat transfer using obstacles of different geometric shapes are compared. For Ha=320, a maximum heat transfer enhancement of 78% is obtained when using the square cylinder at a modes Re=1000, while the triangular cylinder outperformed the various other vortex promoter geometries at Re = 2000 yielding a 75% improvement.. However, at a higher Hartmann number of Ha=2400, a maximum heat transfer augmentation of 16% and 40% is obtained for the triangular cylinder at Re=1000 and 2000, respectively. This suggests that for a duct flow under the influence of a strong magnetic field, the triangular obstacle is a superior heat transfer promoter geometry compared to the square or circular cylinders. A further net power analysis reveals that the heat transfer enhancement dominates over the pumping power to produce net benefits for even a modest heat transfer enhancement.

Temperature fluctuations accompanying MHD heat transfer of liquid metal downflow in a pipe

The present paper describes the experimental investigation of liquid metal mixed convection characteristics in a vertical pipe affected by strong transverse magnetic field. The influence of thermo-gravitational convection in non-isothermal flows of liquid metal affected by magnetic field is significant. Under certain conditions and configurations of the magnetohydrodynamic (MHD) heat transfer, thermo-gravitational convection, can cause low-frequency temperature fluctuations with abnormally high intensity. The RK-3 loop (HEattransfer Liquid Metal Experimental Facility (HELMEF)) has been designed and put into operation for heat transfer and hydrodynamics investigations during previously unstudied ratios of flow parameters such as Reynolds, Hartman, Grashof numbers, wherein temperature, velocity and their statistical characteristics are measured. New experimental data were obtained and processed by reconstruction of temperature fields and fields of temperature fluctuation characteristics. Modes with low frequency fluctuation of temperature have been studied using new experimental facility and compared to previous results. The boundaries where low frequency temperature fluctuations decay in a strong magnetic field have been found and studied for the first time.

Temperature fluctuations in a nonisothermal mercury pipe flow affected by a strong transverse magnetic field

Liquid metal flows in rectangular ducts and circular pipes under high thermal loads and strong transverse magnetic fields are of great interest in context of developing cooling systems for fusion reactors. In this study, a downward flow of mercury in a uniformly heated pipe affected by a transverse magnetic field is studied experimentally. We show that moderate magnetic fields (0.2–1 T; Hartmann numbers 100–500) not only do not suppress the temperature fluctuations, but lead to the existence of significantly different regimes, characterized by strong temperature fluctuations. At least two different states of fluctuating flow exist. These states generate quantitatively different patterns of temperature fluctuation characteristics in the pipe cross-section. Domains of existence of both regimes are localized in the parameter plane Richardson versus Hartmann numbers. It is shown that these regimes are controlled by the Richardson number and disappear at high Hartmann numbers (above 500).

Observation of electromagnetically induced absorption in 87Rb D2 line in strong transverse magnetic field

We have studied electromagnetically induced absorption in the rubidium-87 D2 line using pump−probe spectroscopy. The experiment was done in the presence of an external transverse magnetic field with low pump intensity. Two cases viz. pump beam polarization parallel and perpendicular with respect to the orientation of the applied magnetic field were investigated (probe beam polarization was perpendicular to probe beam). We observed two well-resolved N-type systems for pump beam polarization parallel to the external magnetic field. For perpendicular orientation, N-type systems became superimposed, and well-resolved N-type systems were not obtained. The observations made have been explained by the effect of optical pumping. The calculated spectra were found to be in good agreement with experimental observation. The Lande-g factor of the ground state and the excited state have been experimentally evaluated and found to be in good agreement with theoretical values up to 43 G.

Confinement of laser plasma expansion with strong external magnetic field

The evolutions of laser ablation plasma, expanding in strong (∼10 T) transverse external magnetic field, were investigated in experiments and simulations. The experimental results show that the magnetic field pressure causes the plasma decelerate and accumulate at the plasma-field interface, and then form a low-density plasma bubble. The saturation size of the plasma bubble has a scaling law on laser energy and magnetic field intensity. Magnetohydrodynamic simulation results support the observation and find that the scaling law (Vmax ∝ Ep/B2, where Vmax is the maximum volume of the plasma bubble, Ep is the absorbed laser energy, and B is the magnetic field intensity) is effective in a broad laser energy range from several joules to kilo-joules, since the plasma is always in the state of magnetic field frozen while expanding. About 15% absorbed laser energy converts into magnetic field energy stored in compressed and curved magnetic field lines. The duration that the plasma bubble comes to maximum size has another scaling law tmax ∝ Ep1/2/B2. The plasma expanding dynamics in external magnetic field have a similar character with that in underdense gas, which indicates that the external magnetic field may be a feasible approach to replace the gas filled in hohlraum to suppress the wall plasma expansion and mitigate the stimulated scattering process in indirect drive ignition.

Using short drive laser pulses to achieve net focusing forces in tailored dual grating dielectric structures

Laser-driven grating type DLA (Dielectric Laser Accelerator) structures have been shown to produce accelerating gradients on the order of GeV/m. In simple β-matched grating structures due to the nature of the laser induced steady-state in-channel fields the per period forces on the particles are mostly in longitudinal direction. Even though strong transverse magnetic and electric fields are present, the net focusing effect over one period at maximum energy gain is negligible in the case of relativistic electrons. Stable acceleration of realistic electron beams in a DLA channel however requires the presence of significant net transverse forces. In this work we simulate and study the effect of using the transient temporal shape of short Gaussian drive laser pulses in order to achieve suitable field configurations for potentially stable acceleration of relativistic electrons in the horizontal plane. In order to achieve this, both the laser pulse and the grating geometry are optimized. Simulations conducted with the Particle-In-Cell code VSim 7.2 are shown for both the transient and steady state/long pulse case. Finally we investigate how the drive laser phase dependence of the focusing forces could affect a potential DLA-based focusing lattice.

Cooling Mechanisms of Switching Arcs Under Transverse Magnetic Fields in Comparison With Arcs Without Magnetic Blast

The importance of low-voltage devices to interrupt dc currents under load and fault conditions has been considerably growing recently. From experimental work, it is known that arcs in strong transverse magnetic fields (e.g., flux density 1 T) can generate arc voltages considerably higher than those generated without field. In a recent study, it was shown by the author in 2-D simulations that in arrangements where the arc is magnetically squeezed against a chamber wall, the dominant mechanism of power removal is convection propelled by the JxB force (“electromagnetic pump”). Immediately in front of the wall the convective heat flux is converted to heat conduction toward the cold wall. This subsequent work is concentrated on the mechanism of free-moving arcs in transverse magnetic fields. Here, a similar effect of voltage increase has been known, but the cold wall as final target of power transport is missing. For differentiation, the cooling mechanism of arcs in chambers without magnetic blast field is also studied. While the temperature profiles and consequently the arc field strength for still arcs in chambers are dominated either by heat conduction or by radiation, depending on the current, free-moving arcs in cross fields are mainly cooled by the convective JxB pump. Heat conduction is negligible. Power is consumed by the heating of the plasma gas masses, which can be visualized by an equivalent flux. Values of the arc field strength, i.e., voltage per arc length, are also presented for dc currents between 4 and 1000 A. They increase in the sequence “chamber without field”, “free arc with cross field”, and “chamber with cross field”. In comparison with air, hydrogen yields clearly higher voltages.