Inhomogeneous External Magnetic Field Research Articles

Modeling and simulation of trickle-bed magnetohydrodynamics in inhomogeneous magnetic fields

External inhomogeneous magnetic fields exert a magnetization body force. This force may prove practical in the case of electrically non-conducting and magnetically permeable fluids and may find positive echo in hydrodynamic intensification of chemical processes. Hence, in acting on paramagnetic or diamagnetic gases and liquids, this force can modify the direction as well as the magnitude of the gravitational force. As an intellectual illustration of the potential of this novel route, the ability to influence two-phase flows through packed bed reactors by application of external inhomogeneous magnetic fields was investigated theoretically. The analysis was carried out using an isothermal unidirectional two-fluid magneto-hydrodynamic model based on the volume–average mass and momentum balance equations for the description of two-phase down-flow throughout a vertically oriented and spatially uniform magnetic-field gradient. The magneto-hydrodynamics effects on trickle flow were displayed in terms of liquid hold-up and pressure drop under paramagnetic gas–diamagnetic liquid, paramagnetic gas and liquid, diamagnetic gas and liquid, diamagnetic gas–paramagnetic liquid combinations.

Nanoscale zeeman localization of charge carriers in diluted magnetic semiconductor-permalloy hybrids.

We investigate the possibility of charge carrier localization in magnetic semiconductors due to the presence of a highly inhomogeneous external magnetic field. As an example, we study in detail the properties of a magnetic semiconductor-permalloy disk hybrid system. We find that the giant Zeeman response of the magnetic semiconductor in conjunction with the highly nonuniform magnetic field created by the vortex state of a permalloy disk can lead to Zeeman localized states at the interface of the two materials. These trapped states are chiral, with chirality controlled by the orientation of the core magnetization of the permalloy disk. We calculate the energy spectrum and the eigenstates of these Zeeman localized states, and discuss their experimental signatures in spectroscopic probes.

Two‐Phase Flow in Porous Media Under Spatially Uniform Magnetic‐Field Gradients

AbstractExternal inhomogeneous magnetic fields exert a body force on electrically nonconducting, magnetically permeable fluids, and this force can be used to compensate or to amplify the gravitational body force. The ability to influence two‐phase flow by the application of external inhomogeneous magnetic field is of potential interest in the operation of trickle‐bed reactors from the point of view of catalytic process intensification. An exhaustive modeling framework was developed to illustrate how external inhomogeneous magnetic fields are able to influence both trickle bed hydrodynamics and reaction performance. The model was based on the volume‐average mass and momentum balance equations and volume‐average species balance equations under a spatially uniform magnetic‐field gradient, µ20H(dh/dz). The catalytic wet air oxidation of phenol in the presence of a catalyst consisting of a mixture of CuO, ZnO and CoO was considered as a case study.

Theory of trickle‐bed magnetohydrodynamics under magnetic‐field gradients

AbstractExternal inhomogeneous magnetic fields exert a body force (magnetization force) on electrically nonconducting and magnetically permeable fluids. The force acts on both paramagnetic and diamagnetic fluids and can be used to compensate for or to amplify the gravitational body force, unlike the Lorentz braking force that manifests only for electrically conducting liquids, such as liquid metals. The ability to influence two‐phase flows through porous media by the application of external inhomogeneous magnetic fields is of interest in the operation of trickle‐bed reactors for catalytic process intensification, particularly in oxidation catalysis, where the O2 paramagnetic properties may be propitious to macrogravity operation for enhancing liquid holdup and wetting efficiency. An isothermal 1‐D two‐fluid magnetohydrodynamics model based on volume‐average mass and momentum balance equations to describe the gas‐liquid downflow under a spatially uniform magnetic‐field gradient. The slit model approximation was used for the derivation of drag force closures intervening in the momentum equations. The evolution of the trickle‐bed magnetohydrodynamics was theoretically investigated in terms of liquid holdup and pressure drop under four gas‐liquid combinations. Advantages of this novel approach of process intensification were rationalized in terms of catalytic reactions.

Effect of an inhomogeneous external magnetic field on the electronic coupling of quantum dot molecule

The electron spins are a candidate for one or two qubits, which, when located in semiconductor double quantum dots. The desired two-qubit coupling is provided by a combination of the Coulomb interaction and the Pauli exclusion principle. The coupling between the spin can be switched on and off via externally controlled a uniform magnetic field. In the experimental environment of the system an inhomogeneous magnetic field inevitably appears and added to the homogenous applied magnetic field. In this work, we study a two-electron quantum dot molecule in an inhomogeneous magnetic field. We consider theoretically the interplay between Zeeman coupling and exchange induced swap action in spin-based double quantum dot quantum computer.

Free convection of a gas induced by a magnetic quadrupole field

To investigate the magnetothermal convection of the paramagnetic materials in an external inhomogeneous magnetic field, the air free convection in a two-dimensional square enclosure confined to a magnetic quadrupole field under microgravity is numerically simulated. The magnetic flux line for the magnetic quadrupole field, the magnetic field intensity and magnetic acceleration in the square enclosure are obtained. The flow and temperature fields as well as the local Nusselt numbers are all presented. The results show that the free convection induced by the centrifugal-form magnetic force exhibits different flow and heat transfer characteristics from the gravitational free convection.

Effect of an inhomogeneous external magnetic field on a quantum-dot quantum computer

We calculate the effect of an inhomogeneous magnetic field, which is invariably present in an experimental environment, on the exchange energy of a double quantum dot artificial molecule, projected to be used as a 2-qubit quantum gate in the proposed quantum dot quantum computer. We use two different theoretical methods to calculate the Hilbert space structure in the presence of the inhomogeneous field: the Heitler-London method which is carried out analytically and the molecular orbital method which is done computationally. Within these approximations we show that the exchange energy J changes slowly when the coupled dots are subject to a magnetic field with a wide range of inhomogeneity, suggesting swap operations can be performed in such an environment as long as quantum error correction is applied to account for the Zeeman term. We also point out the quantum interference nature of this slow variation in exchange.

Microwave generation features in a vircator with an inhomogeneous magnetic field in the interaction region

A vircator with an inhomogeneous external magnetic field superimposed onto the cathode-anode-virtual cathode potential well was studied by methods of computer modeling. It is shown that the application of such additional field allows a significant (almost fivefold) increase in the microwave generation efficiency, which is explained by the gyrotron effect development.

“Bloch lines” in domain walls of yttrium orthoferrite

Images of Bloch lines and variations of the brightness of the domain wall images were observed by means of a dark field method in thin plates of orthoferrites cut perpendicularly to the optical axis. A connection between the brightness variations and changes in the domain wall inclinations is discussed. The domain wall structure changes with the external inhomogeneous magnetic field and depends on the preceeding domain wall state.

One-loop finite temperature effective potential in QED in the worldline approach

The one-loop finite temperature effective potential of QED in an external electromagnetic field is obtained using the worldline method. The general structure of the temperature dependent part of the effective action in an arbitrary external inhomogeneous magnetic field is established. The two-derivative effective action of spinor and scalar QED in a static magnetic background at T≠0 is derived.

Quantum charged spinning particles in a strong magnetic field (a quantal guiding centre theory)

A quantal guiding centre theory is presented which allows a systematical study of the separation of the different time scale behaviours of a quantum charged spinning particle moving in an external inhomogeneous magnetic field. A suitable set of operators adapting to the canonical structure of the problem and generalizing the kinematical momenta and guiding centre operators of a particle coupled to a homogeneous magnetic field is constructed. This allows us to rewrite the Pauli Hamiltonian as a power series in the magnetic length making the problem amenable to a perturbative analysis. The first two terms of the series are explicitly constructed. The effective adiabatic dynamics turns out to be in coupling with a gauge field and a scalar potential. The mechanism producing such magnetic-induced geometric-magnetism is investigated in some detail.

Wave generation by laser-plasma interaction in an oscillating inhomogeneous magnetic field

Nonlinear wave generation and amplication at combined frequencies due to laser-plasma interaction are investigated. The effects of an external inhomogeneous high frequency magnetic field and relativistic plasma electrons are taken into consideration.

Quantized spin currents in two-dimensional chiral magnets.

Two-dimensional quantum magnets in which reflection and time-reversal symmetries are broken may be classified by integers which reflect the topology of the ground state. An analogy with the quantum Hall effect exists: these integers are Chern numbers which describe a quantized spin current response to an inhomogeneous external magnetic field. Implications for spontaneous chiral symmetry breaking are discussed.

Paramagnetic attraction of erythrocyte flow due to an inhomogeneous magnetic field

The effect of an external inhomogeneous magnetic field on the flow of erythrocytes containing paramagnetic hemoglobin was studied systematically, with three experimental setups. (1) The attraction of a narrow stream of erythrocyte suspension towards stronger magnetic field, in a wide laminar flow, was found to be proportional to the magnetic susceptibility of erythrocytes χ, the product of the field strength and its spatial gradient B × d B/d z, and the reciprocal of flow velocity 1/ v, and also to the hematocrit h of the suspension. (2) A model flow of erythrocyte suspension in the vessel showed a small change in the radial distribution of erythrocytes arising from a magnetic field, which is proportional to χ, B × d d B/d z (up to 20 T 2/m), 1/ v, and h (< 5%). However, the attraction saturates at high values of B × d B/d z and h. (3) Acceleration of the sedimentation rate was detected for paramagnetic erythrocytes in an inhomogeneous magnetic field, but not with diamagnetic erythrocytes. In short, the paramagnetic attraction takes place with venous blood, and depends on the product of the field strength and its spatial gradient, the degree of deoxygenation, the flow velocity, and the hematocrit.

In vivo19f spin relaxation in index finger bones

19F free induction decay (FID) signals have been observed from the index fingers of four male and two female adult volunteers using a 27-MHz pulsed nuclear magnetic resonance spectrometer equipped with a split ring resonator probe. The value of the in vivo spin-lattice relaxation time T1 of the fluoride ions naturally accumulated in bone mineral has been determined to be 2.0 +/- 0.3 s. The shape of the observed FID signals in the inhomogeneous external magnetic field indicates that the F distribution is not uniform along the length of the finger. The fluoride ion content of the index finger (within a 65-mm length from its distal end) was measured with a sensitivity of +/- 0.5 mg F within a 30-min observation time.

Anomalous heat flux due to magnetic electron-drift vortex waves in a magnetized plasma.

The magnetic electron-drift vortex mode in a magnetized plasma is investigated using the Braginskii transport equations for the electrons. In a sufficiently inhomogeneous external magnetic field, the mode can become unstable, causing enhanced local magnetic fluctuations. The latter give rise to an anomalous electron thermal flux that significantly affects the transport characteristics of the plasma. Using the conventional quasilinear approach, the corresponding anomalous thermal conductivity is obtained.

Linear theory of a gyrotron with weakly tapered external magnetic field

The procedure for the calculation of the starting current in a gyrotron with a weakly inhomogeneous external magnetic field is described in detail. The results obtained are valid for gyrotrons operating at an arbitrary cyclotron harmonic and for arbitrary functions describing the axial structure of the RF resonator field and the axial distribution of the inhomogeneity of the external magnetic field. Examples are given for a gyrotron with parameters close to those of the KfK experiments.

Dynamics of magnetoacoustic solitons in the case of transition radiation in an inhomogeneous magnetic field

A study is made of the change in the parameters of a magnetoacoustic soliton propagating in an inhomogeneous external magnetic field for both the case of abrupt change in the field and periodic variation of a small correction to a constant external field. It is shown that in the second case the width of the soliton and its velocity vary with a certain effective frequency.

Reduction in the dispersion in the magnetoelastic wave propagation time in yttrium-iron and yttrium-iron-gallium garnet single crystals

The distribution of the internal magnetic field corresponding to a fixed frequency-independent delay is computed in the stage theory approximation. Conditions for the realization of an electrically controlled delay time with minimum dispersion in the whole control range are analyzed. Experimental results obtained on yttrium-iron and yttrium-iron-gallium garnets (YIG and YIGG) magnetized by an inhomogeneous external magnetic field are presented. The mean value of the dispersion was 0.5 μsec/GHz in the 2–5 GHz range.

Transport and free carrier electromagnetic phenomena in semiconductor boundary layers

Standard semiconductor surface transport theory is rewritten in a classical Green function formulation physically equivalent to the path variable method. This makes it possible to account for surface boundary conditions, while improving upon the relaxation time solution of the Boltzmann equation itself. The extension to ac fields is trivial. The complex conductivity is written (in the relaxation time approximation) as a function of frequency, position in the inhomogeneous boundary layer, surface potential, polish parameter and external magnetic field. The way to study long wave electromagnetic properties of carriers in boundary layers is indicated and illustrated with applications to the theory of reflectivity as affected by boundary layers.