Transverse Dc Magnetic Field Research Articles

Accurate determination of alkali atom density based on zero-field magnetic resonance in a single-beam spin-exchange relaxation-free atomic magnetometer

A single-beam spin-exchange relaxation-free (SERF) atomic magnetometer can extract vector magnetic field information by detecting the transmission intensity of a resonant circularly polarized pumping beam, which depends sensitively on the atomic density of the alkali metal. We present a novel scheme to determine atomic density based on zero-field magnetic resonance. The resonance linewidth under different transverse DC magnetic fields is fitted by means of a quadratic function. The atomic density can be extracted from the quadratic coefficients of the fitted function. The experimental results indicate that the deviation of measured density is less than two times compared with the theoretical values between 378 K and 403 K. Furthermore, the influence of modulation field on resonance linewidth is investigated experimentally and theoretically. A miniature single-beam SERF atomic magnetometer with a sensor head volume of only 16.2 cm3 and the measuring sensitivity of 40 fT/ Hz1/2 has been achieved. These results should be beneficial for guiding the development of a chip-scale atomic magnetometer with high sensitivity and spatial resolution for bio-magnetic field imaging applications.

Effect of Transverse DC Magnetic Field on the Impedance Matching and Morphology of a Microwave Atmospheric Pressure Plasma Jet

Effect of Transverse DC Magnetic Field on the Impedance Matching and Morphology of a Microwave Atmospheric Pressure Plasma Jet

In situ observation of directional solidification in Ga-In alloy under a transverse DC magnetic field

The directional solidification of a Ga-25wt%In alloy under the effect of a transverse DC magnetic field is investigated by X-ray radiography. The magnetic field pointing parallel to the X-ray beam is generated by two ring-shaped permanent magnets. The magnetic field reaches values up to ∼0.19 T in the field of view. The dendritic growth and the flow patterns of Ga-rich plumes migrating along the solidification front are captured and analysed. It shows that the local fluctuations of solute concentration are partially damped by the magnetic field. At the temperature gradient of 1 K/mm, the growth velocities of solidification front and plumes are not affected. In the case of higher temperature gradient (∼2 K/mm), the magnetic field causes an increase of the plume velocity in the horizontal direction and a decrease in the vertical direction while the velocity of the solidification front remains constant. Additionally, it is found that the magnetic field damps the fluctuations of tip velocity and refines the primary arm spacing. Above phenomena are discussed based on the thermoelectric magnetic and electromagnetic braking effects. The in situ experimental data are important for verification and improvement of the existing numerical simulations of solidification under the magnetic field.

Magnetically assisted optical gain in Zeeman degenerate two-level systems of cold atoms

Abstract We report on the observation of magnetically assisted narrow band probe beam amplification in Zeeman degenerate closed two-level system associated with cold cesium atoms. In the experiment the closed cesium D 2 hyperfine transition F g = 3 → F e = 2 is coherently excited by a probe and an orthogonally polarized coupling beam in the presence of a transverse dc magnetic field. Single pass gain of 80% is observed for moderate values of the magnetic field and coupling beam power. The gain mechanism can be understood in terms of population transfer between the different Zeeman ground-state levels induced by the external magnetic field. We have developed a simple theoretical model using a simpler atomic transition F g = 1 → F e = 0 , which accounts reasonably for the main experimentally observed results.

Excitation of terahertz radiation generation by obliquely incident beating lasers on a hot magnetized plasma with step density profile

AbstractA scheme of resonant terahertz (THz) radiation generation by non-linear beating of two lasers in hot magnetized plasma with step density profile is investigated. Beating lasers of frequency difference ω1 − ω2 ≈ ωp(~1 THz) is incident obliquely on plasma surface and exerts non-linear ponderomotive force on plasma electrons. The plasma electrons start oscillating in the plane of incidence and give rise to space charge field to maintain plasma neutrality. In turn, both ponderomotive force and space charge field excites a non-linear surface current, responsible for THz radiation generation on the reflection side. The coupling between plasma wave and electromagnetic wave present (inside the plasma as well as on reflection side) becomes stronger in the presence of the transverse DC magnetic field. THz radiation amplitude is optimized at an angle of incidence θ ~ 50–70°.

On the nature of sources of audio-frequency electromagnetic interferences

It has been shown that metal elements of electronic equipment executing mechanical vibrations under exposure to the dc or low-frequency magnetic field could be sources of low-frequency electromagnetic interferences. The general solution of a boundary problem of electrodynamics was obtained that dealt with the electromagnetic field emission by a rod with circular cross-section made of metal of the non-ferromagnetic group under harmonic elastic oscillations in the dc magnetic field. A model example was considered for the situation where a source of low-frequency electromagnetic field was the rod with circular cross-section that executed longitudinal harmonic oscillations of compression–tension at the transverse dc magnetic field. An estimation of numerical values of the intensity of low-frequency magnetic field and the induced voltage of interference were also provided.

Tunable and efficient terahertz radiation generation by photomixing of two super Gaussian laser pulses in a corrugated magnetized plasma

A scheme of terahertz (THz) radiation generation is investigated by photo-mixing of two super Gaussian laser beams having different frequencies (ω1, ω2) and wave numbers (k→1, k→2) in a performed corrugated plasma embedded with transverse dc magnetic field. Lasers exert a nonlinear ponderomotive force, imparting an oscillatory velocity to plasma electrons that couples with the density corrugations (n′=nα0eiαz) to generate a strong transient nonlinear current, that resonantly derives THz radiation of frequency ∼ωh (upper hybrid frequency). The periodicity of density corrugations is suitably chosen to transfer maximum momentum from lasers to THz radiation at phase matching conditions ω=ω1−ω2 and k→=k→1−k→2+α→. The efficiency, power, beam quality, and tunability of the present scheme exhibit high dependency upon the applied transverse dc magnetic field along with q-indices and beam width parameters (a0) of super Gaussian lasers. In the present scheme, efficiency ∼10−2 is achieved with the optimization of all these parameters.

Study of the temperature uniformity of aluminium billets heated by superconducting DC induction heaters

Purpose – The purpose of this paper is to analyse and improve the temperature uniformity of aluminium billets heated by superconducting DC induction heaters. Design/methodology/approach – A 3D electromagnetic model coupled with a heat transfer model is developed to calculate the heating process of the billets which are rotated in uniform transverse DC magnetic field. A laboratory-scale DC induction heater prototype has been built to validate the model. The results from simulation and measurement have a good agreement. The model is used to investigate the factors affecting the temperature uniformity of aluminium billets. Findings – The results from simulation show that lower rotation speeds always mean better temperature uniformity along the radial direction, due to the increase in power penetration. However, the situation is very different for the temperature distribution along the axial direction. When the rotation speed is low, the temperature at the ends is lower than other parts. The situation reverses as the rotation speeds increase. This phenomenon is referred to as the “ending effect” in this paper. Research limitations/implications – Because of the ending effect, a lower rotation speed does not always result in better overall temperature uniformity, especially for billets of smaller sizes. Practical implications – There is an optimal rotation speed that yields the best overall temperature uniformity. Lower rotation speeds are not always preferred. The results and numerical model developed are very useful in the design of a superconducting DC induction heater. Originality/value – The temperature uniformity of aluminium billets heated by DC induction heaters is investigated and optimized.

Probing the laser wakefield in underdense plasmas by induced terahertz emission

Terahertz (THz) radiation can be produced from a laser wakefield driven in underdense plasmas in the presence of a transverse DC magnetic field. It is shown that the radiation usually contains a component at the electron plasma frequency and its harmonics when the wakefield is excited at high amplitudes. In the highly nonlinear bubble/blowout regime, the radiation contains a smooth component peaked at the reduced electron plasma frequency and an irregular spectrum extending to tens of the electron plasma frequency. The latter is due to the broken-wave structure behind the bubble. A theoretical model is presented and validated via two-dimensional particle-in-cell simulations. The measurement of such THz emission may provide a diagnostic of the laser wakefield structure.

Domain structure and giant magneto-impedance effect in FeCuNbSiB films prepared with a magnetic field

Domain structure and giant magneto-impedance (GMI) effects of FeCuNbSiB films, which were prepared by radio frequency sputtering without field and with a magnetic field about 72 kA/m, are investigated. The results show that after annealing at 300℃, the GMI ratios reach the maximum values of about 38% in longitudinal and 27% in transverse dc magnetic fields, at a frequency of 13 MHz. The longitudinal magnetic responses of GMI ratios show very sharp peaks around the transverse anisotropy field, and the sensitivity can reach 47.5%/kA m-1 in the field range 0.4-6 kA/m, at 13 MHz. The transverse anisotropy is 0.4 kA/m, larger than that of 0.08 kA/m in the film prepared without magnetic field, because an additional transverse anisotropy is induced by the field applied in the fabrication process.

Calorimetric AC loss measurement of MgB2 superconducting tape in an alternating transport current and direct magnetic field

Applications of MgB2 superconductors in electrical engineering have been widely reported, and various studies have been made to define their alternating current (AC) losses. However, studies on the transport losses with an applied transverse DC magnetic field have not been conducted, even though this is one of the favored conditions in applications of practical MgB2 tapes. Methods and techniques used to characterize and measure these losses have so far been grouped into ‘electrical’ and ‘calorimetric’ approaches with external conditions set to resemble the application conditions. In this paper, we present a new approach to mounting the sample and employ the calorimetric method to accurately determine the losses in the concurrent application of AC transport current and DC magnetic fields that are likely to be experienced in practical devices such as generators and motors. This technique provides great simplification compared to the pickup coil and lock-in amplifier methods and is applied to a long length (∼10 cm) superconducting tape. The AC loss data at 20 and 30 K will be presented in an applied transport current of 50 Hz under external DC magnetic fields. The results are found to be higher than the theoretical predictions because of the metallic fraction of the tape that contributes quite significantly to the total losses. The data, however, will allow minimization of losses in practical MgB2 coils and will be used in the verification of numerical coil models.

Longitudinal chirality, enhanced nonreciprocity, and nanoscale planar one-way plasmonic guiding

When a linear chain of plasmonic nanoparticles is exposed to a transverse dc magnetic field, the chain modes are elliptically polarized in a single plane parallel to the chain axis; hence, a new chain mode of longitudinal plasmon rotation is created. If, in addition, the chain geometry possesses longitudinal rotation, e.g., by using ellipsoidal particles that rotate in the same plane as the plasmon rotation, strong nonreciprocity is created. The structure possesses a new kind of chirality---longitudinal chirality---and supports one-way guiding. Since all particles rotate in the same plane, the geometry is planar and can be fabricated by printing leaflike patches on a single plane. Furthermore, the magnetic field is significantly weaker than in previously reported one-way guiding structures. These properties are examined for ideal (lossless) and lossy chains.

A new high efficiency technology for the induction heating of non magnetic billets

PurposeLow electrical resistivity metal billets can be heated by the currents induced by the rotation of the billet itself inside a transverse DC magnetic field produced by a superconductive coil. The main drawback of this approach is related to cost of installation that requires an adequate refrigerating system. The purpose of this paper is to propose a more convenient solution, which allows the same high efficiency to be achieved at lower cost. In this solution, the billet is kept still and a series of permanent magnets, positioned in the inner part of a ferromagnetic frame, is rotated.Design/methodology/approachSome results of the new induction system are shown. These results are obtained applying for the electromagnetic solution both an FE commercial code and an analytical method. The analytical code is developed because several parameters of the system need to be optimized.FindingsThe performance of the solution presented is comparable with those of the system with superconductive coils. The results of the two methods applied are in good agreement; thus the analytical code is validated.Originality/valueA new solution for the induction heating of aluminum billets is presented. The analytical code developed requires a very short computational time, also because it gives directly the steady‐state condition of the system and, for this reason, it can be conveniently applied to an automatic design process.

Experimental and numerical modeling of the steel flow in a continuous casting mould under the influence of a transverse DC magnetic field

Experimental and numerical modeling of the steel flow in a continuous casting mould under the influence of a transverse DC magnetic field

Enhancement of Second- and Third-Order Nonlinear Optical Susceptibilities in Magnetized Semiconductors

Using electromagnetic treatment, an expression of effective nonlinear optical susceptibility χe[= χ(2)e + χ(3)e E] is obtained for III–V semiconducting crystals in an applied transverse dc magnetic field under off-resonant transition regime. The origin of nonlinear interaction lies in nonlinear polarization arising from the crystal properties such as piezoelectricity and electrostriction. Numerical estimates have been made by a representative n-InSb crystal at 77K duly irradiated by a pulsed 10.6-μm CO2 laser under off-resonant transition regime. Efforts are dedicated to optimizing doping level and externally applied dc magnetic field to achieve maximum χ(2)e and χ(3)e. The results are found to be in good agreement with the available literature. The analysis shows that χ(2)e and χ(3)e can be significantly enhanced in doped III–V semiconductors by the proper selection of doping concentration and dc magnetic field, which confirms its potential as a candidate material for the fabrication of nonlinear optical devices.

Influence of magnetic induced anisotropy on giant magnetoimpedance effects in FeCuNbSiB films

The giant magnetoimpedance (GMI) effects in radio frequency sputtered FeCuNbSiB films with a dc magnetic field applied in the process of deposition are presented, and the relation between annealing temperature and GMI ratio is investigated. The results show that 300 °C is the optimum annealing temperature, at which the GMI ratios reach the maximum values of about 38% and 27% in longitudinal and transverse dc magnetic fields, respectively, at a frequency of 13 MHz. The longitudinal magnetic responses of GMI ratios show very sharp peaks around the transverse anisotropy field, and the sensitivity can reach a value larger than 47.5%/kA/m in the field range 0.4∼6 kA/m, at 13 MHz. The frequency dependencies of the magnetoresistance and magnetoreactance indicate that at lower frequencies, the giant magnetoinductance effect is dominant, while at higher frequencies, skin effect becomes important, and the GMI effect is determined by the change of both the resistance and reactance components.

Non-uniform columnar defect implantation in YBCO coplanar resonators for the control ofvortex-induced microwave dissipation and nonlinearity

This paper reports on 4.2 GeV Au ion irradiation of YBCO coplanar resonators aimed atcontrolling vortex-induced microwave dissipation and nonlinearity by means of asuitable distribution of columnar defects. Two procedures were used: (i) uniformirradiation at a given dose and (ii) non-uniform irradiation, obtained by means of amoving-target technique, producing a tailored profile of columnar defect density. The lastconfiguration is needed in order to attain an optimal compromise between defect-inducedenhancement of vortex pinning and increase of dissipative scattering. The resonatorswere characterized before and after irradiation in order to evaluate penetrationdepth and surface impedance, by means of a data analysis also suitable to accountfor the substrate properties. It turns out that in the case of uniform irradiationthe induced damage, attributed to the addition of dissipative regions, alwaysprevails over the improvement due to vortex pinning. In contrast, in the case of thetailored dose profile, significant damage is absent, while the onset of non-linearity isshifted towards higher circulating power. The need to control vortex-inducedeffects is confirmed by magneto-optical analysis that puts in evidence that vortexpenetration occurs even at low values of transverse dc magnetic field and temperature.

Domain wall pinning in polycrystalline perovskite La0.7Sr0.3MnO3

Reversible and irreversible domain wall (DW) motions have been investigated in La0.7Sr0.3MnO3 ceramic samples using frequency-response complex permeability with various amplitudes of AC field. We also examine the effects of temperature in the range from 293 to 368K and transverse DC magnetic field with a maximum of 4.40×105A/m on the real part of permeability (μ′). Two relaxations corresponding to reversible wall motions and domain rotations occur in low and high frequency regions, respectively. The irreversible DW displacements can be activated as the amplitude larger than the pinning field of 3A/m, leading to an increase in μ′. The μ′ obeys a Rayleigh law at the temperature below 343K or under DC field of less than 4.22×104A/m. The Rayleigh constant η increases from 5.45×10−2 to 1.54×10−1(A/m)−1 as the temperature rises from 293 to 343K, and η decreases from 5.58×10−2 to 3.67×10−2(A/m)−1 with increasing DC field from 1.99×103 to 4.22×104A/m.

Reversible magnetic behavior of superconductors forced by a small transverse ac magnetic field

Recently it was shown that with thin superconductors placed in a transverse dc magnetic field, the application of a small ac magnetic field perpendicular to the dc field causes rapid relaxation of the irreversible magnetization. We show here that for a thin superconducting strip in a transverse dc field the application of a weak ac field perpendicular to the dc field generates a dc voltage in the strip. This voltage leads to the decay of the critical currents circulating in the strip, and eventually the equilibrium state of the superconductor is established. This relaxation is not due to thermally activated flux creep but to the “walking” motion of vortices in the two-dimensional critical state of the strip with in-plane ac field. Our theory explains the “shaking effect” that was used for detecting phase transitions of the vortex lattice in superconductors with pinning. Some recent experiments are discussed.

Why an ac magnetic field shifts the irreversibility line in type-II superconductors.

We show that for a thin superconducting strip placed in a transverse dc magnetic field--the typical geometry of experiments with high-T(c) superconductors--the application of a weak ac magnetic field perpendicular to the dc field generates a dc voltage in the strip. This voltage leads to the decay of the critical currents circulating in the strip, and eventually the equilibrium state of the superconductor is established. This relaxation is not due to thermally activated flux creep but to the "walking" motion of vortices in the two-dimensional critical state of the strip with in-plane ac field. Our theory explains the shaking effect that was used for detecting phase transitions of the vortex lattice in superconductors.