Axial Static Magnetic Field Research Articles

Electromagnetic energy rotation by azimuthal surface waves along plasma-metal interface around a cylindrical metallic rod placed into infinite magnetized plasma

Electromagnetic energy rotation around a cylindrical metallic rod placed into an infinite plasma, parallel to an external static uniform magnetic field, is studied. Angular phase velocity and angular velocity of energy transport are introduced and analyzed as functions of the plasma waveguide parameters: rod radius, plasma particle density, azimuthal wave number, and external static axial magnetic field. These angular velocities are compared with the case of wave propagation in cylindrical metallic waveguides entirely filled by magnetoactive plasma.

Thermo-electromagnetic effect on weld microstructure in magnetically assisted laser welding of austenite steel

Large microstructure gradients in high-power-density welding usually have negative effects on the mechanical properties of weld joints. In this paper, we found that the width of the weld microstructure in transition region was widened (from 65 μm to 140 μm) by applying a static axial magnetic field of 415 mT on the welding process, which efficiently improved the microstructure gradient. Based on our robust comprehensive model, we quantitatively predicted the distributions of thermoelectric currents (TEC) and thermo-electromagnetic force (TEMF) near the liquid-solid (L–S) interface. The three-dimensional annular TEC can reach about 3.0 × 106 A/m2 near the L–S interface. The TEMF vertical to the L–S interface with applying a magnetic field of 415 mT on welding process can reach 1.0 × 106 N/m3 or even larger, which could influence the fluid flow in weld pool and finally the weld microstructure. Furthermore, an intensive parameter of TEMF RTi (a dimensionless ratio) was proposed to estimate the effect of TEMF on the weld microstructure and the estimated results obtained good agreements with experimental data. Using this method, the critical value of the applied magnetic field which can cause the weld microstructure improvement in transition region obviously was predicted to be about 150 mT. Our findings and method can be served as guidance or reference for weld microstructure and joint quality optimization in laser welding process.

Initial stage of interaction between gyrating relativistic electron beam and long-wavelength electromagnetic surface waves in cylindrical metallic waveguides partially filled with plasma

Azimuthal surface waves are eigen electromagnetic waves of cylindrical metallic waveguides with plasma filling. Azimuthal surface waves of extraordinary polarization are known to be effectively excited by a beam of electrons gyrating in an axial external static magnetic field along large Larmor radii around the plasma column. Beam excitation of azimuthal surface waves was well studied theoretically in both linear and nonlinear approaches. An important limitation of the earlier studies was the approximation of zero axial wavenumber. In the present paper, perturbation theory is applied to account for finite magnitude of the axial wavenumber of electromagnetic surface waves in studying their excitation by an electron beam. The dependence of the wave growth rate on the axial wavenumber and parameters of the plasma waveguide is studied. The results obtained in this paper are of interest for application in plasma electronics.

Effect of an axial high static magnetic field on the crystal orientation and magnetic property of Fe-4.5 wt% Si alloy during bulk solidification

Fe-4.5 wt% Si alloy was solidified in the presence of an axial high static magnetic field (HSMF) to investigate the influence of magnetic field on the crystal orientation and magnetic property. With the increase of magnetic field intensity (MFI), the crystal orientation of the Fe-4.5 wt% Si alloy turns to 〈1 0 0〉 which is the easy magnetization axis. The specimen with strong Goss texture was obtained under a 6 T HSMF. Moreover, the saturation magnetic induction also increases with the increase of the MFI. The above results are attributed to the effect of the HSMF on the magnetocrystalline anisotropy during bulk solidification.

Three dimensional dendritic morphology and orientation transition induced by high static magnetic field in directionally solidified Al-10 wt.%Zn alloy

Effect of high static magnetic field on the dendritic morphology and growth direction in directionally solidified Al-10 wt.%Zn alloy were studied by three-dimensional (3D) X-ray micro-computed tomography, Electron Back-scattered Diffraction (EBSD) and X-ray Diffraction (XRD). The application of high static axial magnetic field (5T) during directional solidification was found to destabilize the solid/liquid interface and cause the growth direction of dendrite deviate from thermal gradient, leading to irregular solid/liquid interfacial shape and cellular to dendritic morphology transition. The thermoelectric magnetic convection (TEMC) caused by the interaction of thermoelectric effect and magnetic field was supposed to be responsible for the transition. In addition, the EBSD and XRD results confirm that the preferred growth direction of α-Al was found to transform from the traditionally expected <100> to <110>. The dendrite orientation transition (DOT) in Al-10 wt.%Zn alloy can be attributed to the effect of applied magnetic field on the anisotropy of crystal during solidification. The result indicates the potential application of high static magnetic field in altering the morphology and preferred growth direction of dendrite during directional solidification.

Electromagnetic energy rotation along plasma-metal interface in cylindrical waveguides initiated by azimuthal surface waves

Energy transfer along finite curvature plasma-metal interfaces in cylindrical metallic waveguides entirely filled by magnetoactive plasma is studied. Angular phase velocity, angular velocity of energy transfer, and angular group velocity are introduced and analyzed as functions of the waveguide parameters: radius, plasma particle density, azimuthal wave number, and external static axial magnetic field.

Effect of Thermoelectric Magnetic Convection on Shrinkage Porosity at the Final Stage of Solidification of GCr18Mo Steel Under Axial Static Magnetic Field

Adopting effective strategies to eliminate the shrinkage porosity at the final stage of steel solidification is one of the most attractive research subjects for metallurgists. Despite the progress in understanding the thermoelectric magnetic convection (TEMC) in the solidification process, utilization of TEMC for modifying the solidification feeding of steel has been ignored. The effects of TEMC on the shrinkage porosity at the final stage of solidification of GCr18Mo steel were investigated under an axial static magnetic field (ASMF). The morphology and porosity of GCr18Mo steel were investigated quantitatively by X-ray microtomography. We found that the amount of porosity first decreases and then increases with increasing intensity of the ASMF. Meanwhile, the shape of the porosity changed from tortuous to elliptic under the ASMF. On the basis of both the experimental and simulated results, the decrease in porosity is attributed to an adequate development of the TEMC leading to sufficient feeding. This study sheds light on an alternative technique for eliminating the shrinkage porosity at the final stage of steel solidification in industrial applications.

Columnar to Equiaxed Transition during Directionally Solidifying GCr18Mo Steel Affected by Thermoelectric Magnetic Force under an Axial Static Magnetic Field

Columnar to Equiaxed Transition during Directionally Solidifying GCr18Mo Steel Affected by Thermoelectric Magnetic Force under an Axial Static Magnetic Field

Higher radial modes of azimuthal surface waves in magnetoactive cylindrical plasma waveguides

Azimuthal surface waves are eigenmodes of cylindrical plasma–dielectric–metal structures both in the presence of and without an axial static magnetic field. They are actively studied due to possible applications in plasma electronics, nanotechnologies and biomedical diagnostics. Higher radial modes are known to propagate at higher frequencies and shorter wavelengths compared to those of the zeroth mode, a feature which is of interest for practical applications. To gain the advantage of the excitation of higher radial modes of azimuthal surface waves one has first to know their dispersion properties. This paper generalizes the results of earlier papers by including a static axial magnetic field and considering the higher radial modes. The presence of the constant axial magnetic field removes the degeneracy in the wave spectrum with respect to the sign of the azimuthal wavenumber.

Effects of axial static magnetic field on columnar to equiaxed transition in directionally solidified low carbon steel

ABSTRACTThe effects of axial static magnetic field (ASMF) on the columnar-to-equiaxed transition (CET) in directionally solidified low carbon steel including peritectic reactions were investigated. Experimental results show that the dendrite morphology of this peritectic steel can be modified under ASMF. The increases of the magnetic field intensity and temperature gradient and the decrease of the growth speed can promote the CET. The columnar dendrites become equiaxed dendrites at a low growth speed. According to numerical simulations, a thermoelectric magnetic convection in the melts and a thermoelectric magnetic force acting on the secondary dendrite neck are induced by the interaction between a thermoelectric current and ASMF. The influence of ASMF on the CET covers the formation of the thermoelectric magnetic convection, transport of the fragments in the melts and detachment of dendritic side arms. In addition, the application of ASMF can extend the process window of equiaxed grains formed.

Magnetic Field Generation in Plasma Waves Driven by Copropagating Intense Twisted Lasers.

We present a new magnetic field generation mechanism in underdense plasmas driven by the beating of two, copropagating, Laguerre-Gaussian orbital angular momentum laser pulses with different frequencies and also different twist indices. The resulting twisted ponderomotive force drives up an electron plasma wave with a helical rotating structure. To second order, there is a nonlinear rotating current leading to the onset of an intense, static axial magnetic field, which persists over a long time in the plasma (ps scale) after the laser pulses have passed by. The results are confirmed in three-dimensional particle-in-cell simulations and also theoretical analysis. For the case of 300fs duration, 3.8×10^{17} W/cm^{2} peak laser intensity we observe magnetic field of up to 0.4MG. This new method of magnetic field creation may find applications in charged beam collimation and microscale pinch.

Fabrication of aluminum alloy functionally graded material using directional solidification under an axial static magnetic field

Aluminum alloy in situ functionally graded materials (FGMs) have been successfully fabricated using directional solidification under an axial static magnetic field. Al-Zn, Al-Ni and Al-Cu alloys with a hypereutectic composition were selected to produce FGMs. Experimental results show that the graded composition of the primary phases (i.e., Zn, Al3Ni and Al2Cu) is obvious along the longitudinal section of the sample. The graded composition of the primary phases could be controlled by the value of the magnetic field, growth rate and temperature gradient. A proposed model and simulations are carried out to explain the origin of the graded composition of the primary phases in FGMs during directional solidification under an axial static magnetic field. It should be attributed to the combined actions of heavier species migration under gravity force and thermoelectric (TE) magnetic convection under magnetic field. Furthermore, it can be found that the magnetic field can induce the columnar FGMs to change into equiaxed FGMs. This work not only presents a new approach to fabricate FGMs using the directional solidification under an axial static magnetic field but also deeply understands the effect of the solute migration and temperature distribution on the crystal growth during directional solidification.

Grain Refinement During Directionally Solidifying GCr18Mo Steel at Low Pulling Speeds Under an Axial Static Magnetic Field

The present work investigates how axial static magnetic field affects the solidification structure and the solute distribution in directionally solidified GCr18Mo steel. Experimental results show that grain refinement and the columnar to equiaxed transition is enhanced with the increases in the magnetic field intensity (B) and temperature gradient (G) and the decrease in the growth speed. This phenomenon is simultaneously accompanied by more uniformly distributed alloying elements. The corresponding numerical simulations verify a thermoelectric (TE) magnetic convection pattern in the mushy zone due to the interaction between the magnetic field and TE current. The TE magnetic convection in the liquid should be responsible for the motion of dendrite fragments. The TE magnetic force acting on the dendrite is one of the driving forces trigging fragmentation.

Influence of sense rotation of circularly polarized laser pulse on the wake-field acceleration in magnetized plasma

The present paper investigates the effect of different sense rotations of circularly polarized (CP) Gaussian laser pulse on wake-field generation and electron acceleration when there is an axial static magnetic field. The nonlinear equation, which explains the interaction of intense CP laser beam with magnetized plasma in the quasi static approximation, is derived. The numerical analysis of our results indicates that the amplitude of plasma wake for both sense of circular polarization is similar, but affected by propagation direction of exterior magnetic field. It is found that the dephasing length is increased for the left-hand polarized laser fields, while decreased for the right-hand ones, which is reinforced as the enhancement of the forward exterior magnetic field. It is shown that the right-hand polarization laser pulse with reversed exterior magnetic field could leads to much larger electron energy (about 2.4GeV) than that of other cases.

Low Frequency Excess Noise Source Investigation of Off-Diagonal GMI-Based Magnetometers

The equivalent magnetic noise spectral densities of off-diagonal giant magnetoimpedance (GMI)-based magnetometers exhibit significant low-frequency excess noise, proportional to 1/f noise. As it represents a serious limitation to the ultimate sensing performances of high sensitivity magnetometers, possible sources of this 1/f noise are under investigation. Low-frequency magnetization fluctuations have been proposed as the noise source in the case of classical GMI-based sensors. Here, we apply this model to off-diagonal GMI-based magnetometers. This requires the inclusion of magnetization fluctuation noise sources, in addition to white noise sources from electronic conditioning in the GMI effect equations. A pessimistic scenario is presented, predicting the upper limit of low-frequency excess noise from material characteristics. The equivalent magnetic noise level is then computed from the sensitivity of each term of the sensing element impedance matrix to the magnetization angle at the static working point (for both axial and circumferential static magnetic field) and to conditioning circuitry. Based on this, it appears that magnetization fluctuations similarly affect all modes of operation of the two-port network sensing element, inducing identical impedance fluctuations. It also appears that this noise depends only upon the static equilibrium condition. This condition is governed by the effective anisotropy of the magnetic wire and by both axial and circumferential static components of the working point.

Strong Terahertz Generation by Beating of Two Super Gaussian Laser Beams in Axially Magnetized Plasma

THz generation by nonlinear mixing of two coaxial Super-Gaussian laser beams, propagating in rippled density plasma with an axially applied static magnetic field, is investigated. The ponderomotive force which arises due to nonuniform spatial intensity profiles of laser beams, imparts a nonlinear oscillatory velocity at beat wave frequency to electrons. This oscillatory velocity couples with the preformed density ripple to generate a nonlinear current density driving a radiation. Frequencies of lasers are chosen such that the beat frequency lies in the terahertz region. The density ripple provides the phase matching for resonant excitation and the axial static magnetic field is utilized to enhance the nonlinear coupling. Numerical results show that the yield of the generated THz amplitude enhances with increase in index of super-Gaussian lasers as well as the magnitude of the axially applied static magnetic field.

Standing Helicon Wave Induced by a Rapidly Bent Magnetic Field in Plasmas.

An electron energy probability function and a rf magnetic field are measured in a rf hydrogen helicon source, where axial and transverse static magnetic fields are applied to the source by solenoids and to the diffusion chamber by filter magnets, respectively. It is demonstrated that the helicon wave is reflected by the rapidly bent magnetic field and the resultant standing wave heats the electrons between the source and the magnetic filter, while the electron cooling effect by the magnetic filter is maintained. It is interpreted that the standing wave is generated by the presence of a spatially localized change of a refractive index.

Thermoelectric Magnetohydrodynamic Flows and Their Induced Change of Solid–Liquid Interface Shape in Static Magnetic Field-Assisted Directional Solidification

Applying static magnetic field can produce flows (thermoelectric magnetohydrodynamic flows, TEMHDF) in the melt by interacting with the thermoelectric currents (TEC) during solidification of metals. A physical model was proposed to interpret how these TEC appear at the solid–liquid interface and verified by a corresponding simulation. The influences of TEMHDF on solidification were investigated through both ex-situ experiments and n situ observations by means of synchrotron X-ray radiography. The 3D numerical simulations of TEMHDF were performed for these two cases, respectively, and suggested that both the change of interface shape with different transverse static magnetic fields demonstrated by the ex-situ experiments and the real time observed interface shape varying under a 0.08 T transverse static magnetic field could attribute to the TEMHDF advanced solid–liquid interface in the static magnetic field-assisted directional solidification. The TEMHDF produced by an axial static magnetic field were also computed along with the interface change predicted based on which is good in line with the published experimental results. This study of TEMHDF and their impacts on the solid–liquid interface shape provides a method to tailor the structure during directional solidification using static magnetic field.

Radiation reaction effect on laser driven auto-resonant particle acceleration

The effects of radiation reaction force on laser driven auto-resonant particle acceleration scheme are studied using Landau-Lifshitz equation of motion. These studies are carried out for both linear and circularly polarized laser fields in the presence of static axial magnetic field. From the parametric study, a radiation reaction dominated region has been identified in which the particle dynamics is greatly effected by this force. In the radiation reaction dominated region, the two significant effects on particle dynamics are seen, viz., (1) saturation in energy gain by the initially resonant particle and (2) net energy gain by an initially non-resonant particle which is caused due to resonance broadening. It has been further shown that with the relaxation of resonance condition and with optimum choice of parameters, this scheme may become competitive with the other present-day laser driven particle acceleration schemes. The quantum corrections to the Landau-Lifshitz equation of motion have also been taken into account. The difference in the energy gain estimates of the particle by the quantum corrected and classical Landau-Lifshitz equation is found to be insignificant for the present day as well as upcoming laser facilities.

A comparative study of triaxial and uniaxial magnetic shields made out of YBCO coated conductors

Persistent current loops of arbitrary size can be made from currently manufactured RE123 coated conductors. Our previous work has shown that an assembly of such loops is able to effectively shield quasi static axial magnetic field due to the absence of a resistive joint. The shielding effectiveness depends on the aspect ratio and the number of layers. In the present work we study experimentally the detailed magnetic response of two different configurations of the magnetic shields for various orientations of the applied field. Using a three-axis Hall probe we determine the amplitude of magnetic field generated by the induced persistent currents and its direction with respect to the applied field. The effectiveness of the uniaxial shield decreases strongly when the applied field is not collinear with its axis. In the triaxial (Polywell type) structure comprised of three pairs of coils whose axes are mutually orthogonal, the field attenuation is shown to be only weakly dependent on the direction of the magnetic field. We discuss the properties of the triaxial shield and the ways to improve its screening performance.