External Axial Magnetic Field Research Articles

Effect of an external axial magnetic field on thermal convection of an electric conducting fluid in a rotating spherical shell

We investigated the interaction between the thermal convection structure in a rotating spherical shell and a uniformly applied magnetic field acting externally toward the axis of rotation. We considered a system that the inner and outer spherical boundaries are heated and cooled both isothermally and the effects of induced magnetic fields are neglected. The convection structure of this system is characterized by the generation of travelling waves around the axis of rotation. We numerically investigated how this is affected under the magnetic field. The characteristics of the disturbance (i.e., growth rate and phase velocity) were estimated from the exponential amplification period using three-dimensional (3D) simulations and compared with those obtained from linear stability analyses (LSA). In particular, the effects of the magnetic field on the kinetic energy and Nusselt number were investigated for a quasi-steady state using 3D simulations. Our numerical results revealed that the applied magnetic field destabilizes the flow field and enhances heat transfer. This is noteworthy, contrary to the common sense that magnetic field stabilizes the flow field and reduces the heat transfer coefficient. However, the magnetic field strengths at which the kinetic energy and heat transfer coefficient took their maximum values differ slightly.

Regulating mechanisms of external axial magnetic field on flow and heat transfer behavior for process optimization in plasma beam polishing

Plasma beam polishing has emerged as an innovative alternative to conventional metal finishing techniques. However, the precision machining ability of plasma arc encounters challenges due to the formation of surface undulations, while previous research has predominantly focused on parameter adjustments. In this paper, the feasibility of optimizing the plasma beam polishing process was validated through magnetic field regulation, as the surface topography and subsurface structure were obviously flattened. Given this, a micro plasma arc model under inhomogeneous axial magnetic field was established to explore the regulating mechanisms of the external axial magnetic field on the flow and heat transfer behavior of the plasma arc. Through this model, the plasma characteristics (magnetic induction intensity, electric potential, arc current, Lorentz force, plasma velocity and electron temperature) were comprehensively discussed. The results indicate that the external shape of the plasma arc is determined by the self-induced magnetic field, while the externally applied axial magnetic field significantly influences the internal flow of the plasma arc. The axial and radial magnetic fields generated by the excitation coil can induce circumferential rotation by dragging the plasma arc with Lorentz force, creating a more suitable energy beam for the polishing process. To verify the model, the arc temperature was measured using spectroscopic diagnostics, and the arc shape was captured using a high-speed camera, both of which were basically consistent with the simulation results.

Elastic-plastic conductor damage evaluation at over 0.4% strain using a high-stress REBCO coil

Recent reports on screening current stress simulations of high-field REBCO magnets frequently present peak stresses over 1 GPa. However, this result is probably an unrealistic artifact of purely elastic calculations, considering the macroscopic yield and fracture stresses of approximately 900 MPa and less than 1.1 GPa for Hastelloy substrate-coated conductors. Here, we evaluate elastic-plastic conductor damage at over 0.4% strain using a high-stress REBCO coil exposed to a high field to explore this elastic-plastic regime. The coil was located off-center in a low-temperature superconductor magnet so as to induce a significant screening current in the enhanced radial field. Voltage taps, a Hall sensor, and two strain gauges were used for the instrumentation. We obtained strains exceeding 0.4% near the outward edge during the coil current charge from 350 A to 390 A, where the coil was exposed to external axial and radial magnetic fields of 13 T and 0.5 T. Post mortem results showed wavy plastic deformation, electrical damage, and REBCO defects. An elastic-plastic simulation reproduced the measured strains and predicted that ∼1 GPa stress is sufficient to induce ∼0.9% strain, thus validating our initial concerns with purely elastic models. This paper provides our experimental and simulation results.

Melt azimuthal rotation in direct current electric arc furnace without external axial magnetic field

This paper considers the azimuthal rotation of melt about the vertical axis of a DC EAF, which is ensured by an inclined installation of the power supply electrodes, which is a patented solution that is the basis for this article. Near inclined electrodes, the Lorentz force has a pronounced azimuthal component, which is the driver of the melt rotation without an external axial magnetic field – thus, additional energy consumption to create an external magnetic field is not necessary. The main design and technological solutions, formulated in the patent and presented by the authors, were obtained using observations and numerical LES study for a laboratory-scale experimental setup (capacity 4.8 kg of GaInSn), as well as using LES computations for an industrial-scale DC EAF (capacity 3.6 t of molten steel). Basic technological solutions studied: the flow of the melt may be controlled by varying the number of vertical and inclined electrodes of DC EAF, choosing the angle of inclination of the electrodes as well as choosing the sequence of turning on and turning off power supply through the electrodes.

Laser wakefield acceleration of electrons in a magnetically controlled plasma

In the present study, the laser wakefield acceleration of electrons in a magnetically-controlled plasma is investigated. The results indicate that by employing a linearly-chirped laser pulse propagating through magnetized plasma with a reversed external magnetic field, higher energy electrons are obtained than with unmagnetized plasma and/or a nonchirped laser pulse. By considering an appropriate constant chirping magnitude and an axial external magnetic field, one can obtain remarkable GeV electron energies. It is also found that the effect of the external magnetic field direction on the electron energy is not sensed much in the mildly relativistic regime, while the reversed magnetic field increases the peak of electron energy gains compared with the forward magnetic field in the highly relativistic regime. For the mildly relativistic case, a peak energy of 412 MeV and a relative energy spread of 7% is obtained. In addition, for the highly relativistic case, a peak energy of 1.55 GeV is obtained.

Effect of a dynamic axial magnetic field on a preconditioned single-wire Z-pinch

In this study, the effect and mechanism of a dynamic axial magnetic field on a preconditioned single-wire Z-pinch were investigated experimentally and theoretically. Optical diagnostic methods, including shadowgraphy, interferometry, Faraday rotation, and Thomson scattering, have been used to measure the parameters of magnetized plasmas. Compression of the azimuthal and axial magnetic fields was observed, and the suppression of the plasma instability was recorded and analyzed. The results showed that an external axial magnetic field could reduce the plasma instability and non-uniformity, but prolong the implosion time and weaken the compression ratio. In the implosion process with an axial magnetic field, the plasma rotated at a speed similar to that of imploding, which could be regarded as a stabilization method. A simplified model of the diffusion and compression processes of a dynamic axial magnetic field was developed to investigate the conditions for maximizing the amplitude of the axial magnetic field. Subsequently, the snowplow model was used to calculate the effect of axial magnetic fields on the implosion process and energy conversion.

Enhancement of NOx production in water by combining an air bubble plasma jet and an external magnetic field

Production of NOx (NO2− + NO3−) in water with an air bubble discharge plasma jet under the influence of an external axial steady magnetic field was investigated experimentally. The gas phase plasma parameters, rotational (Tr), vibrational (Tv) and electronic excitation (Tx) temperatures, and electron density (ne), as well as the liquid phase pH and the concentrations of nitrite (NO2−) and nitrate (NO3−), were measured as a function of treatment time and magnetic field strength. It was found that Tr, Tv, Tx, and ne slightly increased as a function of magnetic field strength in the gas phase plasma. The pH decreased both with treatment time and magnetic field strength. In the maximum field strength of 290 mT, the concentrations of NO2− and NO3− were ∼82% and ∼74%, respectively, greater than with B=0. With B=290 mT, the energy cost for producing NOx was ∼78% lower than with B=0. The energy cost may likely be reduced due to decreasing radial diffusion loss of charged species in the discharge with increasing magnetic field strength.

Edge effects on the electromagnetic response of inhomogeneous type-II superconductors

From a macroscopic point of view, the edges of a type II superconductor are degraded non-homogeneous regions compared with the bulk of the sample. This paper presents numerical simulations of a long superconducting bar with square cross section subjected to an axial external magnetic field, the edges effect on its electromagnetic properties is studied. The edges of the sample are modeled as finite width regions with lower critical current density than the rest of the material. The simulations are based on a continuum electrodynamics model which describes the magnetic induction dynamics of partially penetrated states. Unlike a simpler homogeneous superconductor, in an inhomogeneous material rough flux fronts are formed. It was established that the stochastic profile of the current density produces jets of magnetic flux near a macrodefect.

Morphology Control of Metal Pool and Eutectic Carbides in Electroslag Remelted M2 HSS with an External Axial Static Magnetic Field

This study investigates the influence of a superimposed axial static magnetic field (ASMF) on the morphology of metal pool and eutectic carbides (ECs) in electroslag remelted M2 high-speed steel (HSS). The application of ASMF caused the metal pool to become shallower, and the solidified structure to expand axially, along with finer (i.e., more uniformly distributed and crystallographically oriented) ECs. Lorentz force-driven unidirectional circulation in slag pool was the primary cause of the metal pool’s morphological adjustment; this resulted in a more homogenous temperature distribution in slag pool. Thus, the heat transfer from the slag pool to the metal pool became more uniform, creating a metal pool that is shallower. Additionally, local solidification time (LST) became shorter, while the number of (Ti, V)N-Al2O3 inclusions serving as heterogeneous nuclei for EC formation increased due to ASMF, enabling finer EC with more crystallographic orientations in ESR ingots.

Structure and local parameters of self-compressed plasma streams in external magnetic field

Abstract The influence of the external axial magnetic field on pinching plasma flows generated by a magnetoplasma compressor (MPC) has been studied using magnetic and electric probes. In the presence of an external magnetic field, temperature measurements show two groups of electrons with different temperatures near the plasma stream core. The external magnetic field leads to a noticeable increase in the electric current in the plasma stream, electron temperature, and the formation of the current-sheet-like structure observed in the MPC for the first time.

Numerical simulations of flow around dual tandem circular cylinders under a strong axial magnetic field

This work is dedicated to understand the magnetohydrodynamic (MHD) flow around two identical tandem circular cylinders confined in a duct under external axial magnetic field. The influences of magnetic field (represented by the Hartmann number Ha), inlet velocity of conducting fluid (represented by the Reynolds number Re), and the gap ratio (L/d, the ratio of the distance between the centers of two cylinders to the diameter of the circular cylinder) on the flow regimes, vortex shedding frequency, pressure coefficients, drag coefficients, and pressure drop are investigated. The simulations are conducted in the parameter ranges 2≤L/d≤8, 180≤Re≤1440, and 101≤Ha≤1818, respectively. Four flow modes are observed, namely, no vortex shedding, single body, reattachment regime, and vortex shedding (VS) modes, with various Ha/Re2 and L/d values. For L/d>5, the flow field presents the VS flow model, a vortex street with synchronous periodic shedding appears behind the two cylinders, and the vortex shedding frequency matches well with that of the single one. The pressure coefficient shows different tendencies because of the arrangement of two circular cylinders. It can be found that the mean drag coefficients for the upstream circular cylinder and the downstream circular cylinder vary with Re/Ha0.8. For Re=361, Ha=504 and Re=722, Ha=1212, the flow field remains almost unchanged, which implies the balance between inertial force and Lorentz force because the inertial force would promote the flow, while the electromagnetic force leads to the reverse effect. Furthermore, the effects of magnetic field and inlet velocity of metal fluid on pressure drop ΔP between inlet and outlet can be fitted as a linear relation ΔP∼ReHa.

Магнитное поле внутри и вне тороидального плазменного шнура с током, удерживаемого в равновесии внешним аксиальным полем

We provide a description of magnetic field inside and outside of a circular plasma ring carrying an electric current. The equilibrium is maintained by an external axial magnetic field. The provided solution is close to that by Titov and Demoilin (1999), but our solution is more general and accurate

Long-wavelength electromagnetic waves of surface type in circular metal waveguides partially filled by plasma in presence of an axial static magnetic field

The method of successive approximations is applied to solve the Maxwell equations in cylindrical plasma waveguide geometry for electromagnetic waves with arbitrary azimuthal wave index and small axial wavenumber. The theory of surface flute waves is used as zeroth approximation. This study generalizes previous investigations whose results are utilized for verification of newly obtained conclusions. The influences of several plasma waveguide parameters such as magnitude and sign of the azimuthal wave index, the width of the dielectric layer between a plasma and a waveguide wall and the magnitude of its dielectric constant, the radii of the plasma column and the metal wall, and the external axial static magnetic field on the wave dispersion properties are analyzed.

Low Frequency Giant Magneto-Impedance Effect of Co-Rich Ribbons Induced by Joule Annealing Treatment

The giant magneto-impedance (GMI) effect of Co83.2Fe5.2Si8.8B2.8 ribbons at frequencies of <1 MHz was analyzed. To improve the GMI response, a Joule annealing treatment was conducted with a direct current, and the domain structure of the ribbon surface was investigated via magneto-optical Kerr effect microscopy. The annealed ribbons show larger impedance changes under external magnetic fields, and higher field sensitivity is obtained by certain current annealing treatments. The field sensitivity of 418 and 782%/(kA/m) at 0.2 MHz and 0.8 MHz are achieved after annealing at 0.8 A for 20 min. The annealing treatment under direct electric current induces stress relaxation, and domain rearrangement, and the crystallization process gradually increases with the increasing current density, which gives rise to anisotropic reformation. The release of stresses due to Joule heating below the crystallization temperature causes the homogenous distribution of stress induced by rapid solidification and influences the elastic anisotropy, causing the domain structures to become much more regular. The crystallization, along with the precipitation of hard magnetic phases, increases the crystal anisotropy and induces the intense magnetic coupling action. Consequently, the magnetic domains in the annealed ribbons are rearranged with reformed anisotropy by Joule annealing heat and by the transverse magnetic field induced by the current. The irregular domains, with complex anisotropy in the as-cast ribbons corresponding to the weak GMI response, are transformed into regular and strip-like domains, with transverse easy magnetization after annealing at 0.4 A. After annealing at 0.8 A, the domains are further transformed into fine axial fingerprint-like domains, which are much more sensitive to the change in the axial external magnetic field, allowing for the best GMI response. These results indicate that the Joule annealing treatment is an optional method to optimize the soft magnetic properties and the GMI effect of these Co-rich ribbons at low frequencies.

GeV Electron Acceleration by Trapezoidal Laser Pulse Envelope Under Combined Effect of Frequency Chirp and Axial Magnetic Field in Vacuum

In the present article, we have investigated efficient electron acceleration by employing trapezoidal laser pulse envelope under the combined effect of linear chirp and external axial magnetic field in vacuum. In this envelope, the electric signal linearly rises to a certain level, stay there, and then falls back to its initial level. So, there will be an opportunity to increase the interaction time between laser pulse and electron for efficient energy transfer, which has been reflected in this study. A few megaelectronvolt electrons have been injected axially to the front of a trapezoidal short laser pulse. In all calculations, the front end of each laser pulse captures electron at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$t= 0$ </tex-math></inline-formula> . The dynamics arise from analytical and numerical solutions of Newton–Lorentz force equations of motion. Electron energy gain has been calculated for initial laser phase varying from 0 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\pi $ </tex-math></inline-formula> . For the optimum set of laser parameters, we have observed electro’s energy gain of the order of GeV.

Radiation Power of a Short High-Current Vacuum Arc at High Current Densities

The radiation of a vacuum arc was measured in the spectral region <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$200~\text {nm} \le \lambda \le 1100$ </tex-math></inline-formula> nm. The arc was initiated in the center of the cathode by breaking the current in the auxiliary circuit and was fed by the rectangular current pulse with a duration of 10 ms. The interelectrode gap was fixed at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$h = 4$ </tex-math></inline-formula> mm, and the butt <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2r =30$ </tex-math></inline-formula> -mm CuCr30 electrodes were used. The arc was stabilized by an external uniform axial magnetic field (AMF). The radiation was coming out of the vacuum chamber through quartz ultraviolet (KU)-1 quartz window. The radiation detector was a silicon photodiode with a diameter of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d =1.2$ </tex-math></inline-formula> mm. It was located on an axis intersecting the axis of symmetry of the discharge in the center of the interelectrode gap. The distance from the diode to the axis of symmetry was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L =1340$ </tex-math></inline-formula> mm. After amplification, the signal from the diode was recorded on an oscilloscope. The measurements were made in a range of currents from 10 to 25 kA in order to cover the modes with the anodic activity. It was found that under the conditions of these experiments, the anode activity begins to manifest itself at the end of the pulse, starting with currents exceeding 15 kA. As the current increases, the activity begins closer to the beginning of the pulse. Considering the type of the spectral sensitivity of the diode, two series of measurements were made—measurements without a filter and through a yellow filter ZhS-10 that cuts off radiation with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda \le400$ </tex-math></inline-formula> nm. The results obtained made it possible to analyze the dependence of the radiation power on the arc current. The results showed that at high current densities in the developed vacuum arc with anodic activity, a significant part of the power is transferred by radiation. At the end of a pulse with a current of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 25$ </tex-math></inline-formula> kA, the radiation power is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P_{\mathrm {uv}}~\approx ~150$ </tex-math></inline-formula> kW, while in the discharge, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P_{a} ~\approx ~800$ </tex-math></inline-formula> kW is released. That is, at high current densities at the end of the pulse, the radiation power reaches almost 20% of the power released in the arc at this time. Estimates of lateral losses due to radiation are made.

Optical Investigation of Excited Species Distribution in Low-Current Vacuum Discharges

It is well-known that when high currents flow through a vacuum gap, anode spots are formed on the anode surface. However, anode flare phenomena are also observed in low-current vacuum discharges. The objective of this article is to experimentally observe the spatial distribution of excited Cu atoms and ions generated from a single cathode spot (CS) and to interpret the mechanism of the anode flare at low currents (40–80 A). Optical emission spectroscopy (OES) and laser-induced fluorescence (LIF) techniques have been used for investigating low-current vacuum arc discharge confined to axial magnetic fields (AMFs). The effects of AMFs and arc currents on the axial distribution of the radiation intensity were investigated for different copper ionization states (Cu I and Cu II). The highest luminance was found near the electrodes for both excited neutral copper atoms and ions, and only weak spectral lines of excited Ni atoms were detected in the near-anode region. With increasing the external AMF, a bright “spot” appeared near the anode surface. In the visible light range, excited atoms were the main contributor to anode flare in low-current vacuum discharge. As measured by LIF, the density of copper atoms near the anode increased from 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{17}$ </tex-math></inline-formula> to 14 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10^{17}\,\,\text{m}^{-3}$ </tex-math></inline-formula> when the applied magnetic fields increased from 31 to 74 mT. The anode flare contains far more excited atoms from the cathode material than the anode material. They are the results of cathodic plasma expansion to reach the anode after charge neutralization and reflection on the anode surface. For the single CS, the observed fact that a tiny number of atoms leave the anode surface and participate in the anode flare is more likely the result of a sputtering effect rather than thermal evaporation by cathode electron stream.

Numerical simulation of low-current vacuum arc jet considering anode evaporation in different axial magnetic fields

As the main source of the vacuum arc plasma, cathode spots (CSs) play an important role on the behaviors of the vacuum arc. Their characteristics are affected by many factors, especially by the magnetic field. In this paper, the characteristics of the plasma jet from a single CS in vacuum arc under external axial magnetic field (AMF) are studied. A multi-species magneto-hydro-dynamic (MHD) model is established to describe the vacuum arc. The anode temperature is calculated by the anode activity model based on the energy flux obtained from the MHD model. The simulation results indicate that the external AMF has a significant effect on the characteristic of the plasma jet. When the external AMF is high enough, a bright spot appears on the anode surface. This is because with a higher AMF, the contraction of the diffused arc becomes more obvious, leading to a higher energy flux to the anode and thus a higher anode temperature. Then more secondary plasma can be generated near the anode, and the brightness of the ‘anode spot’ increases. During this process, the arc appearance gradually changes from a cone to a dumbbell shape. In this condition, the arc is in the diffuse mode. The appearance of the plasma jet calculated in the model is consistent with the experimental results.

Tornados and cyclones driven by Magneto-hydrodynamic forces

A numerical analysis is carried out to explore the variety of flow structures induced inside a conducting fluid emerging from the interaction of an electric current source with an external axial magnetic field. Results are obtained for Shercliff (S) numbers up to 1010 and Hartmann (Ha) numbers up to 115, for a single magnetic Prandtl number of Pm=8.6× 10−7. Depending on these numbers, very different swirling flow patterns are predicted. Transitions seem to be solely controlled by two dimensionless ratios Γ=Ha/(PmS)1/2 and N=Ha6Pm/S.For low Γ, vortex-breakdown, tornados and cyclones are obtained. These MHD tornados are found to be very similar in structure to atmospheric tornados. A slight increase in Γ results in the transformation of the tornado into a cyclone which rotates at relatively constant angular velocity. Surprisingly for further increase in Ha, the cyclone is restructured into an inverted tornado. The induced currents are found to be at the origin of this phenomenon. For large N, a Hartmann swirling flow occurs which is associated with almost vertical electric current lines. In this regime, both poloidal and azimuthal flows are damped, and remain mainly present in the forcing region at the edge of the electric current column. These results may be of relevance for the understanding of flow structures in the atmosphere, in aluminum reduction cells, electro-metallurgical processes, and in liquid metal batteries.

Confinement of intense proton beams by an applied axial magnetic field in large-scale plasma

Stable and efficient transport of particle beams through plasma is a frequent topic in particle–matter interactions. In plasma, intense ion beams can focus and flap because of the self-generated electromagnetic fields and soon diverge if no restrictions are imposed. In this study, the transport of a slab beam in large-scale plasma with a uniform applied axial magnetic field is simulated and analyzed using a newly developed kinetic particle-in-cell code. The simulation results show that the applied axial magnetic field intensifies the Lorentz force acting on the beams and is effective at preventing ion-beam divergence. This confinement effect from the external magnetic field influences the beam flapping more than it does the focusing, and with increasing applied magnetic field, more beam particles converge and more energy is transferred into the transverse direction in the flapping region. In the present scenario, the beam behavior is affected synthetically by both the self-generated electromagnetic field and the external axial magnetic field. Also shown is that the applied field exerts little control over the total beam energy, which the present theoretical analysis explains well. Beam confinement by an external magnetic field is likely to have a major impact on nuclear fusion, astrophysics, and beam control.