Transverse Static Magnetic Field Research Articles

Effect of Transverse Static Magnetic Field on Microstructure of Al-12%Si Alloys Fabricated by Powder-BlowAdditive Manufacturing

Effect of Transverse Static Magnetic Field on Microstructure of Al-12%Si Alloys Fabricated by Powder-BlowAdditive Manufacturing

Time-integrated optical emission studies on laser-produced copper plasma in the presence of magnetic field in air ambient at atmospheric pressure

We report on the effect of laser fluence variation on ablation rate and the time-integrated optical emission of copper plasma in the presence of static transverse magnetic field in air. In the presence of magnetic field, the ablation rate at 14 Jcm−2 laser fluence is around two times higher than that in the absence of magnetic field. It is attributed to an increase in melt ejection caused by recoil pressure of ablated mass in the presence of magnetic field. We also simulated the laser ablation rate using finite element method to justify that the ablation rate is higher in the presence of magnetic field. The intensity of atomic/ionic transition is enhanced in the presence of magnetic field. The enhancement in the intensity of spectral line depends on laser fluence and it is pronounced at fluence 14 Jcm−2. The intensity enhancement is due to an increase in electron-impact excitation and recombination process as a result of magnetic confinement of plasma. The ionic line also showed intensity enhancement in the presence of the magnetic field. It is due to the increase in ionization of atom in the presence of magnetic field. The ionization potential is lowered in the presence of magnetic field which results in more ionization. The electron density and temperature initially increased with fluence and then decreased slightly at higher fluence. In the presence of magnetic field, the electron density and temperature followed the same trend; however, higher compared with that in the absence of magnetic field. It is attributed to magnetic confinement of plasma. The spatially resolved optical emission measurements showed that the intensity with magnetic field decreased with lateral distance. However, it is higher than that without magnetic field up to 2 mm and beyond that the intensity with and without magnetic field remained almost the same.

Tunable terahertz radiation generation by nonlinear photomixing of cosh-Gaussian laser pulses in corrugated magnetized plasma

AbstractThis paper presents a scheme of THz generation by nonlinear photomixing of two cosh-Gaussian lasers pulses having different frequencies (ω1, ω2) and wave numbers $(\vec k_1, \vec k_2 )$ and same electrical field amplitude in a corrugated plasma embedded with transverse static magnetic field. Cosh-Gaussian laser pulses have steep gradient in intensity profile along with wider cross-section, which exerts a stronger nonlinear ponderomotive force at ω1 − ω2 and $\vec k_1 - \vec k_2 $ on plasma electrons imparting a nonlinear oscillatory velocity to plasma electrons. Oscillatory plasma electrons couple with the density ripple n′ = nα0eiαx to produce a nonlinear current, which is responsible for resonant THz radiation at frequency $\sim\left( {{\rm \omega} _{\rm c}^2 + {\rm \omega} _{\rm p}^2} \right)^{1/2} $. The amplitude, efficiency and beam quality of THz radiation can be optimized by choosing proper corrugation factor (α of the plasma), applied magnetic field (ωc), decentered parameter (b), and beam width parameter a0 of cosh-Gaussian lasers. An efficiency of $\sim\!10^{ - 2} - 10^{ - 1} $ is achieved for laser electric field E = 3.2 × 109 V/cm.

Influences of the Transverse Static Magnetic Field on the Droplet Evolution Behaviors during the Low Frequency Electroslag Remelting Process

Influences of the Transverse Static Magnetic Field on the Droplet Evolution Behaviors during the Low Frequency Electroslag Remelting Process

Effect of lens focusing distance on laser-produced copper plasma in air in the presence of static transverse magnetic field

We report on the role of lens focusing conditions on laser-produced copper plasma in air at atmospheric pressure in the presence of magnetic field using optical emission spectroscopy. From the time integrated and temporal optical emission spectroscopy, we observed that the copper neutral/ionic line showed a higher intensity when the focal point of the lens was below the target surface in the absence of the magnetic field. In the presence of magnetic field, significant intensity enhancement was observed when the focal point was below the target surface which is attributed to an increase in the plasma-magnetic field interaction at this focusing condition. When the focal point was above the target surface, the intensity of neutral line was quite low and the effect of magnetic field was insignificant. This is because of air breakdown which caused lesser laser-matter interaction and eventually plasma-magnetic field interaction. Based on heat conduction equation, we also simulated the laser heating of copper target using a finite element method to predict the expansion velocity at different laser focusing conditions and the stopping radius of plasma and found that it depends on the distance between the focal point and the target surface. The intensity enhancement in the presence of magnetic field occurred at a stopping radius where the density of plasma was high due to considerable deceleration of the plasma by magnetic field.

Effect of Current Frequency on Droplet Evolution During Magnetic-Field-Controlled Electroslag Remelting Process Via Visualization Method

A transparent physical model was set up to investigate the influence of the remelting current frequencies on droplet evolution during the magnetic-field-controlled electroslag remelting process. Physical simulation experiments were done under the remelting current of 8 A with frequencies ranging from 10 to 500 Hz, and a transverse static magnetic field (TSMF) of 0.7 T was superimposed simultaneously. The high-speed camera was used to record the evolution behavior of the droplet. Representative processes of formation and detachment of the droplets were observed under different conditions. The results showed that there was little influence of the current frequencies on the evolution behavior of the droplet without the external magnetic field. Nevertheless, if a TSMF was introduced, the liquid droplet’s neck would be smashed into a lot of smaller droplets when the remelting current frequencies were lower than 100 Hz, while the smashing effect disappeared when the frequencies were higher than 100 Hz. The mechanism of the smashing effect was discussed. Statistical work was done to obtain the quantitative data to give a clear result revealing the influence of the remelting current frequencies on droplet evolution. The decrease in the diameter of the liquid droplets would remarkably increase the interface area and shorten the migrating distance of the inclusions in the droplets, which meant that a higher purifying efficiency could be expected.

Spin noise of electrons and holes in (In,Ga)As quantum dots: Experiment and theory

The spin fluctuations of electron and hole doped self-assembled quantum dot ensembles are measured optically in the low-intensity limit of a probe laser in absence and presence of longitudinal or transverse static magnetic fields. The experimental results are modeled by two complementary approaches based either on semiclassical or quantum mechanical descriptions. This allows us to characterize the hyperfine interaction of electron and hole spins with the surrounding bath of nuclei on time scales covering several orders of magnitude. Our results demonstrate (i) the intrinsic precession of the electron spin fluctuations around the effective nuclear Overhauser field caused by the host lattice nuclear spins, (ii) the comparably long time scales for electron and hole spin decoherence, as well as (iii) the dramatic enhancement of the spin lifetimes induced by a longitudinal magnetic field due to the decoupling of nuclear and charge carrier spins.

Influence of a Static Magnetic Field on Laser Induced Tungsten Plasma in Air

In this work, laser induced tungsten plasma has been investigated in the absence and presence of 0.6 T static transverse magnetic field at atmospheric pressure in air. The spectroscopic characterization of laser induced tungsten plasma was experimentally studied using space-resolved emission spectroscopy. The atomic emission lines of tungsten showed a significant enhancement in the presence of a magnetic field, while the ionic emission lines of tungsten presented little change. Temporal variation of the optical emission lines of tungsten indicated that the atomic emission time in the presence of a magnetic field was longer than that in the absence of a magnetic field, while no significant changes occurred for the ionic emission time. The spatial resolution of optical emission lines of tungsten demonstrated that the spatial distribution of atoms and ions were separated. The influence of a magnetic field on the spatial distribution of atoms was remarkable, whereas the spatial distribution of ions was little influenced by the magnetic field. The different behaviors between ions and atoms with and without magnetic field in air were related to the various atomic processes especially the electrons and ions recombination process during the plasma expansion and cooling process.

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.

Effect of transverse static magnetic field on microstructure and properties of GCr15 bearing steel in electroslag continuous casting process

The present study investigated the influence of transverse static magnetic field (TSMF) on microstructure and properties of GCr15 bearing steel remelted by electroslag continuous casting (ESCC) method. The microstructure, inclusions, contents of elements (sulfur, phosphorus and oxygen) and mechanical properties in terms of tensile test, impact toughness and hardness had been analyzed. The overall results reveal that the presence of static magnetic field is beneficial for microstructure refinement as well as inclusion and harmful elements removal, thus achieving considerable improvement in mechanical properties of GCr15 bearing steel.

Laser beat wave resonant terahertz generation in a magnetized plasma channel

Resonant excitation of terahertz (THz) radiation by nonlinear mixing of two lasers in a ripple-free self created plasma channel is investigated. The channel has a transverse static magnetic field and supports a THz X-mode with phase velocity close to the speed of light in vacuum when the frequency of the mode is close to plasma frequency on the channel axis and its value decreases with the intensity of lasers. The THz is resonantly driven by the laser beat wave ponderomotive force. The THz amplitude scales almost three half power of the intensity of lasers as the width of the THz eigen mode shrinks with laser intensity.

Suppression of decay instability of the non-resonant beat wave excited by two counter-propagating x-mode lasers in magnetized plasma

The decay instability of non-resonant beat mode is investigated in homogeneous, hot, and collision less plasma having transverse static magnetic field. Two counter-propagating X-mode lasers with frequency difference ω1∼ω2≥2ωp and wave numbers k→1 and k→2 drive a non-resonant space charge beat wave at phase matching conditions of frequency ω0=ω1∼ω2 and wave numbers k→0=k→1+k→2. The driven beat wave acts as a pump for decay instability and parametrically excites a pair of lower hybrid wave (ω,k→) and sideband upper hybrid wave (ω3,k→3) propagating in sideward direction so that momentum remains conserved. The sideband wave couples with the driver beat wave to exert ponderomotive force on plasma electrons at frequency ω=ω0+ω3. The oscillatory motion of plasma electrons due to ponderomotive force and lower hybrid wave causes density perturbation in plasma, which couples with oscillating beat mode by feedback mechanism and gives rise to a sideband wave at resonance. The maximum growth rate is achieved at scattering angels θs∼30° and θs∼150°. The growth rate becomes half by changing applied magnetic field from ∼90 T to ∼270 T. The suppression of decay instability can be beneficial for parametric excitation of fast plasma wave (coupled with slow plasma wave) by two counter-propagating lasers for electron acceleration.

Influence of varying magnetic order in an external magnetic field on the electronic structure and Fermi surface within the t–J model

We investigate the electronic structure of the two-dimensional t–J model in a transverse external static magnetic field with canted long-range magnetic order using cluster perturbation theory. The distribution of the spectral weight in the whole range of fields from zero to ferromagnetic saturation is explored. We demonstrate the possibility of a sharp change in a distribution of spectral weight at the Fermi level associated with the magnetic correlations when varying magnetic field.

Multi-structured temporal behavior of neutral copper transitions in laser-produced plasma in the presence of variable transverse static magnetic field

We report on the effect of variable magnetic field on temporal behavior of neutral copper (Cu I) transitions in laser-produced copper plasma at atmospheric pressure using optical emission spectroscopy. In the presence of magnetic field, the intensity of copper atomic lines at 510.5, 515.3, and 521.8 nm gets enhanced due to increase in electron-impact excitation rate. The enhancement factor of the neutral lines is different due to different electron-impact excitation rates. We observed that the Cu I profile consists of two components recorded in the absence of magnetic field and at 0.1 T. At magnetic field of 0.3 T, the appearance of third slow component at delayed time, i.e., 122, 130, and 140 ns for Cu I (521.8, 515.3, and 510.5 nm) is also observed. We demonstrate that the generation of slow component is related to electron-impact excitation of Cu I atom rather than backflow particles and instabilities at atmospheric pressure. The instabilities generated during the plasma deceleration by magnetic field can be reduced in the presence of air at atmospheric pressure which will be applicable to enhance the sensitivity of laser-induced breakdown spectroscopy. The fast component is least affected by the magnetic field due to longer magnetic diffusion time.

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.

Coupled spin-vortex pair in dipolar spinor Bose-Einstein condensates

We investigate the ground-state and magnetic properties of a dipolar spin-1 Bose-Einstein condensate trapped in a symmetric double-well potential. In particular, we focus on the spin-vortex states by assuming that each potential well is highly pancake shaped. We show that the presence of the double-well potential gives rise to two different spin configurations for the spin-vortex pair states. We also study the response of the coupled spin-vortex pair to static transverse magnetic fields.

Stimulated Brillouin side-scattering of the beat wave excited by two counter-propagating X-mode lasers in magnetized plasma

The stimulated Brillouin scattering (SBS) of nonresonant beat mode in the presence of static magnetic field is investigated in a plasma. Two counter-propagating lasers of frequencies (ω1 and ω2) and wave vectors (k1 and k2) drive a nonresonant space charge beat mode at the phase matching condition of frequency ω0≈ω1∼ω2 and wave number k→0≈k→1+k→2. The driver wave parametrically excites a pair of ion acoustic wave (ω,k→) and a sideband electromagnetic wave (ω3,k→3). The beat wave couples with the sideband electromagnetic wave to exert a nonlinear ponderomotive force at the frequency of ion acoustic wave. Density perturbations due to ion acoustic wave and ponderomotive force couple with the oscillatory motion of plasma electron due to velocity of beat wave to give rise to a nonlinear current (by feedback mechanism) responsible for the growth of sideband wave at resonance. The growth rate of SBS was reduced (from ∼1012s−1 to 1010s−1) by applying a transverse static magnetic field ∼90 T. The present study can be useful for the excitation of fast plasma waves (for the purpose of electron acceleration) by two counter-propagating laser beams.

Shielding from external magnetic fields by rotating magnetic conducting cylindrical shells

Purpose – In many different engineering fields often there is a need to protect regions from electromagnetic interference. According to static and low-frequency magnetic fields the common strategy bases on using a shield made of conductive or ferromagnetic material. Another screening technique uses solenoids that generate an opposite magnetic field to the external one. The purpose of this paper is to discuss the shielding effect for a magnetic and conducting cylindrical screen rotating in an external static magnetic field. Design/methodology/approach – The magnetic flux density is expressed in terms of the magnetic vector potential. Applying the separation of variables method analytical solutions are obtained for an infinitely long magnetic conducting cylindrical screen rotating in a uniform static transverse magnetic field. Findings – Analytical formulas of the shielding factor for a cylindrical screen of arbitrary conductivity and magnetic permeability are given. A magnetic Reynolds number is found to be an appropriate indication of the change in magnetic field inside the screen. Useful simplified expressions are presented. Originality/value – This paper treats in a qualitative way the possibility of static magnetic field shielding by using rotating conducting magnetic cylindrical screens. Analytical solutions are given. If the angular velocity is equal to zero or the relative permeability of the shield is equal to one the shielding factor has forms well known from literature.

Abstract T P173: Cerebral Oxygen Extraction Fraction MRI to Assess Metabolic Changes in Acute Ischemic Stroke

Background: In acute cerebral ischemia, there is an imbalance between oxygen tissue consumption and delivery and oxygen extraction fraction (OEF) from the circulation increases to maintain normal oxygen metabolism and neuronal function. Detecting such metabolic changes may provide important data to inform decisions on therapeutic interventions such as patient selection for reperfusion or identification of futile therapeutic situations. We used serial OEF MRI to assess metabolic changes in brain tissue associated with reperfusion interventions. Methods: Serial OEF acquired in 10 consecutive cases of acute anterior circulation ischemic stroke at a single center during a 12 month period. Transverse relaxation and static magnetic field inhomogeneity driven relaxation was mapped using multislice gradient echo planar imaging. Data are fitted on a voxel-by-voxel basis to the Yablonskiy and Haacke model to map deoxyhemoglobin concentration and cerebral blood volume. Results: 10 cases (median age 72 years (IQR 62-77), 5 women) were studied. Median NIHSS score 15.5 (IQR 9-25). 3 patients received no intervention as last known well time exceeded a treatment window. 3 patients received intravenous thrombolysis (IV), 3 patients received IV and mechanical thrombectomy (MT), and 1 patient received MT only. In the IV treated patients, median time to therapy 110. 5 minutes (IQR 80-132). In the MT treated patients, median time to therapy 257 minutes (IQR 234-331). In the untreated patients, the OEF was elevated in the ischemic territory and this appeared to persist at 24 hours. In the treated patients, those who had at least partial recanalization, demonstrated improved OEF in the ischemic territory at 24 hours. Conclusions: OEF provides a novel method for non-invasive detection of tissue with impaired oxygen delivery in acute stroke patients and is a novel tool for imaging metabolic changes in pathophysiology and disclosing therapeutic opportunities.

Targeted inductive heating of nanomagnets by a combination of alternating current (AC) and static magnetic fields

The conversion of electromagnetic energy into heat by nanomagnets has the potential to be a powerful, non-invasive technique for cancer therapy by hyperthermia and hyperthermia-based drug release, while temperature controllability and targeted heating are challenges to developing applications of such magnetic inductive hyperthermia. This study was designed to control the hyperthermia position and area using a combination of alternating current (AC) and a static magnetic field. MnZn ferrite (MZF) nanoparticles which exhibited excellent hyperthermia properties were first prepared and characterized as an inductive heating mediator. We built model static magnetic fields simply using a pair of permanent magnets and studied the static magnetic field distributions by measurements and numerical simulations. The influence of the transverse static magnetic fields on hyperthermia properties was then investigated on MZF magnetic fluid, gel phantoms and SMMC-7721 cells in vitro. The results showed a static magnetic field can inhibit the temperature rise of MZF nanoparticles in an AC magnetic field. But in the uneven static magnetic field formed by a magnet pair with repelling poles face-to-face, the heating area can be restricted in a central low static field; meanwhile the side effects of hyperthermia can be reduced by a surrounding high static field. As a result we can position the hyperthermia area, protect the non-therapeutic area, and reduce the side effects just by using a well-designed combination of AC and static field.