External Longitudinal Magnetic Field Research Articles

Driving positron beam acceleration with coherent transition radiation

Positron acceleration in plasma wakefield faces significant challenges, as the positron beam must be pre-generated and precisely coupled into the wakefield and, most critically, suffers from defocusing issues. Here we propose a scheme that utilizes laser-driven electrons to produce, inject, and accelerate positrons in a single setup. The high-charge electron beam from wakefield acceleration creates copious electron–positron pairs via the Bethe–Heitler process, followed by enormous coherent transition radiation due to the electrons’ exiting from the metallic foil. Simulation results show that the coherent transition radiation field reaches up to tens of GV m−1, which captures and accelerates the positrons to cut-off energy of 1.5 GeV with energy peak of 500 MeV (energy spread ~ 24.3%). An external longitudinal magnetic field of 30 T is also applied to guide the electrons and positrons during the acceleration process. This proposed method offers a promising way to obtain GeV fast positron sources.

Field-induced freezing in the unfrustrated Ising antiferromagnet.

We study instantaneous quenches from infinite temperature to well below T_{c} in the two-dimensional square lattice Ising antiferromagnet in the presence of a longitudinal external magnetic field. Under single-spin-flip Metropolis algorithm Monte Carlo dynamics, this protocol produces a pair of magnetization plateaus that prevent the system from reaching the equilibrium ground state except for some special values of the field. We explain the plateaus in terms of local spin configurations that are stable under the dynamics.

Anisotropic Effect on Local Magnetic Properties of 3D Extended Ising Model

The magnetic properties of anisotropic 3D Ising model on a cubic lattice are studied by Monte Carlo simulation. In particular, we have considered an extended 3D Ising model with spatially uniaxial anisotropic bond randomness on the simple cubic lattice parameterized by exchange interaction parameter [Formula: see text], anisotropy parameter [Formula: see text] and external longitudinal magnetic field [Formula: see text]. The obtained numerical data clearly point out a second-order ferromagnetic-paramagnetic phase transition belonging in the same universality class with the 3D random Ising model at critical temperature [Formula: see text] that is strongly correlated to [Formula: see text] and [Formula: see text]. Especially, in the limit, [Formula: see text], the spin ½ cubic lattice becomes a collection of noncorrelated Ising chains, whereas in the other limit, [Formula: see text], the system becomes a stack of noncorrelated Ising square lattice.

Dynamic magnetic properties of the mixed spin (1/2, 3/2) Ising system in the presence of magnetic field within the path probability method

The dynamic magnetic properties of the mixed spin (1/2, 3/2) Ising system with bilinear nearest-neighbor exchange interactions and a crystal-field term under the presence of the external longitudinal magnetic field (ELMF) are investigated by the path probability method. First we briefly examine the equilibrium magnetic properties of the system within the cluster variation method in order to clearly understand the dynamic behavior. In particular, we investigate thermal behavior of the magnetization in detail to search whether or not the metastable and unstable branches of magnetizations occur in the system. We find that the metastable and unstable branches of magnetization only exist if the system is in the presence of the ELMF. We also presented the metastable phase diagrams in the ELMF. Then, we apply the path probability method to the system and obtain the dynamic equations. Numerical solutions of these equations are given in the form of the flow diagrams (FDs). FDs explicitly illustrate the stable, metastable and unstable states as well as the role of the unstable states and initial conditions. FDs also display how one can obtain the metastable state more easily.

Non-classically Controlled Sign in a 1.6 Tesla Magnetic Circularly Polarized Luminescence of Three Pyrenes in a Chloroform and a PMMA Film

Molecular pyrene, 1-hydroxylpyrene, and 2-hydroxypyrene in chloroform and embedded in poly(methyl methacrylate) film reveal mirror-image magnetic circularly polarized luminescence (MCPL) when south (S)-up or north (N)-up direction of the longitudinal external magnetic field of 1.6 T to unpolarized incident light (Faraday geometry) is employed. The |gMCPL| magnitude is on the order of 10−3 T−1 in 374–416 nm. The sign of MCPL is controlled by the S-up and N-up geometry and by the position (1- or 2-) of the hydroxy group.

Experimental Investigation of AC Loss Characteristics under HTS Cable Electromagnetic Conditions

In a high-temperature superconducting (HTS) multi-layer AC cable, each layer of HTS tape is twisted cylindrically and exposed to the AC magnetic field produced by the AC transport current in the other layers. The magnetic field consists of a circumferential magnetic field and a longitudinal magnetic field. The magnetic field of each layer serves as an external magnetic field to the other layers. We measured the AC losses in twisted cylindrical samples comprising single layers of HTS cable transporting AC currents in external circumferential and longitudinal magnetic fields. The amplitude and direction of the combined magnetic fields were controlled via modifying the balance between the circumferential and longitudinal magnetic fields. We investigated the influence of the twist angle, and amplitude and direction of the magnetic field on the AC loss characteristics.

Water-based helical laser hole-cutting in nickel super-alloy GH4049 assisted by longitudinal and transverse magnetic fields with/without ultrasonic assistance

A novel helical laser hole-cutting (HLHC) technology based on coupled assistance of water-based ultrasonic vibrations and external longitudinal/transverse magnetic fields is reported for cutting holes in nickel super-alloy GH4049 sheets. Effects of external unidirectional transverse and longitudinal magnetic fields on HLHC performance were correspondingly analyzed and compared with and without water-based ultrasonic assistance. The influence of the loading direction of an external magnetic field and the magnetic field intensity of an external longitudinal/transverse magnetic field on HLHC performance is first reported based on systematic corresponding comparisons with and without water-based ultrasonic assistance. The effects of external longitudinal/transverse magnetic fields and beam expanding ratios on actual laser focal spot sizes were analyzed and discussed. It was demonstrated that the latest reported HLHC technology assisted by water-based ultrasonic vibrations and external longitudinal magnetic fields could generate deeper holes with relatively high cutting efficiency, smaller hole taper, higher hole aspect ratio, smaller hole circularity deviation, better hole wall quality, much better performance for relieving residual stress, thinner recast layer, more uniform refinement of grains, and better performance for improving micro hardness. Generally, a stronger external longitudinal magnetic field coupled by water-based ultrasonic vibrations was more helpful for cutting deeper and higher-quality holes. It was shown that the change of loading direction of an external magnetic field with/without ultrasonic assistance did not greatly alter recast layer formation and micro hardness performance for the workpiece helically cut. Compared to the transverse magnetic assistance, a corresponding longitudinal magnetic field coupled with water-based ultrasonic vibrations was more effective for micro hardness improvement for this reported HLHC technology. When using the same water-based ultrasonic vibration assistance, a stronger longitudinal magnetic field was more effective for relieving residual stress for the workpiece helically cut (a stress reduction percentage of around 96.5% was reported in this work). The average laser focal spot size obviously decreased with laser beam expanding ratio, but the external longitudinal/transverse magnetic assistance did not greatly alter the actual laser focal spot size.

Магнитно-резонансная силовая спектроскопия колебаний магнитного вихря

We present the results of micromagnetic modeling and experimental magnetic resonance force spectroscopy studies of forced magnetization oscillations in a circle NiFe disk in an external longitudinal magnetic field. The main attention is paid to the low-frequency resonance associated with the gyrotropic motion of the magnetic vortex core. It is shown that the applying external magnetic field in the plane of the sample leads to a significant shift in the resonance frequency of the gyrotropic mode. The influence of the inhomogeneous magnetic field of the probe on the peculiarities of the magnetization oscillations is discussed.

Magnetic field effect on the free induction decay of hydroxyl radicals (OH) in the terahertz region.

The effect of an external longitudinal magnetic field on the optical free induction decay from a free radical is observed for the first time. The experiments were performed on a rotational transition of the hydroxyl radical, OH (2Π3/2(J = 1) ← 2Π3/2(J = 0) at 83.8 cm-1), using a terahertz free electron laser. In contrast to the results of the experiments with a stable paramagnetic molecule, NO, the observed effect of an external magnetic field on the free induction decay from hydroxyl radicals is more complicated. A longitudinal magnetic field leads to the rotation of the polarization plane of the FID radiation and to an additional modulation of the signal intensity. The angle of the rotation of the plane of polarization is large, in agreement with the theoretical predictions. The observed FID kinetics in the time domain are in semi-quantitative agreement with the modeling. This observation opens an opportunity for the selective detection of weak signals of short-lived reactive paramagnetic free radicals from overwhelming signals that originate from stable non-paramagnetic species by polarization discrimination.

Spin inertia and polarization recovery in quantum dots: Role of pumping strength and resonant spin amplification

Spin inertia measurements are a novel experimental tool to study long-time spin relaxation processes in semiconductor nanostructures. We develop a theory of the spin inertia effect for resident electrons and holes localized in quantum dots. We consider the spin orientation by short optical pulses with arbitrary pulse area and detuning from the trion resonance. The interaction with an external longitudinal magnetic field and the hyperfine interaction with the nuclear spin bath is considered in both the ground and excited (trion) states of the quantum dots. We analyze how the spin inertia signal depends on the magnetic field (polarization recovery) and on the modulation frequency of the helicity of the pump pulses as well as on their power and detuning. In particular, we elaborate how approaching the saturation limit of the spin polarization influences the measurements. The quantitative description of spin inertia measurements will enable the determination of the parameters of spin dynamics such as the spin relaxation times in the ground and excited states and the parameters of the hyperfine interaction. Finally, we predict the emergence of resonant spin amplification due to the transverse components of the nuclear spin fluctuations, which manifests itself as oscillations of the spin polarization as a function of the longitudinal magnetic field.

Formation and time development of spark channel in strong magnetic field

The characteristics of the spark channel plasma formed in the external longitudinal magnetic field are studied experimentally. In experiment, the current-voltage, optical, and spectral characteristics of the discharge were measured and analyzed. Synchronization of the pulsed electric and magnetic fields was performed. The effect of the magnetic field on the radial expansion of the spark channel, the shock wave formation, as well as the changes in the balance of energy and temperature in the ionized plasma of the high-pressure gas discharge were analyzed. It was found out that, in the strong longitudinal magnetic field, the temperatures of electrons and ions in the spark channel (ne ∼ 1018 cm−3) become equal. The current density, the plasma channel conductivity of the spark channel, and the specific energy deposition into the discharge increased. The channel plasma parameters were analyzed as functions of the external magnetic field.

An integrated three-dimensional micro-solenoid giant magnetoimpedance sensing system based on MEMS technology

In this paper, we proposed an integrated three-dimensional (3D) micro-solenoid giant magnetoimpedance (GMI) sensing system. The sensing system is composed by a 3D micro-solenoid inside which patterned Cobalt-based ribbon is kept as magnetic core and this system was fabricated by Micro-electromechanical Systems (MEMS) technology. The system was measured by non-contact method, which has the advantages of high sensitivity, low power consumption and widespread application prospect. The influence of external magnetic field direction and excitation frequency on micro-solenoid GMI sensing system has been discussed respectively. The fabricated system exhibits an optimum magnetoimpedance (MI) ratio of 4360±40 % at the excitation frequency of 13 MHz under longitudinal external magnetic field. In consequence, 3D micro-solenoid GMI sensing system offers great magnetic performance and this integrated sensing system has potential applications in the field of weak magnetic field detection.

Enhancement of terahertz field in the relativistic coupling of high power laser with magnetized plasma

In this article, the relativistic nonlinear self-focusing of a right circular polarized laser beam propagated through magnetized plasma was investigated. The high power of laser beam which resulted from the self-focusing effect was exploited to excite an electromagnetic wave to the terahertz frequency range. By controlling the external longitudinal magnetic field, a high stabilized terahertz field was observed. At terahertz frequency (1THz) where the initial laser beam radius (x0=20μm) and initial laser beam intensity (I0=1017W/cm2), the highest terahertz radiation power attained was about tens of gigawatts that associated with best stabilization seen.

Magnetic behaviors in a ternary metallic nanoisland with bilayer hexagonal core-shell structure

The Monte Carlo simulation has been applied to study the magnetic behaviors of a ferrimagnetic mixed-spin (1, 2, 5/2) Ising nanoisland with bilayer hexagonal core-shell structure in an external longitudinal magnetic field. We have investigated the influences of the anisotropies, the exchange couplings, the longitudinal field and temperature on magnetization, susceptibility, internal energy and hysteresis loop of the present nanoisland. The behavior of the X parameter (the magnetization, susceptibility, internal energy and blocking temperature) as a function of the parameter is studied. Particularly, the multiple hysteresis loop behaviors have been discovered, such as triple, quintuple, septuple, ninefold and elevenfold hysteresis loops under certain physical parameters.

Chiral mixed phase in disordered 3d Heisenberg models under magnetic fields

Using Monte Carlo simulations, we studied the effect of uniform external longitudinal magnetic field on the 3d isotropic Heisenberg disordered model. Simulations are performed for a variety of fields. Our results confirm that the mixed chiral phase must belong to the same class of universality with those of zero fields and with same group of critical exponent.

Improvement of ion acceleration in radiation pressure acceleration regime by using an external strong magnetic field

AbstractTwo-dimensional particle-in-cell (PIC) simulations have been used to investigate the interaction between a laser pulse and a foil exposed to an external strong longitudinal magnetic field. Compared with that in the absence of the external magnetic field, the divergence of proton with the magnetic field in radiation pressure acceleration (RPA) regimes has improved remarkably due to the restriction of the electron transverse expansion. During the RPA process, the foil develops into a typical bubble-like shape resulting from the combined action of transversal ponderomotive force and instabilities. However, the foil prefers to be in a cone-like shape by using the magnetic field. The dependence of proton divergence on the strength of magnetic field has been studied, and an optimal magnetic field of nearly 60 kT is achieved in these simulations.

Radiation from Hot Plasma of High-Pressure Discharge in Magnetic Field

We have considered the effect of the bulk energy density on the formation of a high-current discharge plasma in Ar (P = 2280 Torr) in the presence of preionization and the application of longitudinal magnetic field. The formation of the initial stages of a high-current discharge changes with increasing bulk density of energy, switching from a continuous form to the brightness pulsation. We have detected that the external critical magnetic field accelerates the process of formation of all stages of the discharge with a simultaneous increase in the maximum of the bulk density of energy, concentration, and temperature. It should be specially noted that the appearance of new spectral lines at the stage of the spark channel formation in an external longitudinal magnetic field.

Electron spin dynamics in Ce3+ : YAG crystals: Hyperfine interaction of 4f and 5d electrons

The electron spin dynamics of $\mathrm{C}{\mathrm{e}}^{3+}$ ions in YAG crystals is studied by time-resolved photoluminescence spectroscopy with an alternating left and right circularly polarized continuous-wave laser modulation technique. The electron spin relaxation due to hyperfine interaction with randomly oriented nuclear spins can be efficiently suppressed by a weak external longitudinal magnetic field. The suppression of the hyperfine-induced spin relaxation makes the electron spin polarization increase from 0.001 to 0.016 for the lowest $4f$ state, and from 0.08 to 0.32 for the lowest $5d$ state. The suppression magnetic fields for the $4f$ electrons are several times weaker than that for the $5d$ electrons, dependent on the crystal orientation. The dispersion of the local hyperfine field distribution $\mathrm{\ensuremath{\Delta}}B$ is isotropic and equal to 4.0 mT for the $5d$ electrons, and anisotropic for the $4f$ electrons with $\mathrm{\ensuremath{\Delta}}{B}_{[001]}=0.8\phantom{\rule{0.16em}{0ex}}\mathrm{mT},\mathrm{\ensuremath{\Delta}}{B}_{[110]}=1.1\phantom{\rule{0.16em}{0ex}}\mathrm{mT},\mathrm{\ensuremath{\Delta}}{B}_{[1\overline{1}0]}=2.4\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$. The hyperfine coupling strength for the $4f$ electrons is \ensuremath{\sim}4 times weaker than that for the $5d$ electrons.

Simulation and analysis of heat transfer and fluid flow characteristics of arc plasma in longitudinal magnetic field-tungsten inert gas hybrid welding

In the present work, an axisymmetrical model based on the magnetohydrodynamics (MHD) is established to investigate the effect of external longitudinal magnetic field (LMF) on arc characteristics during the gas tungsten arc welding (GTAW) process. The profiles of temperature and voltage drop, distributions of axial velocity, shear stress, and arc pressure, etc., in the cases of different applied LMF strengths ranging from 0 to 0.06 T are simulated by utilizing the fluid dynamic theory coupled with Maxwell equations. In order to achieve more accurate values of heat transfer and fluid flow of arc plasma, we take the boundary layer of electrodes into consideration. The results show that the applied LMF could drive particles to rotate and expand the arc, and a negative pressure area appears at the center and induces an upward streaming of gas (i.e., anti-gravity flow) through the arc core with the effects of centrifugal force, concentrating the anodic energy to the cathode. When the magnetic induction strength is 0.06 T, vortexes are dramatically formed around the arc axis by the interaction between the anti-gravity flow from the arc center and outside downward flow from the arc fringes. Thus, the distribution of current density, anodic heat flux, and arc pressure shifts from the arc center to the periphery and forms a bimodal pattern. The various thermal fluxes and subsequent thermal efficiency are also quantitatively investigated for a better understanding of the effects of LMF on arc behaviors and the theoretical predictions show good agreement with the experimental results.

Vibration of initially stressed carbon nanotubes under magneto-thermal environment for nanoparticle delivery via higher-order nonlocal strain gradient theory

In this paper, the forced vibration of a single-walled carbon nanotube (SWCNT) under a moving nanoparticle is investigated based on the higher-order nonlocal strain gradient theory. The SWCNT is subjected to thermo-mechanical stresses and an external longitudinal magnetic field. The influences of higher-order stress gradients in conjunction with the strain gradient nonlocality are taken into account. Using Hamilton’s principle and Maxwell’s equations, the higher-order differential equations of motion are derived. An analytical solution is obtained for the dynamic deflection of SWCNTs using the Galerkin method. Furthermore, the governing differential equation is solved numerically using the precise integration method. The results of the two solution procedures are compared and an excellent agreement is found between them. Finally, the influences of various scale parameters, the velocity of the moving nanoparticle, the initial axial stress, the temperature change and longitudinal magnetic field on the dynamic response of SWCNTs are investigated.