Axial Component Of Magnetic Field Research Articles

An Insight into Creeping Electromagnetic Waves around the Human Body

The electromagnetic wave propagation around human body torso is modelled by considering elementary electric and magnetic dipoles over an infinite muscle-equivalent cylinder. The poles in the spectral domain Green’s function with smallest imaginary part are found to correspond to creeping wave propagation coefficients which predict the general trend in propagation around human body. In addition, it was found that axial magnetic field component is crucial for communication via creeping waves since it generates modes with smaller field decay compared to axial electric field. The developed model may thus serve as a practical guideline in design of on-body wearable antennas. The theoretical considerations are verified with simulations and measurements on the prototype of PIFA antenna placed on the human body.

Annular structures formed in a beam of ions during their collective acceleration in a system with dielectric anode

We have studied the collective acceleration of protons and deuterons in an electron beam emitted from plasma formed at the surface of a dielectric anode insert. The experiments were performed with a pulsed electron accelerator operating at an accelerating voltage up to 1 MV, current amplitude up to 40 kA, and pulse duration of 50 ns. Reduction of the accelerating voltage pulse front width and optimization of the diode unit and drift region ensured the formation of several annular structures in the electron beam. As a result, up to 50% of the radioactivity induced in a copper target was concentrated in a ring with 4.5-cm diameter and 0.2-cm width. The formation of high energy density in these circular traces and the appearance of an axial component of the self-generated magnetic field of the electron beam are related with the increasing efficiency of acceleration of the most intense group of ions.

The effects of magnetic field topology on secondary neutron spectra in Magnetized Liner Inertial Fusion

The Magnetized Liner Inertial Fusion (MagLIF) concept involves the compression of a magnetized fuel such that the stagnated fuel contains a magnetic field that can suppress heat flow losses and confine α particles. Magnetic confinement of α particles reduces the fuel ρR required for ignition. Recent work [1,2] has demonstrated that the magnitude of the magnetic field in deuterium fuel can be inferred from the yields and spectra of secondary DT neutrons. In this work we investigate the potential for using the shape of the secondary neutron spectra to diagnose the magnetic field topology in the stagnated fuel. Three different field topologies that could possibly occur in MagLIF experiments are studied: (1) a cylindrical fuel column containing axial and azimuthal magnetic field components, (2) a fuel column which is pinched at the ends to form a magnetic mirror and (3) a fuel column that has a helical tube shape with magnetic field lines following the helical path of the tube’s axis. Each topology is motivated by observations from experimental or simulated MagLIF data. For each topology we use a multi-physics model to investigate the shapes of the secondary neutron spectra emitted from a steady-state stagnated fuel column. It is found that the azimuthal and helical topologies are more suitable than the mirror topology for reproducing an asymmetry in the axial spectra that was observed in experiments. Gorgon MHD simulations of the MagLIF implosion in 1D are also carried out. These show that sufficient azimuthal magnetic field can penetrate from the liner into the fuel to qualitatively reproduce the observed spectral asymmetry.

Neutral and non-neutral collisionless plasma equilibria for twisted flux tubes: The Gold-Hoyle model in a background field

We calculate exact one-dimensional collisionless plasma equilibria for a continuum of flux tube models, for which the total magnetic field is made up of the “force-free” Gold-Hoyle magnetic flux tube embedded in a uniform and anti-parallel background magnetic field. For a sufficiently weak background magnetic field, the axial component of the total magnetic field reverses at some finite radius. The presence of the background magnetic field means that the total system is not exactly force-free, but by reducing its magnitude, the departure from force-free can be made as small as desired. The distribution function for each species is a function of the three constants of motion; namely, the Hamiltonian and the canonical momenta in the axial and azimuthal directions. Poisson's equation and Ampère's law are solved exactly, and the solution allows either electrically neutral or non-neutral configurations, depending on the values of the bulk ion and electron flows. These equilibria have possible applications in various solar, space, and astrophysical contexts, as well as in the laboratory.

A Longitudinal Mode Electromagnetic Acoustic Transducer (EMAT) Based on a Permanent Magnet Chain for Pipe Inspection.

A new electromagnetic acoustic transducer (EMAT) design, employing a special structure of the permanent magnet chain, is proposed to generate and receive longitudinal guided waves for pipe inspection based on the magnetostriction mechanism. Firstly, a quantitative analysis of the excitation forces shows the influence of the radial component can be ignored. Furthermore, as the axial component of the static magnetic field is dominant, a method of solenoid testing coils connected in series is adopted to increase the signal amplitude. Then, two EMAT configurations are developed to generate and receive the L(0,2) guided wave mode. The experimental results show the circumferential notch can be identified and located successfully. Finally, a detailed investigation of the performance of the proposed EMATs is given. Compared to the conventional EMAT configuration, the proposed configurations have the advantages of small volume, light weight, easy installation and portability, which is helpful to improve inspection efficiency.

The structure of steady, relativistic, magnetized jets with rotation

We present equilibrium models of relativistic magnetized, infinite, axisymmetric jets with rotation propagating through an homogeneous, unmagnetized ambient medium at rest. The jet models are characterized by six functions defining the radial profiles of density, pressure, and the toroidal and axial components of velocity and magnetic field. Fixing the ambient pressure and the jet rest-mass density and axial components of the flow velocity and magnetic field, we analyse the influence of the toroidal magnetic field and several rotation laws on the structure of the equilibrium models. Our approach excludes by construction the analysis of the self-consistently magnetically launched jet models or the force-free equilibrium solutions. Several forbidden regions in the magnetic pitch angle/magnetization plane are found where models of the class considered in our study could not be settled. These forbidden regions are associated with the existence of maximum axial and toroidal magnetic field components compatible with the prescribed equilibrium condition at the jet surface, and/or an excess of centrifugal force producing gaps with negative pressures in the jet. The present study can be easily extended to jet models with different transversal profiles and magnetic field configurations. In the last part of the paper, we test the ability of our relativistic magnetohydrodynamics (RMHD) code to maintain steady equilibrium models of axisymmetric RMHD jets in one and two spatial dimensions. The one-dimensional numerical simulations serve also as a consistency proof of the fidelity of the analytical steady solutions discussed in the first part of the paper. The present study allows us to build initial equilibrium jet models with selected properties for dynamical (and emission) simulations of magnetized relativistic jets with rotation.

Kelvin–Helmholtz instability in solar cool surges

We study the conditions for onset of Kelvin–Helmholtz (KH) instability in a cool solar surge observed in NOAA AR 8227 on 1998 May 30. The jet with speeds in the range of 45–50kms−1, width of 7Mm, and electron number density of 3.83×1010cm−3 is assumed to be confined in a twisted magnetic flux tube embedded in a magnetic field of 7G. The temperature of the plasma flow is of the order of 105K while that of its environment is taken to be 2×106K. The electron number density of surrounding magnetized plasma has a typical value for the TR/lower corona region of 2×109cm−3. Under these conditions, the Alfvén speed inside the jet is equal to 78.3kms−1. We model the surge as a moving magnetic flux tube for two magnetic field configurations: (i) a twisted tube surrounded by plasma with homogeneous background magnetic field, and (ii) a twisted tube which environment is plasma with also twisted magnetic field. The magnetic field twist in given region is characterized by the ratio of azimuthal to the axial magnetic field components evaluated at the flux tube radius. The numerical studies of appropriate dispersion relations of MHD modes supported by the plasma flow in both magnetic field configurations show that the Kelvin–Helmholtz instability can only occur for MHD waves propagating in axial direction, but with high negative azimuthal mode numbers, and the instability occurs at sub-Alfvénic critical flow velocities in the range of 24–60kms−1.

Axial magnetic field and toroidally streaming fast ions in the dense plasma focus are natural consequences of conservation laws in the curved axisymmetric geometry of the current sheath

Direct measurement of axial magnetic field in the PF-1000 dense plasma focus (DPF), and its reported correlation with neutron emission, call for a fresh look at previous reports of existence of axial magnetic field component in the DPF from other laboratories, and associated data suggesting toroidal directionality of fast ions participating in fusion reactions, with a view to understand the underlying physics. In this context, recent work dealing with application of the hyperbolic conservation law formalism to the DPF is extended in this paper to a curvilinear coordinate system, which reflects the shape of the DPF current sheath. Locally unidirectional shock propagation in this coordinate system enables construction of a system of 7 one-dimensional hyperbolic conservation law equations with geometric source terms, taking into account all the components of magnetic field and flow velocity. Rankine-Hugoniot jump conditions for this system lead to expressions for the axial magnetic field and three components of fluid velocity having high ion kinetic energy.

Precise ages of the Réunion event and Huckleberry Ridge excursion: Episodic clustering of geomagnetic instabilities and the dynamics of flow within the outer core

The Réunion event is one of the earliest recognized periods of normal polarity within the reversed Matuyama chron. Named for the site at which it was first discovered on Réunion Island, it has since purportedly been found globally in both volcanic rocks and sediments, and thus has become a key chronostratigraphic marker. However, geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration, but also the number of Réunion events. New 40Ar/39Ar ages from eight Réunion Island lavas in three distinct sections are indistinguishable from one another, thereby placing the event at 2.200±0.007/0.010 Ma (±2σ analytical/total uncertainty, note this format is used throughout the paper). The paleomagnetic behavior recorded at two of the island sites shows that the opposite (normal) polarity was reached and sustained for a period during which several lava flows were erupted. Whether this can be classified as a very short subchron bounded by a rapid set of back-to-back reversals, or as a special case of a geomagnetic excursion, is unclear. Hence, we choose to continue labeling the dynamo activity recorded by these Réunion Island lavas as an “event”. This event preceded a ∼38 kyr period of normal polarity that we name the Feni subchron after its locality of discovery at ODP site 981. The Feni subchron was succeeded by the Huckleberry Ridge excursion for which 40Ar/39Ar sanidine and U–Pb zircon ages of 2.077±0.001/0.003 Ma and 2.084±0.012/0.013 Ma, respectively, from member B of the Huckleberry Ridge tuff in Idaho, are in agreement. These findings suggest that the full normal polarity recorded on Réunion Island is a singular brief period of unstable field behavior at the onset of a ∼125 kyr bundling of dynamo instabilities from 2.20 to 2.07 Ma. Disturbances to the axial dipole component of earth's magnetic field during this period, and by analogy similar periods of temporally-clustered excursions during the early and late portions of the Brunhes chron, may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner-core or core–mantle boundaries; a proposition that might be tested by future numerical dynamo simulations.

Dense Plasma Focus: A question in search of answers, a technology in search of applications

Diagnostic information accumulated over four decades of research suggests a directionality of toroidal motion for energetic ions responsible for fusion neutron production in the Dense Plasma Focus (DPF) and existence of an axial component of magnetic field even under conditions of azimuthal symmetry. This is at variance with the traditional view of Dense Plasma Focus as a purely irrotational compressive flow. The difficulty in understanding the experimental situation from a theoretical standpoint arises from polarity of the observed solenoidal state: three independent experiments confirm existence of a fixed polarity of the axial magnetic field or related azimuthal current. Since the equations governing plasma dynamics do not have a built-in direction, the fixed polarity must be related with initial conditions: the plasma dynamics must interact with an external physical vector in order to generate a solenoidal state of fixed polarity. Only four such external physical vectors can be identified: the earth's magnetic field, earth's angular momentum, direction of current flow and the direction of the plasma accelerator. How interaction of plasma dynamics with these fields can generate observed solenoidal state is a question still in search of answers; this paper outlines one possible answer. The importance of this question goes beyond scientific curiosity into technological uses of the energetic ions and the high-power-density plasma environment. However, commercial utilization of such technologies faces reliability concerns, which can be met only by first-principles integrated design of globally-optimized industrial-quality DPF hardware. Issues involved in the emergence of the Dense Plasma Focus as a technology platform for commercial applications in the not-too-distant future are discussed.

Development of a polarization resolved spectroscopic diagnostic for measurements of the vector magnetic field in the Caltech coaxial magnetized plasma jet experiment

In the Caltech coaxial magnetized plasma jet experiment, fundamental studies are carried out relevant to spheromak formation, astrophysical jet formation/propagation, solar coronal physics, and the general behavior of twisted magnetic flux tubes that intercept a boundary. In order to measure the spatial profile of the magnetic field vector for understanding the underlying physics governing the dynamical behavior, a non-perturbing visible emission spectroscopic method is implemented to observe the Zeeman splitting in emission spectra. We have designed and constructed a polarization-resolving optical system that can simultaneously detect the left- and right-circularly polarized emission. The system is applied to singly ionized nitrogen spectral lines. The magnetic field strength is measured with a precision of about ±13 mT. The radial profiles of the azimuthal and axial vector magnetic field components are resolved by using an inversion method.

The pinch-type instability of helical magnetic fields

To find out whether toroidal field can stably exist in galaxies the current-driven instability of toroidal magnetic fields is considered under the influence of an axial magnetic field component and under the influence of both rigid and differential rotation. The MHD equations are solved in a simplified model with cylindric geometry. We assume the axial field as uniform and the fluid as incompressible. The stability of a toroidal magnetic field is strongly influenced by uniform axial magnetic fields. If both field components are of the same order of magnitude then the instability is slightly supported and modes with m>1 dominate. If the axial field even dominates the most unstable modes have again m>1 but the field is strongly stabilized. All modes are suppressed by a fast rigid rotation where the m=1 mode maximally resists. Just this mode becomes best re-animated for \Omega > \Omega^A (\Omega^A the Alfven frequency) if the rotation has a negative shear. -- Strong indication has been found for a stabilization of the nonaxisymmetric modes for fluids with small magnetic Prandtl number if they are unstable for Pm=1. For rotating fluids the higher modes with m>1 do not play an important role in the linear theory. In the light of our results galactic fields should be marginally unstable against perturbations with m<= 1. The corresponding growth rates are of the order of the rotation period of the inner part of the galaxy.

Measurements of velocity shear and ion viscosity profile in a magnetohydrodynamic plasma jet

Time-dependent, two-dimensional profiles of the axial flow velocity, density, electron temperature, and magnetic field components are measured at two axial locations in a screw pinch plasma column of the reconnection scaling experiment. The results show that the ion momentum flux for a given column radius is dissipated by the ion-ion Coulomb scattering viscosity due to a significant radial shear of the axial velocity. By comparing the terms of the magnetohydrodynamic momentum balance equation, radial profile of ion viscosity is determined. Chord-integrated ion temperature measurements performed at several radial locations using Doppler broadening spectroscopy show ion temperature of about 1 eV. Measured ion viscosity agrees within a factor of 2 with the classical Braginskii expectations.

Influence of inhomogeneous magnetic field on the characteristics of very high frequency capacitively coupled plasmas

The effect of inhomogeneous magnetic field on the spatial structure of very high frequency (VHF) plasmas is investigated for different coil configurations, gas pressures, high frequency bias powers, and degrees of electronegativity. The simulation results show that the electron density peaks in the center of the chamber for VHF plasmas due to the standing electromagnetic wave effect. On application of a magnetic field, the density increases near the wafer edge and decreases at the chamber center. The radial magnetic field component is found to limit electron loss to the electrodes and locally enhance the electron density. The axial magnetic field component limits plasma diffusion in the radial direction helping preserve the effect of improved electron confinement by the radial magnetic field. The peak electron density decreases with increasing magnetic field as the plasma moves toward the electrode edge occupying a larger volume. The effect of magnetic field becomes weaker at higher pressure due to the increased electron-neutral collisions which reduce the effectiveness of electron confinement around the magnetic field lines. The impact of magnetic field on plasma profile is somewhat weaker in an electronegative Ar/CF4 plasma because of the presence of less mobile and unmagnetized negative ions.

Network numerical analysis of hydromagnetic squeeze film flow dynamics between two parallel rotating disks with induced magnetic field effects

We study numerically the hydromagnetic squeeze film between two rotating disks using the numerical network simulation method. The external magnetic field, H, generates an induced magnetic field, B, with radial (Br), tangential (Bθ) and axial (Bz) components between the two disks, which rotate with different angular velocities, Ω1 and Ω2, and at time t are separated by a distance D(1−αt)1/2. The applied magnetic field at the lower disk is assumed to be zero. The conservation equations for mass, momentum and induced magnetic field are reduced to a set of ordinary differential equations using a series of transformations, in terms of four dependent variables, f (axial velocity), g (azimuthal velocity), m (axial magnetic field component) and n (azimuthal magnetic field component) and a single independent variable, η (dimensionless disk separation), with appropriate boundary conditions. The transformed ordinary differential equations have collective order of 10 and are shown to be controlled by rotational Reynolds number (R1), squeeze Reynolds number (R2=Rem/Bt), dimensionless parameter based on the magnetic force in the axial direction (R3), dimensionless parameter based on magnetic force strength in the azimuthal (tangential) direction (R4), magnetic Reynolds number (Rem), disk rotational velocity ratio (S) and Batchelor number (Bt). In the present study we examine the flow regime at various Batchelor numbers (for the case of unity value of the squeeze Reynolds number, Rem=Bt). Excellent comparison of NSM solutions is achieved with earlier analytical and shooting solutions. The present study finds applications in hydromagnetic lubrication of braking devices, slider bearings, rotating machinery, etc. Applications also arise in hydraulic shock absorbers employing electrically conducting liquids such as sodium where electro-magnetical braking of streams can be achieved in liquid metal cooled nuclear reactors for arresting control rods. Finally in the context of astronautical vehicles, the present study has applications in electromagnetic braking for potential spacecraft in planetary orbits.

Pseudo–magnetorotational instability in a Taylor-Dean flow between electrically connected cylinders

We consider a Taylor-Dean-type flow of an electrically conducting liquid in an annulus between two infinitely long perfectly conducting cylinders subject to a generally helical magnetic field. The cylinders are electrically connected through a remote, perfectly conducting endcap, which allows a radial electric current to pass through the liquid. The radial current interacting with the axial component of magnetic field gives rise to the azimuthal electromagnetic force, which destabilizes the base flow by making its angular momentum decrease radially outwards. This instability, which we refer to as the pseudo-magnetorotational instability (MRI), looks like an MRI although its mechanism is basically centrifugal. In a helical magnetic field, the radial current interacting with the azimuthal component of the field gives rise to an axial electromagnetic force, which drives a longitudinal circulation. First, this circulation advects the Taylor vortices generated by the centrifugal instability, which results in a traveling wave as in the helical MRI (HMRI). However, the direction of travel of this wave is opposite to that of the true HMRI. Second, at sufficiently strong differential rotation, the longitudinal flow becomes hydrodynamically unstable itself. For electrically connected cylinders in a helical magnetic field, hydrodynamic instability is possible at any sufficiently strong differential rotation. In this case, there is no hydrodynamic stability limit defined in the terms of the critical ratio of rotation rates of inner and outer cylinders that would allow one to distinguish a hydrodynamic instability from the HMRI. These effects can critically interfere with experimental as well as numerical determination of MRI.

Discussions About Measurement Results of Levitation Force and Its Time Relaxation Between GdBCO Bulk Superconductors and Superconducting Magnet With a High Magnetic Field Gradient

High temperature bulk superconductors have significant potential for various engineering applications such as a flywheel energy storage system. This system is expected to decrease the energy loss by using bulk superconductors for the bearing. Recently, the authors have developed a new superconducting magnet to realize the large levitation force. The magnet has two coils on the same axis and each coil's current flows to the opposite direction. In this system, the axial component of magnetic field is canceled each other but the radial component of magnetic field is expected to be enhanced. In this paper, the authors measured the levitation force and its time relaxation at different bulk positions in both cases of one and two bulks by using this new magnet. From the results, it was made clear that the levitation force of two bulks was twice as large as that of one bulk and its time relaxation was decreased. In addition, Hall sensors were attached on the sample surface to measure the surface magnetic field distribution. The behavior of the magnetic field was discussed.

Comparison of simulation and experiment on levitation force between GdBCO bulk superconductor and superconducting magnet

High temperature bulk superconductors have significant potential for various engineering applications such as a flywheel energy storage system. This system is expected to decrease the energy loss by using bulk superconductors for the bearing. Recently, the authors have developed a new superconducting magnet to realize large levitation force. In this system, the axial component of magnetic field is canceled each other but the radial component of magnetic field expects to be enhanced. Thus, it was expected that the large levitation force can be realized and its time relaxation will be decreased. And in the previous paper, the levitation force and its time relaxation were measured under the various conditions by using this new magnet. But it is difficult to consider what phenomenon has happened in the bulk from only experimental results. In addition the quantitative evaluation cannot be done only by the experimental results, for example, the influence of the magnetic field penetration and magnetic distribution around a bulk superconductor on the maximum force and so on. Thus, in this paper, the authors simulated the levitation force of bulk superconductor by using ELF/MAGIC, which is a three-dimensional electromagnetic analytical software. In the simulation the bulk was considered as a rigid body and the simulation was executed under the same conditions and model with the experiment. The distribution of magnetic field and the levitation force were obtained and discussed.

An Improvement of a Thin Wire Representation for FDTD Electromagnetic and Surge Calculations

In this paper, we have shown that FDTD calculations for a conductor system having a radius smaller than 0.15Δs or larger than 0.65Δs (Δs is the lateral side length of cells employed), represented using arbitrary-radius-wire representation techniques proposed by Noda and Yokoyama, and Railton et al., with a time increment determined from the upper limit of Courant's stability condition, result in numerical instability. The reason for this numerical instability is that the speed of waves propagating outward in the radial direction from the wire in the immediate vicinity of the wire exceeds the speed of light, and therefore, Courant's condition is not satisfied there. Furthermore, we have improved the arbitrary-radius-wire representation proposed by Noda and Yokoyama. In representing a wire whose radius is smaller than the equivalent radius (r0 =0.230Δs) using the improved technique, the permeability for calculating the axial magnetic field components closest to the wire and for calculating the circulating magnetic field components closest to and half cell away from the tip of the wire is modified in addition to the permeability and the permittivity for calculating the circulating magnetic field components and the radial electric field components, closest to the wire, respectively. In representing a wire whose radius is larger than r0 using the technique, the permittivity for calculating the axial electric field components closest to the wire is modified in addition to the permittivity and the permeability for calculating the radial electric field components and the circulating magnetic field components, respectively. The improved technique is effective in representing a wire whose radius ranges from 0.0001Δs to 0.9Δs.

Coupled transverse modes of coaxial metal waveguides completely filled with magnetoactive current-carrying plasma

The propagation of ordinary bulk modes coupled with extraordinary surface modes in coaxial metal waveguides completely filled with cold magnetoactive plasma is investigated theoretically. The interaction between modes propagating across the waveguide axis in the presence of the axial and azimuthal components of the external magnetic field is examined. The effect of the azimuthal magnetic field on the dispersion properties of these modes is thoroughly studied for the case of a uniform plasma.