Axial Magnetic Field Strength Research Articles

Globally Optimal Superconducting Homogeneous Magnet Design for an Asymmetric 3.0 T Head MRI Scanner

This paper describes a hybrid numerical method with a combination of linear programming (LP) and nonlinear programming to design an asymmetric magnetic resonance imaging (MRI) system for head imaging. The method was successfully employed to a 0.7 T open-style MRI. Recently, we have updated the LP mathematical model in the hybrid optimization strategy by adding maximum axial and radial magnetic field strength limitation to make the magnet safe. The whole magnet consists of eight coaxial coils asymmetry about the z-axis. The six innermost coils are the main coils with positive current direction and the two outermost coils are shielding coils with negative current direction. All coils contribute their efforts to produce a central magnetic field strength of 3.0 T over a 20 cm diameter of spherical volume, and the peak to peak homogeneity is 12 ppm. The temperature bore for the magnet is formed with two different diameters cylindrical bores which makes head and shoulder access to the imaging volume easier. A 5 Gauss footprint area is restricted larger than an elliptical region with the sizes of 3.5 m in radial and 4.0 m in axial direction. The results show that the methodology is very flexible and efficient for asymmetric MRI magnet design.

Double layer-like structures in the core of an argon helicon plasma source with uniform magnetic fields

A hot (Te ≈ 10 eV) electron population is observed in the core of a 3 mTorr argon helicon plasma source at 500 W RF power and 900 G uniform axial magnetic field strength, 12 cm from the edge of the helicon antenna. A double layer-like structure consisting of a localized axial electric field of approximately 8 V/cm over 1–2 cm is observed adjacent to the hot electron population. The potential step generated by the electric field is shown to be large enough to trap the hot electrons. To our knowledge this is the first observation of these structures in the core of a helicon discharge.

Permanent magnet electron beam ion source/trap systems with bakeable magnets for improved operation conditions

The magnetic system of a Dresden electron beam ion source (EBIS) generating the necessary magnetic field with a new type of permanent magnet made of high energy density NdFeB-type material operable at temperatures above 100 °C has been investigated and tested. The employment of such kind of magnets provides simplified operation without the time-consuming installation and de-installation procedures of the magnets for the necessary baking of the ion source after commissioning and maintenance work. Furthermore, with the use of a new magnetization technique the geometrical filling factor of the magnetic Dresden EBIS design could be increased to a filling factor of 100% leading to an axial magnetic field strength of approximately 0.5 T exceeding the old design by 20%. Simulations using the finite element method software Field Precision and their results compared with measurements are presented as well. It could be shown that several baking cycles at temperatures higher than 100 °C did not change the magnetic properties of the setup.

A homogeneous superconducting magnet design using a hybrid optimization algorithm

This paper employs a hybrid optimization algorithm with a combination of linear programming (LP) and nonlinear programming (NLP) to design the highly homogeneous superconducting magnets for magnetic resonance imaging (MRI). The whole work is divided into two stages. The first LP stage provides a global optimal current map with several non-zero current clusters, and the mathematical model for the LP was updated by taking into account the maximum axial and radial magnetic field strength limitations. In the second NLP stage, the non-zero current clusters were discretized into practical solenoids. The superconducting conductor consumption was set as the objective function both in the LP and NLP stages to minimize the construction cost. In addition, the peak–peak homogeneity over the volume of imaging (VOI), the scope of 5 Gauss fringe field, and maximum magnetic field strength within superconducting coils were set as constraints. The detailed design process for a dedicated 3.0 T animal MRI scanner was presented. The homogeneous magnet produces a magnetic field quality of 6.0 ppm peak–peak homogeneity over a 16 cm by 18 cm elliptical VOI, and the 5 Gauss fringe field was limited within a 1.5 m by 2.0 m elliptical region.

High-quality multi-GeV electron bunches via cyclotron autoresonance

Autoresonance laser acceleration of electrons is theoretically investigated using circularly polarized focused Gaussian pulses. Many-particle simulations demonstrate feasibility of creating over 10-GeV electron bunches of ultra-high quality (relative energy spread of order 10^-4), suitable for fundamental high-energy particle physics research. The laser peak intensities and axial magnetic field strengths required are up to about 10^18 W/cm^2 (peak power ~10 PW) and 60 T, respectively. Gains exceeding 100 GeV are shown to be possible when weakly focused pulses from a 200-PW laser facility are used.

Axial Magnetic Field Strength Needed for a 126-kV Single-Break Vacuum Circuit Breaker During Asymmetrical Current Switching

The objective of this paper is to investigate how strong an axial magnetic field (AMF) is needed in test duty T100a for a 126-kV single-break vacuum circuit breaker (VCB) when interrupting a rated short circuit current of 40-kA rms in light of arcing contact gap, arc duration and phase shift effect. The AMF is generated by a pair of 2/3 coil-type AMF contacts. First, the accuracy of a 3-D finite element simulation of AMF is validated by an AMF measurement at a contact gap 30 mm and the error is <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${&lt;}{10\%}$</tex></formula> . After that test duty T100a of the 126-kV single-break VCB are carried out 18 times with arc duration from 3.3 to 19 ms at 40-kA rms. With a given opening displacement curve, the contact gap as arc extinguishes within a range from 12 to 50 mm corresponding to the arc duration from 3.3 to 19 ms. Then the AMF distribution of the 126-kV single-break VCB at each contact gap at which arc extinguished is simulated to analyze the current interrupting results. The results show that there are successful interruptions at the short-circuit current 40-kA rms in test duties T100a, with the shortest arcing contact gap window from 18 to 23 mm and the longest arcing contact gap of 44 mm. The shortest arcing contact gap window corresponds to the maximum AMF at the intermediate plane of the contact gap per kilo-ampere of the breaking peak current from 9.3 to 8.5 mT/kA. In addition, the longest arcing contact gap corresponds to the value of 6.1 mT/kA. The phase shift time at the center of intermediate plane of the contact gap ranged from 1.36 to 0.75 ms corresponding to an increase of arcing contact gap from 12 to 50 mm in the tests.

Axial Magnetic Field Strength Needed for a 126-kV Single-Break Vacuum Circuit Breaker During Asymmetrical Current Switching

The objective of this paper is to investigate how strong an axial magnetic field (AMF) is needed in test duty T100a for a 126-kV single-break vacuum circuit breaker (VCB) when interrupting a rated short circuit current of 40-kA rms in light of arcing contact gap, arc duration and phase shift effect. The AMF is generated by a pair of 2/3 coil-type AMF contacts. First, the accuracy of a 3-D finite element simulation of AMF is validated by an AMF measurement at a contact gap 30 mm and the error is ${ . After that test duty T100a of the 126-kV single-break VCB are carried out 18 times with arc duration from 3.3 to 19 ms at 40-kA rms. With a given opening displacement curve, the contact gap as arc extinguishes within a range from 12 to 50 mm corresponding to the arc duration from 3.3 to 19 ms. Then the AMF distribution of the 126-kV single-break VCB at each contact gap at which arc extinguished is simulated to analyze the current interrupting results. The results show that there are successful interruptions at the short-circuit current 40-kA rms in test duties T100a, with the shortest arcing contact gap window from 18 to 23 mm and the longest arcing contact gap of 44 mm. The shortest arcing contact gap window corresponds to the maximum AMF at the intermediate plane of the contact gap per kilo-ampere of the breaking peak current from 9.3 to 8.5 mT/kA. In addition, the longest arcing contact gap corresponds to the value of 6.1 mT/kA. The phase shift time at the center of intermediate plane of the contact gap ranged from 1.36 to 0.75 ms corresponding to an increase of arcing contact gap from 12 to 50 mm in the tests.

HIGH-RESOLUTION OBSERVATIONS AND THE PHYSICS OF HIGH-VELOCITY CLOUD A0

The neutral hydrogen structure of high-velocity cloud A0 (at about -180 km/s) has been mapped with a 9.1 arcmin resolution. Gaussian decomposition of the profiles is used to separately map families of components defined by similarities in center velocities and line widths. About 70% of the HI gas is in the form of a narrow, twisted filament whose typical line widths are of order 24 km/s. Many bright features with narrow line widths of order 6 km/s, clouds, are located in and near the filament. A third category with properties between those of the filament and clouds appears in the data. The clouds are not always co-located with the broader line width filament emission as seen projected on the sky. Under the assumption that magnetic fields underlie the presence of the filament, a theorem is developed for its stability in terms of a toroidal magnetic field generated by the flow of gas along field lines. It is suggested that the axial magnetic field strength may be derived from the excess line width of the HI emission over and above that due to kinetic temperature by invoking the role of Alfven waves that create what is in essence a form of magnetic turbulence. At a distance of 200 pc the axial and the derived toroidal magnetic field strengths in the filament are then about 6 microGauss while for the clouds they are about 4 microGauss. The dependence of the derived field strength on distance is discussed.

Effect of Axial Magnetic Field Strengths on Radial Velocity Field in Liquid Bridge under Microgravity

This article studies on the effect of magnetic field strengths on the flow field in a liquid bridge under zero gravity. The mass conservation level set method is used to track the two-phase interface. The results show that inhibitory effect of additional axial magnetic field on thermocapillary convection within liquid bridge is obvious, and this kind of inhibitory effect increasing as the magnetic field strength is strengthened.

Sheared-flow induced confinement transition in a linear magnetized plasma

A magnetized plasma cylinder (12 cm in diameter) is induced by an annular shape obstacle at the Large Plasma Device [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)]. Sheared azimuthal flow is driven at the edge of the plasma cylinder through edge biasing. Strong fluctuations of density and potential ({delta}n/n{approx}e{delta}{phi}/kT{sub e}{approx}0.5) are observed at the plasma edge, accompanied by a large density gradient (L{sub n}={nabla}lnn{sup -1}{approx}2cm) and shearing rate ({gamma}{approx}300kHz). Edge turbulence and cross-field transport are modified by changing the bias voltage (V{sub bias}) on the obstacle and the axial magnetic field (B{sub z}) strength. In cases with low V{sub bias} and large B{sub z}, improved plasma confinement is observed, along with steeper edge density gradients. The radially sheared flow induced by ExB drift dramatically changes the cross-phase between density and potential fluctuations, which causes the wave-induced particle flux to reverse its direction across the shear layer. In cases with higher bias voltage or smaller B{sub z}, large radial transport and rapid depletion of the central plasma density are observed. Two-dimensional cross-correlation measurement shows that a mode with azimuthal mode number m=1 and large radial correlation length dominates themore » outward transport in these cases. Linear analysis based on a two-fluid Braginskii model suggests that the fluctuations are driven by both density gradient (drift wave like) and flow shear (Kelvin-Helmholtz like) at the plasma edge.« less

Numerical Study on Magnetic Field Characteristics of Typical Permanent Magnet

Selecting a single piece of typical rectangular open planar magnet，with its axial magnetization, to build a finite element model. Based on ANSYS software, the article used NdFeB45 material properties, giving 100×100×10mm to fit the relationship of three-dimensional size. In the air zone as 10 times as model size, it adopted a static simulation analysis of the effect on relationship between axial magnetic distance and magnetic field strength. The magnetic field contours as simulation results, after the Origin software fitting process, obtained an equation is: ; Comparing with the formula: , which reflected the basic characteristics of permanent magnetic field, and with the numerical results by Jing-tian LI using equivalent magnetic charge method, they all appeared: the magnetic field strength exponentially decrease along with the magnetic distance increases, the magnetic field non-uniformity coefficient is of 4.628% deviation, the fitting correlation coefficient R is 0.99924; As a result of the numerical verification results, it also visually response to the magnetic field parameters of open permanent magnet.

Whistler Wave Resonances in Laboratory Plasma

Standing whistler wave patterns have been investigated in the Naval Research Laboratory's Space Physics Simulation Chamber. In the original experimental configuration, partial reflection of the antenna-launched whistler waves from the chamber end boundaries occurs, setting up a combination of standing and traveling waves. By controlling the axial magnetic field strength profile, cyclotron absorption of the whistler waves can be induced before reflection occurs, leaving only the forward propagating waves. By comparing standing-wave amplitudes to that when the wave is prevented from reflecting, cavity Q's in excess of 30 have been observed. Under uniform axial magnetic field conditions, the addition of planar conducting grids across the vacuum chamber cross section at the ends of the plasma column provides improved reflecting surfaces and corresponding increases in the value of Q.

Influence of a homogeneous axial magnetic field on Taylor–Couette flow of ferrofluids with low particle–particle interaction

Experimental results concerning the stability of Couette flow of ferrofluids under magnetic field influence are presented. The fluid cell of the Taylor–Couette system is subject to a homogeneous axial magnetic field and the axial flow profiles are measured by ultrasound Doppler velocimetry. It has been found that an axial magnetic field stabilizes the Couette flow. This effect decreases with a rotating outer cylinder. Moreover, it could be observed that lower axial wave numbers are more stable at a higher axial magnetic field strength. Since the used ferrofluid shows a negligible particle–particle interaction, the observed effects are considered to be solely based on the hindrance of free particle rotation.

Experimental and theoretical investigations on Taylor–Couette flow of ferrofluids subject to magnetic fields

The effect of magnetic fields on flows of ferrofluids enters the basic hydrodynamic equations by an expression for the relaxation of magnetization of the fluid. Despite intense investigations of ferrohydrodynamics, the question of the appropriate approach to describe the magnetization outside equilibrium is still open.By a comparison of experimental and theoretical data of a Taylor–Couette system as a model system, this question is highlighted in this paper. The fluid cell of the Taylor–Couette system is subject to a homogeneous axial magnetic field and the flow profiles are measured by ultrasound-Doppler velocimetry. In this paper, experimental results concerning the transition from circular Couette flow to Taylor vortex flow at different axial magnetic field strengths are measured, compared to theory and discussed.

Experimental Investigation on the Influence of Axial Magnetic Field Distribution on Resisting the Constriction of a High-Current Vacuum Arc

Effect of the axial magnetic field (AMF) on resisting the constriction of a high-current vacuum arc is studied in this paper. Two typical AMF distributions were investigated, i.e., the traditional bell-shaped AMF, and the saddle-shaped AMF. Experiments were conducted in a detachable vacuum chamber with a rms arc current in the range of 10 kA to 25 kA. The arc column was photographed by a high-speed digital camera with an exposure time of 2 microseconds. The constriction of the vacuum arc was compared by processing the images of the arc column under the two different field configurations and numerically determining the dimensions of the arc column near the electrodes. It was also confirmed that the AMF distribution had a significant influence on its effectiveness in resisting arc constriction. Furthermore, the AMF strength near the periphery of the arc is more influential than that at the centre of the electrodes in resisting arc constriction.

Free-electron lasers with magnetized ion-wiggler

Significant progress has been made using laser ionized channels to guide electron beams in the ion focus regime in a free-electron laser. Propagation of an electron beam in the ion focusing regime (IFR) allows the beam to propagate without expanding from space-charge repulsion. The ninth-degree polynomial dispersion relation for electromagnetic and space-charge waves is derived analytically by solving the electron momentum transfer and wave equations. The variation of resonant frequencies and peak growth rates with axial magnetic field strength has been demonstrated. Substantial enhancement in peak growth rate is obtained as the axial field frequency approaches the gyroresonance frequency.

Study on High-Current Vacuum Arc Characteristics Under Self-Generated Axial Magnetic Field of Contact at a Long Contact Gap for High-Voltage Vacuum Interrupters

In this paper, the high-current vacuum arc characteristics of the single coil-type AMF contacts, under axial magnetic field (AMF) generated by a sinusoidal current following, were investigated at a long contact gap of 40 and 60 mm. The distributing characteristics of AMF in contact gap were studied by a finite element method, taking eddy current effect into account. Vacuum arcs appearance was observed by a high-speed charge-coupled device video camera. Vacuum arc voltage was measured using a high-voltage probe. Dependence of arc voltage and vacuum arc appearance upon the self-generated AMF characteristics was investigated. The research results show that anode melting phenomena without arc voltage noises could occur both in an unstable arc and in a diffuse arc in a wide range of arc current and AMF strength at a long contact gap. Also, both many small-scale transient spots and several small luminous melting regions on anode could be observed with a diffuse arc column and a quiescent arc voltage.

The effect of magnetic field strength on the time evolution of high energy bremsstrahlung radiation created by an electron cyclotron resonance ion source

An electron cyclotron resonance (ECR) ion source is one of the most used ion source types for high charge state heavy ion production. In ECR plasma the electrons are heated by radio frequency microwaves in order to provide ionization of neutral gases. As a consequence, ECR heating also generates very high electron energies (up to MeV region) which can produce a vast amount of bremsstrahlung radiation causing problems with radiation shielding and heating superconducting cryostat of an ECR ion source. To gain information about the time evolution of the electron energies in ECR plasma radial bremsstrahlung measurements were performed. JYFL 14 GHz ECR ion source was operated in pulsed mode and time evolution measurements were done with different axial magnetic field strengths with oxygen and argon plasmas. Bremsstrahlung data were analyzed with a time interval of 2 ms yielding information at unprecedented detail about the time evolution of high energy bremsstrahlung radiation from an ECR ion source. It was observed, for example, that reaching the steady state phase of the plasma bremsstrahlung requires several hundred milliseconds and the steady state time can be different with different gases.

The cold and hot electron populations, temperatures and their transports in the edge plasma of the ORNL CAPRICE ECR ion source

The edge plasma of the ORNL CAPRICE ECR ion source is studied by directly measuring, with electrical probes, its local plasma parameters such as plasma density, temperature and electron energy distribution characteristics at different rf power levels, at various pressures, at various axial magnetic field strengths and different distances from the resonant zone. It is found that the edge plasma can be approximated to be bi-Maxwellian, whose characteristics become more pronounced at the more distant positions from the ECR zone, at lower source pressures and at increased magnetic field strengths. These trends are consistently explained in terms of long-range electron–electron and electron–ion Coulomb scattering collisions that occur during the transport of hot electrons from the ECR plasma to the probe and their frequency dependence on electron temperature and density.

Time-Dependent 2D Modeling of Magnetron Plasma Torch in Turbulent Flow

A theoretical model is presented to describe the electromagnetic, heat transfer and fluid flow phenomena within a magnetron plasma torch and in the resultant plume, by using a commercial computational fluid dynamics (CFD) code FLUENT. Specific calculations are presented for a pure argon system (i.e., an argon plasma discharging into an argon environment), operated in a turbulent mode. An important finding of this work is that the external axial magnetic field (AMF) may have a significant effect on the behavior of arc plasma and thus affects the resulting plume. The AMF impels the plasma to retract axially and expand radially. As a result, the plasma intensity distribution on the cross section of torch seems to be more uniform. Numerical results also show that with AMF, the highest plasma temperature decreases and the anode arc root moves upstream significantly, while the current density distribution at the anode is more concentrated with a higher peak value. In addition, the use of AMF then induces a strong backflow at the torch spout and its magnitude increases with the AMF strength but decreases with the inlet gas velocity.