HYDROMAGNETIC INSTABILITY OF LIQUID METALS IN AC MAGNETIC FIELDS AND AUGMENTATION OF HEAT TRANSFER

ABSTRACT

Liquid metals stressed by high frequency magnetic fields, as they are in levitation systems, generally display interfaces that undulate in an erratic manner. A model has been developed that pictures these unsteady motions as the result of an instability in which elements of the fluid behave as the rotors of single phase induction machines in the inductance dominated regime. This lumped parameter model not only gives physical insight to the electromechanical origins of the instability observed as the magnetic field is raised, but also includes thermal effects to account for the observed rapid growth rate. According to both the lumped parameter model and a continuum model developed elsewhere, incipience of instability is determined by the parameter M = |B_O|² / μηω, where |B_O| is the peak magnetic flux density at the interface, μ = 4pi;& × 10^?7, η is the viscosity and w is the angular frequency of the magnetic field. The model supports the premise that the undulations derive their energy from heating but make and have an appearance determined in large part by the electromechanics.     

HIGH SPEED INDUCTION MOTOR DRIVE WITH ACTIVE MAGNETIC BEARINGS AND SLIDING MODE DECENTRALIZED CONTROL

ABSTRACT

Active magnetic bearings are credited with high performance in terms of low energy consumption, high speeds, increased reliability and low maintenance costs.

This paper discusses a high-speed induction motor with active magnetic bearings: both radial and axial. The decentralized control system of active magnetic bearings is based on sliding mode variable structure principle to provide robustness to parameter detuning and perturbations.

Test results on a laboratory model of 300 W and 25000 rpm motor designed and built for the scope are very promising for applications to high speed machine tools, vacuum pumps etc.     

Development of anisotropic phantoms using wood and fiber materials for diffusion tensor imaging and diffusion kurtosis imaging

Objective

Several studies have demonstrated that anisotropic phantoms can be utilized for diffusion magnetic resonance imaging. The purpose of our study was to examine whether wood is suitable as an anisotropic phantom material from the viewpoints of affordability and availability. In the current study, wood was used for restricted diffusion, and fibers were used for hindered diffusion.

Materials and methods

Wood and fiber phantoms were made. Diffusion kurtosis images were acquired with three magnetic resonance scanners. Fractional anisotropy, radial diffusivity, axial diffusivity, radial kurtosis and axial kurtosis values were measured. The wood phantom was imaged, and its durability was confirmed. The phantoms were imaged in varying orientations within the magnetic field. The wood was observed using an optical microscope.

Results

Ten kinds of wood and the fiber had a diffusion metrics. The wood diffusion metrics suggested low variation over a period of 9 months. Changing the orientation of the phantoms within the magnetic field resulted in changes in diffusion metrics. Observation of wood vessels and fibers was conducted.

Discussion

Wood and fibers have anisotropy and are promising as phantom materials. The development of anisotropic phantoms that anyone can use is useful for diffusion magnetic resonance imaging research and clinical applications.

     

Relaxation measurements of an MRI system phantom at low magnetic field strengths

Objective

Temperature controlled T₁ and T₂ relaxation times are measured on NiCl₂ and MnCl₂ solutions from the ISMRM/NIST system phantom at low magnetic field strengths of 6.5 mT, 64 mT and 550 mT.

Materials and methods

The T₁ and T₂ were measured of five samples with increasing concentrations of NiCl₂ and five samples with increasing concentrations of MnCl₂. All samples were scanned at 6.5 mT, 64 mT and 550 mT, at sample temperatures ranging from 10 °C to 37 °C.

Results

The NiCl₂ solutions showed little change in T₁ and T₂ with magnetic field strength, and both relaxation times decreased with increasing temperature. The MnCl₂ solutions showed an increase in T₁ and a decrease in T₂ with increasing magnetic field strength, and both T₁ and T₂ increased with increasing temperature.

Discussion

The low field relaxation rates of the NiCl₂ and MnCl₂ arrays in the ISMRM/NIST system phantom are investigated and compared to results from clinical field strengths of 1.5 T and 3.0 T. The measurements can be used as a benchmark for MRI system functionality and stability, especially when MRI systems are taken out of the radiology suite or laboratory and into less traditional environments.

     

Magnetic Circuit Analysis of a Tubular Permanent Magnet Linear Alternator

ABSTRACT

A simplified magnetic circuit analysis of a tubular permanent magnet linear alternator (PMLA) is presented. The analysis accounts for saturation, leakage and armature reaction. Under these conditions the fields in every portion of the linear alternator (LA) are determined. The field determination leads to the alternator induced voltage and inductances.     

Commissioning of the Iseult CEA 11.7 T whole-body MRI: current status,gradient–magnet interaction tests and first imaging experience

Objectives

The Iseult MRI is an actively shielded whole-body magnet providing a homogeneous and stable magnetic field of 11.7 T. After nearly 20 years of research and development, the magnet successfully reached its target field strength for the first time in 2019. This article reviews its commissioning status, the gradient–magnet interaction test results and first imaging experience.

Materials and methods

Vibration, acoustics, power deposition in the He bath, and field monitoring measurements were carried out. Magnet safety system was tested against outer magnetic perturbations, and calibrated to define a safe operation of the gradient coil. First measurements using parallel transmission were also performed on an ex-vivo brain to mitigate the RF field inhomogeneity effect.

Results

Acoustics measurements show promising results with sound pressure levels slightly above the enforced limits only at certain frequency intervals. Vibrations of the gradient coil revealed a linear trend with the B₀ field only in the worst case. Field monitoring revealed some resonances at some frequencies that are still under investigation.

Discussion

Gradient-magnet interaction tests at up to 11.7 T are concluded. The scanner is now kept permanently at field and the final calibrations are on-going to pave the road towards the first acquisitions on volunteers.

     

HOMOPOLAR LINEAR SYNCHRONOUS MOTOR: STEADY STATE ANALYSIS AND EXPERIMENTAL RESULTS

ABSTRACT

The main electrical and mechanical performances of a homopolar linear synchronous motor (HLSM) are calculated using classical theories which are usually applied to rotating synchronous machines. An experimental study on a low-power test motor permits the verification of theoretical results. It cranes to evidence that simple theories are applicable under certain conditions to such a motor of original conception. A full scale operational motor proposed for a combined propulsion and magnetic suspension system for high speed ground transportation is designed through extrapolation.     

The Magnetic Pressure Exerted on a Metal Sheet Due to a Parallel Current-Carrying Rod When Both Have a Finite Skin Depth

ABSTRACT

Analytic expressions for the magnetic pressure exerted on a metal sheet due to a rod carrying an AC current are derived. They include the effects of the rod diameter and the finite skin depth in both the sheet and the rod.     

ASYNCHRONOUS PERFORMANCES OF HYSTERESIS MOTORS UNDER UNBALANCED CONDITIONS

ABSTRACT

The torque-speed curves of an hysteresis motor under unbalanced conditions are computed by a numerical method. The magnetic characteristic of the rotor ring is represented by the Preisach-Néel's model. This method takes into account the minor loops which appear on the B(H) characteristic in the case under consideration. Computed and experimental curves are compared. This work shows that eddy currents are of no importance on the shape of these curves     

IMPEDANCE CALCULATION OF SOLID SALIENT-POLE MACHINE AT ASYNCHRONOUS OPERATION

ABSTRACT

This paper calculates the impedance of salient-pole machine, using the finite element technique, also the induction torque calculated for asynchronous operation. The flux distribution and Poynting vector are also calculated by finite element method (FEM).

The impedance calculation formula of the solid pole which has nonlinear magnetic characteristics is obtained analytically by using the solution of the two dimensional problem taking into consideration the magnetic saturation of the solid rotor induction motor     

THERMAL ANALYSIS OF LARGE OIL-IMMERSED TRANSFORMER WINDINGS

ABSTRACT

A method of numerical thermal analysis of large oil-immersed transformer windings is presented. The analysis consists of two steps: first, the leakage magnetic field is solved in the windings; second, the basic and additional losses distribution is calculated simultaneously with temperature rise distribution in the cross-sections of individual coils allowing for special geometries and cooling conditions of different types of windings. An example of the analysis is given and the values calculated are compared with the results of factory tests.     

Specific absorption rate (SAR) simulations for low-field (< 0.1 T) MRI systems

Objective

To simulate the magnetic and electric fields produced by RF coil geometries commonly used at low field. Based on these simulations, the specific absorption rate (SAR) efficiency can be derived to ensure safe operation even when using short RF pulses and high duty cycles.

Methods

Electromagnetic simulations were performed at four different field strengths between 0.05 and 0.1 T, corresponding to the lower and upper limits of current point-of-care (POC) neuroimaging systems. Transmit magnetic and electric fields, as well as transmit efficiency and SAR efficiency were simulated. The effects of a close-fitting shield on the EM fields were also assessed. SAR calculations were performed as a function of RF pulse length in turbo-spin echo (TSE) sequences.

Results

Simulations of RF coil characteristics and B₁⁺ transmit efficiencies agreed well with corresponding experimentally determined parameters. Overall, the SAR efficiency was, as expected, higher at the lower frequencies studied, and many orders of magnitude greater than at conventional clinical field strengths. The tight-fitting transmit coil results in the highest SAR in the nose and skull, which are not thermally sensitive tissues. The calculated SAR efficiencies showed that only when 180° refocusing pulses of duration ~ 10 ms are used for TSE sequences does SAR need to be carefully considered.

Conclusion

This work presents a comprehensive overview of the transmit and SAR efficiencies for RF coils used for POC MRI neuroimaging. While SAR is not a problem for conventional sequences, the values derived here should be useful for RF intensive sequences such as T_1ρ, and also demonstrate that if very short RF pulses are required then SAR calculations should be performed.

     

TORQUE PRODUCTION IN ac MOTORS WITH CONSTANT CURRENT SUPPLY

ABSTRACT

Basic theory is presented concerning the production of torque in synchronous motors and induction motors under conditions of constant current supply. Simple methods are described for treating the effects of magnetic saturation, which are important at high currents. It is shown that, for a given stator current, a synchronous motor produces more torque than an induction motor, albeit with higher air-gap flux. Examples illustrate the variation, due to saturation, of the optimum torque angle of synchronous motors with cylindrical and salient rotors.     

NUMERICAL ANALYSIS OF TUBULAR INDUCTION MOTOR WITH COMPOSITE ROTOR

ABSTRACT

A numerical solution is presented for the fields and forces in tubular induction motor having a rotor consisting of alternate rings of copper and steel mounted on a solid steel core when excited by a sinusoidally distributed current sheet. The edge effect due to the finite length of the current sheet and the inhomogeneity in the rotor material are taken into consideration.

The numerical solution of the field problem is obtained using the finite difference technique. The field equations are formulated in terms of the magnetic vector potential. The corresponding difference equations are derived for the different media, different surface boundaries and the different corner nodes.

The magnetic vector potential distribution, the flux density distribution and the generated force for different values of copper to steel ratio in the rotor are computed. The computed results are compared with the experimental results published by the authors [ 4 ].     

MAGNETIC FIELD DIFFUSION IN FAST DISCHARGING HOMOPOLAR MACHINES

ABSTRACT

The unusually high mechanical and thermal stresses occuring in fast discharging homopolar machines require accurate prediction of high magnetic fields accompanying their operation. Linear methods and ideal configurations are no longer acceptable as simplifying assumptions in designing such devices used in controlled thermonuclear fusion experiments, laser applications, etc.

A finite element method - Galerkin technique is used for solution of Maxwell's equations for a moving medium. The transient skin effect in the system is described in terms of a magnetic vector potential and an electric scalar potential. Lagrange multipliers are used to impose the necessary constraint on the vector potential ā. The formulation for the steady-state magnetic fields in nonlinear media results as a particular case of the method.

This approach was used for predicting the parameters for the very fast discharging homopolar machine (FDX) designed by the Center for Electromechanics at The University of Texas at Austin. FDX is in an advanced state of fabrication.     

The Application of the Duality of Equivalent Circuits to Modelling Cylindrical Solid-Iron Rotors of Synchronous Machines at Asynchronous

ABSTRACT

By means of the duality of magnetic and electric equivalent circuits, an equivalent circuit is derived for the machine with slotted solid-iron rotor at asynchronous speeds. All impedances are calculated from geometrical data. The saturation, the magnetic field in slots and the impedance of the rotor ends are taken fully into consideration.     

An integrated target field framework for point-of-care halbach array low-field MRI system design

Objective

Low-cost low-field point-of-care MRI systems are used in many different applications. System design has correspondingly different requirements in terms of imaging field-of-view, spatial resolution and magnetic field strength. In this work an iterative framework has been created to design a cylindrical Halbach-based magnet along with integrated gradient and RF coils that most efficiently fulfil a set of user-specified imaging requirements.

Methods

For efficient integration, target field methods are used for each of the main hardware components. These have not been used previously in magnet design, and a new mathematical model was derived accordingly. These methods result in a framework which can design an entire low-field MRI system within minutes using standard computing hardware.

Results

Two distinct point-of-care systems are designed using the described framework, one for neuroimaging and the other for extremity imaging. Input parameters are taken from literature and the resulting systems are discussed in detail.

Discussion

The framework allows the designer to optimize the different hardware components with respect to the desired imaging parameters taking into account the interdependencies between these components and thus give insight into the influence of the design choices.

     

STATIC AND DYNAMIC PERFORMANCE OF ELECTRODYNAMIC (REPULSION) LEVITATION SYSTEMS (EDS)

ABSTRACT

Based on a new simplified approach this paper shows theoretically that the sheet secondary is net inferior to discrete secondary in repulsive magnetic levitation systems.

The dynamic behavior is then explored and the control coil data are obtained. Numerical example of practical interest are also provided to get a feeling of magnitudes.     

Non-parametric deconvolution using Bézier curves for quantification of cerebral perfusion in dynamic susceptibility contrast MRI

Objective

Deconvolution is an ill-posed inverse problem that tends to yield non-physiological residue functions R(t) in dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI). In this study, the use of Bézier curves is proposed for obtaining physiologically reasonable residue functions in perfusion MRI.

Materials and methods

Cubic Bézier curves were employed, ensuring R(0) = 1, bounded-input, bounded-output stability and a non-negative monotonically decreasing solution, resulting in 5 parameters to be optimized. Bézier deconvolution (BzD), implemented in a Bayesian framework, was tested by simulation under realistic conditions, including effects of arterial delay and dispersion. BzD was also applied to DSC-MRI data from a healthy volunteer.

Results

Bézier deconvolution showed robustness to different underlying residue function shapes. Accurate perfusion estimates were observed, except for boxcar residue functions at low signal-to-noise ratio. BzD involving corrections for delay, dispersion, and delay with dispersion generally returned accurate results, except for some degree of cerebral blood flow (CBF) overestimation at low levels of each effect. Maps of mean transit time and delay were markedly different between BzD and block-circulant singular value decomposition (oSVD) deconvolution.

Discussion

A novel DSC-MRI deconvolution method based on Bézier curves was implemented and evaluated. BzD produced physiologically plausible impulse response, without spurious oscillations, with generally less CBF underestimation than oSVD.

     

MAGNETIC CIRCUIT STUDIES FOR AN AXIAL-GAP RELUCTANCE MOTOR

ABSTRACT

The analysis of the magnetic circuit of an axial air-gap, battery-excited reluctance motor is presented. Permeance coefficients from which machine inductance can be calculated are derived. Inductance values calculated from these permeance coefficients are compared with values determined by magnetic analogs and by studies of air gap magnetic potential distribution using relaxation techniques and the Finite Element technique.