Flat Spiral Coil Research Articles

Numerical simulation of electromagnetic acoustic transducers using distributed point source method

In spite of many advances in analytical and numerical modeling techniques for solving different engineering problems, an efficient solution technique for wave propagation modeling of an electromagnetic acoustic transducer (EMAT) system is still missing. Distributed point source method (DPSM) is a newly developed semi-analytical technique developed since 2000 by Placko and Kundu (2007) [12] that is very powerful and straightforward for solving various engineering problems, including acoustic and electromagnetic modeling problems. In this study DPSM has been employed to model the Lorentz type EMAT with a meander line and flat spiral type coil. The problem of wave propagation has been solved and eddy currents and Lorentz forces have been calculated. The displacement field has been obtained as well. While modeling the Lorentz force the effect of dynamic magnetic field has been considered that most current analyses ignore. Results from this analysis have been compared with the finite element method (FEM) based predictions. It should be noted that with the current state of knowledge this problem can be solved only by FEM.

Cryogenic receive coil and low noise preamplifier for MRI at 0.01 T

We have investigated the design and construction of liquid nitrogen cooled surface coils made from stranded (litz) copper wire for low field MRI applications. If designed correctly, cooled litz coils can provide a competitive alternative to high temperature superconducting (HTS) coils without the complications associated with flux trapping. Litz coils can also be produced with a wider range of shapes and sizes, and at lower cost. Existing models were verified experimentally for flat spiral coils wound from solid and litz wires, operated at room temperature and 77 K, and then used to design and optimise a cooled receive coil for MRI at 0.01 T (425 kHz). The Q-factor reached 1022 when the coil was cooled to 77 K, giving a bandwidth of just 0.42 kHz, so a low noise JFET preamplifier was developed to provide active damping of the coil resonance and thus minimise image intensity artefacts. The noise contribution of the preamplifier was determined using a method based on resistive sources and image noise analysis. The voltage and current noise were measured to be 1.25 nV/ Hz 1/2 and 51 fA/ Hz 1/2, respectively, and these values were used to estimate a noise figure of 0.32 dB at the resonant frequency of the cooled coil. The coil was used to acquire 0.01 T spin echo images, first at room temperature and then cooled to 77 K in a low noise liquid nitrogen cryostat. The measured SNR improvement on cooling, by a factor of 3.0, was found to correspond well with theoretical predictions.

A simple model to simulate electromagnetic sheet free bulging process

Electromagnetic sheet forming is a high-velocity forming process driven by the coupled electromagnetic and mechanical phenomena. The deformation of the workpiece is governed by the body forces (Lorentz forces) that results from a pulsed magnetic field produced by a flat spiral coil. Formability can be increased using this high-velocity forming technique due to the inertial forces and high strain rates. In this study, we consider the electromagnetic and the mechanical aspect of the process as two independent problems. The finite difference method has been employed to solve the electromagnetic equations. The pressure acting on the sheet and due to the Lorentz forces is estimated neglecting the influence of the sheet velocity on the magnetic field. Then it has been treated as a load in the mechanical problem. Numerical simulations of the mechanical problem have been performed with the commercial finite element code ABAQUS/Explicit. The magnetic pressure has been introduced in ABAQUS/Explicit as an analytical pressure distribution. The general objective of this study is to better understand the complex phenomenon of deformation and the influence of viscoplastic material behaviour during the simulation of a free bulging electromagnetic sheet forming process.

Controlling welding hot cracking based on electromagnetic force

Controlling welding hot cracking based on electromagnetic force was carried out. The results show that the welding cracking can be inhibited with this method, which lies on the voltage of capacitor and the frequency of applying force. The sheet bears the downward axial force and the less radial force of flat spiral coil. The radial force points at the centre of the sheet, so it is able to compress the weld metal. Moreover, the axial force also has the ability to compress the weld metal by means of the backing plate under weldment. Controlling welding hot cracking is mainly due to the improving of mechanical condition of weld metal within brittle temperature range.

Formability of steel sheet in high velocity impact

High velocity deformation can be quite effective in increasing the forming limits of metal sheets as well as effectively treating some other common metal forming problems, such as wrinkling and distortion. This study examines the high velocity formability of cold rolled sheet steel as developed in impact with a curved punch. To study forming over large areas, a steel sheet sample was electromagnetically launched, at a shaped punch at velocities of 50–220 m/s, using a flat spiral electromagnetic coil and an aluminum driver sheet. Estimates of impact velocity were obtained with the help of a high-speed camera. It was observed that the impact velocity of the samples increased nearly linearly with the energy of launch. The failure strains of all the steels were dramatically increased beyond those obtained in tensile tests as expected from the forming limit diagram. The forming limits of steels with both very low and high quasi-static ductility were similar in high velocity forming. Formability seemed to depend largely on local boundary conditions as dictated by the punch/tool geometry used. The strain distribution obtained depended on the shape of the punch used for impact. Thus it appears that in high velocity impact, the quasi-static ductility of the material is not of primary importance to the material's formability. At high velocities, high strains to failure can be obtained with steels having very low quasi-static ductilities. Based on these observations, one may be able to develop forming operations that take advantage of this dramatic improvement in formability. This could enable the use of high-strength low-cost materials in a number of complex components.

Diagnostics and two-dimensional simulation of low-frequency inductively coupled plasmas with neutral gas heating and electron heat fluxes

This article presents the results on the diagnostics and numerical modeling of low-frequency (∼460 KHz) inductively coupled plasmas generated in a cylindrical metal chamber by an external flat spiral coil. Experimental data on the electron number densities and temperatures, electron energy distribution functions, and optical emission intensities of the abundant plasma species in low/intermediate pressure argon discharges are included. The spatial profiles of the plasma density, electron temperature, and excited argon species are computed, for different rf powers and working gas pressures, using the two-dimensional fluid approach. The model allows one to achieve a reasonable agreement between the computed and experimental data. The effect of the neutral gas temperature on the plasma parameters is also investigated. It is shown that neutral gas heating (at rf powers⩾0.55 kW) is one of the key factors that control the electron number density and temperature. The dependence of the average rf power loss, per electron–ion pair created, on the working gas pressure shows that the electron heat flux to the walls appears to be a critical factor in the total power loss in the discharge.

Inductively coupled plasmas sustained by an internal oscillating current

A global electromagnetic model of an inductively coupled plasma sustained by an internal oscillating current sheet in a cylindrical metal vessel is developed. The electromagnetic field structure, profiles of the rf power transferred to the plasma electrons, electron/ion number density, and working points of the discharge are studied, by invoking particle and power balance. It is revealed that the internal rf current with spatially invariable phase significantly improves the radial uniformity of the electromagnetic fields and the power density in the chamber as compared with conventional plasma sources with external flat spiral inductive coils. This configuration offers the possibility of controlling the rf power deposition in the azimuthal direction.

Effect of an internal rotating current on low-frequency inductively coupled plasmas.

The effect of an internal low-frequency rotating current on inductively coupled plasmas in cylindrical chambers is studied. The electromagnetic field structure, power density distribution, the plasma density, and the operating regimes of the discharge are investigated using electrodynamic, power, and particle balance equations. It is shown that the rotating current sheet can dramatically improve the uniformity of the electromagnetic fields and the power transferred to the plasma electrons from that of conventional low-frequency inductively coupled plasma sources with external flat spiral inductive coils.

Electromagnetic forming of thin metal sheets

The present paper describes a numerical method to solve the problem of the electromagnetic forming of clamped thin circular metal sheets by using a flat spiral coil. The method involves circuit analysis, electromagnetic field calculation and the dynamic plastic deformation of the workpiece. A two-dimensional axisymmetric model based on the integral representation of the electromagnetic vector potential is proposed for the calculation of the magnetic field, eddy currents and the electromagnetic force density in the metal sheet. Equations for the motion of the loaded work-piece are used assuming plane stress condition.

Analysis of the performances of heat exchangers used in hot-water stores

The thermal response of a hot-water store and the thermal discharge characteristics from heat exchanger coils placed inside have been experimentally investigated. The analysis of the experimental results shows that it is possible to describe the thermal behaviour of these systems by the use of a convenient set of dimensionless parameters. Two different geometric configurations of the heat-exchanger coils were investigated, namely the classic cylindrical coil and the flat spiral coil, this last one presenting a greater heat transfer efficiency. The hypothesis of a lumped heat capacity for the thermal evolution of the water store enables the construction of a simplified theoretical model. The solution of which, obtained by a numerical method, predicts with reasonable accuracy the time evolutions of both the average temperature of the water tank and of the inlet-outlet temperature difference of the circulating water.

Helicon plasma source excited by a flat spiral coil

We have characterized a new helicon plasma source with an end-launch antenna configuration. Power at 13.56 MHz is coupled via a four-turn flat spiral coil into an m=0 helicon mode with the application of a weak (B≳5 G) axial magnetic field. Plasma parameters were measured with Langmuir probes, and the structure and absorption of the helicon wave fields were determined with magnetic induction probes. Plasma densities of 1011–1012 cm−3 were produced in argon for pressures in the 1–100 mTorr range with a 5–60 G magnetic field. Radio frequency power absorption occurs primarily via collisionless Landau damping for pressures below 5–10 mTorr and collisional damping dominates at higher pressures. Single pass absorption occurs for magnetic fields less than 20 G. This source does not require a separate source chamber, and thus combines the compactness of flat coil inductively coupled sources with the advantages of remote plasma generation found in wave supported sources. The source can be easily optimized for a variety of operating conditions, because the helicon parallel wavelength is not constrained.

Investigation of electron source and ion flux uniformity in high plasma density inductively coupled etching tools using two-dimensional modeling

Inductively coupled plasma (ICP) sources are being developed as reactors for high plasma density (1011–1012 cm−3), low-pressure (<10–20 mTorr) etching of semiconductors and metals for microelectronics fabrication. Transformer coupled plasmas (TCPs) are one variant of ICP etching tools which use a flat spiral coil having a rectangular cross section powered at radio frequencies (rf) to produce a dense plasma in a cylindrical plasma chamber. Capacitive rf biasing of the substrate may also be used to independently control ion energies incident on the wafer. The uniformity of generating the plasma and the uniformity of the flux of reactants to the substrate are functions of the geometry and placement of the coil; and of the materials used in the construction of the chamber. In this article, we use results from a two-dimensional model to investigate design issues in TCPs for etching. We parametrize the number of turns and locations of the coil; and material properties of the reactor. We find that at low pressure, designs which produce ionization predominantly at larger radii near the edge of the wafer produce more uniform ion fluxes to the substrate. This results from a ‘‘converging’’ ion flux which compensates for losses to lateral surfaces. Careful attention must be paid to metal structures in the vicinity of the coils which restrict the azimuthal electrical field. This situation results in reduced power deposition at large radii, which can be compensated by over sizing the coil or by using auxiliary solenoidal coils. The plasma and neutral transport, dominated by diffusion, treats the advective flow from the gas inlets and pump port as local sources and sinks which are rapidly volume averaged.

High-temperature superconducting gradiometric SQUIDs and flux transformers

Gradiometric DC superconducting quantum interference devices (SQUIDs) have been constructed from thin-film YBa 2Cu 3O 7− x on MgO substrates. The SQUIDs were fabricated as forst-order gradiometers with PrBa 2Cu 3O 7− x insulator providing electrical isolation at the crossovers. Highly symmetrical first-order gradiometer flux transformers have also been fabricated. These couple to the SQUIDs and enhance the sensitivity to magnetic-field gradients. Each flux transformer has two 20 turn flat spiral SQUID input coils and a pair of larger-area balanced pickup coils. Flux transformers with both parallel and series configured pickup loops have been fabricated. When coupled to a SQUID as a flip chip, one gradiometer was operated at 77 K with a gradient sensitivity of (0.79±0.03) nT/m/ √Hz at 10 Hz and (2.0±0.1) nT/m/ √Hz at 1 Hz.

Two-dimensional modeling of high plasma density inductively coupled sources for materials processing

Inductively coupled plasma sources are being developed to address the need for high plasma density (1011–1012 cm−3), low pressure (a few to 10–20 mTorr) etching of semiconductor materials. One such device uses a flat spiral coil of rectangular cross section to generate radio-frequency (rf) electric fields in a cylindrical plasma chamber, and capacitive rf biasing on the substrate to independently control ion energies incident on the wafer. To investigate these devices we have developed a two-dimensional hybrid model consisting of electromagnetic, electron Monte Carlo, and hydrodynamic modules; and an off line plasma chemistry Monte Carlo simulation. The results from the model for plasma densities, plasma potentials, and ion fluxes for Ar, O2, Ar/CF4/O2 gas mixtures will be presented.

Characteristics and simulations of the planar multi-layer spiral coil and the miniature opto-electric transformer

The characteristics of a miniature optoelectric transformer (MOET) and simulations for the MOET based on the simple improved model for L, C and R of the multilayer coil with the outer closed core are shown. The circuit simulations are performed by a PSPICE simulator, and experimental results can be explained by these simulations. Simulations for a very small MOET of about 200 mu m are performed and discussed. A new method to form the substantially flat spiral coil, in which anodic oxidation of the deposited Al film is used, is proposed to overcome the unevenness problem arising from increasing the etched Al coil thickness.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Percolative reconciliation between resistive and magnetic measurements of the transition temperature in HTS thin films

Several Bi-Sr-Ca-Cu-O thin films have been produced ex situ by sequential thermal evaporation. Characterizing the transition temperatures Tc of these films, a constant discrepancy between resistive and inductive measurements is observed. The resistive van der Pauw technique indicates Tc (onset) typically above 90 K and Tc (zero) at around 78 K. However, the Tc (onset) for these same films is not seen by an inductive measurement, in which the film surface is in physical contact with a flat spiral coil, inducing changes in the coil inductance during the superconductivity transition. Conversely, the inductive transition starts around 5 K above the resistively obtained Tc (zero). Percolation theory is employed in an attempt to explain this discrepancy and to establish generally how Tc (zero) can be found from the resulting curve in the inductive measurement technique.

Eddy-current transducer based on flat spiral coils : Soviet journal of nondestructive testing, Vol. 24, No. 4, pp. 235–238 (Dec. 1988)

The article deals with the experimental investigation of flat-coil eddy current transducers with spiral windings. It is shown that it is possible to use them for detecting discontinuities on electrically conducting products with different configuration of the surface. The instrument is described. The use of crossed flat coils makes it possible effectively to detect flaws in electrically conducting products of any shape. A transducer containing crossed flat coils helps, in addition to detecting flaws, in evaluating anisotropy (magnetic and mechanical) from which conclusions as to the physicomechanical properties of the product can be drawn.

Eddy-current transducer based on flat spiral coils Soviet journal of nondestructive testing, Vol. 24, No. 4, pp. 235–238 (Dec. 1988)

Eddy-current transducer based on flat spiral coils Soviet journal of nondestructive testing, Vol. 24, No. 4, pp. 235–238 (Dec. 1988)

High-speed forming of metal sheets by electromagnetic force.

A numerical method to simulate the high-speed free forming of a clamped circular disk in an electromagnetic forming system using a flat spiral coil is presented. The method combines field penetration into the disk and dynamic elasto-plastic deformation of the disk. The spiral coil is approximated by coaxial circular loops carrying the discharge current from a capacitor bank. The penetration of the magnetic field of the coil into the disk is formulated as a boundary-value problem for the diffusion equation. Magnetic field, eddy currents and electromagnetic force density in the disk are calculated. An equation of motion for the disk loaded by the magnetic force is combined with the field equation as well as an equivalent circuit equation. Plane stress condition is assumed. Strain-rate effect on the work-hardening law of the disk material is taken into account. An experiment on the free bulging of annealed aluminum disks is also presented, showing fairly good agreement with the numerical solution.

Fabrication of multi-turn thin film head

A new fabrication method has been developed for multi-turn thin film heads. In this method, a multi-turn coil is formed by piling two layers of flat surfaced bifilar spiral coils. A bevel-etching technique has been developed to form the flat surface of the bifilar coil. A 9-turn, 18-track thin film head has been fabricated and tested on the conventional particulate disk. The read efficiency was 88% and the MMF required for 30 dB of overwrite was 0.32AT.