Permanent Magnet Assembly Research Articles

A comparison of internal plasma parameters in a conventional planar magnetron and a magnetron with additional plasma confinement

This paper deals with Langmuir probe measurements in a planar magnetron sputtering system in which the plasma confinement near a target can be improved by means of an assembly of permanent magnets placed above the target. The electron density, electron temperature, plasma and floating potentials and electron distribution function are measured in various positions between the target and a substrate under various conditions in the discharge. Our experiments proved that the additional magnetic confinement leads to approximately two times higher electron density and strong enhancement (by about an order of magnitude) of the electron temperature in almost all positions of the deposition system investigated.

Planar magnetron with additional plasma confinement

This paper reports on a small, planar, circular magnetron with target of 60 mm diameter for low pressure sputtering of thin films. A stable discharge can be sustained in the case when the plasma confinement near the target surface is improved by means of an assembly of permanent magnets placed above the target. It is shown that the polarity of permanent magnets plays a crucial role in the plasma confinement. The plasma confinement is characterized by measurements of the pressures for the sputtering discharge ignition p st and extinction p ex. The discharge characteristics of this magnetron were measured in detail. Also, it is shown that the efficiency of the target utilization increases considerably at pressures below 0.1 Pa. At argon pressures of about 0.05 Pa and lower, target utilization greater than 80% can be reached.

Irreversible demagnetization loss for electric machines with arc-shaped permanent magnets stabilized in air

For electric machines with their permanent magnet assemblies stabilized in air, a certain amount of irreversible loss of magnetization will occur in permanent magnets when magnet material with knee points in the demagnetization curve is used. To estimate quantitatively the irreversible demagnetization loss in permanent magnet machines, the maximum loss of magnetization, the remanent flux loss and the demagnetization area are defined as their three figures of merits. A series of design curves for predicting the irreversible demagnetization loss for arc-shaped permanent magnets stabilized in air is presented in the paper. The effects of material characteristics, magnet geometry and its magnetic orientation on these merits are also discussed. Design curves are obtained from field computation results by using the multi-hysteresis-loop PM model recently proposed by the first author. These design curves are useful for permanent magnet machine designers.

Single-stage 5 GHz ECR-multicharged ion source with high magnetic mirror ratio and biased disk

A low-cost, single-stage 5 GHz electron cyclotron resonance (ECR) multicharged ion source (MCIS) has been constructed for various atomic collision experiments. It features an axial magnetic field with a mirror ratio of up to five, and a magnetic hexapole field produced by a simple Nd–Fe–B permanent-magnet assembly. A disk probe axially mounted near the ECR resonance zone opposite to the ion extraction, and negatively biased with respect to the ECR plasma potential, permits reduction of the appropriate neutral feeding gas pressure by an order of magnitude, resulting in greatly improved ion charge state distributions, as normally offered by two-stage ECR–MCIS only. We present performance data for multicharged ion production from Ar and N2, including measured ion current emittances.

Improvement of the Field Homogeneity with a Permanent Magnet Assembly for MRI

In the last few years, MRI (magnetic resonance imaging) has become one of the most useful and important radiological and diagnostic methods. MRI requires a strong uniform magnetic field in the area of the patient to be examined. The requirement of a highly homogeneous field grows more stringent with rapid progress in tomographic techniques. At the same time, low-cost magnets are also desired. We previously developed and mass-produced a permanent magnet-type circuit using high-energy Nd-Fe-B meterial. This paper describes a newly developed 15-plane measuring method to replace the previous 7-plane method, for precise evaluation of the homogeneous field. Using this method of analysis and a linear programming method, a pole piece with a new shape has been developed. As a consequence, the homogeneity was doubled and the magnetic weight was reduced by 10% as compared with existing pole pieces.

Development of a Permanent Magnet Assembly for MRI

A permanent magnet assembly for MRI (magnetic resonance imaging) has been developed using a high-energy product sintered Nd-Fe-B magnet. There are three types of magnet assembly for use in MRI: superconductive electromagnets, conductive electromagnets, and permanent magnets. The last does not require liquid helium, electricity or cooling water, and can be installed in small spaces. Despite these advantages, however, permanent magnet assemblies have not been widely adopted, mainly because of the difficulty of attaining uniformity on the order of ppm, and the tremendous weight of such magnets. However, a high-energy product Nd-Fe-B magnet developed in 1983 has made it possible to realize MRI based on permanent magnets. Using such a magnet and investigating the optimum configuration of pole pieces with shims, we obtained a Pk/Pk deviation of 30 ppm over a 35 cm diameter spherical volume at the center of the air gap.

永久磁石方式ＭＲＩ用磁気回路の開発

A permanent magnet assembly for MRI (magnetic resonance imaging) has been developed using a high energy sintered Nd-Fe-B magnet. There are three types of magnet assembly for MRI: superconductive magnet, resistive conductive magnet and permanent magnet. Permanent magnet type does not require any liquid helium, electricity or cooling water, and can be intsalled in a small space. In spite of these advantages, however, the permanent type has not been widespread. The main reasons have been the difficulty of getting ppm order homogeneity and the tremendous magnet weight. However, a high energy Nd-Fe-B magnet developed in 1983 has made it possible to realize the permanent magnet type MRI. Using this magnet and investigating optimum configuration of pole pieces with shims, we obtained 30 ppm of Pκ/ Pκ deviation value in 35 cm diameter of spherical volume at the center of an air gap.

A development of a permanent magnet assembly for MRI devices using Nd-Fe-B material

A permanent-magnet assembly for whole-body magnetic resonance imaging (MRI) has been developed, using a high-energy Nd-Fe-B magnet material. The developed assemblies have the following features: (1) by using shimmed pole pieces in addition to the new material, a compact assembly is realized compared with currently used magnets. (2) The signal/noise ratio can be improved about 40% because the field is oriented vertically with respect to the patient. (3) Since the assembly has a four-column configuration, patients feel comfortable during examination. (4) Small leakage flux makes possible installation in a small room. (5) Operating cost is drastically reduced by using a permanent magnet. A typical assembly and its characteristics are shown. It is confirmed that the quality is almost at the same level as that of a mid-field superconducting magnet. >

A novel self-shielding permanent-magnet rotor assembly

The use of permanent magnets in brushless motors and generators is highly desirable in that they have great potential for reducing weight and increasing efficiency. A self-shielding cylindrical permanent-magnet assembly has been designed and was found to produce high fields at the outer magnet surface and very little flux leakage into the interior rotor space. Construction of this assembly is simplified because it is composed of magnets of simple triangular cross sections, which have only four distinct orientations. The self-shielding nature of the design obviates any need for ferromagnetic material for flux shaping or shielding, thus simplifying greatly the mathematical analysis of the design and reducing its weight and bulk. Finite element methods are used to analyze a hypothetical permanent-magnet rotor assembly with regard to various design parameters.

High gradient magnetic capture of cells and ferritin-bound particles

The magnetic retention of biological cells is of interest as a convenient system in which essentially identical particles can be used to study HGMS and, conversely, as an area in which the HGMS technique can be applied to make biologically important cell separations. Previously, red blood cells have provided data which confirmed theoretical predictions concerning the dependence of particle capture on field, flow rate, particle magnetic susceptibility and diameter of wire in the magnetized matrix. In these experiments the technique has been extended to examine the retention of more weakly paramagnetic (ferritin-labelled) particles and the use of weaker magnetic fields. Latex particles with the iron storage protein ferritin attached and a 7 Tesla magnet have been used to model the expected HGMS capture of ferritin-labelled cells. Red cell retention in weak fields was demonstrated using .3 - .45 Tesla permanent magnet assemblies and stainless steel wire of three diameters. A decrease in HGMS capture efficiency for the finest wire was observed.

Miniature magnet assembly for NMR–ESR spectroscopy

A compact and inexpensive permanent magnet assembly, well suited for educational NMR–ESR demonstrations, field use, and possibly space flight applications is described. For NMR a field of 3.2 kG, homogeneous over a 4-mm sample to within 4 ppm, has been achieved (at 0.44-in. air gap). The assembly, mounted with a different set of polecaps designed for X-band ESR, provides a 3.4-kG field (at 0.6-in. air gap), uniform to within 40 ppm. A simple, mechanical means of sweeping the field through 70 G without significantly affecting the homogeneity is also described.

Effect of Radius of Yoke Curvature on Leakage Flux outside the Yoke in Permanent Magnet Assemblies

The amount of the leakage flux outside the yoke was measured for five kinds of barrel type magnet assemblies with various curvature radii at yoke corners in order to find a suitable yoke shape which provides a stable magnetic field between pole pieces. The path length of the leakage flux which was expressed as the product of the magnetic circuit length of yoke and the ratio of the magnetic fields in and outside the yoke, was exponentially increased with increasing curvature radius of an intermediate plane of the yoke. The leakage flux was calculated for various shapes of the yoke using the experimental results.

Magnetic Shield for Stabilizing Air Gap Fields between Pole Pieces of Permanent Magnets

A permanent magnet assembly was covered with a magnetic shield in order to study experimentally the effect of such a shield on stabilizing the magnetic field between the pole pieces. This experiment was carried out by changing the yoke-shield spacing g and by changing the material used for the shield. A shielding ratio of 10∼60 was obtained with a high permeability shield placed at 0.1 dy<g<0.2 dy, where dy is the yoke diameter. The data were compared with the results of some analysis.

Calculation of equipotentials and flux lines in axially symmetrical permanent magnet assemblies by computer

Two equivalent theoretical models of permanent magnets are used to develop algorithms for numerically computing the magnetic scalar potential and the magnetic vector potential in the vicinity of an axially symmetric array of pole pieces and permanent magnets. A computer program based on these algorithms calculates equipotential surfaces and flux lines in and around the magnets and pole pieces. In deriving the algorithm for numerically calculating the vector potential a relationship between the magnetic scalar potential and the vector potential was found which enables the program to calculate the vector potential from the scalar potential distribution and thus generate equipotentials and flux lines with only one iterative calculation. An algorithm which calculates the scalar potential of a "floating" pole piece, that is, one on which the scalar potential has not been specified, is developed. The vector potential around the pole piece is determined from the scalar potential calculation, and this information is used to calculate the vector potential and the flux lines within the pole piece. The computer program calculates the coordinates of all points at which the equipotential lines and flux lines cross the Liebmann net. This information is fed to a cathode ray tube plotter which generates a field plot. To deal with systems in which macroscopic currents are present as well as permanent magnets, the iterative Liebmann net calculation of the vector potential is developed, and a method of applying Neumann boundary conditions to the vector potential at high-permeability surfaces is described.

Surveyor V: Magnet Experiment

During the Surveyor V landing, a footpad with an attached permanent magnet assembly slid for about a meter through lunar surface material at a depth of about 10 centimeters. Subsequent pictures showed material adhering to the magnetic pole edges, where the magnetic field strength is greatest. Comparison of these pictures with those made under simulated laboratory conditions permits three conclusions. (i) Iron is present on the lunar surface in one of the forms attracted to a 500-gauss magnet. (ii) A 1-percent addition by volume of powdered free iron to a powdered terrestrial rock represents an upper limit for the lunar results. (iii) The lunar results are most similar to a terrestrial plateau basalt with no addition of free iron.

Flexure Mounted Beam Balance for Long-Term Magnetic Stability Measurements

A beam balance was designed for the long-term measurement of permanent magnet stability. The balance measures an air-gap magnetic field change with a precision of one part in 30 000. The balance is supported on crossed flexures and operates on the torque-balance servo principle. A fixed torque is applied to the balance arm. Servo action causes the permanent magnet assembly to rotate in such a way as to create an opposition torque which is equal to that applied. The magnet speed required to establish torque balance depends on the square of the magnetic field. This speed is measured with great precision by a preset counter. A large number of magnet assemblies are measured.

System for the Precision Measurement of Long-Time Change of an Inhomogeneous Magnetic Field

The long-time stability of an inhomogeneous magnetic field is measured with a precision of 3×10−5. The system is instrumented for short-time dependence on component stability while referencing on a precision homogeneous field with NMR. An InAs Hall effect sensor is positioned in the gap of a permanent magnet assembly. The Hall voltage is balanced by a low voltage reference source. Without changing the reference voltage the Hall probe is placed in the field of a 6-in. electromagnet whose field is varied to again obtain null, then is measured with an NMR gaussmeter. The time requirement on component stability is about one minute. The Hall probe is temperature controlled to ±0.01°F.

Temperature compensation of periodic permanent-magnet assemblies for focusing traveling-wave tubes

Temperature compensation of periodic permanent-magnet assemblies for focusing traveling-wave tubes