Patent Grant
6856224
This invention relates to permanent magnet devices. More particularly, this invention relates to useful devices and methods that eliminate or lessen the effects of transverse fields in the cylindrical cavity of hollow cylindrical magnets.
Many devices that employ magnetic fields have heretofore been encumbered by massive solenoids with their equally bulky power supplies. There has been increasing interest in the application of permanent-magnet structures for such uses as electron-beam focusing and biasing fields. The current demand for compact, strong, static magnetic field sources requiring no electrical power supplies has created needs for permanent magnet structures of unusual form. Many of these permanent magnet structures have been developed for electronic beam guidance in millimeter or microwave tubes, for millimeter wave filters, circulators, isolators and strip lines, nuclear magnetic resonance imagers and other similar devices for which a relatively large uniform magnetic field is desired.
Many of these permanent magnet structures provide a relatively high uniform magnetic field and have embodied the principles of a “magic” ring, cylinder, hemisphere sphere. An example of a “magic sphere” or hollow spherical flux source is disclosed in Leupold, U.S. Pat. No. 4,835,506, entitled “Hollow Substantially Hemispherical Permanent Magnet High Field Flux Source,” issued May 30, 1989. Methods of making the “magic ring” and “magic sphere” are also disclosed in Leupold, U.S. Pat. No. 5,337,472, entitled “Method of Making Cylindrical and Spherical Permanent Magnet Structures,” issued on Aug. 16, 1994. Magnets creating transverse magnetic fields have been disclosed in Leupold, U.S. Pat. No. 5,319,339, entitled “Tubular Structure Having Transverse Magnetic Field With Gradient,” all of which are incorporated herein by reference. Particularly promising for such purposes is the configuration based on the hollow cylindrical flux source (“HCFS”), also known as a magic ring, which is a cylindrical permanent-magnet shell offering a magnetization vector that is primarily constant in magnitude and produces a field greater than the remanence of the magnetic material from which it is made.
The magic ring concept and its polygonal approximations have proven useful for applications requiring relatively high transverse fields in tubular working spaces, such as mm/microwave radiation sources and amplifiers. The magic ring has also been particularly useful as a common permanent magnet configuration to confine transverse magnetic fields to cylindrical magnets. For example, see H. A. Leupold and E. Potenziani, An Overview of Permanent Magnet Design. U.S. Army T.R. SLCET-TR-90-6, August 1990. However, the magic ring is not without its drawbacks, shortcomings, limitations and difficulties. The magic ring can exhibit field distortions because of end effects, and in order to achieve a fairly uniform biasing field, each end of the ring must be elongated by an amount approximately equal to its structural diameter, so that the central portion of the field which is uniform is long enough for the user's purposes. However, the elongated structure's wastefully long length-to-length ratio, as well as the end regions having the field attenuating because of the significant amounts of wasted space in the structure, causes a number of undesirable increases in a device's mass and bulk, making the elongated magic ring unsuitable for a number of applications.
Thus there has been a long-felt and unsatisfied need for a uniform magnetic field within an internal cavity in a far more compact structure that does not suffer from the drawbacks, shortcomings, limitations and difficulties associated with the elongated magic ring. Up until now, there is no compact magnetic structure that also provides the much-needed magnetic field uniformity. The present invention solves the long-felt need for field uniformity within a more compact structure by providing a permanent magnetic structure that produces a uniform biasing field by capping the end of the magic ring with a hemispheric section of a magic sphere having a cavity diameter and shell axis equivalent to the magic ring. In accordance with the present invention, a permanent magnetic structure with a capped end of a magic ring and a hemispheric magic sphere section, as further described herein, produces a magnetic field on an axis of the magic ring's end because the magic sphere caps now produce a transverse magnetic field within the structure's hollow cavity.
Accordingly, it is a primary object of the present invention to achieve a highly uniform biasing field within a very compact magnetic structure and a high, uniform magnetic field that is undistorted because of end effects.
It is yet another object of the present invention to achieve a highly effective and uniform magnetic field within a magic ring by capping each end with a hemispheric section of a magic sphere having a cavity diameter and shell axis equivalent to the magic ring.
It is still a further object of the present invention to achieve a highly effective and uniform magnetic field within a magic ring by capping the end of a magic ring with a hemispheric section of a magic sphere having a cavity diameter and shell axis equivalent to the magic ring to produce magnetic field on an axis of the magic ring's end because the magic sphere caps now produce a transverse magnetic field within the structure's hollow cavity.
These and other objects and advantages are achieved in accordance with the permanent magnetic structure producing a uniform and undistorted biasing magnetic field, comprising a magic ring structure with its ends advantageously capped with a portion of a magic sphere having both a cavity diameter and shell axis equivalent to the magic ring in order to produce a transverse magnetic field. The capped magic ring structure of the present invention achieves a compact, uniform magnetic flux source having a transverse magnetic field without suffering from any of the disadvantages, shortcomings, limitations and difficulties previously associated with the elongated magic ring and other prior art devices.
The magic ring structure is a cylindrical shell of permanent magnet material in which the direction of magnetization, γ, will vary, as depicted in
γ=2φ
where φ is the azimuthal angle of the cylindrical coordinate system with its origin at the magic ring's center. Such a ring of infinite length, or a length much larger than its diameter, produces a magnetic field, H, in its interior cavity according to the formula:
where Br is remanence, and ro and ri are the outer and inner radii of the ring, respectively. Theoretically, there is no limit to the magnitude attainable for magnetic field, H. However, because of the logarithmic dependence of H on the outer radius, such a structure would be prohibitively large for magnetic fields much greater than about 2 Br.
The present invention discloses permanent magnetic structures that solve this long-felt and heretofore unsatisfied need for field uniformity within a more compact structure by utilizing a portion of the HCFS, or magic sphere structure depicted in
and the magic ring produces at that point a field of only:
it is clear that the remanence, Br, of the sphere must be reduced so that:
Thus, the HS and HC of the preceding equations (2 and 3) are equal with the sphere and ring each contributing one half of the desired field. These and other objects, advantages and features will become readily apparent in view of the drawings and more detailed description.
In order to satisfy the long-standing need for a uniform magnetic field within an internal cavity in a compact magnetic structure without suffering from the drawbacks, shortcomings, limitations and difficulties associated with prior art devices, the present invention provides compact magnetic structures that provide the much-needed magnetic field uniformity by capping the end of the magic ring with a hemispheric section of a magic sphere having a cavity diameter and shell axis equivalent to the magic ring. The permanent magnetic structures of the present invention further comprise a compact parametric capped permanent magic ring structure and a compact geometric capped permanent magic ring structure without suffering the drawbacks, shortcomings, limitations and difficulties associated with prior art devices.
In accordance with the present invention, it is now possible to configure magic ring structures in such a way that the outer radius of the sphere can be reduced so that H and HC in equations (2) and (3) above are equal despite the remanence values of the sphere and the cylinder being the same. This approach will save weight, but will also require additional tooling, because if the diameters of the sphere and the cylinder are the same, the component rings of each structure can be made with the same die, although parts of the sphere need to be subjected to additional treatment in either case. See the inventor's U.S. Pat. No. 5,337,472, entitled “Method of Making Cylindrical and Spherical Permanent Magnet Structures,” which is also incorporated herein by reference.
Variations of the parametric capped permanent magic ring structure 30 include 13 the cylindrical magnetic flux source 31 being composed of a plurality of circular magnetic segments, the cylindrical magnetic flux source 31 being a magic ring, the first magnetic flux source 36 being composed of a plurality of magic sphere segments and the second magnetic flux source being composed of said plurality of magic sphere segments.
The compact geometric capped magic ring structure 50 tends to be lighter than the parametric capped permanent magic ring structure 30. The parametric capped permanent magic ring structure 30 is also somewhat easier to fabricate than the geometric capped magic ring structure 50. The cylindrical magnetic flux source 31 could be a magic ring structure composed of a number of magnetic segments with the appropriate dimensions, which are not shown in
It is also noted that the addition of the hemispherical caps of the present invention to the ends of the cylinder will result in correct fields at the junctures, but this configuration will not completely accomplish the correct field within the cylinder because of the fall-off of the spherical field with distances from the juncture are not quite the same as the rise in the cylindrical field over the same distance. Therefore, the sum of the hemispherical and cylindrical fields will not add up to exactly the same field at all points. For most purposes this variation is insignificant, however, where a more precise approximation is needed, this can be effected through the variation of the cylindrical field by slight variations of the remanence value along the cylindrical axis similar to the methods disclosed in the inventor's U.S. Pat. No. 5,319,339, entitled “Tubular Structure Having Transverse Magnetic Field With Gradient,” which is also incorporated herein by reference. Based on this, it should therefore be possible to cancel the mismatch between the hemisphere and cylinder by an appropriate manipulation of the remanence value as well as the outer radii of the cylinder, if needed.
It is also noted that results of this invention may also be accomplished through the use of polygonal and polyhedral approximations to the magic rings and magic spheres, respectively.
The present invention also encompasses a method for producing a uniform magnetic biasing field in a compact permanent magnet structure, comprising the steps of forming a cylindrical magnetic flux source with a cylindrical axis, a hollow central cavity, an inner diameter and a remanence perpendicular to said cylindrical axis; capping a first end of the cylindrical magnetic flux source with a first magnetic flux source, the first magnetic flux source having a first cavity diameter; capping a second end of the cylindrical magnetic flux source with a second magnetic flux source, the second magnetic flux source having a second cavity diameter; dimensioning the inner diameter to be equivalent to the first cavity diameter and the second cavity diameter and generating a transverse magnetic field from the first magnetic flux source and the second magnetic flux source, the transverse magnetic field assisting in producing the uniform biasing magnetic field perpendicular to the cylindrical axis and confined within the hollow central cavity. The variations to the parametric capped permanent magic ring structure 30, geometric capped magic ring structure 50 may also be applicable to the method for producing a uniform magnetic biasing field in a compact permanent magnet structure.
It is to be further understood that other features and modifications to the foregoing detailed description are within the contemplation of the present invention, which is not limited by this detailed description. Those skilled in the art will readily appreciate that any number of configurations of the present invention and numerous modifications and combinations of materials, components, geometries, arrangements and dimensions can achieve the results described herein, without departing from the spirit and scope of this invention. Accordingly, the present invention should not be limited by the foregoing description, but only by the appended claims.
The invention described herein may be manufactured, used and licensed by or for the United States of America without the payment to me of any royalties thereon.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4835506 | Leupold | May 1989 | A |
| 4839059 | Leupold | Jun 1989 | A |
| 5126713 | Leupold | Jun 1992 | A |
| 5319339 | Leupold | Jun 1994 | A |
| 5337472 | Leupold | Aug 1994 | A |
| 5805044 | Leupold | Sep 1998 | A |
