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.