Patent Publication Number: US-6989730-B1

Title: Adjustable toroidal magnet

Description:
GOVERNMENT INTEREST 
   The invention described herein may be manufactured, used, imported, sold, and licensed by or for the Government of the United States of America without the payment of any royalty thereon or there for. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to permanent magnets and, more particularly, to permanent magnets capable of being adjusted to vary a magnetic field strength. 
   2. Related Art 
   Permanent magnets for use where a highly uniform magnetic field is required, such as in magnetic resonance imaging devices and traveling wave tubes, are known. Examples of such permanent magnets may be found in U.S. Pat. No. 4,837,542 to Leupold, entitled “Hollow Substantially Hemispherical Permanent Magnet High-Field Flux Source For Producing a Uniform High Field”and U.S. Pat. No. 5,103,200 to Leupold, entitled “High-Field, Permanent Magnet Flux Source”. The permanent magnets described in those patents produce unusually high fields in their interiors. In the latter example patent, a hollow magic cylinder comprises an annulus that is magnetized so that the direction angle of magnetization γ is twice the local coordinate θ or γ=2θ. 
   It is also known to modify a magic cylinder to obtain a twister or a wiggler. For example, U.S. Pat. No. 4,862,128 to Leupold, entitled “Field Adjustable Transverse Flux Sources”, and incorporated hereby by reference, concerns a permanent magnet with a variable field strength created by an axial sequence of two concentric magic rings of equal magnetic field output. Individual rings of each concentric pair are mutually rotatable with respect to each other whereby the magnetic fields of each concentric magic ring add by vector addition or subtraction to augment or diminish the combined magnetic field strength. 
   SUMMARY OF THE INVENTION 
   In accordance with one embodiment of the present invention, a permanent magnet, comprises a body that has a body axis and that, in turn, comprises a plurality of discreet components. Each of the components may be radially spaced from the body axis approximately an equal distance and each component may be circumferentially spaced an approximately equal distance apart. Also, each of the components may comprise a cavity and a component axis and wherein each of the components may comprise an inner body, that, in turn, comprises a magnetic substance and that is rotatable about the component axis and a plurality of inner segments each having an inner segment magnetic field. The components may also comprise an outer body that, in turn, comprises a magnetic substance and that is rotatable about the component axis. The outer body may also comprise a plurality of outer segments each having an outer segment magnetic field and the outer body being located in proximity to the inner body whereby each of the inner segment magnetic fields and the outer segment magnetic fields interact. 
   In accordance with another embodiment of the present invention, a permanent magnet comprises a toroidal body having a body central axis. The toroidal body may comprise an inner body that includes a magnetic substance and that is rotatable about an inner body central axis. The inner body may comprise a plurality of inner segments that each include an inner segment magnetic field. The toroidal body may also comprise an outer body that, in turn, comprises a magnetic substance and that is rotatable about an outer body central axis. The outer body comprises a plurality of outer segments that each include an outer segment magnetic field and the outer body is located in proximity to the inner body whereby each of the inner segment magnetic fields and the outer segment magnetic fields interact. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description is made with reference to the accompanying drawings, in which: 
       FIG. 1  is a top view of a toroidal permanent magnet in accordance with an embodiment of the present invention; 
       FIG. 2  is a top view of a portion of the toroidal magnet of  FIG. 1 ; 
       FIG. 3  is a front view of the portion of the toroidal magnet of  FIG. 2 ; 
       FIG. 4  is a diagram showing a permanent magnet comprising a plurality of components in accordance with another embodiment of the present invention; 
       FIG. 5  is a diagram showing an end view of one component of the permanent magnet of  FIG. 4 ; 
       FIG. 6  is a diagram showing the component of  FIG. 5  with an inner body of the component rotated about an axis c with respect to an outer body; and 
       FIG. 7  is a diagram showing a permanent magnet comprising a plurality of components in accordance with a further embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   One embodiment of the present invention concerns a permanent magnet that is adjustable to provide for the selection of various field strengths along a cavity of thereof. The permanent magnet may comprise a generally toroidal outer configuration and may comprise a plurality of individual components each of which may be rotatable about a transverse axis thereof. 
   A permanent magnet having a generally toroidally shaped body is illustrated generally at  10  in  FIG. 1 . Referring also to  FIGS. 2 and 3 , the permanent magnet  10  may comprise a central cavity  12 , an inner body  14  and an outer body  16 . The inner body  14  and the outer body  16  may each comprise a flexible and a magnetic material such as a cobalt-rare earth or a rare earth-iron. Each of the inner body  14  and the outer body  16 , may be separably rotatable about an axis A in a manner similar to that described in U.S. Pat. No. 4,862,128 incorporated herein by reference and described below in connection with  FIG. 5 . The inner body  14  has an outer radius R O  that is substantially equal in length to an inner radius R I  of the outer body  16 , although, it will be understood that a slight difference may be provided to allow for the rotations of the inner and outer bodies. It will be understood that a transverse magnetic field, represented by an arrow  18  within the central cavity  12 , may remain almost the same as that in a corresponding unbent cylindrical magnet, such as that described in U.S. Pat. No. 4,862,128, although, the field may taper slightly from a smaller value on an inner extreme  20  of the central cavity to a slightly larger one on an outer extreme  22 . Although not shown as such, it will be understood that, in practice, a portion of the permanent magnet  10 , may be removed in order to provide access to the central cavity  12 . 
   The inner body  14  of the permanent magnet  10  has a direction of magnetization represented by arrows  24  and an outer body  16  having a direction of magnetization represented by arrows  26 . It will be understood that each direction of magnetization varies in a known manner such that an angle of magnetization γ is twice a local coordinate θ or γ=2θ. 
   In operation, rotation of the inner body  14  with respect to the outer body  16  allows for variation in field strength within a portion of the central cavity  18  as a, e.g., a magnetic particle travels through the central cavity  12 . Reference may be had below to the description provided in connection with  FIG. 5  which functions in a similar manner to the permanent magnet  10  and to U.S. Pat. No. 4,862,128, incorporated herein by reference. 
   Referring now to  FIG. 4 , another embodiment of a permanent magnet in accordance with the present invention, which provides for additional adjustment to an internal magnetic field while retaining a generally toroidal configuration, is illustrated generally at  100 . The permanent magnet  100  may function as a radial particle accelerator and comprises a plurality of discreet components  102  radially spaced from a central axis (a) and generally equally spaced apart. Each component  102  may be interconnected via a moldable substance such as a plastic (not shown) and may comprise a configuration that, it will be appreciated, provides for rotation of each about an axis (c). The axis (c) may form a central axis of the components as illustrated, although, an eccentric axis is contemplated by the present invention. 
   Referring now to  FIG. 5 , which shows an end view of each component  102 , it will be appreciated that each component may comprise a generally disk-like shape and a magnetic material such as that comprising a rare earth material. Each component may also comprise a central cavity  104 , an inner body  106  having a direction of magnetization represented by arrows  108  and an outer body  110  having a direction of magnetization represented by arrows  112 . The inner body  106  and the outer body  110  are each separably rotatable about the axis (c) and each cylinder comprises a number of segments  114 , 116  each having a generally trapezoidal configuration. The inner body  106  has an outer radius R O  that is substantially equal in length to an inner radius R I  of the outer body  110 , although, it will be understood that a slight difference may be provided to allow for the rotations of the inner and outer bodies. It will also be understood that the inner and outer bodies  106 , 110  may be generally tubular in configuration. 
   In operation, rotation of the inner body  106  with respect to the outer body  110  allows for variation in field strength, represented by arrow  118 , within portions of the central cavity  104 . In particular, through vector addition, each adjacent segment of each inner and outer body may be combined to provide a particular magnitude and direction of the combined magnetic field. 
   Referring now to  FIG. 6 , one example adjustment in field strength through the central cavity  104  possible with the component  102  of the permanent magnet  100  is shown. In this example, the inner body  106  is rotated with respect to the outer body  110  whereby, through vector addition, a slight variation in the direction of the magnetic field, represented by arrow  118 , through the central cavity  104  results. It will be appreciated that when multiple components  102  of the permanent magnet  100  are adjusted to provide varying field directions, a magnetic particle traveling through each central cavity  104  may be caused to “wiggle”. 
   Referring now to  FIG. 2 , a trapezoidal portion  28  of permanent magnet  10  is shown in comparison with a rectangular component  102  of the permanent magnet  100 . A difference in volume of magnetic material is apparent from this comparison which, correspondingly, results in a loss of magnetic field strength. This loss may be determined in rough proportion to the difference in quantity of matter between the trapezoidal portion and the rectangular component  102 . This is, in turn, roughly proportional to the disparity between the inner and outer arcs as approximated by equation (1).
 
 V   M   /V   T =2/(1 +R )  (1)
 
Where V M , V T  and R are the volume of the pared toroid (permanent magnet  100 ), the volume of the complete toroid (permanent magnet  10 ) and the ratio of outer to inner toroidal radius respectively.
 
   Another embodiment of a permanent magnet, in accordance with the present invention, is illustrated generally at  200  in  FIG. 7 . As illustrated, the permanent magnet  200  comprises a generally linear configuration rather than the generally toroidal configuration of the permanent magnet  100 , described above. The permanent magnet  200  comprises a plurality of components  202 , each of which may be similar to the components  102  and thus reference may be had above for further details thereof. Each of the components  102  may be connected together via beams  204  comprising, e.g., an insulating and moldable plastic substance. 
   While the present invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to these herein disclosed embodiments. Rather, the present invention is intended to cover all of the various modifications and equivalent arrangements included within the spirit and scope of the appended claims.