Abstract:
A permanent magnet comprises a shell surrounding a cavity. The shell has a magnetic remanence B r (θ) configured such that a magnetic field taper extends through the cavity and wherein the shell includes a non-distortive access region that is substantially magnetic field.

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 therefor. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to permanent magnets and, more particularly, to high intensity permanent magnets having gradient fields and to methods of making such magnets. 
   2. Related Art 
   Permanent magnets that are capable of producing high intensity magnetic fields and that have a compact structure are known and are used, e.g., in miniaturized electrical components including disk drives for laptop and palmtop computers. In particular, permanent magnet materials that are highly remanent and coercive, such as those of the rare earth type, are produced to make compact flux sources of extraordinary strength. Examples of high-intensity, compact permanent magnets, which may employ these materials, may be found in the following patents. 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”; U.S. Pat. No. 4,839,059 to Leupold, entitled “Clad Magic Ring Wigglers”; U.S. Pat. No. 5,103,200 to Leupold, entitled “High-Field Permanent Magnet Flux Source”; U.S. Pat. No. 5,216,401 to Leupold, entitled “Magnetic Field Sources Having Non-Distorting Access Ports”; U.S. Pat. No. 5,382,936 to Leupold et al., entitled “Field Augmented Permanent Magnet Structures”; U.S. Pat. No. 5,426,338 to Leupold, entitled “High-Power Electrical Machinery with Toroidal Permanent Magnets”; U.S. Pat. No. 5,434,462 to Leupold et al., entitled “High-Power Electrical Machinery”; and U.S. Pat. No. 5,523,731 to Leupold, entitled “Simplified Method of Making Light Weight Magnetic Field Sources Having Distortion-Free Access Ports. The entire contents of each of the foregoing patents is hereby incorporated herein by reference to the extent necessary to make and practice the present invention. 
   The basic configuration from which the magnetic arrangements described above are derived may be referred to as a Halboch Structure or a magic cylinder or ring. The magic ring is a permanent magnet which is magnetized in accordance with the configuration shown in FIG.  1 . The orientation of magnetization at any point (P) is at an angle (γ) from a vertical axis (z) and is equal to twice the polar coordinate of P,(θ) or according to equation (1) as follows:
 
γ=(2)(θ)  (1) 
 
where:
         (θ) is a polar angle between the x and z axes that may vary from θ=0° to θ=±0° as shown.
 
Such a magnetization pattern produces in an internal cavity (c) a uniform magnetic field (represented by arrow h). Ideally the change in direction of magnetization should be continuous but this is not technically feasible. Instead an approximation to the ideal magic ring structure can be formed by a method as described in U.S. Pat. No. 5,523,731, previously incorporated herein by reference. The method may comprise uniformly magnetizing a cylindrical shell along a plane defined by radii of the shell and cutting the shell into thin washer shaped pieces. Each of the washer-shaped pieces may then be cut in a radial manner to form truncated pie shaped pieces that may be then reversed 180° (turned upside down) and transposed to proper locations along the circumference of the ring to form a thin magic ring slice having the magnetization approximately as shown in FIG.  1 . The formation of a magic cylinder may be accomplished by stacking the magic rings together in an elongated fashion. It will be recognized that the ideal structure can be approached as closely as desired by increasing the number of truncated pie shaped pieces per magic ring.
       

   It may be desired in particular applications employing magic rings or cylinders that access ports of various sizes, shapes and locations extend through the shell and communicate with the internal cavity (c). However, removal of magnetic material to provide an access port to the interior through the magnetic shell will distort the interior field especially in the vicinity of the port. To overcome this drawback, a further method is proposed, as also described in U.S. Pat. No. 5,523,731, wherein some of the thin washer-shaped pieces that are sliced from the uniformly magnetized cylinder are interleaved with the magic ring slices to form a cylinder that allows for non-distorting access ports. In such a case, removal of magnetic material results in a non-distorting access port, since a uniformly magnetized ring produces no field in its interior cavity and superposition of such a magnetization pattern upon that of ae magic ring would result in no change in the field located in the cavity of the magic ring. 
   It is also sometimes desired to produce permanent magnets having a shell and a cavity wherethrough a high intensity magnetic field extends which is tapered, or has a gradient. For example, electron-beam tubes often require gradient fields, which typically vary along a beam axis, for use in focusing and guiding the beam. Microwave and millimeter wave sources require an axial field variation of a longitudinal magnetic field in order to produce a crisp waveform and storage rings and particle accelerators may require transverse magnetic fields including field tapering in the direction of a longitudinal axis of a beam in order to compensate for changes in a velocity of the beam. In spectroscopic analysis, magnetic fields with a linear taper in the direction of the field are often used to produce a spectral distribution of absorbed or emitted electromagnetic energy. 
   U.S. Pat. No. 5,216,400 to Leupold (below referred to as the “400 Patent”), the entire contents of which is hereby incorporated herein by reference to the extent necessary to make and practice the present invention, describes a permanent magnet having a cavity and producing a magnetic field that varies in intensity and in the direction of its orientation to produce a tapered or gradient magnetic field within a cavity thereof. As generally described therein, to provide a linear taper along the z axis of a magnetic ring or cylinder where the south pole is at z=0 at an inner edge of a cavity, the remanence is tapered along the polar angle θ according to equation (2) as follows:
 
B r (θ)=mθ+B r   Min (0°)  (2) 
 
where:
         B r (θ) is the desired magnet remanence at θ;   B r   Min  is the minimum remanence appropriate to produce a field H(Min) at the low end of the taper located in the cavity of the magnetic ring or cylinder and H(Max) being the field at the high end of the taper; and   where m is found according to equation (3) as follows:
 
m=[B r   Max (90°)−B r   Min (0°)]/π  (3) 
 
where:
   B r   Max  is the maximum remanence at z=d l  at the north pole of the cavity and d i  is the diameter of the cavity.       

   The permanent magnets described in the above patents and documents have numerous applications and advantages, however, it is desired to provide a permanent magnet including a cavity having both a gradient magnetic field and a distortion free access port. 
   SUMMARY OF THE INVENTION 
   In accordance with an embodiment of the present invention, a permanent magnet comprises a shell surrounding a cavity. The shell comprises a magnetic remanence B r (θ) configured such that a magnetic field taper extends through the cavity and wherein the shell also comprises a non-distortive access region that is substantially absent any magnetic field. 
   Another aspect of the present invention concerns a permanent magnet that comprises a shell surrounding a cavity and wherein the shell comprises a magnetic remanence configured whereby a magnetic field taper extends through the cavity. The remanence B r (θ) of the shell varying according to the formula:
 
B r (θ)=[(B r   X (θ)) 2 +(B r   Z (θ)) 2 ] 1/2  
 
where:
         B r   X (θ)={[(B r   Max −B r   Min )/90°](θ)−B r   Min }cos(90°−2θ);   B r   Z (θ)={[(B r   Max −B r   Min )/90°](θ)−B r   Min }sin(90°−2θ)+(B r   Max +B r   Min )/2;   (θ) is a polar angle between θ=0° to θ=±180°;   B r   Max  is a maximum remanence desired; and   B r   Min  is equal to a minimum remanence appropriate to produce a minimum magnetic field H(Min) of the magnetic field taper. The shell also comprises a pair of opposing non-distortive access regions substantially absent any magnetic field.       

   A further aspect involves a method of making a permanent magnet having a cavity, comprising providing at least one first segment having a first magnetic field that has a single predetermined direction of magnetization and that has a uniform first remanence; providing at least one second segment having a second magnetic field that has a direction of magnetization (γ) that varies circumferentially along the segment according to the formula γ=(2)(θ) where θ is a polar angle from θ=0° to θ=360° and wherein the second magnetic field comprises a second remanence which increases in magnitude from θ=0° to θ=180° and decreases in magnitude from θ=180° to θ=360°; and combining the at least one first segment and the at least one second segment to form a permanent magnet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description is made with reference to the accompanying drawings, in which: 
       FIG. 1  is diagram showing a magnetic ring in accordance with the prior art; 
       FIG. 2  is a series of diagrams showing how magnetic fields of magnetic rings may be combined in accordance with an embodiment of the present invention; 
       FIG. 3  is a top view of a cylindrical permanent magnet showing various directions of magnetization in accordance with an embodiment of the present invention; 
       FIG. 4  is a perspective view of a magnetic ring having a uniform magnetic field; 
       FIG. 5  is a perspective view of a magnetic ring having a magnitude and direction of magnetization that varies with a change in polar angle; 
       FIG. 6  is a perspective view of a cylindrical permanent magnet in accordance with an embodiment of the present invention; 
       FIG. 7  is a perspective view of a first cylindrical blank magnetized with a uniform magnetic field and comprising a plurality of washer-shaped pieces; 
       FIG. 8  is a perspective view of a second cylindrical blank having a magnitude and direction of magnetization that varies with a change in polar angle and that comprises a plurality of magnetic ring slices; 
       FIG. 9  is an exploded view showing assembly of the first segments of FIG.  7  and the second segments of  FIG. 8  to form the cylindrical permanent magnet of  FIG. 6 ; 
       FIG. 10  is an end view of the cylindrical permanent magnet of  FIG. 6  showing the various magnitudes and directions of magnetization; 
       FIG. 11  is an end view of the cylindrical permanent magnet of  FIG. 6  showing the lines of flux extending through a bore of the magnet; and 
       FIG. 12  is an exploded view showing assembly of a spherical permanent magnet in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   One embodiment of the present invention concerns a permanent magnet that has both a cavity comprising a gradient magnetic field and a distortion free access port. In another embodiment of the invention, a permanent magnet may comprise a plurality of first and second segments that may be easily assembled together to provide the desired magnetic field parameters. 
   Referring to  FIG. 2 , and in accordance with an embodiment of the present invention, a combination of magnetic rings, each having a particular magnetization, may be combined to form a magnetic ring having a particular resultant magnetization, the particulars of which may be found by vector analysis. For example, a first magnetic ring A including a magnetization direction and magnitude represented by arrows a may be combined with a magnetic ring B including a magnetization represented by arrows b (of equal magnitude to those of arrows a and in tandem at poles p where θ=0° and θ=180°) to form a magnetic ring C that has a resulting field reflected in arrows c 1  and voids or nulls in points c 2 , located at θ=±90°. It will be understood that since a magnetization of zero is present at the nulls, represented at points c 2 , material may be removed from the magnetic ring C at points c 2  in order to provide non-distortive access ports as described in more detail below. Within the magnetic ring C the magnetization (M) components vary in the x and z directions according to the following equations (4) and (5):
 
M x =M 0 sin(2θ);  (4) 
 
M z =M 0 (cos2θ+1);  (5) 
 
where: 
 
   M 0  is the magnetization of the material used and is equal to the remanence (B r ) divided by 4π (M 0 =B r /4π). It will be recognized that where a desired B r  exceeds that of the best available material, an increase in the radius of the ring may be used to compensate for this. 
   In another example, by combining magnetic ring D including magnetization represented by arrows d with magnetic ring E including a magnetization represented by arrows e (of equal magnitude to those of arrows e and in opposition at poles p), a magnetic ring F is formed with a resulting field reflected in arrows f 1  and voids in points f 2 , located at θ=0° and 180°. Accordingly, material may be removed from the magnetic ring F and non-distortive access ports may be provided at points f 2 . Within the magnetic ring C the magnetization (M) components vary in the x and z directions according to the following equations (6) and (7):
 
M x =M 0 cos(2θ);  (6) 
 
M z =M 0 (sin θ+1).  (7) 
 
   Referring now to  FIG. 3 , a cylindrical permanent magnet having both-a gradient magnetic field in a cavity thereof and a distortion free access port in accordance with an embodiment of the present invention is shown generally at  200 . The permanent magnet  200  may comprise a shell  202  and a plurality of sections  204 . Each section  204  may be magnetized, as described in more detail below, such that both a direction of magnetization and a magnitude of magnetization may vary section by section. This is represented by the direction and length of arrows  206 . 
   Further in accordance with this embodiment, a non-distortive access region or notch  208  is provided for access to a cavity  210 . Also, a gradient magnetic field, represented by arrow  211 , resides within the cavity  210 . 
   It has been found that when a magnetization of two separate magnetic rings or cylinders, such as those illustrated in  FIGS. 4 and 5 , are combined, a magnetization that results is consistent with the magnetization represented by the arrows  206  of the permanent magnet  200  of FIG.  3 . 
   Referring now also to  FIG. 4 , a magnetic ring  210  may be magnetized by a uniform magnetic field to form a uniform remanence (B r1 ) as represented by the direction and length of arrows  212 . As is evident to one of ordinary skill in the art, generally no magnetic field is present within a cavity  213  of the magnetic ring  210 . 
   Referring now further to  FIG. 5 , a magnetic ring  214 , which may be similar to that described in U.S. Pat. No. 5,216,400, above incorporated herein by reference, is shown which may be magnetized such that the direction of magnetization (γ) varies circumferentially along the segment according to the equation (8) as follows:
 
γ=(2)(θ)  (8) 
 
where:
         (θ) is a polar angle between the x and z axes that may vary from θ=0° to θ=±180° as shown.       

   Arrows  216  are oriented in a manner to illustrate the direction of magnetization (γ) which varies about the circumference of the magnetic ring  214 . Arrows  216  also illustrate the magnitude of a remanence (B r2 ) which also varies about the circumference of the magnetic ring  214 . In particular, the remanence (B r2 ) generally increases from θ=0° to θ=180° and decreases from θ=180° to θ=0°. More specifically, the remanence (B r2 ) varies according to equation (9) as follows:
 
B r2 (θ)=mθ+B r   Min   (9) 
 
where:
         θis a polar angle from θ=0° to θ=±180°; and   m varies according to equation (10) as follows:
 
m=(B r   Max −B r   Min )/90°  (10) 
 
where:
   B r   Max  is the maximum remanence required to generate a maximum H(Max) magnetic field strength at the high end of a resulting tapered or gradient field represented by arrow  218  which is located within a cavity  220  of the magnetic ring  214 ; and   B r   Min  is equal to the minimum remanence appropriate to produce a magnetic field H(Min) at the low end of a resulting tapered or gradient field represented by arrow  218 . B r   Min  is also equal to B r1  of the magnetic ring  210 .       

   Combining the uniform magnetization of the magnetic ring  210  with the varying magnetization arrangement of the magnetic ring  214  results in a varying magnetization such as that of the permanent magnet  200  of FIG.  3 . Since generally no magnetic field is present in the cavity  213  of the magnetic ring  210 , combining magnetic rings  210  and  214  results in no change to the tapered or gradient magnetic field of the magnetic ring  214  and thus is represented in the magnetic ring  200  by arrow  211 . Also since the remanences (B r1 ) and (B r2 ) are equal but opposite in direction where θ=0°, a non-distortive access region exists at θ=0° and notch  208  may be provided. Further, the direction of magnetization (γ), illustrated by the direction of arrows  206 , may be found in accordance with vector analysis as exemplified above in connection with FIG.  2 . 
   In accordance with vector analysis, the resulting remanence in the permanent magnet  200  for θ=0° to θ=±180° for each vector component of the remanence along the x direction may be found from equation (6) as follows:
 
B r   X (θ)={[(B r   Max −B r   Min )/90°](θ)−B r   Min}cos( 90°−2θ)   (11) 
 
Each vector component of the resulting remanence along the z direction for θ=0° to θ=±180° may be found from equation (12) as follows:
 
B r   Z (θ)={[(B r   Max −B r   Min )/90°](θ)−B r   Min }sin(90°−2θ)−B r   Min   (12) 
 
The vector components B r   X (θ) and B r   Z (θ) may be combined to form a resultant remanence via equation (13) as follows:
 
B r (θ)=[(B r   X (θ)) 2 +(B r   Z (θ)) 2 ] 1/2   (13) 
 
The direction of magnetization (γ) for each of B r (θ) may be found in accordance with equation (14) as follows:
 
tan(γ)=B r   Z /B r   X   (14) 
 
   It will also be appreciated that the particular location (θ) of the notch  208  (or slot in the case of a cylinder) may be varied depending upon a desired location for distortion free access. For example, distortion free access may be provided in the permanent magnet  200  at θ=90° and at θ=270° by modifying the uniform remanence (B r1 ) of the magnetic ring  210  to equal (B r   Max +B r   Min )/2 whereby the following equations (15) and (16) for vector components of the remanence are obtained.
 
B r   X (θ)={[(B r   Max −B r   Min )/90°](θ)−B r   Min }cos(90°−2θ)  (15) 
 
B r   Z (θ)=[((B r   Max −B r   Min )/90°)(θ)−B r   Min ]sin(90°−2θ)+(B r   Max +B r   Min )/2  (16) 
 
The vector components B r   X (θ) and B r   Z (θ) may be combined to form a resultant remanence via equations (13) and (14) above.
 
   It will be appreciated that the above-described equations may be used in connection with a sphere, although, the resulting distortion free access ports are cylindrical tunnels at the poles and an equatorial slot at the equator. 
   Optional Embodiment For Simple Assembly 
   Referring now to  FIG. 6 , a permanent magnet, in accordance with another embodiment of the present invention, is illustrated generally at  10 . In this embodiment, the permanent magnet  10  comprises a plurality of segments  12 ,  14  each having an aperture  16 . As illustrated, each of the segments  12 ,  14  are magnetized, as described in more detail below, and may be concatenated together to create a magnetic field within the apertures  16  which is tapered or increases in strength in the direction of the arrow  18  and which comprises a non-distortive access region or notch  19 . 
   The segments  12 , or washer-shaped pieces and shown also in  FIG. 7 , may be cut (illustrated by dashed lines  19 ) from a cylindrical blank  20  that may comprise any suitable material capable of high remanence and thereby producing a high strength magnetic field such as a composition that includes a rare earth element. The cylindrical blank  20  may be magnetized by a uniform magnetic field to form a uniform remanence (B r1 ) as represented by the direction and length of arrows  22 . 
   Referring to  FIG. 8 , a cylindrical blank  24  is shown from which segments  14 , or magnetic ring slices, may be cut (as illustrated by dashed lines  26 ). The cylindrical blank  24  may comprise a similar material to that of the cylindrical blank  20 , although, it will be appreciated that other materials capable of high remanence may be employed. The cylindrical blank  24  may be magnetized such that the direction of magnetization (γ) varies circumferentially along the segment similar to that described above in connection with FIG.  5  and according to the equations (9) and (10) above. Accordingly, arrows  28  are oriented in a manner to illustrate the direction of magnetization that varies about the circumference of the cylindrical blank  24 . Arrows  28  also illustrate the magnitude of the remanence (B r2 ) which also generally increases from θ=0° to θ=180° and decreases from θ=180° to θ=360° as described above. 
     FIG. 9  illustrates assembly of the permanent magnet  10 , whereby segments  12  and  14  are interleaved together to form an elongated structure. It will be appreciated that the permanent magnet  10  may comprise a notch  30  ( FIG. 5 ) which may be formed prior to or after assembly of the segments  12  and  14 . 
   Referring now also to  FIGS. 10 and 11 , after assembly of the permanent magnet  10 , the permanent magnet may have a resultant magnetic field remanence (represented by arrows  32 ) that includes a general null at the notch  30  and an increasing field strength (H) along a z axis through cavity  16 . As shown, H(min) represents a minimum field strength within the cavity  16  while H(max) represents a maximum field strength. 
   Another embodiment of a permanent magnet in accordance with the present invention is illustrated generally at  110  in FIG.  12 . The permanent magnet  110  may be similar to permanent magnet  10  in many aspects except that, instead of a generally cylindrical configuration, the permanent magnet  110  comprises a generally spherical configuration. Accordingly, similar components are labeled with similar reference numbers excepting that a one is included in the reference number for those referring to permanent magnet  110 . 
   The permanent magnet  110  comprises segments  112  and  114  each of which comprise a magnetic remanence B r1  and B r2  (represented by arrows  122 ,  128 , respectively) which may be similar to that described above in connection with  FIGS. 2 and 3 . Therefore, reference may be had above to segments  12  and  14  for further details concerning segments  112  and  114 . 
   Segments  112  and  114  may be cut from spherical blanks (not shown) which have been magnetized appropriately, as described above, and then assembled together by interleaving the segments  112  and  114  as shown. Also, the segments  112  and  114  may comprise a notch  130  that may be formed, e.g., prior to assembly thereof. 
   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.