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Adjustable Multi-Tesla Permanent Magnet Field Sources

Herbert A. Leupold, Anup S. Tilak and Ernest Poteruiani 11 US Army Research Lab, AMSRLEP-EC-H, Fort Monmouth, New Jersey 07703-5601 USA

Absfract - Permanent magnet flux murces of the enhanced “magic” ring I “magic” sphere types with an outer to Inner radius ratlo of 6 can be made to be mechanically field adJustable h m 0.0 T to i4.0 T In working gaps transverse to the applled field. b a d j ~ t p b k BoUreeS Of S h h Z ’ S h e dth gap bngltudlnal to the field, the somewhat smaller field range of -3.0 to 3.4 T is attainable. The cylindrical sources pose no dimCultles of access or adjustpblllty and ways of provldlng these for the spheres 8s well are discussed.

INTRODUCIION

Recent work at ARL has resulted in the design of permanent magnet configurations that can produce fields of up to 4.0 T in disc-shaped cavities 2.5 cm in diameter and a half a cm in gap width with structures 15 cm in diameter [1821.

Somewhat smaller fields are also attainable in narrow cavities with their large dimensions parallel to the field, such as needle-like cylindrical cavities drilled through the axes of the structures or disc-shaped cavities with their principal

planes through meridional planes. Figure 1 illustrates some of the more useful possibilities. Further consideration of the longitudinally and transversely placed cavities has revealed that both are easily made field adjustable. Except where otherwise stated, the permanent magnet materials used in the calculations have remanence BR and coercivity ~ H c both equalto 12T.

DISCUSSION

Figure 1 shows cross sections of assorted “magic” rindsphere configurations (spherical and cylindrical cross sections are identical) and illustrates how these can be made fEld-varying. This process is the same as for unaugmented “magic” spheres and rings. The ring is split into two annular ”maghf’ rings (or spheres) so that each piece produces the same field in the interior. These fields then add vectorially and the field resultant can be. varied by rotation of the two rings through the same angle 8 in opposite directions 131.

If the field in a transversely oriented cavity is augmented by an iron insert, that insert is subjected to a uniform field

0018-9464/93$03.00 8 1993 IEEE

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4

3

n 0 20 40 60 80

Angular Shell Displacement (Degrees)

Field adjustment as described above presents no problem with cylindrical structures, as one need only attach the axles of adjusting motors to the cylindrical rings at the ends of the structure. Since a magic cylinder possesses no net magnetic moment, the inner cylinder may be rotated freely without application of torque in the field of the outer cylinder. The same torque-free rotatability is also enjoyed by the outer cylinder.

Only for rotations of the shells with iron inserts need a torque L be supplied equal to

dM L = H R - de

where HR is the internal field supplied by the rotated ring and dM/d8 is the rate of change of magnetic moment M of the iron with the rotation angle 8. For field above saturation,

Fig 2. Fields in the cavities of “magic” spheres with cross sections of Figs. IE-IH versus adjustment angle 8.

perpendicular to the gap that forms the working space. The field will magnetize, and if strong enough, saturate the iron thereby adding to the field in the working space. Figure 2 shows the field dependence upon the displacement angle 8 for “magic” spheres with a 0.4 T maximum gap field. Also shown in Fig. 2 is the same dependence for a structure with a permanent magnet insert with its direction of magnetization parallel to the field of the surrounding magic sphere. In this case, the total field is not symmetric about the 8 axis because the inserted permanent magnet does not depend upon the magic sphere for its magnetization and therefore continues to provide the same field even when the field due to the spherical shells is zero. If the field due to the insert is greater than the maximum provided by the surrounding shells, the field in the working space cannot even be reduced to zero. For structures of the same size, those with iron inserts will provide higher fields than those with permanent magnet inserts if the field due to the outer shell structure is sufficient to saturate the iron. This is because of the greater magnetic moment of the iron. For lower fields, permanent magnet inserts are better. Figure 3 shows the field dependence on 8 for cylindrical structures.

If the cavity walls are to be aligned with the field as in Fig. lD, the field cannot be augmented with an iron insert. If a permanent magnet insert is to be used, it must be aligned antiparallel to the field so that the resulting demagnetization field will augment that of the outer sphere by one-third of the magnetization which is typically from 0.3 to 0.4 T. This is only half as much as the 0.6 to 0.8 T augmentation for the case of thin cavities transverse to the field as in Figs. 1B and 1C where the magnetization of the insert is aligned with the field from the sphere. For cylinders, these augmentations are one half the remanence for both cavity orientations and have a typical range of 0.45 to 0.6 T.

the torque is zero.

4 ’ : ’ ; . : ’ ! I . , 1 . 1 .

......... 4 ........... I ....................... 4 ....................... 4 ........... < ......................

0 20 40 60 80 Angular Shell Displacement (Degrees)

Fig 3. Fields in the cavities of magic cylinders with cross sections of Figs. IE-1H versus adjustment angle 0.

For spheres, rotation is slightly more difficult as the inner sphere must either be accessed through a narrow slot cut along a meridian of longitude or through a hole drilled along an equatorial diameter. In both cases, cavity access must also be through either or both of these penetrations of the spherical shells.

Since a “magic” sphere has a magnetic moment, d, equal to one third the product of magnetization and shell volume, a torque must be applied to rotate either with respect to the other. The magnitude of the torque in an unaugmented “magic” sphere is given by

L H R X G

For the case of the unaugmented “magic” sphere, the maximum torque at a 8 of ld2 is 50 newton-meters. For the augmented sphere, the fields of the inserts must be added to

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HR to obtain the total field acting on the rotated shell. with a permendur insert. With a permanent magnet insert, the maximum flux density is approximately 3.5 T. In a needle- shawd cavity with axis parallel to field or a disc-shaped

CONCLUSIONS

In various type cavities in which the largest dimension is 2.5 cm, modified “magic” sphere structures can apply

cavity with i& bases parailel to the field, the peak field is about 3.2 T.

REFERENCES adjustable flux densities as large as 4.0 T with an outer diameter of only 15 cm and a mass of approximately 14 kg. The value of 4.0 T is attainable in a disc shaped cavity with its base planes transverse to the field in a spherical structure

[ l ] H. A. Leupold. E. Potenzia@ II and A. S. T U . to be published,J. Appl. Phys., March 1993 [2] H. A. Leupold and E. PotenZiani II, J . Appl. Phys.,vol. 70, p. 6621,1991 [3] H. A. Leupold. E. Potenziani 11, and M. G. Abele, J . Appl. Phys., vol. 64, p.5994.1988