US3919678A

US3919678A - Magnetic field generation apparatus

Info

Publication number: US3919678A
Application number: US456663A
Authority: US
Inventors: Alan S Penfold
Original assignee: Telic Corp
Current assignee: Dart Industries Inc
Priority date: 1974-04-01
Filing date: 1974-04-01
Publication date: 1975-11-11
Anticipated expiration: 1992-11-11

Abstract

A device for generating an easily accessible, substantially uniform and parallel magnetic field. A number of identical solenoid posts are arranged symmetrically about a central axis and secured by corresponding ends to a pair of end-plates of magnetic material, to form an open, cagelike structure. Equal magnetomotive forces are applied by separate windings on each post, the resulting magnetic field in the space between the endplates being substantially uniform and parallel to the axis, any non-axial flux components within the structure tending to cancel each other.

Description

i United States Patent [191 Penfold 5] Nov. 11, 1975 [5 MAGNETIC FIELD GENERATION 3.439305 4/1969 Klein 335/300 x APPARATUS 35l3.422 5/1970 Watson et al 335/296 [75] Inventor: Alan S. Penfold, Playa' Del Rey,

Calif.

[73] Assignee: Telic Corporation, Santa Monica,

Calif. 22 Filed: Apr. 1, 1974 [21] Appl. No.: 456,663

[52] US. Cl. 335/296; 335/298; 335/300 [51] Int. Cl.'- l-lOlF 3/00 [58] Field of Search 335/296, 297, 298. 299. 335/300 [56] References Cited UNITED STATES PATENTS 3,030,556 4/l962 Watson 335/298 3.4l2.352 1l/l968 Watson 335/298 Primary Examiner-G. Harris Attorney. Agent. or FirmFulwider. Patton. Rieber. Lee and Utecht [57] ABSTRACT A device for generating an easily accessible. substantially uniform and parallel magnetic field. A number of identical solenoid posts are arranged symmetrically about a central axis and secured by corresponding ends to a pair of end-plates of magnetic material. to form an open, icagelike structure. Equal magnetomotive forces are applied by separate windings on each post, the resulting magnetic field in the space between the end-plates being substantially uniform and parallel to the axis, any non-axial flux components within the structure tending to cancel each other.

17 Claims, 8 Drawing Figures U.S. Patent Nov. 11,1975 Sheet1of2 3,919,678

f l I I I l US. Patent Nov. 11, 1975 Sheet20f2 3,919,678

BACKGROUND OF THE INVENTION This invention relates generally to apparatus for the generation of uniform magnetic fields, and, more particularly, to a magnetic cage structure for the generation of an accessible and substantially uniform magnet field of relatively large volume.

Uniform magnetic fields are required for a variety of industrial, manufacturing and experimental processes, as for example, in some cathode sputtering processes. In the past, such a uniform field would usually be obtained by means of a large, hollow solenoid. In the latter case, relatively uniform magnetic field is obtained near the longitudinal axis of the solenoid, so long as the solenoid is long enough with respect to its diameter to minimize end effects tending to distort the field.

In addition to the aforementioned end effects, such a solenoid has the further disadvantage that it consumes a relatively large amount of power to produce a fairly modest magnetic field, and it therefore requires a correspondingly large bulk of copper in its windings. More importantly, the uniform region of the field generated by the solenoid is not easily accessible, since it is enclosed by the solenoid itself. Therefore, any processing of wide, continuous strips of material is not possible. Additional problems are raised if a process has to be performed not only in a uniform magnetic field, but in a near-vacuum environment. If a vacuum chamber is constructed within the solenoid, it must be of nonmagnetic material, and if the solenoid is wound inside the chamber, there are other problems, such as cooling of the windings in the vacuum.

It will be appreciated, then, that there is a continuing need for an improved magnetic system which overcomes these disadvantages and is capable of generating a substantially uniform, and readily accessible magnetic field. The present invention fulfills this need.

SUMMARY OF THE INVENTION The present invention resides in apparatus for the generation of a magnetic field ofa predetermined, and, in the usual case, uniform configuration, with a relatively low power consumption. Briefly, and in general terms, the apparatus of the invention includes a number of magnetizable elements spaced generally symmetrically about a central axis, means for applying a magnetomotive force of desired magnitude and direction to each of the elements, and means for connecting corresponding ends of the elements to form a cage-like structure in which the desired field is generated.

In a presently preferred embodiment, by way of example, each of the magnetizable elements is a post of magnetic material, and the means for applying a magnetomotive force is an outer electrical winding on the surface of each post. It will be appreciated, however, that, in some applications requiring a uniform field of relatively low flux density, the elements may be permanently magnetized. The windings are preferably connected electrically in series and have equal numbers of turns, so that each post is subjected to an equal magnetomotive force. The direction of current flow in the windings is such that the direction of magnetic flux in each post is the same. The flux contributions from the several posts combine in the yokes, and flux paths are produced in the space between the yokes, both within and outside of the cage structure. Within the cage structure, components of flux in non-axial directions with respect to the central axis tend to be cancelled by oppositely directed components from other posts. Therefore, the resulting field within the cage structure is generally parallel to the central axis and substantially uniform throughout the structure.

If the posts and their end connections through the yokes provide identical magnetic circuit elements, there will be no flux flow in the yokes from one post to another. However, any slight differences in the posts could produce wasteful flux flow from one post to another in the yokes. Therefore. it is preferable to minimize this flux flow by means of radial gaps in the yokes, which may be filled with a non-magnetic structural material, or bridged by removable straps for ease of dissassembly.

The posts may be tapered to a smaller cross-section toward their ends, to apply fine corrections to the field. Additional control may be obtained by placing thin, non-magnetie shims between the posts and the yokes.

In the presently preferred embodiment of the invention, the yokes are generally circular end-plates. and the posts are straight rods arranged in parallel relation and secured to each end-plate at uniformly spaced points near its outer peripheral edge. The end-plates may have central openings therethrough to provide additional access to the field without significantly affecting its uniformity, and may be thinner in cross-section toward their centers to take advantage of lower flux densities in those regions. "An alternate embodiment includes an additional post located along the central axis, with a winding to provide a magnetomotive force equal to that applied to each ofthe other posts. The current around the central post may be separately controllable to provide another fine adjustment in minimizing radial components in the field.

If the invention is to be used in a near vacuum environment, the posts may be each surrounded by a vacuum-tight tube sealed to the end-plate. Cooling fluid may then be passed through the tubes and around the windings.

It will be appreciated from the foregoing that the present invention represents a significant advance in magnetic devices for the production of uniform magnetic fields in relatively large volumes. In particular, the invention provides a uniform field for a relatively low power consumption, and is readily accessible, even for continuous-sheet processing, between the posts of the structure or through additional access openings. Furthermore, the field produced by the device may be adjusted to minimize any non-uniformities, and the device is relatively uncomplicated to manufacture and assemble. Other aspects and advantages of the invention will become apparent from the following more detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified perspective view of one embodiment of the present invention, showing a magnetic cage structure with four solenoid posts;

FIG. 2 is an enlarged partial view of a typical one of the solenoid posts, taken substantially in the area 2 of FIG. 1;

FIG. 3 is a diagrammatic view of a magnetic cage structure. showing only two solenoid posts for clarity. and showing the principal flux paths by broken lines;

FIG. 4 is a diagrammatic view showing the manner in which the windings on four solenoid posts of the'invention are electrically interconnected:

FIG. 5 is an enlarged. more detailed and elevationa view of the embodiment shown in simplified form. in FIG. 1;

FIG. 6 is a side-elevational view. partly in section. taken generally along the line 6-6 in FIG. 5'.

FIG. 7 is a simplified perspective view of an alternate embodiment of the invention utilizing five solenoid posts; and

FIG. 8 is a simplified. fragmentary view, partly in section. of another embodiment of the invention. having end-plates of non-uniform thickness.

DETAILED DESCRIPTION As shown in the drawings for purposes of illustration. the present invention is embodied in a magnetic cage structure for the generation of a substantially uniform and parallel magnetic field for use in a variety of industrial or laboratory processes. In the past, uniform magnetic fields were usually generated by large, hollow solenoids. but those have a number of inherent disadvantages; in particular. significant non-uniformities at tthe solenoid ends. and difficulty of access to the uniform field.

In accordance with this invention. and with particular regard to FIG. I, a magnetic cage structure (indicated generally by the numeral 10) generates a substantially parallel and uniform magnetic field which is readily accessible through openings in the cage structure. As shown in the simplified embodiment illustrated in FIGS. 1 and 2, the cage structure 10 comprises a plurality of posts 11 arranged symmetrically about a central axis 12. There are four posts 11 in the embodiment'illustrated, but it will be appreciated from the description that follows that any number of posts may be used, although three or more are required to produce a field that is uniform in three dimensions.

The posts 11 are fabricated of a magnetic material, such as iron. and are substantially identical in size and shape. Corresponding remote ends of the posts 11 are secured to two yokes 13, preferably taking the form of circular end-plates, thus forming the cage-like structure 10. The end-plates 13 are also of magnetic material. and each of the posts 11 is wound with an electrical winding 14, as indicated diagrammatically in FIG. 2.

The windings 14 have equal numbers of turns and are electrically connected so that a magnetic flux of like direction and magnitude is generated in each of the posts 11. This can be best appreciated from FIG. 4, which shows four posts Ila-11d having four corresponding windings l4al4d. The windings are connected electrically in series, as shown, to ensure that an equal magnetomotive force is applied to each post 11. As shown in FIG. 4, the windings l4a-l4a' may be physically wound in alternating directions to avoid trailing winding interconnections from end to end of the cage structure 10.

FIG. 3 shows, for purposes of explanation only, two posts 11a and 11b of the magnetic cage structure 10. It will be appreciated, however, that a two-post structure is impractical in the sense that it will produce a field substantially uniform only along the common plane in which the two posts 11 lie. In FIG. 3. the identical windings 14a and 14h apply equal magnetmotive forces along the posts 11a and 11b. and flux paths are produced in the posts in like directions. as shown by the broken lines at 15a and 15b. respectively.

The flux paths 15a and 15h continue into the endplates 13 and form a large number of flux paths in the space separating the two end-plates 13. The flux paths 15a and 1511 from the posts 11a and 111; can be thought of as diverging within one of the end plates I3, and each forming two distinct paths in the space between the end-plates. These are shown as flux paths 16a and 161; formed within the cage 10, and flux paths 17a and 171; formed outside the cage. Flux flow within the endplates 13 is indicated by the numerals 18a and 1812.

Provided the posts 11 are identically formed, and assembled to end-plates 13 which are also identical. the magnetic flux paths indicated by reference numerals ending in a should be entirely separate from the flux paths indicated by reference numerals ending in b. In other words, all of the flux generated in the post 11a should return through the space between the end-plates l3, and there should be no flux flow from the post 11a to the other post llb, since such a flow should be inhibited by an opposing magnetomotive force from post 1 lb.

The direction of flux generated by each of the posts 11a and 11b considered separately, will have radial components as well as a predominantly axial component between the end-plates 13. If the fields produced by the two posts 11a and 11b are substantially symmetrical, however, the radical components inside the cage structure 10 will buck each other and cancel out, leaving a substantially uniform axial field within the structure. The same theory applies to-the more general, three-dimensional case illustrated in FIG. 1. Non-axial components of flux produced within the cage structure by one of the solenoid posts 11 will tend to be cancelled by equal, oppositely-directed components from another of the posts.

It should also be apparent that, in the threedimensional case illustrated in FIG. 1, if there is perfect symmetry and identity of the solenoid posts 11 and their associated magnetic circuits, then there will be no flux flow in the end plates 13 between one of the posts 11 and another. Any flux flow around one of the endplates 13 from one of the posts 11 to the next must result from an imbalance in the magnetic reluctance of the circuits associated with the respective posts. A useful analogy can be seen in the parallel connection of several electric batteries with slightly different voltages or internal resistances. The imbalance here results in reduced efficiency and distortion of the uniform magnetic field. It will be appreciated that, while a cage structure with identical solenoid posts 11 is presently preferred. an equivalent structure employing nonidentical posts and non-identical magnetomotive forces could be used to provide equal flux contributions from the several posts.

'In the presently preferred embodiment of the invention, flux flow from one post to another in the endplates 13 is minimized by the inclusion of radial gaps 21 in the end-plates. In the form of the invention shown in FIG. 1, there are four such gaps 21, each between adjacent posts 11, and each end-plate 13 is thus divided into four equal sectors 22. The gaps 21 may be filled with a nonmagnetic structural material such as aluminum. or they may be merely air or vacuum spaces, with the sectors 21 joined only by straps of non-magnetic material.

The end-plates 13 may also include a central aperture 23 (FIG. 1) through each ofthem, to provide additional access to the generated magnetic field. It has been found that the aperture 23, provided it is not excessively large, has only a minimal effect on the uniformity of the field within the cage structure 10.

The preferred embodiment illustrated in simplified form in FIG. 1 is shown in greater detail in FIGS. 5 and 6, in which the same reference numbers are retained for corresponding parts also shown in FIG. I. The solenoid posts 11 are securely joined to the end-plates 13, as by bolts 24, to provide a proper interface for the flow of flux between the posts and the end-plates. Each post 11 is surrounded by an outer jacket 25, preferably of high-reluctance material. through which a cooling fluid may be circulated to cool the windings 14. This may be necessary if the invention is to be practiced in an evacuated environment. for example. The cooling jackets 25 are, of course, secured in a pressure-tight manner to the end-plates 13. Holes 26 in the end-plates 13 provide access for the cooling fluid and for electrical con nections to the windings 14. Junction boxes 27 are sealably secured to the outside faces of the end-plates 13 at positions aligned with the solenoid posts 11 and covering the holes 26. In the junction boxes 27, electrical interconnection of the windings 14 is effected, as shown at location 28, and exit holes 29 are provided to allow electrical and cooling-fluid communication between junction boxes.

In the illustrative embodiment, a pair of nonmagnetic straps 31 is'employed to maintain two of the radial gaps 21 in the end-plates 13. The straps 31 are used only on two, diametrically-aligned gaps 21, and are secured to the end plates by bolts or screws 32. The other two diametrically aligned, radial gaps 21 are maintained by means of non-magnetic pins 33 extending from one side of each gap 21 and seated within corresponding holes 34 on the other side of each gap 21, thereby locating one half of the entire structure with respect to the other half. This arrangement has the advantage of ease of disassembly of the structure 10 for rapid access to the space in which the uniform magnetic field is produced. This may be useful, for example, if an article to be processed in the field cannot be conveniently inserted between the posts 11. Usually, the two halves of the structure 10 will be held together by the weight of the upppermost half, and no additional securing straps will be needed.

The magnetic flux density in each of the solenoid posts 11 will be maximum at its mid-point. Consequently, each of the posts 11 may be tapered at its ends, as shown in FIG. 6. The density, i.e., turns per unit length, of the windings 14 may also be varied to make fine adjustments in the field. Furthermore, the flux density in the end-plates 13 decreases towards their centers, and the end-plates may be made thinner toward their centers to save material, reduce weight, or for'fine field adjustments. This is illustrated in FIG. 8, in which the end-plate sectors 22 are tapered to a smaller crosssectional thickness toward the central aperture 23'. As in the previous embodiments ll; 13' and 21 respectively shown the solenoid pests, end plates and radial gaps. Any dissimilarities in the solenoid posts 11 and 6 their associated magnetic circuits may also be compensated for, at least in part. by adding thin shims 40 (FIG. 6) of any appropriate non-magnetic material between the posts and the end-platds I3 as required.

FIG. 7 illustrates a modified embodiment of the invention. including four. symmetrically spaced solenoid posts 11. as in the aforedescribed embodiment. but also including a fifth. central solenoid post 36. The central post 36 is also wound to produce a magnetic flux in the same direction as the four outer posts 11. The post 36 has flanged ends 37 lying in planes coincident with the respective end-plates 13, each of the ends being secured to its respective adjacent end-plate by an annular ring of non-magnetic material 38.

The central solenoid post 36 may be separately supplied with magnetizing current, as it is in the preferred embodiment, and the current and resulting magnetic field may be varied independently to provide a convenient fine adjustment on the uniformity ofthe magnetic field generated within the overall magnetic cage structure.

The electrical power requirements of the magnetic cage structure are quite modest. For example, a flux density of 45 gauss can be produced in a four-post structure having an interior cylindrical volume 8 inches in diameter and 4 feet long, with less than 600 watts of electrical power. Larger structures may be constructed. of course.

It will be appreciated from the foregoing that the present invention represents a significant advance over devices previously available for the production of uniform magnetic fields. In particular, a substantially uniform and parallel magnetic field may be produced for a modest power consumption in an open cage structure which provides easy operational access to the field.

It will also be appreciated that, although particular embodiments of the invention have been described in detail for purposes of illustration. various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.

I claim:

1. For use in a cathode sputtering process in a nearvacuum environment. apparatus for generating a magnetic field of predetermined configuration, comprising:

a plurality of elongated magnetizable elements arranged in a generally symmetrical array about a central axis;

means for applying a magnetomotive force of a desired magnitude and direction to each of said magnetizable elements; and means for connecting corresponding ends of said magnetizable elements to form a cage-like structure substantially longer than the maximum spacing of said elongated magnetizable elements, and to complete a plurality of magnetic flux paths between said magnetizable elements and the magnetic field generated within the structure;

whereby each of said magnetizable elements produces within the structure paths of magnetic flux having both axial and non-axial components with respect to said central axis, and the non-axial components of flux are minimizable by appropriate selection of said magnetizable elements and said meansfor applying a magnetomotive force to generate non-axial components which substantially cancel each other.

2. Apparatus as defined in claim I, wherein:

said means for connecting correponding ends of said magnetizable elements are end-plates: and

at least one of said end-plates includes a plurality of sectors separated by generally radial, highreluctance gaps located between adjacent ones of said magnetizable elements. to minimize non-radial flux flow caused in said end-plates by unequal flux contributions from said magnetizablc elements;

3. For use in a cathode sputtering process in a nearvacuum environment apparatus for generating a substantially uniform and parallel magnetic field therein, comprising:

two yokes fabricated principally of a magnetic material; and

a plurality of magnetized bars spaced generally symmetrically with respect to a central axis, said mag netized bars being secured by corresponding ends to said yokes to form a cage structure substantially longer than the maximum spacing of said magnetized bars, and said magnetized bars having like magnetomotive forces associated with each of them to produce magnetic flux paths directed toward the same one of said yokes;

whereby each of said magnetized bars produces within said cage structure paths of magnetic flux having both axial and non-axial components with respect to said central axis. and the non-axial components of flux substantially cancel each other to leave a resultant uniform magnetic field parallel to said axis.

4. Apparatus as defined in claim 3, wherein:

said yokes are generally circular end-plates; and

at least one of said end-plates includes a like plurality of sectors separated by generally radial, highrcluctance gaps located between adjacent ones of said bar magnets;

whereby said radial gaps minimize non-radial flux flow caused in said end-plates by dissimilarities among said magnetized bars, and thereby produce a more uniform field more efficiently.

5. Apparatus as defined in claim 4, and further including a plurality of straps of non-magnetic material secured to outside faces of said sectors to maintain at least one of said radial gaps and to facilitate disassembly of the cage structure for access to the enclosed fields.

6. Apparatus as defined in claim 3, wherein:

said structure further includes at least one nonmagnetic shim between one of said magnetized bars and one of said yokes to balance dissimilarities among said bars and yokes; and

said yokes have central apertures through which additional access to the uniform field within said structure is provided.

7. For use in a cathode sputtering process in a nearvacuum environment, a magnetic cage structure for generating a substantially uniform and parallel magnetic field therein, said structure comprising:

two yokes of magnetic material;

a plurality of at least three posts of magnetic material spaced generally symmetrically with respect to a central axis, said posts being secured by corresponding remote ends to respective ones of said yokes to form said cage structure, said structure being substantially longer than the maximum spacing of said posts; and

a like plurality of electrical windings wound on re- 1 spective ones of said posts and connectable to apply a magnetomotive force of like magnitude and direction in each of said posts;

whereby each of said posts produces within said cage structure paths of magnetic flux having both axial and non-axial components with respect to said central axis. and the non-axial components of flux from said posts substantially cancel each other to leave a resultant uniform magnetic field parallel to said axis;

8. A structure as defined in claim 7, wherein:

said yokes are generally circular end-plates; and

each of said end-plates comprises a like plurality of sectors separated by generally radial, highreluctance gaps, located between adjacent ones of said posts;

whereby said radial gaps minimize non-radial flux flow caused in said end-plates by dissimilarities among said posts and end-plates, and thereby produce a more uniform field more efficiently.

9. A structure as defined in claim 7, and further including a plurality of straps of non-magnetic material secured to outside faces of said sectors for maintaining at least one of said radial gaps and for facilitating disassembly of said cage structure for access to the enclosed field.

10. A structure as defined in claim 8, and further including non-magnetic location means fitted between at least one pair of adjacent sectors, for removably engaging corresponding holes in at least one of said sectors to maintain the radial gap between said sectors and to locate said sectors laterally with respect to each other.

. 11. A structure as defined in claim 8, wherein each of said end-plates is of non-uniform thickness to maintain a substantially uniform flux density in said endpiates for economy of construction.

12. A structure as defined in claim 7, wherein:

said structure further includes at least one nonmagnetic shim betwecn adjacent components of magnetic material, to balance any dissimilarities among said posts and yokes; and

said yokes have central aperturcs'therethrough to provide additional access to the uniform field within said structure.

13. A structure as defined in claim 7, wherein said electrical windings have equal numbers of turns and are connected in series to apply the equal magnetomotive forces to said posts. v

14. A structure as defined in claim 7, wherein said posts are tapered to a smaller cross-sectional area at their ends to maintain a substantially uniform flux density in said posts for economy of construction.

15. A structure as defined in claim 7, and further including a like plurality of cooling jackets surrounding said posts and permitting the flow of a cooling fluid between said posts and said cooling jackets to cool said electrical windings in the near-vacuum environment.

16, A structure as defined in claim 7, and further including:

a central post of magnetic material secured to said yokes by spacers of non-magnetic material; and an additional electrical winding on said central post to subject said central post to a magnetomotive force in the same direction as that produced by each of said plurality of windings.

17. A structure as defined in claim 16, wherein said additional electrical winding is separately controllable for use as a fine adjustment to the uniformity of the generated magnetic field.

Claims

1. For use in a cathode sputtering process in a near-vacuum environment, apparatus for generating a magnetic field of predetermined configuration, comprising: a plurality of elongated magnetizable elements arranged in a generally symmetrical array about a central axis; means for applying a magnetomotive force of a desired magnitude and direction to each of said magnetizable elements; and means for connecting corresponding ends of said magnetizable elements to form a cage-like structure substantially longer than the maximum spacing of said elongated magnetizable elements, and to complete a plurality of magnetic flux paths between said magnetizable elements and the magnetic field generated within the structure; whereby each of said magnetizable elements produces within the structure paths of magnetic flux having both axial and nonaxial components with respect to said central axis, and the non-axial components of flux are minimizable by appropriate selection of said magnetizable elements and said means for applying a magnetomotive force to generate non-axial components which substantially cancel each other.

2. Apparatus as defined in claim 1, wherein: said means for connecting correponding ends of said magnetizable elements are end-plates; and at least one of said end-plates includes a plurality of sectors separated by generally radial, high-reluctance gaps located between adjacent ones of said magnetizable elements, to minimize non-radial flux flow caused in said end-plates by unequal flux contributions from said magnetizable elements.

3. For use in a cathode sputtering process in a near-vacuum environment apparatus for generating a substantially uniform and parallel magnetic field therein, comprising: two yokes fabricated principally of a magnetic material; and a plurality of magnetized bars spaced generally symmetrically with respect to a central axis, said magnetized bars being secured by corresponding ends to said yokes to form a cage structure substantially longer than the maximum spacing of said magnetized bars, and said magnetized bars having like magnetomotive forces associated with each of them to produce magnetic flux paths directed toward the same one of said yokes; whereby each of said magnetized bars produces within said cage structure paths of magnetic flux having both axial and non-axial components with respect to said central axis, and the non-axial components of flux substantially cancel each other to leave a resultant uniform magnetic field parallel to said axis.

4. Apparatus as defined in claim 3, wherein: said yokes are generally circular end-plates; and at least one of said end-plates includes a like plurality of sectors separated by generally radial, high-reluctance gaps located between adjacent ones of said bar magnets; whereby said radial gaps minimize non-radial flux flow caused in said end-plates by dissimilarities among said magnetized bars, and thereby produce a more uniform field more efficiently.

6. Apparatus as defined in claim 3, wherein: said structure further includes at least one non-magnetic shim between one of said magnetized bars and one of said yokes to balance dissimilarities among said bars and yokes; and said yokes have central apertures tHrough which additional access to the uniform field within said structure is provided.

7. For use in a cathode sputtering process in a near-vacuum environment, a magnetic cage structure for generating a substantially uniform and parallel magnetic field therein, said structure comprising: two yokes of magnetic material; a plurality of at least three posts of magnetic material spaced generally symmetrically with respect to a central axis, said posts being secured by corresponding remote ends to respective ones of said yokes to form said cage structure, said structure being substantially longer than the maximum spacing of said posts; and a like plurality of electrical windings wound on respective ones of said posts and connectable to apply a magnetomotive force of like magnitude and direction in each of said posts; whereby each of said posts produces within said cage structure paths of magnetic flux having both axial and non-axial components with respect to said central axis, and the non-axial components of flux from said posts substantially cancel each other to leave a resultant uniform magnetic field parallel to said axis;

8. A structure as defined in claim 7, wherein: said yokes are generally circular end-plates; and each of said end-plates comprises a like plurality of sectors separated by generally radial, high-reluctance gaps, located between adjacent ones of said posts; whereby said radial gaps minimize non-radial flux flow caused in said end-plates by dissimilarities among said posts and end-plates, and thereby produce a more uniform field more efficiently.

11. A structure as defined in claim 8, wherein each of said end-plates is of non-uniform thickness to maintain a substantially uniform flux density in said end-plates for economy of construction.

12. A structure as defined in claim 7, wherein: said structure further includes at least one non-magnetic shim between adjacent components of magnetic material, to balance any dissimilarities among said posts and yokes; and said yokes have central apertures therethrough to provide additional access to the uniform field within said structure.

13. A structure as defined in claim 7, wherein said electrical windings have equal numbers of turns and are connected in series to apply the equal magnetomotive forces to said posts.

16. A structure as defined in claim 7, and further including: a central post of magnetic material secured to said yokes by spacers of non-magnetic material; and an additional electrical winding on said central post to subject said central post to a magnetomotive force in the same direction as that produced by each of said plurality of windings.