U.S. patent number 3,919,678 [Application Number 05/456,663] was granted by the patent office on 1975-11-11 for magnetic field generation apparatus.
This patent grant is currently assigned to Telic Corporation. Invention is credited to Alan S. Penfold.
United States Patent |
3,919,678 |
Penfold |
November 11, 1975 |
Magnetic field generation apparatus
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 end-plates being substantially
uniform and parallel to the axis, any non-axial flux components
within the structure tending to cancel each other.
Inventors: |
Penfold; Alan S. (Playa Del
Rey, CA) |
Assignee: |
Telic Corporation (Santa
Monica, CA)
|
Family
ID: |
23813645 |
Appl.
No.: |
05/456,663 |
Filed: |
April 1, 1974 |
Current U.S.
Class: |
335/296;
204/298.16; 335/300; 335/298 |
Current CPC
Class: |
H01F
7/20 (20130101); H01J 37/3402 (20130101) |
Current International
Class: |
H01J
37/32 (20060101); H01J 37/34 (20060101); H01F
7/20 (20060101); H01F 003/00 () |
Field of
Search: |
;335/296,297,298,299,300 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Harris; G.
Attorney, Agent or Firm: Fulwider, Patton, Rieber, Lee and
Utecht
Claims
I claim:
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 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 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.
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 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.
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 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.
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.
Description
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
non-magnetic 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 of a 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-magnetic 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 of the 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 elevational 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. 1, 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 11a-11d having four
corresponding windings 14a-14d. 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 14a-14d 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 14b 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 15b continue into the end-plates 13 and form
a large number of flux paths in the space separating the two
end-plates 13. The flux paths 15a and 15b from the posts 11a and
11b can be thought of as diverging within one of the end plates 13,
and each forming two distinct paths in the space between the
end-plates. These are shown as flux paths 16a and 16b formed within
the cage 10, and flux paths 17a and 17b formed outside the cage.
Flux flow within the end-plates 13 is indicated by the numerals 18a
and 18b.
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 13, and
there should be no flux flow from the post 11a to the other post
11b, since such a flow should be inhibited by an opposing
magnetomotive force from post 11b.
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 three-dimensional 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 end-plates 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 non-identical 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 end-plates 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 of them, 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. 1. 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 connections 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 non-magnetic 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 10
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
cross-sectional thickness toward the central aperture 23'. As in
the previous embodiments 11; 13' and 21' respectively shown the
solenoid posts, end plates and radial gaps. Any dissimilarities in
the solenoid posts 11 and 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-plates 13 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 of the
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.
* * * * *