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Description  |
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TECHNICAL FIELD
The present invention relates in general to permanent magnet structures for
use in electronic devices and, more particularly, to magnet structures for
devices which act as high energy radiation sources, e.g. wigglers,
undulators, free electron lasers.
BACKGROUND OF THE INVENTION
Many devices that employ magnetic fields have heretofore been encumbered by
massive solenoids with their equally bulky power supplies. Thus, there has
been increasing interest in the application of permanent magnet structures
for such uses as electron-beam focusing and biasing fields. The current
demand for compact, strong, static magnetic field sources that require no
electric power supplies has created needs for permanent magnet structures
of unusual form. A number of configurations have been designed and
developed for electron beam guidance in millimeter wave microwave tubes of
various types; for dc biasing fields in millimeter wave filters,
circulators, isolators, striplines; for field sources in NMR (nuclear
magnetic resonance) imagers; and so on. Especially promising for such
purposes is the configuration based upon the hollow cylindrical flux
source (HCFS) principle described by K. Halbach in "Proceedings of the
Eighth International Workshop on Rare Earth Cobalt Permanent Magnets",
Univ. of Dayton, Dayton, Ohio, 1985 (pp. 123-136). A HCFS, sometimes
called a "magic ring", is a cylindrical permanent magnet shell which
produces an internal magnetic field that is more or less constant in
magnitude. The field is perpendicular to the axis of the cylinder, and
furthermore the field strength can be greater than the remanence of the
magnetic material from which the ring is made.
The ideal hollow cylindrical flux source (HCFS) is an infinitely long,
annular cylindrical shell which produces an intense magnetic field in its
interior working space. The direction of the magnetic field in the working
space interior is perpendicular to the long axis of the cylinder. The
aforementioned Halbach publication discloses a structure with an octagonal
cross section which closely approximates the performance and field
configuration of an ideal HCFS (which has a circular cross section). In
both the ideal and Halbach configurations, no magnetic flux extends to the
exterior of the ring structure (except at the ends of a finite cylinder).
The terms HCFS and "magic ring" as used herein encompasses not only the
ideal cylindrical structure but also octagonal, sixteen sided,
thirty-two-sided and even higher order polygonal-sided structures which
approximates the ideal HCFS structure.
Unfortunately, the ideal HCFS structure is theoretically infinitely long.
Thus, achievement of the desirable high uniform magnetic field in the
interior of the magic ring structure demands that the structure be made
extremely long (theoretically infinite). If the structure is not long
enough, distortion of the interior fields will result.
Those concerned with the development of high power devices such as wigglers
have continually searched for means to create intense uniform magnetic
fields in confined spaces with lightweight devices.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a permanent magnet
structure with compact external dimensions and a high uniform interior
magnetic flux.
It is a further object of this invention to provide a permanent magnet
structure with minimal external flux leakage.
It is another object of this invention to provide a permanent magnet
structure with minimal internal field distortion.
The above and other objects are achieved in accordance with the present
invention, which makes advantageous use of the HCFS structure uniquely
combined with superconducting plates or sheets.
In one embodiment of the present invention an HCFS producing a uniform high
field in the central cavity is truncated to produce a magnetic structure
of finite length. When an HCFS is so truncated, the field no longer
remains uniform due to the distortions caused by flux leakage from the
open ends of the structure. To eliminate flux leakage a pair of
superconducting plates or sheets, at least peripherally coextensive with
the truncated flux source, are placed abutting the end faces of the same.
A uniform field within the central cavity is thereby produced. The cavity
provides a working space for applications requiring a uniform high field.
To gain access to the interior of the structure, holes may be drilled
through either the superconducting sheet(s) or the permanent magnet
structure depending on purpose. As an additional advantage either or both
superconducting sheets can be removed to provide access to the central
cavity. One use for this embodiment of the present invention is as an
oratron.
In another embodiment of the present invention a plurality of truncated
HCFS structures are arranged in an axially aligned linear array so that
their interior working space magnetic fields are 180.degree. apart. That
is, the interior magnetic fields of successive HCFS in the array alternate
in direction. Superconducting sheets are sandwiched between adjacent
truncated HCFS structures and also cover the end faces of the array. The
superconducting sheets abutting the end faces of each truncated HCFS
confine the flux or magnetic field to the interior of each ring, establish
a uniform field in the interior, and isolate each ring from its oppositely
oriented nearest neighbors thereby preventing distortion of the field by
neighbor-induced counterfields.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully appreciated from the following detailed
description when the same is considered in connection with the
accompanying drawings in which:
FIG. 1 is a perspective view of a truncated octagonal hollow cylindrical
flux source structure with superconducting sheets abutting the end faces
according to the present invention; and
FIG. 2 is a perspective view of a plurality of the FIG. 1 structures
arranged in a stack so that the interior magnetic fields continually
alternate in direction, that is, the successive fields are 180.degree.
apart.
DETAILED DESCRIPTION
FIG. 1 shows an embodiment of the present invention which comprises a
truncated hollow cylindrical flux source (HCFS) or "magic ring" 101 and a
pair of superconducting sheets 102 and 103 covering the end faces of the
truncated HCFS. The plates or sheets 102 and 103 are shown in FIG. 1 as
being peripherally coextensive with the HCFS. However, as will be evident
to those skilled in this art, the plates 102, 103 can extend beyond the
flux source 101 in one or more directions. It is only important that they
be not less in extent than the source 101.
As previously indicated, an ideal HCFS or "magic ring" is an annular
cylindrical shell that produces an intense magnetic field in its central
cavity. It is not feasible to construct an ideal HCFS, however, and so in
practice a segmented approximation is resorted to. In such a configuration
the magnetization is constant in both magnitude and direction within any
one segment. Fortunately, even with as few as eight segments, more than 90
percent of the field of the ideal structure is obtainable.
In fact, an octagonal approximation to the ideal magic ring, such as shown
in FIG. 1, appears suitable for almost all applications. The segmented and
octagonal approximations to an ideal HCFS are disclosed in the article
"Impact of the High-Energy Product Materials on Magnetic Circuit Design"
by H. A. Leupold, et al, Materials Research Society Symposium, Proc. Vol.
96 (1957), pp. 279-306, esp 297.
A central cavity 104 exists in the interior of the HCFS/superconducting
sheets pyx which contains a strong magnetic field designated by arrow 105.
Magnet segments 106, 107, 108, 109, 110, 111, 112 and 113 form sections of
a truncated octagonal HCFS or "magic ring". If the ring formed by the
eight segments were infinitely long, the magnetic field 105 in the
interior of the HCFS would be substantially uniform within the interior.
However, because each of the segments has finite length, there is
distortion of the magnetic field in the interior. Unfortunately use of the
magic ring in various electronic devices demands that the length of the
magic ring be limited.
Each of the magic ring segments has a unique magnetic orientation or
magnetization, M. For convenience, segments 106 and 110 will be referred
to as magnetically oriented in a northerly direction. Thus, in the figure,
segments 106 and 110 have arrows pointing in a northerly direction.
Segments 108 and 112 are oriented in a southerly direction. Segments 107
and 111 are oriented in an easterly direction, while segments 109 and 113
are oriented in a westerly direction. (The aforementioned compass
directions serve merely to provide a convenient frame of reference and
should not be confused with magnetic north and south poles).
The magnetic field strength H.sub.w represented by arrow 105 within the
working space of the HCFS is assumed to be known. Design procedures known
to those skilled in the art permit one to calculate the magnetic field
strength within the interior of a magic ring when the inner and outer
radii of the ring are known, together with the remanence, B.sub.r, of the
magnetic material comprising the ring. For example, for an ideal,
infinitely long magic ring, the magnetic field strength is given by
H.sub.w =B.sub.r ln (r.sub.2 /r.sub.1)
where
r.sub.2 =outer radius of ring
r.sub.1 =inner radius of ring
B.sub.r =remanence of ring material
Of course, the truncated HCFS illustrated in FIG. 1 has an octagonal cross
section and so the above ideal formula does not give an entirely correct
result. However, it is a close approximation. A more accurate value for
H.sub.w is given by: H.sub.w =[B.sub.r sin (2.pi./M)]/(2.pi./M) where M is
the number of segments in the approximation.
Since the magic ring is truncated, the field inside the working space is
not uniform. However, the superconducting plates 102, 103 act as
diamagnetic mirrors to the field abutting the plate surface. Thus, the
image of the cavity field in the superconducting planes "continues" the
cavity field longitudinally in both directions in space. Therefore, an
infinitely long HCFS cavity appears (magnetically). Thus, the cavity field
is made uniform.
There is normally flux leakage from the interior to the exterior of an open
ring. However, a magnetic field cannot penetrate a superconducting plate.
Therefore the addition of superconducting plates 102 and 103 to the end
faces of the HCFS structure will prevent magnetic flux from escaping the
interior of the pyx structure.
The plates or planar sheets 102 and 103 can, in bulk form, be composed of
tin, lead, niobium, tantalum, etc. Each of these materials, and others,
are known to be superconducting below a distinct critical temperature.
Moreover, recent developments in the field of superconductivity have
produced a large variety of new ceramic-type materials which are capable
of achieving the superconducting state at critical temperatures above
77.degree. K., the boiling point of liquid nitrogen. By way of example, an
entire class of superconducting compounds with the chemical composition
RBa.sub.2 Cu.sub.3 O.sub.9-y (where R stands for a transition metal or a
rare earth ion and y is a number less than 9, preferably 2.1.+-.0.05) has
demonstrated superconductive properties above 90.degree. K. The
superconducting ceramics are formed by plasma-spraying techniques, or
sputtered or evaporated onto a substrate (magnesium oxide), or produced by
growing epitaxial films (RBa.sub.2 Cu.sub.3 O.sub.7-x) on a substrate
(e.g. SrTiO.sub.3), etc. It is to be understood, however, that the present
invention is in no way limited to the superconductivity material selected
for the superconductor planar sheets or, if the same is a
superconductive-ceramic, the manner in which the same is formed.
There are various techniques known to those skilled in the art for bringing
(i.e., cooling) the superconductor plate material to its distinct critical
temperature. For example, the use of cryorefrigerator means such as
disclosed in the co-pending application of L. J. Jasper, Jr., Ser. No.
068,389, filed June 12, 1987, can be advantageously utilized to cool the
superconducting plates. However, since the same comprises no part of the
present invention and one or more of these known techniques will suffice
for present purposes, further detailed discussion herein would not appear
to be necessary.
This embodiment of the present invention can be used in any application
requiring a uniform high field. Access holes can be drilled into either
the superconducting sheets or into the HCFS depending on the purpose
proposed for use of the invention. In addition the superconducting sheets
may be removed for access into the central cavity. One example of the use
of this embodiment is an oratron.
FIG. 2 shows a series of truncated HCFS structures 201 arranged to form a
sequence of fields 202 and 203 of alternating up-down-up-down
magnetization. The truncated HCFS structures are cut away for illustrative
purposes to reveal the central cavity. These truncated HCFS structures are
separated by superconducting sheets 204 which separate the adjacent
cavities. The superconducting plates or sheets shown in the figure are
typically quite thin. In practice, the only requirement is that the plates
or sheets be thicker than the penetration depth of the specific
superconducting material used. By way of example, the superconducting
sheets might comprise a thick film one or more millimeters in thickness.
Alternatively, if the superconducting plates are also intended to serve as
"spacers" between adjacent permanent magnets, they will, of course, be
thicker (e.g., several centimeters). Accordingly, it is to be understood
that the present invention is in no way limited to any specific
superconducting material or even the relative thickness of the same.
A bore hole 205 is drilled through each superconducting plate along the
central axis of the array. This series of bore holes permits passage of an
electron beam through the working central cavities of the HCFS array. This
embodiment as described functions as a wiggler; a wiggler is a high power
(megawatt) radiation source. In wiggler operation, an electron beam is
injected into a drift region which is surrounded by a periodic magnet
source. The periodic magnet source creates a magnetic field which changes
in direction (by 180.degree.) at fixed intervals, yet is always
perpendicular to the principal direction of electron beam travel. When
such a wiggler is constructed according to the present invention it can be
termed a "pyx wiggler".
It is a feature of this invention to provide a permanent magnet structure
with minimal internal field distortion within each segment of the pyx
wiggler array. There are two sources of distortion effects, both of which
are corrected by the present invention. These sources are (1) distortion
caused by the bending of the field lines at the end faces of an open HCFS
and (2) distortion caused by interference with incoming flux leaking from
neighboring open segments. The addition of superconducting sheets 204
overcomes both these problems.
A superconducting surface does not permit a magnetic field to penetrate.
The addition of the superconducting sheets confines outward flux leakage
from the working cavity of the segment and thereby prevents the bending of
the field lines at the end faces which would have occurred without the
addition of the sheets. Therefore, the field is made substantially
uniform. An alternate way to consider this effect utilizes the concept of
diamagnetic mirrors. The superconducting sheets 204 magnetically mirror
the field abutting the surfaces of the sheets. Thus, the field of each
cavity appears to continue infinitely long in both directions when viewed
from within each cavity. A theoretical HCFS is infinitely long. Therefore,
a theoretical HCFS is created magnetically and in such an HCFS the field
is uniform. The addition of superconducting sheets also prevents
penetration of flux into neighboring cavities because no flux is capable
of exiting each of these neighboring cavities. In this manner, the effect
of interference from adjacent segments is eliminated, leaving the field
within each pyx cavity unaffected by its neighbors. Both sources of
distortion effects, that is, those caused by outward leakage and also
those caused by incoming interference effects are eliminated by the
addition of superconducting sheets. The goal of creating a uniform high
field within each cavity is achieved.
Of course, truncated HCFS segments with circular cross sections or
sixteen-sided cross sections, or thirty-two sided cross sections might
also be employed in accordance with the present invention. Other
components of the wiggler well known to those skilled in the art of design
of such devices have been eliminated from the discussion. In a typical
embodiment of the present invention, the array might consist of a series
of ten magnetic pyxes. However, it should be borne in mind, that greater
or fewer magnetic pyxes may be desirable in any given application.
Accordingly, having shown and described what is at present considered to be
several preferred embodiments of the invention, it should be understood
that the same has been shown by way of illustration and not limitation.
And all modifications, alterations and changes coming within the spirit
and scope of the invention are hereby meant to be included.
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