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TECHNICAL FIELD
The present invention relates in general to permanent magnet structures for
use in electronic devices which act as radiation sources and, more
particularly, to magnet structures in wigglers and twisters.
BACKGROUND OF THE INVENTION
Many devices that employ magnetic fields have been encumbered by massive
solenoids accompanied by bulky power supplies. However, permanent magnet
structures can provide compact, strong, static magnetic fields that do not
require the use of additional power supplies. Thus, there has been
increasing interest in applications using permanent magnet structures.
Often these permanent magnet structures must be designed in unusual
configurations. A number of configurations have been designed and
developed for electron beam guidance in electron beam tubes of various
types. Especially promising for such purposes is the configuration based
upon the hollow cylindrical flux source (HCFS) 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
hollow cylindrical flux source (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 across the central
cavity. The field is perpendicular to the central, longitudinal 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. No
magnetic flux extends to the exterior of the HCFS structure except at the
ends of a finite cylinder. The ideal HCFS is an infinitely long, annular
cylindrical shell with a circular cross section. However, the
aforementioned Halbach publication discloses an HCFS structure with an
octagonal cross section which closely approximates the performance and
field pattern of an ideal HCFS. The "HCFS structure" as used herein
encompasses not only the ideal cylindrical structure but also other
polygonal sided structures which behave with the characteristics of an
HCFS.
Recently, HCFS structures have been applied to the design of wigglers and
twisters. Reference may be had to "Applications of yokeless flux
confinement", J. Appl. Phys. 64(1)), 15 Nov. 1988 for example. More
specifically of interest in this regard is applicant's U.S. Pat. No.
4,862,128 entitled "Field Adjustable Transverse Flux Sources" on which the
invention of the instant application is an improvement. A wiggler is a
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 varies in direction
by 180.degree. at fixed intervals, yet is always perpendicular to the
principal direction of electron beam travel. A twister is also a radiation
source. In twister operation, an electron beam is injected into a drift
region in which there is a transverse magnetic field of constant magnitude
whose direction changes continuously with progression along the axis,
thereby forming a helical field configuration with either constant or
progressive pitch. The central cavity of the HCFS structure functions as a
drift region in those wigglers and twisters using these HCFS
configurations.
SUMMARY OF INVENTION
It is an object of this invention to provide a permanent magnet structure
which by simple mechanical adjustment of the apparatus may be used to
individually produce both the necessary magnetic field patterns required
in either a wiggler or a twister.
It is another object of the invention to provide a permanent magnet
structure wherein the interior magnetic flux can be varied continuously
from zero to a maximum field when used in the wiggler configuration.
This object and other objects are achieved in accordance with the present
invention, which makes advantageous use of multiple HCFS structures
uniquely combined to create an adjustable apparatus with dual functioning
capabilities.
The present invention comprises a linear array wherein each of the
components of such array are composed of a plurality of truncated,
concentric, hollow cylindrical flux source structures such that the HCFS
structures have a common longitudinal central axis in the central cavity
and such that the outer radius of each HCFS structure beginning with the
innermost structure substantially equals the inner radius of the
immediately adjacent larger structure. Structures in this configuration
will be termed "nested structures." The present invention is so
constructed that each HCFS structure can be rotated about the central axis
and can be displaced linearly parallel to the direction of the central
axis.
The HCFS structures are characterized by their ability to add magnetic
fields vectorially. Therefore, the resultant field in the central cavity
is the vector sum of the fields generated in the central cavity by each of
the component HCFS structures working independently.
In the preferred embodiment of the present invention, the plurality of
truncated, nested HCFS structures are limited to a pair of nested HCFS
structures comprising an inner and an outer HCFS structure. Nested pairs
of HCFS structures are arranged in a linear array along a longitudinal
central axis. Each HCFS structure is designed to be of a width equaling
one half the period of the desired modulatition waveform, i.e. .lambda./2.
Each HCFS structure is designed to independently generate a field of a
magnitude equal to that of each of the other HCFS structures in the linear
array. There are two configurations of the preferred embodiment which
allow the present invention to function both as a wiggler and as a
twister. Configuration changes which afford the two preferred modes of
operation are achieved by simple mechanical adjustment of the device.
In one configuration, the inner HCFS structures of the array are oriented
so that the magnetic vector components in the central cavity caused by
adjacent HCFS structures are 180.degree. apart. That is, the interior
magnetic field components of adjacent inner HCFS structures alternate in
direction. The outer HCFS structures are positioned directly above the
inner segments, that is, relative linear displacement between inner and
outer segments in each pair is zero. The outer segments of the array are
also oriented so that the magnetic vector components in the central cavity
caused by adjacent HCFS structures are 180.degree. apart. That is,
interior magnetic vector components of adjacent outer segments alternate
in direction. In this configuration, the present invention is composed of
a pair of nested wigglers. The resultant function for the device as a
whole is wiggler operation. By rotation of either all inner or all outer
HCFS structures or both the field may be made to vary continuously from
zero to a maximum for the device. Thus, the apparatus when in this
configuration can function as a field variable wiggler. To maximize the
magnetic fields in the central cavity, the magnetic field components of
inner and outer rings can be aligned.
It is an advantageous feature of the present invention that mechanical
adjustment permits the present invention to also function as a twister.
Assuming that the apparatus as described has been oriented to produce
maximum magnetic fields in the central cavity by alignment of the magnetic
vector components in each of the pairs, then a simple, specific relative
linear displacement and rotation will convert the apparatus from operating
in a wiggler mode into operating in a twister mode. More specifically, by
displacing either all the outer HCFS structures or all the inner HCFS
structures by one quarter period in a direction parallel to the central
axis of the linear array and by rotating either all the outer HCFS
structures or all the inner HCFS structures by 90.degree. around the
central axis of the linear array, the structure may be configured to
function in the twister mode. In this configuration of the preferred
embodiment, the pattern of the magnetic field components in the central
cavity is one of two substantially sinusoidal waves whose vibrational
directions are at right angles to each other and with the same direction
of propagation. The sine waves are 90.degree. out of phase and have the
same amplitude. Whenever two sine waves are so defined, the resultant
vector forms a helix pattern, which is the field pattern in the drift
region of a twister. Therefore this invention allows for conversion
between a wiggler mode and a twister mode by simple mechanical adjustment.
It is this simple mechanical adjustment enabling the invention to be
readily changed from a wiggler to a twister which comprises the primary
advantage over that invention described in applicant's U.S. Pat. No.
4,862,128. This simplicity of adjustment is brought about by this
invention's ability to provide relative linear displacement of the inner
and outer HCFS structures. In the preferred embodiment, two nested
wigglers are used. Thus the device can be termed an "adjustable biwiggler
twister.
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 an exploded schematic diagram of a preferred embodiment of the
present invention showing pairs of nested, truncated, hollow cylindrical
flux sources structures arranged in a linear array. A common longitudinal
central axis is assumed to run the length of the central cavity of the
linear array but is not shown. FIG. 1 illustrates a wiggler mode
configuration of the present invention. The drawing shows the magnetic
field orientations internal to the HCFS structures and also the magnetic
field orientations in the central cavity.
FIG. 2 is an exploded diagram of the twister mode configuration of the
present invention.
FIG. 3 is an compacted diagram of the exploded view of the twister mode
configuration as shown in FIG. 2.
FIG. 4 is a vector diagram of the magnetic field components of the inner
and outer HCFS structures positioned in the twister mode configuration
illustrated in FIGS. 2 and 3.
FIG. 5 is the vector diagram of the resultants of the components of the
inner and outer HCFS structures positioned in the twister mode
configuration as illustrated in FIGS. 2 and 3.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an exploded view of the preferred embodiment of the present
invention in one of two preferred configurations of that embodiment. The
preferred embodiment comprises a linear array of nested pairs of truncated
hollow cylindrical flux source structures, 101 and 102. Inner HCFS
structures are denoted by 101; outer structures by 102. Each HCFS
structure has a width equal to one half period of the modulation waveform,
i.e. .lambda./2. Arrows 103 illustrate the magnetic orientations of the
fields interior to the HCFS structures. The resultant fields in the
central cavity are represented by arrows 104.
The magnetic field strength H.sub.w independently produced by each HCFS
structure is assumed to be known. Design procedures known to those skilled
in the art permit one to calculate the magnetic field strength within the
central cavity of an HCFS structure when the inner and outer radii of the
structure are known, together with the remanence, B.sub.r, of the magnetic
material comprising the HCFS structure. For example, for an ideal,
infinitely long HCFS, 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 the HCFS
r.sub.1 =inner radius of the HCFS
B.sub.r =remanence of the HCFS material
In accordance with the present invention, the magnetic field strengths
H.sub.w.sbsb.1 and H.sub.w.sbsb.2 add vectorially. That is, H.sub.w
=H.sub.w.sbsb.1 +H.sub.w.sbsb.2.
In the preferred embodiment of the present invention, the magnitudes of the
field strengths of the structures are designed to be equal.
Each inner HCFS structure, 101, is oriented such that the pattern of vector
components in the central cavity from the inner HCFS structures varies by
180.degree. going from inner structure to inner structure. The outer HCFS
structures, 102, are also arranged to alternate in direction. Therefore
the present configuration comprises a pair of nested wigglers. The
magnetic field components of the HCFS structures of each nested pair add
as vectors. Therefore, the resultant in the central cavity also alternates
by 180.degree. with progression along the length of the linear array.
Thus, the present invention can function as a wiggler. Since the HCFS
structures can rotate, the magnitude of the field in the central cavity
can vary. In FIG. 1 the inner and outer HCFS structures are positioned by
appropriate rotations to cause maximum field magnitudes by aligning the
components of inner and outer structures.
The terms HCFS, HCFS structure, and "magic ring" encompass not only the
ideal cylindrical structure in which mathematically the azimuthal field
dependence is assumed continuous but also segmented approximations in
which each segment has the magnetization constant in both amplitude and
direction within any one segment. The invention is not limited to any
specific number of segments and the greater the number of segments the
closer the approximation to the ideal case. The "HCFS structure" as used
herein encompasses not only the ideal cylindrical structure but also
polygonal sided structures.
As is evident to those skilled in the art, there are many ways to
structurally mount the magnets of the present invention. For example, the
magnetically active structures may be mounted on hollow cylinders of
non-magnetic material such as stainless steel or brass. The ends of these
cylinders can project from either/or both ends of the magnetic array. At
the ends, they may be hollow or solid. The cylinder may be attached to
some means of rotation such as a motor. A ball detent may be used as a
locating mechanism to set the array either circularly and/or
longitudinally at the correct positions.
Although circular and polygonal sided HCFS structures are described, as
will be evident to those skilled in the art, the principle does not depend
on HCFS structures for its application. Any magnetic slice that produces a
transverse magnetic field and is free of iron or other soft magnetic
material can be used.
FIG. 2 illustrates an exploded view of the preferred embodiment in another
preferred configuration. As in FIG. 1, each inner HCFS structure, 201, is
oriented such that the field components in the central cavity of the inner
HCFS structures alternate in direction going from inner structure to inner
structure. A similar pattern exists for the set of outer HCFS structures
202. Therefore, this configuration is also characterized as a pair of
nested wigglers. However, there is a relative displacement and a relative
rotational difference between inner and outer structures which
distinguishes the position shown in FIG. 2 from that shown in FIG. 1.
There is a relative displacement of one quarter period (.lambda./4) as
shown and a relative rotational change of 90.degree..
Displacement and rotation form the basis for variation of the field
magnitude and direction in the central cavity of the present invention.
Displacement by .lambda./4 and rotation by 90.degree. affords the
conversion of the device from the wiggler mode to the twister mode and the
reverse. To understand how the configuration shown in FIG. 2 functions as
a twister, reference should be made to FIGS. 4 and 5 and to the
accompanying discussion.
FIG. 3 illustrates a cut-away view of the configuration pictured in FIG. 2.
The magnetization orientations, 301, within the HCFS structures are shown.
The magnetic field orientations created in the central cavity by the inner
HCFS structure are given by vectors, 302, and those of the outer HCFS
structures by vectors, 303.
For each HCFS structure a maximum field in the central cavity is achieved
at the midpoint of the structure. The field tapers to a minimum at the
ends of the structures. This variation in the magnitude of the magnetic
fields occurs because the HCFS structures have been truncated to a finite
length and end effects are present, and there is also interference between
adjacent structures. The variation of the field strength in the central
cavity from each HCFS structure traces a sine wave. Both inner and outer
HCFS structures exhibit this sine wave pattern. The component fields
superimpose.
FIG. 4 is the vector diagram of the field components of inner and outer
sets of HCFS structures in the configuration shown in FIGS. 2 and 3.
Vectors, 401, are components of the inner segments; vectors, 402, are
those of the outer structures. Two sine waves are traced. The sine waves
have vibrational directions at right angles to each other and with the
same direction of propagation. They are 90.degree. out of phase and have
the same amplitude.
FIG. 5 shows the resultants, 501, of the components described in FIG. 4. A
helix is traced.
Therefore a twisted helically oriented magnetic field of constant magnitude
is formed along the length of the central axis. If an electron beam is
passed through the central cavity, the device is a "twister." For a more
detailed description of a twister, reference can be had to my co-pending
application, Ser. No. 316,374, filed Feb. 24, 1989.
To summarize, using the preferred embodiment of the present invention as
described, a relative linear displacement of .lambda./4 and a relative
rotational displacement of 90.degree. between inner and outer HCFS
structures enables the present invention to alternate reversibly between
wiggler and twister modes.
Other and different approximations to the adjustable twister may occur to
those skilled in the art. Accordingly, having shown and described what is
at present considered to be a preferred embodiment 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 herein meant to be
included.
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