Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
A rare earth magnet such as samarium cobalt or cerium cobalt has
been used as magnets for microwave tubes, for example as shown in United
States Patent No. 3~781J592~ December 25, 1973, W. J. ~larrold. Ilowever~ such
devices have generally been positioned sufficiently far from sources of heat
in the microwave tubes so that a relatively low temperature such as 125C was
not exceeded. As a result, additional weight of material for the pole piece
and an additional amount of permanent magnet material was generally required.
When rare earth permanent magnet material was used over an extended period of
time in air even at temperatures somewhat below 125C, the magnet properties
of the rare earth magnet were altered generally reducing ~he energy product
and changing the operating characteristics of devices such as microwave tubes.
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Summary of the Invention
In accordance with this invention, there is disclosed
the discovery that rare earth magnets can be operated at
substantially higher temperatures in a protected environment
such as a vacuum or inert gas for extended periods of time
without permanent alteration of the rnagnetic properties.
More specifically, tests have shown that temperatures in excess
of 250C may be used for extended periods of time without
any substantial permanent change of the magnet material.
In accordance with this invention, a variety of applications
o rare earth permanent magnets to microwave tubes may utilize
the magnet material directly in the desired region without
additional pole pieces for field concentration and/or
magnetic flux return paths. Such benefits are achieved by
reason of the high 0nergy product of the rare earth magne~
ma~erial and the fact that such energy product is not
permanently al~ered by R~ ~ields in devices such as magnetro~s,
amplitrons, or travelling wave ~ubes using heated cathodes
having a transverse magnetic field of a few thousand gauss
produced by the rare earth permanent magnet system.
In one embodiment of the invention, a travelling wave
tube of the O-type has a beam directed down an interaction
path produced, for example, by slow wave structure such as
helix while providing permanent magnets in regions outside
the helix to supply magnet bias for ferrite material oriented
to present a minimal insertion loss to signals travelling
on the helix in the same direction as the electron beam and
a substantially greater insertion loss to signals travelling
on the helix in a direction opposite to the beam, to prevent
oscillation of the tube when used as an amplifier due ~o
reflections from the impedance mismatches at the output of the helix and/or
at the input of the helix. The close proximity of rare earth magnets to the
slow wave structure which is being heated by impingement of stray electrons
from the beam is possible without the temperature of the magnet material ex-
ceeding temperatures such as 250C.
In addition, this invention further discloses that the magnet mate-
rial may be cooled by thermal conduction through the support structure to an
outside surface structure of the tube so that thermal energy radiated to the
rare earth magnet material by hot portions of the tube such as the cathode or
anode is conducted away at a rate causing thermal equilibrium of the magnet
material at a temperature below its long-term degradation temperature.
This invention further discloses that the permanent magnet material
may be encapsulated in a thin layer of conductive, substantially thermal,
radiation reflective material such as copper which further prevents heat of
the magnet material, such conductive layer being in general insuficient in
thickness to provide the wall between an evacuated area and atmospheric pres-
sure between being of sufficient thickness to concluct heat away from the
region which may be generated due to impi-ngement o stray electrons thoreon
or to thermal radiation.
In accordance with the invent:i.on there is provided a microwave tube
comprising: an evacuated envelope containing an anode structure having a
plurality of resonators formed therein and surrounding a central bore contain-
ing a cathode; and means for producing a magnetic field transverse to the
direction of motion of electrons from said cathode to said anode comprising
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~permanent magnets supported wholly within the vacuum in said envelope adjacent
the ends of said cathode; said magnets being shielded :Erom electrons emanating
from said cathode; and said cathode having end shields with annular grooves
providing substantially field free regions adjacent the ends of said cathode.
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Brief Description of the Drawings
Other and further objects and advantages o~ this
invention will become apparent as the description thereof
progresses, reference being made to the drawings wherein:
Fig, 1 illustrates a longitudinal sectional view, taken
along line 1-1 of Fig. 2 and of a magnetron embodying the
invention;
Fig. 2 illust~ates a transverse sectional view of the
em~odiment illustrated in Fig. 1 taken along line 2-2 o-f Fig. l;
Fig~ 3 illustrates a diagram of the characteristics of some
permanent magnets including rare earth types useful in this
inve~ntion;
Fig. 4 illustrates a longitudinal sectional YieW of a
travelling wave arnplifier taken along line 4-4 of Fig. 5~ and
illustrating an alternate em~odiment of the invention; and
Fig. 5 illustrates a transverse section view o~ the
embodiment of the invention illustrated in Pig. 4 taken along
line 5-5 of Pig. 4.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Figs. 1 and 2, there is shown a magnetron
10 comprising an anode cylinder 12 made, for example, of a
material having high permeability such as steel coated with
copper. A plurality of anode members 14 extend radially inwardly
from cylinder 12 to a central bore 16 containing a cathode 18
of the directly heated type utilizing a carbonized tungsten
filament 20, which is helically coiled and is attached at its
upper end to a central support rod 22.
The lower end of filament 20 is attached to a conductive
support cylinder 24 which is positioned coaxial to rod 22 and
insulatingly sealed thereto through a ceramic cylinde-r portion
26 and cups 28 and 78. Upper and lower cathode end shields
30 and 32 are attached respectively to the upper end of suppoTt
rod 22 and the upper end of support cylinder 24.
In accordance with this invention, upper and lower annu:Lclr
rare earth permanent magnet members 34 and 36, such as SmCos,
are positioned coaxi.al with cathode 1~ and respecti.vely above and
below end shields 30 and 32
~s shown hereln, by way of example only, permanent magnet
members 34 and 36~ which preferably are both poled in the same
direction axailly to cathode 18 to produce a magnetic field,
are positioned substantially coaxial with the cathode 18 and
extend radially from a point inside the diameter of filament 20
to a point outside the diameter of bore 16.
In accordance with this invention, magnets 34 and 36 are
positioned as close as practicable to the interaction space
between the inner ends of anode members 14 and the filament 20
in order that the amount of magnet material required to produce
the desired magnetic field density is minimized.
In accordance with this invention, it is disclosed
that rare earth magnets in an inert environment such as a vacuum
can withstand high temperatures while maintaining stable
magnetic characteristics. ~or example, temperatures in the
range between 150C and 250C during continuous operation as
well as higher temperatures up to 500C d-uring short periods
of hours to days can be achieved. The magnetron, as
illustrated herein, may be used, for example, in a microwave
oven operating with a voltage between the filament 20 and
the anode members 14 of around 4,000 volts. .~t an average
current of around 300 mils will result in heating oE the
tips of the anode ~embers 14 to several hundred degrees. In
addition, filament 20 is preferably heated to temperatures in
the range of lj400C to 1,700C. Ileat from the tips of the
anode members 14, which produces no useful function, ls conduc~ed
away from the tips o-f vanes 14 to the anode cylinder 12 where
it may be dissipated, :Eor example, by f:ins (not shown)
contacting the outside oE cylinder 12. I-lowever, thcrmal
radiation :Erom inner ends o anod0 members 14 as well as thermal
radiation from :fi.l.ament 20, which may be reflected by the
shiny copper surfaces o:E anode members 14 and cylinder 12,
can be radiated toward magnets 34 and 36. In addition, some
stray electrons, which can escape from the interaction
region of bore 16 between the end shields 30 and 32 may move
toward the magnets 34 and 36. Therefore, thermal energy
absorbed by the magnets 34 and 36 is preferably dissipated to
prevent such magnets from exceeding temperatures during
operation of, for example, 150C to 2S0C.
Upper and lower cups 44 and 46 of material having high
thermal reflectivity and thermal conductivity, such as copper,
are positioned around magnets 34 and 36, respectively, to
reflect such thermal energy as may be radiated toward magnets
34 and 36, and to intercept such stray electrons as escape
~rom the interaction region and impinge on cups 44 or 46
rather than the surfaces oE magnets 34 or 36. Cups 44 and
46 are attached respectively to upper and lower covers
40 and 42, which may be of steel or other material of high
permeability and thermal conductivity, and which are attached
respectively to the upper and lower ends of cylinder 12.
Cups 44 and 46 are of su~icient strength to hold magnets
34 and 36 tight].y in p].ace against covers 40 and 42 and have
spaces 33 for gases in magnets 34 and 36 to escape during
evacuation and bake out of the magnetron.
Since cylinder 12 and covers 40 and 42 are of high
permeability material, a low reluctance magnetic path is
formed therethrough and a major portion of the magnetic
flux produced by the magnets 3~ and 36 and passing through
the electron interaction sapce between the tips oE the anode
members 14 ancl cathode l8 returns through anode cylinder 12
and covers 40 and 42. ~s a result, an interaction space .Elux
l z~ o~D 7, o~
density of, for example ~ to ~ gauss may be achieved
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with the relatively s~all.~are earth permanent magnets, which
being positione~ inside a magnet return path structure produce
extremely low stray magnetic fields outside the magnetron.
Retaining cups 44 and 46 are preferably attached to covers
40 and 42 by means, such as spot welding, in regions spaced
from the magnets 34 and 36, for example as at points 48 and
50, to avoid overheating magnets 3~ and 36. It should be
understood that the size, shape, and spa.cing oE the magnets
34 and 36 may be adjusted to produce any desired intensity
of magnetic field in the interaction space and -that such
intensity may be tapered in the region o-E the end shields to
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interact with stray clectrons moving axially of the cathode.
Referring now to Fi~ure 3, there is shown a graph of
the second quadrant hysterisis characteristics of various
magnetic materials in which magnetizing force H is oersted and
the flux density B in gauss. Curve 50 shows a rare earth
cobalt such as SmCo5 which is preferably formed of grains of
SmCo5 the majority of which have a size less than that which
will support two domains hereafter referred to as single
domain grains of SmCoS. Such grains are preferably bonded
together by materials which may include samarium oxide or
other samarium cobalt compounds which prevent grain growth.
At a flux density of, for example, 1,800 gauss as shown
by point 52 on curve 50, a coerslve force of approximately
7,500 oersteds will be present. Thus since the primary
reluctance is in the interaction space between the magnets
such as a gap can be on tha order of ive times the total
axial distance through the magneks 34 and 36. It shvuld thus
be noted that such magnet material could in fact be utilized
without a maglletic return path by utilizing a greater welght of
magnet material. ~lowever, since the envelope 12 and the
magnet suppor~ covers 40 and 42 are preferably of materials
having a large strength to weight ratio such as steel, whose
inner surfaces are preferably plated with high conductivi*y
material such as copper for a thickness OfJ for example, one
mil it becoDIes economically advantageous to use these members
as a magnetic flux r0turn path. The substantial improvement
of rare earth magnets over permanent magnets of alnico 5,
alnico 8, ferrite, and platinum cobalt is shown by curves
54, 56, 58, and 60 respecti.vely. ,~t 1,800 gauss, alnico 5,
as shown by curve 54, has a coersive force of less than
500 oersteds so that the total magne-t length must be four
to five times the air gap distance thereby substanti.ally
increasing the total path length of the alnico magnets as
well as requiring a substantially additional weight. .~err:ite,
alnico 8, and p].atinum cobalt similarly required larger
magnets, best of the group being platinum cobalt which is
extremely expensive and, hence, economically impractical.
It should be clearly understood that SmCo5 is shown by
way of example only and other rare earth cobalts such as
cerium cobalt could be used for the magnet material. By
maintaining the material of the rare earth ~agnets 34 and
36 below 250C, the tube can be operated for thousands of
hours without sufficient shift in the characteristics of
the magnets 34 and 36 t.o substantially aEfect the ef:Eiciency
of the magnetron. In addition, a:fter iassernbly o:E the tube,
i.t may be heated to 400-150C or even 500C during evacuation
and bake out o:E the i.nter.i.or o:E the tube.
During operation, microwave energy generated by the
magnetron is extracted from the resonant anode structure 14
by an output probe 62 connected to the upper edge of one of
the anode members 14 and extending through an aperture 64 i.n
upper cover 40 and upwardly through a metal cylinder 66
coaxial with the axis of the tube to pinch off seal tubulation
68 through which the tube is evacuated. Tubulation 68 is
attached to cylinder 66 through a ceramic cylinder 70 to
provide a vacuum seal, in which tubulation 68 is insulated
from cylinder 66, and to provide an output aperture through
which microwave energy is radiated by probe 62 to a microwave
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energy load such as a microwave oven. Tubulation 68 is
covered by a c~p 72 to proteck the tubulation 68 and to
provide a smooth outer surface radiation~
In order to prevent molding of the magnetron,
straps 82 alternately connect the upper and lower edges
of the inner ends of anode member 14 in accordance with
well-known practice. If desired, end shields 30 and 32 may
have grooves 84 therein to suppress axial mode oscillations
during tube warm-up in accordance with the teaching of
Canadian Patent No. 1,084,121, August 19, 1980, John M.
Osepohuk.
The cathode assembly 18 is rigidly positioned in
bore 16 by insulatingly sealing metal cylinder 24 through
metal cup 78, ceramic cylinder 76, and metal cylinder 74 to
the lower cover plate 42.
While the magnets retaining cups 44 and 46 are
illustrated herein with apertures 33 to expose the magnets
to the vacuum withi.n the magnetroT~ desired, the magnets
34 and 36 may be ~ncapsulated between cups 44 and 46 and
covers 40 and 42 respectively.
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~escription of ~
Referring now to Figs. 4 and 5, there is shown a
travelling wave tube 11~ embodying the invention. Tube
11~ comprises a tubular envelope 112 of conductive metal such
as copper containing a helical slow wave structure 114
supported by three insulating supports 116 and connected at
one end to a signal input structure 118 and at the other
end to a signal output structure 120.
A cathode 122 is positioned at the end o-f the helix 11
which is connected to the input structure 118. Cathode 122
is supported from an insulated sleeve 12~ sealed to tubular
member 112 and a grid structure 12~ is insulated from both
the cathode and the tubular member. Cathode 122 is heated
by a heater 128 sealed through an insulating seal supported
by sleeve 12~. The other end o the helix has positioned
adjacent thereto a load in-to which electrons emitted ~rom
the cathode 122 and passing through helix 11~ are dlr~cted to
be absorbecl. ~Sw~h a travelling wave tube as is well known
can be made to arnplify microwave signals over a wide band of,
for example, an octave by directing a beam of electrons past
the helix while introducing a signal wave at one end which
travels along the helix substantially in synchronism with
the electron beam and is extracted in amplified form at the
other end of the helix. ~lowever, reflections from the output
end of the helix to the input, due for example to mismatched
signal input and output loads~ can be re-reflected from the
output to the input to cause the device to oscillate or
produce undesirable ampli~ication characteristics. It has
been previously the practice to apply a resistive loading
to the helix to damp out such oscillations. Such loading
V~ 3
may be, for example, aquadag applied to portions of the
helix or as lumped constant loadlng surrounding the
helix.
In accordance with this invention, ferr;te structures
are positioned outside the helix in fringing microwave
fields with unidirectional magnetic fields applied thereto
by rare earth permanent magnets positioned inside the
vacuum envelope to produce magnetic field components in a
circumferential direction about the helix. Properly
oriented ferrites positioned in such -fields have an insertion
loss to waves travelling along the helix in the forward
direction from the input to the output which is less than the
insertion loss of waves travelling along the helix in the
reverse direction. As a result, less power is absorbed from
the amplified wave moving in the forward direction than would
otherwise be necessary if an isotropic loss medium were
used and less heating thereby generated.
In accordance w:ith ~his inv0ntion, there is shown in
~lg. 5 a plurality of Eerrite slabs 130 positioned in the
spaces within the tubular memher 112 between the support
structures 116. Such ferrite slabs 130 are positioned between
permanent magnet slabs 132 of a rare earth co~alt in
accordance with this invention and the outer surfaces of~
permanent magnet 132 are covered by a metal support 134 which
is welded to the inner surface of tubular member 112. ~ietal
supports 134, in accordance with this invention, are
preferably of material having high thermal conductivity such
as copper and consists of a tab extending substantially
-radially inwardly. The magnetic members 132 and ferrite 130
are slightly tapered so that they are retained adjacent the
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surface of tubular member 112. While as shown here "na~nets
132 are positioned on either side of ferrite 130 and
magnetically poled in the same direction to produce the
circumferential magnetic ield component, any desired
con-figuration of magnet could be used.
In accordance with this invention, the electron beam
may be focussed, -for example, by a solenoid 140 surrounding
tubular member 112 to produce an a.xial focussing ield; however,
a substantial portion of the electrons of the beam will still
hit the heli~ 114 thereby producing heat which will be
transferred out of the tube by radiation to the walls and
by conduction through supports 116 to wall 112. The thermal
energy impinging on the magnets 13Z raises the sur-face tempera-
ture thereof but in accordance with this invention it has
been discovered that such surface temperature can, i.n a
vacuum, be raised to as hi~h as Z50C Eor extendecl periocls
of tube operatiorl wi.thout observable magnetic :Ei,eld
deteriorat:ion .
This completes the description o~ the embodiments of t'he
invention illustrated herein; however, many modifications
therao will be apparent to persons skil'led in the art without
departin,g from the spiri~ and scope of this invention. For
example, an axial permanent magnet of rare earth cobalt could
be used in the travelling wave tube enve'lope in place of the
external solenoid in the embodiments of Figs. 4 and 5, the
transverse magnetic ield device o~ Figs. 1 and 2 could be an
amplitron or ather cross field device and the principles o-~
this invention could be applied to ~ubes other than the micro-
wave oscillators and amplif:iers disclosed herein. Accordingly,
it is intendecl that this invention be not limitecl ~,o the
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particular details disclosed herein except as deined by
the appended claims.
