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"rates arm
Federated Mar. 27, l9§2
Raymond ll’. Tuinila, Beverly, and William Caithness,
West Acton, Mass, assignors to Raytheon Company,
a corporation of Delaware
Filed Dec. 15, 1958, Ser. No. ‘780,449
13 Claims. (ill. 313-1il7)
Such space charge limited operation is also feasible in
some pulse magnetrons in which back bombardment oc—
curs only during the culse time, provided that the duty
cycle is so low that the cathode has an opportunity to
cool between pulses. However, for C.W. magnetrons,
and in many other hi?h frequency devices, the back bom
bardment effect is so pronounced that appreciable noise
is generated within the magnetron tube and such noise
This invention relates to temperature-limited cathodes
is too great to be tolerated. Although the complete
for electron discharge devices which are particularly suit 10 theory of noise generation is not fully understood, it is
able for use in magnetrons and other devices wherein the
a well known phenomena in space charge limited tubes
cathode is subject to back bombardment by either elec
and possibly may be related to the interaction between
trons or ions, as well as in devices, such ‘as magnetrons,
electrons in the space charge cloud and also to unfavor
in which noise generation must be held to extremely
able interaction between ions and space charge electrons
low limits.
15 in the interaction space. This undesirable noise can be
Existing cathodes, particularly those containing oxide
reduced apperciably by operating the cathode temperature
bearing materials, often are unsatisfactory for electron
discharge devices, such as magnetrons, wherein electrons
limited. With this type of operation, the space charge
or ions move in a high frequency electromagnetic ?eld,
since the noise level of noise generated in such devices
may well exceed acceptable limits. In devices such as
emitted by ‘the cathode are able to reach the anode or the
those having transverse electric and magnetic ?elds, many
e?ect is no longer predominant and most of the electrons
vicinity of the anode.
The cathode emission now is
governed largely by the cathode temperature and is sub
stantially independent of the cathode-to~anode voltage.
As already mentioned, the back bombardment of the
electrons emitted from the cathode return to the cathode;
some of these electrons, when impinging upon the cath
cathode causes the cathode temperature to become quite
ode, cause secondary electrons to be emitted and this 25 high and to vary over a relatively wide range at different
process may be repetitive. This phenomena is referred
places on the cathode surface. it has been found that,
to commonly as back bombardment and is not limited to
under such conditions of cathode temperatures, there is
a pronounced tendency for the cathode emission to in
electron movement, but also may arise from the move
crease to the point at which space charge limited oper
ment of ions within the tube. It should be noted that,
even though magnetrons and other electron discharge 30 ation occurs. This tendency is quite noticeable in the
cathodes of the prior art. A certain number of electrons
devices are rather highly evacuated, there still may be
per second is required for operation in the space charge
many gas ions in the tube enclosure. Electrons and ions
limited region; this emission must be kept below a certain
striking the cathode cause the latter to heat up con
value if one is to avoid such operation. The emission
siderably and the temperature of the cathode ?uctuates
tremeudously——being different at dill'erent portions of the 35 value at which the transition from space charge limited
cathode. Indeed, instantaneous spot temperatures Well
above the integrated average temperature of the cath
operation to temperature limited operation takes place
is substantially independent of the cathode composition.
ode have been observed owing to the back bombardment
However, in cathodes, such as oxide cathodes, the tem
perature at which this transition point is reached is rela
ration rate of the material to increase considerably, 40 tively low, being of the order of about 800‘ degrees C.
thereby proportionately shortening the cathode life.
Because of the back bombardment, already mentioned, the
magnetron temperature usually is considerably in excess
Bombarding ions striking the cathode have relatively high
Bombardment of tie cathode causes t e evapo
tend to remain on the cathode to react with
of such a value. Consequently, it is necessary to derive
a cathode material which will allow operation at a much
the cathode material and cause poisoning of the cathode.
In accordance with the invention, the cathode mate
rial includes the element rhenium which has a very low
secondary emission ratio-of the order of l.l--so that
higher temperature before the above transition point is
attained. This transition point may be quite high if
rheniurn is included in the cathode composition. In
the adverse effects of cathode bombardment are cut down
other words, the tendency of the cathode emission to in
crease to the point at which space charge limited oper
appreciably. The cathode, according to the invention, is
composed of a highly refractory material, such as tung 50 ation occurs is substantially reduced by the use of rheni
um, since a change in cathode temperature does not pro
sten, which carries a composition comprising substantially
duce as pronounced an effect upon cathode emission as a
25 percent of tungsten, 25 percent thorium tetraboride,
similar change in cathode temperature in the cathodes
and 50 percent rhenium, all by weight. These percent
or" the prior art. The ability of rhenium to absorb ad
ages are for ambient temperature conditions. The com
position and crystal structure of the ?nished cathode will 5.” ditional thermal energy without emitting more electrons
contributes to this eii’ect and is associated with the fact
depend to some extent upon the processing temperature,
that the energy bands of rhenium are Widely separated.
that is, the temperature at which the powdered mixture
Another disadvantage of space charge limited opera
is heated. The mixture may be applied to a metal mesh
tion, as applied to such devices as magnetrons, is that
in the form of a suspension of the rhenium, tungsten and
thorium boride in a suitable liquid vehicle, such as nitro 60 the space charge radius increases with anode voltage and
the front of the space charge approaches the anode with
cellulose. The purpose of the vehicle is to facilitate ap
increasing anode-to-cathode voltage. In effect, there
plication of the powdered mixture and is such as to
fore, the space charge constitutes a virtual cathode of
evaporate soon after the suspension has been deposited
varying radius. Since the frequency of a magnetron, as
upon the mesh.
well as its mode spectrum, is a direct function of the ratio
In many types of electron discharge devices, space
of the cathode radius to the anode radius, and since the
charge limited operation of the cathode is feasible. In
effective cathode radius varies with the con?guration of
other words, for a given operating temperature of the
the space charge, the frequency of operation of the mag
cathode, the cathode emission increases as the voltage
netron and the moding chharacteristics thereof tend to
between anode and cathode is increased. However,
the space charge cloud in the region adjacent the oath 70 vary with changes in anode-to-cathode voltage. Conse
quently, temperature limited cathode operation in mag
ode eventually assumes such proportions as to limit the
netrons is often highly desirable. The use of a cathode
number of electrons capable of reaching the cathode.
composition containing rhenium, therefore, contributes
to greater frequency stability and mode stability of mag
netron-type devices.
ing cylinder 27 to which one end of a cathode sleeve 28
is attached. The supporting cylinder 27 is supported
from the pole piece 25, and electrically insulated there
from, by means (not shown) which are familiar to those
skilled in the magnetron art. For example, the support
ing cylinder 27 may be attached to a glass bushing which
magnetrons, since the oxygen escaping into the region of
also is secured to a metal ring, said ring being sealed
RF. ?elds causes considerable noise to be generated.
hermetically to the pole piece 25. The upper end. of the
Furthermore, the thorium evaporates rapidly from such
supporting cylinder 27 is enlarged in order to serve as one
cathodes, especially when subjected to back bombard
ment of electrons and ions. The life of such cathodes, 10 of the cathode end shields. The upper end shield is
formed by a tubular member 25‘ which is brazed to the
consequently, is relatively short. In the case of cathodes
upper end of the cathode sleeve 28. The cathode sleeve
which use thorium boride and tungsten only, the rate of
28 is made of a highly refractory material, preferably
production of thorium depends upon the rate of diffusion
tungsten, which is capable of withstanding high cathode
of boron into the tungsten; that is, the rate of diffusion
operating temperatures and which has no appreciable
of the boron into the tungsten determines the rate of avail
chemical effect upon the cathode emissive material which
ability of the free thorium. If the rate of dissociation
The usual thorium~~bearing cathodes, particularly,
thorium oxide cathodes, are unsatisfactory for low noise
of thorium is too ‘fast, more than one monomolecular
layer of thorium is formed and Van der Waal’s force, that
is, the intermolecular forces of attraction between the
outermost evaporating layer of thorium and tungsten
(which decreases as the number of monomolecular layers
increases, and vice versa) decreases. After about ?ve
is deposited thereon.
or six monomolecular layers of thorium have been de
attached to a metallic insert 33 affixed to the cathode
posited upon the surface of the thorium boride-tungsten
cathode, the evaporation rate approaches that ‘from a body
of pure thorium. It is important, therefore, that the rate
sleeve 28 adjacent the upper end of the cathode sleeve.
The electron emissive portion 37 of the cathode 13
of dissociation of the thorium boride not be too rapid, con
sistent, of course, with sufficient dissociation ‘for proper
It has been found that the diffusion rate of boron into 30
rhenium is substantially less than that of boron into tung
sten, and, since his rate of diffusion of boron determines
the rate of availability of free thorium, the generation of
A heater coil 31 is supported within an elongated central
bore in the cathode sleeve 28. One end of heater 31 is
aii‘ixed to a heater lead-in conductor 32 which passes
through the cathode supporting cylinder 27 and external
ly of the tube envelope. The other end of heater 31 is
preferably includes a wire mesh 35 which is made of a
highly refractory material, such as tungsten, and having
the same general characteristics as mentioned previously
in connection with cathode sleeve 28. This wire mesh
surrounds the cathode sleeve 28 and is secured thereto,
for example, by spot welding or by sintering with a re
fractory metal powder such as tungsten. The interstices
in the wire mesh 35 are ?lled with an electron-emissive
material 36 which comprises thorium tetraboride, tungsten
free thorium is slower when rhenium is added to the thori
um boride and tungsten. The number of monomolecular -'V4 Us and rhenium. The electron-emissive material 36 may be
layers ‘of thorium formed on the surface of a rhenium
a mixture of comminuted tungsten, thorium tetraboride,
based cathode, therefore, is less than the number formed
on the surface of tungsten alone. Consequently, Van der
Waal’s force is greater, and the tendency for thorium
sion may be brushed or sprayed into the cathode mesh
and rhenium suspended in a suitable binder; this suspen
35. The binder may, for example, be a nitrocellulose
to evaporate from a rhenium-based cathode is reduced. 40 binder or any similar material which evaporates rapidly
after the comminuted material has been applied to the
This may be stated in another way, namely, that the life
mesh. The electron-emissive material 36 need not be ap
time of free thorium molecules on a rhenium based cath
plied to a mesh, however. For example, the powdered
ode is much longer than that of free thorium molecules on
mixture referred to above may be compacted to form a
a refractory material such as either tungsten or molyb
denum alone.
45 tubular element which may be a?ixed to cathode sleeve
28. The cathode next is ?red at an elevated temperature
Other features, objects and advantages of the invention
to harden the electron-emissive material 36. The tem
will be better understood from the following description,
taken in conjunction with the accompanying drawing,
FIG. 1 is a cross-sectional view of a magnetron in
corporating a cathode according to the invention;
FIG. 2 is a view illustrating the detailed construction
of the cathode of the magnetron shown in FIG. 1; and
perature at which the cathode is processed will determine
the exact composition and the electron emissive level of
the ?nished cathode. A mixture, by weight, of 25 per
cent tungsten, 25 percent thorium tetraboride and 50
percent rhenium, at ambient temperature, has provided
extremely quiet operation and long cathode life in con
tinuous wave magnetrons. Other ternary compounds of
IG. 3 is a fragmentary section view of an electron
discharge device showing a typical electron gun assembly 55 tungsten, thorium tetraboride and rhenium may be used;
for example, a mixture of 15 percent tungsten, 15 percent
according to the invention.
thorium tetraboride and 70 percent rhenium has been
In the drawings, the reference numeral 10 designates
found satisfactory. The amount of tungsten and thorium
generally a cavity resonator type of magnetron which
bor-ide generally is of about the same order of magnitude,
comprises an anode having a plurality of vanes 1.2. A
cathode 13 is located with its axis at the center of the anode 60 although it is not essential that this relationship exist.
There are, of course, certain limits to the percentage of
vanes 12. The magnetron lltl includes a cylindrical outer
each material used in the cathode. It is obvious that as
wall 14 and a circular top plate 15. A pole piece 16 is
the percentage of rhenium approaches 100 percent, the
inserted coaxially with the cathode 13 through an open
amount of thorium tetraboride available would be so
ing in the plate 15 Strapping 18 of one of the well-known
types is provided adjacent the upper and lower ends of
the vanes 12.
The output is obtained from one of the
cavity resonators between an adjacent pair of vanes 17, by
a coaxial probe 26 comprising an inner conductor 21
terminating in a loop and an outer conductor 22. The
magnetron further includes a bottom plate 24 which is
hermetically sealed to the cylindrical outer wall 14. A
second pole piece 2.5 is inserted in an opening formed at
the center of the bottom plate
This pole piece 25
contains a central bore for receiving a portion of the
cathode assembly 13. The cathode 1.3 includes a support
small as to reduce the electron-emissive thorium to a
value below that required for adequate emission; it should
be noted that rhenium alone is not a good electron
emitting material.
There must always be su?icient
thorium tetraboride present in the mixture to provide a
minimum electron emission. On the other hand, if the
supply of rhenium is too limited, say below about 15
percent, the advantages accruing to a rhenium-based cath
ode begin to disappear, that is, the advantages of low
secondary emission ratio, the relatively slow rate at which
rhenium is released from the cathode body, etc. It has
been found that a certain amount of tungsten is required
in order to maintain the rate of reaction which forms free
thorium su?iciently fast to provide proper emission levels
for temperature-limited operation. For effective opera
tion under the conditions already mentioned, it is im
portant that the cathode material be substantially free of
oxygen. The thorium boride may be replaced by a
thorium compound not comprising oxygen, such as a
5. A cathode composition comprising substantial quan
tities of rhenium and oxide-free thorium.
6. A cathode composition consisting of approximately
25 percent thorium boride and 50 percent rhenium, by
weight, at ambient temperatures.
7. A cathode composition consisting of approximately
15 percent tungsten, 15 percent thorium boride and 70
percent rhenium, by weight, at ambient temperatures.
thorium nitride or a thorium sul?de. The percentage of
8. A cathode comprising a refractory metal support
the latter thorium compounds used with rhenium and 10 ing member and a coating of mixed particles of tungsten,
tungsten is approximately the same as for thorium boride.
thorium boride, and rhenium disposed on said member.
Although the cathode so far has been described in con
9. A cathode comprising a refractory metal supporting
nection with a magnetron, the rhenium based cathode
is also of great value in other types of electron discharge
member and a coating of mixed particles of tungsten, a
devices, such as electron microscopes using ions for focus
on said member.
sing, X-ray equipment, and other high frequency devices
wherein the cathode is subject to considerable bombard
ment, either by ions or by electrons. A typical cathode
for devices of this type is shown in FIG. 3, wherein only
a portion of a tube envelope is shown. The tube envelope
may consist of an elongated metallic portion 41 and an
end portion 42 made of glass or other electrically insu
lated material through which the necessary heater and
cathode leads and other electron gun leads, if any, may
be brought out of the tube.
compound of oxygen-free thorium, and rhenium disposed
10. A cathode comprising a refractory metal support
ing member and a coating of mixed particles of tungsten,
thorium nitride, and rhenium disposed on said member.
11. A cathode comprising a refractory metal support
ing member and a coating of mixed particles of tungsten,
thorium sul?de, and rhenium disposed on said member.
12. A cathode for electron discharge devices including
a refractory metal supporting member, a refractory metal
mesh disposed on said member, and an electron-emis
For simplicity, only the 25 sive material permeating said mesh, said material con
cathode and heater are shown in FIG. 3. The heater 44
sisting of mixed particles of tungsten, thorium boride, and
is located adjacent a generally cup-shaped cathode 45
which contains a central aperture 47 into which the elec
13. A cathode for electron discharge devices including
tron-emissive material 48 may be inserted. The emissive
a refractory metal supporting member, a refractory metal
material 48 may be of the same composition as the ma 30 mesh disposed on said member, and an electron-emissive
terial 36 previously described in connection with FIGS.
1 and 2. An appropriate heater-to-cathode voltage is sup
plied by means of an external source 49.
This invention is not limited to the particular details
of construction, materials and processes described, as 35
many equivalents will suggest themselves to those skilled
in the art. For example, the con?guration and size of
the cathode is not limited to that shown in the ?gures.
The shape of the cathode will depend upon the particular
electron discharge device which utilizes the cathode. For 40
X-ray tubes, the cathode would be much larger and
heavier than a cathode used in a small magnetron, for
It is accordingly desired that the appended
claims be given a broad interpretation commensurate with
the scope of the invention within the art.
What is claimed is:
1. A cathode composition comprising an oxide-free
electron-emissive material combined with a substantial
amount of rhenium.
material permeating said mesh, said material consisting of
approximately 25 percent tungsten, 25 percent thorium
boride, and 50 percent rhenium, by weight, at ambient
References Cited in the ?le of this patent
Spencer ____________ __ Aug. 17, 1948
Kurtz ______________ __ Apr. 19, 1949
Kurtz ______________ __ Dec. 20, 1949
Brown ______________ __ July 28, 1953
Warin _______________ __ Oct. 28, 1958
Great Britain ________ __ Mar. 10, 1954
Macksoud ___________ __ Dec. 8, 1959
2. A cathode composition consisting principally of 50 Boride Cathodes, by La?erty, .1’. App. Physics, March
thorium boride, tungsten and rhenium.
1951, pages 299-309.
3. A cathode composition consisting of substantially
Rhenium Metal, Its Properties and Future by Kotes
equal amounts of tungsten and thorium boride and a sub
in Materials and Methods, March 1954, vol. 39, No. 3,
stantial amount of rhenium.
page 88.
4. A cathode composition consisting of substantial 65 Rare Metals Handbooks, edited by Hampel, published
quantities of thorium boride and rhenium.
by Rheinhold Publishing Corp. in 1954.
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