Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2034919
CATH~E AND HhATER A'SEh~LY FOR ELECTRON-BEAM
DEVICES
This invention relates to electronics and more
specifically to the cathode and heater assemblies
of electron-beam devices.
~ he invention can be success~ully used in the
electronics industry in the production o~ TV, oscilloscope,
display, and other electron-Geam devices (CR~s), whe-
rein high electron beam density has to be provided
simultaneously with high resolution, long service li~e,
short readiness time and low power consumption.
The cathode and heater assembly is a crucial compo-
nent of modern electron-beam tubes, which determines
such per~ormance parameters as brightness (luminance),
resolution, service life, reliability, power consump-
tion, readiness time,etc.
Currently, electron-beam tubes use as the source
of electrons cathode and heater assemblies with oxide
indirectly heated cathodes, whose emissivity is limited
and does not allo~s current densities above units
of amperes per square centimetre to be obtained in
the CW mode, such a density in certain cases being
inadequate provide the required perlormance charac-
teristics. ~hus, for instance, to provide the luminance
speci~ied for modern kinescopes the oxide cathode has
to be provide a current density in the beam o~ a value,
impairing the cathode's li~espan and, therefore, that
o~ the entire device.
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Furthermore, oxide cathodes are inertial, i.e.
a certain warmup time is required to attain the opera-
ting temperature, and in certain cases this readiness
time i9 a critical per~ormance parameter.
These features of oxide cathodes predetermined
the trend of further ca-thode devel~pments towards directly
heated thermionic emit-ters based on emissive metals and
their alloys. Along vJith short warmup time, such cathodes
feature higher electron current density and longer life
than oxide cathodes. However, the design of such directly
heated cathode and heater assemblies is as yet inade-
quately developed due to the contradictory requirements
of high reliability and low power consumption. Thus,
the problem of creating a cathode and heater assembly
meeting the requirements of reliability and efficiency,
is at present the most urgent one in electron-beam electro-
nics .
Another currently urgent pro~lem is that of simul-
taneously obtaining the picture and accompanying sound
on turning on a ~V receiver, also solvable by using di-
rectly heated cathodes.
Known in the art i9 the simplest design of a cathode
and heater assembly (K.M. Tisher. Einige Probleme direct
geheizter Eatoden fur Fernseh-Bildrohren. ~unk-Technik,
S.F.& E., b.33 No. 1, 1978, pp. 1-6. In German), wherein
the heater element is rectilinear section of metal band
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fitted by its ends to current-conducti~g buses and moun-
ting the thermionic emitter at its middle.
When turned on, the cathode heater current heats
this thin metal band, this leading to its warping, buck-
ling, reduced elasticity Qnd resulting in the cmitter
being uncontrollably shifted from the slectron-optical
axis of the device. The overall result is a low reliabi-
lity and a low repeatability of the performance parameters.
~ his problem cannot be solved by introducing stretching
means to componsate the heater element's thermal expan-
sion, becauso of the design complications and the stret-
ching elements losing their elasticity with time. Further-
more, these stretching elements lead to microphonics,
i.e. the heater become,s capable of vibrating under the
effect of various mechanical loads on the cathode as
sembly.
Known in the art is a directly heated cathode for
use as a source o~ electrons (US, A, 4193~13), comprising
a thermionic emitter in the form of a bar of lanthanum
hexaboride ~itted to the central part of the graphite
heater of an arced configuration.
~ his design features high electric power requirements
~about 8W) to heat the heater element due to the latter's
considerable cross-section area required to exclude displa-
cement of the thermionic emitter from its original position.
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Also widel7 used in the art is a cathode and heater
assembly for electron-beam devices (EP, B, 02~7772),
comprising a thermionic emitter having a longitudi~al
geometrical axis and fitted to the middle sections of
at least two filamentary heater elements, the peripheral
parts whereof are o~ equal length and are ~itted to
current-conducting leads rigidly mounted in the base.
This known in the art cathode and heater assembly uses
two heater elements, the peripheral sections whereo~
are parallel to the emitter axis.
Such an embodiment of the cathode and heater as-
sembly features a low reliability due to the loss
of shapo stability when the filamentary heating elements
aro heated, leading to displacement o~ the emitter
~rom the elèctron-optical 3XiS 0~ the device during
operation. Thus, the reliability of such an a~sembly
is determined by the a-qsembly retaining its original
positioning in the device, rather than by the properties
o~ the emitter itself.
An objectivo of this in~ention is to provide a
cathode and heater assembly for electron-beam devices,
having a high reliability.
Another objective i9 to improve the efficiency.
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~ his is achieved by that the cathode and heater as-
sembly for electron-beam devices, comprising a thermionic
emitter having a lcngitudinal geometrical axis and fitted
to the middle sections of at least two filamentary heater
elements, the per~pheral sections whereof are of equal
length and are fitted by t~eir ends to current-conducting
leads rigidly fitted to the base, accordin~ to the
invention the peripheral sections of the filamentary
heaters are positioned at an acute angle to the longitu-
dinal geometrical axis of the thermionic emitter with
their ends located at the vertices of a polygon with
two orthogonal symmetry axes, with one axis passing
through the current-conducting leads axes, and with
the symmetry axes' intersect on point lying on the
longitudinal axis of the thermionic emitter.
It is expedient to design the cathode and heater
assombly for electron-beam devices with two groups of
h~ater holders, each such group having a number of hol-
ders equal to the number of filamentary heating elements,
with one holder end connected to the end of the peripheral
section of the filamentary heating element and with the
other holder end rigidly fitted to one of the current-
conducting leads, with holders of one group positioned
symmetrioally relative to holders of the other group and
in plane passing through the longitudinal geometrical
axis of the thermionic emitter and through the symmetry
axis of the polygon orthogonal to the symmetry axis passing
through the axes of the current-conducti~g leads.
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It is desirable that the holders be of a current-
conducting material with an electric resistivity higher
than tha-t of the filamentary heating el~ments, and that
the ratio of the c~oss-sectional areas o~ each hol~er
and the filamentary heater be at least equal to the
ratio of their electriv resistivi-ties.
It is reasonable that the length o~ each holder
be determined by the relation:
l2 = l1 r(1 ~ ~2/~1) /2 _ 1 ~ ,
where: 11 is the length of the peripheral section of the
filamentary heating element;
12 is the ho~ er length;
T1 is the melting temperature of the filamentary
heating element; and
T2 is the melting temperature of the holder material.
It is highly advantageous that each holder be of
arched configuration.
~ he cathode and heater assembly for electron-beam
devices of this invention is characterized hy a high
reliability during its entire lifespan. ~he shape stabi-
lity of this design arrangement, constituting one of the
critical factora affecting assembly reliability, is achie-
ved by selecting a proper geometry of positioning the
filamentary heater elements.
Holders of high electric resistance also improves
the shape stability, at the same time facilitating heat
sink via them and thus reducing the temperature gradient
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betweon the "cold" current-conducting leads and the "hot"
heater el~men-ts and reducing heat transfer ~rom the
thermionic e~itter to the current-conducting leads.
~ hus, the cathode and heater assembly of the in-
vention features, on the one hand high mechanical rigidity
and temperature stability, and on the other hand low
power consumption and quick warmup to the operating
temperature.
~ he invention will now be described in greater detail
with referencc to specific embodiments thereof and to the
acaompanying drawings, wherein:
Fig. 1 shows the general view of the cathode and
heater assembly for electron-beam devices, according to
the invention;
Fig. 2 shows the plan view of tne cathode and heater
assembly o~ the invention shown in Fig. 1;
Fig. 3 shows the axonometric projection of -the cathode
and heater a~sembly of Fig. 1 with four holders of recti-
linear shape, accordi~g to the invention;
Fig. 4 shows the axonometric projection of the cathode
and heater assembly of ~ig. 3 with three filamentary heating
elements and six rectilinear holders, according to the
invention;
Fig. 5 shows the plan view of the cathode and heater
assembly of Fig. 3 with arched holders, according to the
invention;
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Fig. 6 shows the te~perature distribution along
the holders and filamentary heating element.
The cathode and heater assembly for electron-beam
devices comprises thermionic emitter 1 (Fig. 1) having
longitudinal geometrical axis 2 and fitted to the
middle sections 3 (conventionally shown by dashed lines)
of at least two filamentar~ hea-ting elements 4.
In this embodiment two heating elements 4 are used,
with peripheral sections 5 of each having an equal length
11 and positioned at equal acute angles ~ relative to
the longitudinal geometrical axis 2 of thermionic emitter 1.
~he ends of peripheral sections 5 are fitted, i.e. welded
by resistance spot welding, to current-conducting leads 6
rigidly fitted to base 7 of an electrically insulating
material, e.g. ceramics.
Such a con~iguration of heating elemen~s 4 as if
constitutes the cage of a tetrahedral, pyramid which i9
a rigid structure, providin~ shape stability and constancy
o~ emitter 1 positioning when heated during service.
~ he amount of thermal displacement of emitter 1
along axis 2 is constant and can easily be defined for
ea¢h speci~ic con~iguration and, consequently, taken
into account during precise emitter 1 positioning in
the electron-beam device. The ends of peripheral sec-
tions 5 are in a plane normal to geometrical axis 2 and
inoluding the vortices on the polygon (in this embodiment -
the rectangl~) with two orthogonal symmetry axes 8 (Fig. 2),
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one such axis passing through the axes of current-conduc-
-ting lea~s 6, ànd VJith the point of axes 8 intersection
lying on geometrical axis 2 (Fig. 1).
Increasing the number of heating elements improves
the shape stability, and consequently - the reliability,
but results in a more complex structure1 so that it proves
practically expedient to restrict the number of heating
elements to two or three.
Heating elements 4 are of a refractory metal, e.g.
of tungsten wire, and thermionis emitter 1 is of rare
earth borides, e.g. LaB6.
~ o ~urther improve the structure rigidity and the
efficiency of the cathode and heater assembly, it is
complemented with holders 10 (Fig. 3) supporting heating
elements 4. Xolders 10 are of a high resistivity current-
-conducting material, higher than that of heater ele-
ments 4, e.g. of chromic alloys, such as nickel-chrome,
nickel-tungsten-zirconium alloys etc.
Holders 10 are combined into two groups, each group
having a n-~mber of holders 10 equal to that of heating
elements 4, with one end of each holder 10 connected to
the end of a peripheral section 5 of heater element 4
and with the other end o~ each holder 10 rigidly fitted
to one of the current-conducting leads 6. Holders 10
of one group are positioned symmetrically relative to
those of the other group and ~o the plane passing through
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geometric axis 2 of emitter 1 and that of polygon sym-
metry axes 8, 9 (in this embodiment - axis 8) normal
to axis 9, 8 passing through the axes of current-conducting
leads 6
In this embodiment holders 1~ are rectilinear sec-
tions of conductors connecting the ends of peripheral
se¢tions 4 o~ heatin~ elements 4 to current-conducting
leads 6.
In the embodiment shown in ~ig. 4 there are three
heater elements 4 and, respectively, six holders 1
positioned radially relative to current-conducting
leads 6. ~his structure of the cathode and heater as-
sembly is si~ilar to the cage o~ a hexagonal pyramid
with peripheral sections 5 of heat.er elements 4 consti-
tuting the pyramid edges.
An embodiment, wherein holders 1J are arched, rather
than rectilinear is shown in Fig. 5, this shape further
enhancing emitter 1 positioring stability durin, heating
by that the thermal expansion (elongation) of holders 10
is reduced to their displacement along the directrix
o~ a cone enclosing the pyramid, rather than being trans-
ferred to e~itter 1 via the edges of the pyramid cage,
and this, as is well known, does not cause displacement
of the pyramid apex, wherein emitter 1 is positioned.
The use of holders 1~, independent of their con~igu-
ration and positioning, allows utilizing heater elements 4
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of shorter length and smaller cross-~ection area~ The
cross-section are o~ holders 10 and heater elements 4
are selected in accordance with their electric resistivity.
As is well known, least heat transfer from heater element 4
to current-conducting leads ~ ~ill be at a constant
power dissipated along the integrated heater co~posed
of two holders 10 and the heater element 10 itsel~,
~his condition will be sa-tisfied at a constant electric
resistance per unit length of the integrated h~ater,
this in the embodiment being described practically meaning
an equality between the ratio of cross-sectional areas
of holders 10 and heater elements 4 and the ratio o~
-their resistivitles, or slioh-t exceed of this value.
Optimization o~ lengths l1, l2 ~ peripheral sec-
tion 5 o~ heater element 4 and holder 1~, respectively,
takes into account melting temperatures T1~ T2 of their
materials.
Obviously, holder 10 length 12 may be increased
till the temperature at its point of contact with heater
element 4 does not exceed T2, with the necessity to main-
tain a constant electric resistance per unit length
o~ the integrated heater (to maintain a constant heat
emission) taken into account. Heat losses due to thermal
condu¢tivity Ph at any point of the in-tegrated heater
depend on the distance from this point to the site o~
holder 10 mounting to the massive current-conducting
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leads 6 (the origin x=0 co~responds to one of the leads 6)
and can be described as:
Ph = Pi( x t i-x ) = Pi x~1-x) (1)
where Pi i5 th~ emitted he~t power and 1 = 2(11 + 12),
neglecting -the length of the middle part 3 of heater
element 4 as insignicant.
The steady-state ~emperature, ~, in any point of
the integrated heater under these conditions is in-
versely proportional to the heat lossea, i.e.:
~(x) = k ~ =_~ ~(1 1 x) (2)
where k is the proportionality factor.
~ he ratio k/Pi ¢an be obtained from the condition
o~ equality of temperatures T(x) at the middle of heater
element 4, i.e. at x = 1/2, to the operating temperature,
To~ of thermionic emitter 1:
~(l/e) = ~0 = ~ ~ = pk
whence
k = ~ 4
~ o r
Thus, the ~inal equation decribing T(x) will be:
~ (x) = To4x(l2~-x) , (4)
desoribing the dynamic equilibrum between heat emission
and heat losses due to heat sinking established in the
integrated heater.
Fig. 6 shows the temperature distribution along the
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length of the integrated heater (holder 1~ and heater
element 4), with the temperature of current-conducting
leads 6 conventionally as~med to be zero.
Coordinates of the point~ of holder 10 ¢onnection
to the ends of peripheral sections 5 of heater elements 4
can be deter~ined from the boundary condition that the
current temperature T(x) at this poin~ be equal to ~2 and
the tempera~ure at the middle of heater element 4 be
T1 = To~ With this taken into account,
T2 = 4x(1 - x)
T1 1 2 (5)
Hence x ~ +(1 ~ T2 )1/2~ (6)
Thus, the optimal point of heater element 10 joint
to holder 10, i.e. the length l1, is symmetrically
spaced from the middle o~ heater element 4 by
x1 = 2 (1 ~ T1 ) / (7)
~ he optimal ratio of holder 10 length 12 to the
length 11 of peripheral se¢tion 5 of heater element 4 will
th~re~ore be:
2 X2 ~-- L1 - (1 - T2/~1)1/
(1 - T2/T1)1/2
- 1,
( 1 - T2 /~1 ) /
whence the length, 12, of holder 1~ can be obtained a9:
2 = 11 ~ 2/T1) 1/2 _ 1~ (8)
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An increase in length 12 above this analytical
optimal value can lead to the danger of softening or
melting of holders 1~ at the point of fitting to heater
elements 4, a shorter than opt~mal length l2 is inexpe-
dient because of the lower shape stability and higher
heat losses due to shorter holders 1~.
The cathode and heater assembly of the invention
functions as follows.
Application of volta~e to current-conducting leads
(Figs. 1, 2) causes heating of heater elemen-ts 4, thus
heating thermionic emit~er 1 to its operating temperature ~0.
Emitter 1 emits electrons, bunched into an electron
beam in the electron-beam device, wherein the cathode
and heater assembly is installed.
~ he cathode and heater assembly of the herein above
described design configuration fea-tures a short warmup
time (o~ about 1 second) due to the short length and
thinness of heater elements 4.
Embodiments of the cathode and hea-ter assembly shown
in Figs. 3-5 mainly function in a similar manner, di~-
fering in that after voltage application to current-
conduoting leads 6, the temperature whereof rises to
about 100C, the ends of holders 10 at the points of their
fitting to heater elements 4 are heated to about 7~0C,
whereas the heater elements 4 raise the temperature of
emitter 1 to its operating temperature of about 1400C
to provide thermionic emission. Thus, introduction of hol-
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ders 10 reduces the temperature gradient across the
junction between heater elements 4 and leads 6 there-
fore reduces heat losses, thus improving the cathode
and heater assembly efficiency.
On the whole, the cathode and heater assembly of
the invention, due to the herein above described geometry
of filamentary heater elements positioning and to the
introduction of holders of a specified length and cross-
section to constitute a rigid structure, shape-stable
under high-temperature operating conditions, is cha-
racterized by a high reliability and a high efficiency.
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