US3473073A

US3473073A - Electron tube having an improved direct-heated cathode structure

Info

Publication number: US3473073A
Application number: US631464A
Authority: US
Inventors: Pierre Gerlach
Original assignee: Compagnie Francaise Thomson Houston SA
Current assignee: Compagnie Francaise Thomson Houston SA
Priority date: 1966-05-18
Filing date: 1967-04-17
Publication date: 1969-10-14
Anticipated expiration: 1986-10-14
Also published as: DE1589608A1; GB1179731A; NL6705621A; FR1490577A; CH456776A

Description

Oct. 14, 1969 .GERLACH 3,473,073

- ELECTRON TUBE HA G AN IMPROVED DIRECT-HEATED V CATHODE RUCTU Filed Apr' l7, 7

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a Z-ZIEQA United States Patent Int. 'Cl. H01j ]/15 19/08; H01k 1/02 US. Cl. 313-341 Claims ABSTRACT OF THE DISCLOSURE The cathode in the form of a tubular wire mesh (1) is supported between end supporting plates (2, 3) which serve to apply heating current thereto. The end support plates have increased resistance to radial current flow in an annular region adjacent the periphery thereof, as by having the plates taper in thickness towards their periphery, so that the heating current will heat the plate peripheries and thus reduce the sharpness of the temperature drop in the end regions of the cathode wires. Further, annular reflecting surfaces associated with the support plates reflect back heat radiated from the midsections of the cathode wires towards their end sections.

This invention relates to directly-heated cathode structures for electron discharge tubes. In high-power electron tubes it is customary to provide the cathode in the form of a tubular member which may consist of a plurality of filament wires, eg of thoriated tungsten, frequently disposed in two helical arrays of opposite pitch joined by solder at their crossover points. The invention however is also applicable to other forms of tubular cathode. For supporting the tubular cathode and connecting it to the heating current circuit there are generally provided two end support members in the form of plates or discs having the respective ends of the cathode wires attached to their peripheries. These end support plates in turn are supported by way of rods extending to the cathode connecting terminals of the tube. In operation, heating current applied from the terminals through the rods and end plates heats the cathode wires or filaments to a high temperature, e. g. 1600" C. or higher, whereupon they emit electrons by thermionic effect.

It is important that the cathode should produce uniform thermionic emission throughout its length and, therefore, that it should be of uniform high temperature throughout. However, it is found in practice that the end regions of the tubular cathode have a substantially lower temperature than the intermediate section, and emit substantially less electrons or none at all. While various devices have been suggested in the form of anti-radiant shields, heat baffles and the like, for conserving the heat in the various sections of the tubular cathode, none of these conventional means has been found effective :in overcoming the defect just referred to. In fact, it has been common practice, in view of the said defect, to provide the tubular cathode substantially longer in axial extent than would be otherwise required for the rated performance of the tube, so that the relatively low-temperature end regions of the cathode wires are, in effect, inactive.

The presence of thermionically-ineflicient or inactive regions at both ends of the cathode is obviously a nuisance in any case, but is is especially objectionable in the case of ultra-high frequency and the like high-power tubes, because it is important in such tubes that the length of the electrodes and connectors be kept as short as possible in order to reduce the internal imped- Patented Oct. 14, 1969 ance and increase the upper limiting frequency of the frequency band the tube is able to handle.

Objects of this invention are to overcome the indicated difficulties in an efficient way, to reduce greatly or eliminate entirely the thermionically-inefficient end regions in a direct-heated tubular cathode, and thereby to permit the construction of electron tubes, especially in the highpower range, having shorter length and improved internal impedance and frequency characteristics.

This invention provides an improved construction of the end supporting and connecting means for a directlyheated tubular mesh cathode in an electron tube. The cathode supporting and connecting means comprises at least one member in the general form of a plate or disc having the related end of the tubular cathode attached to its periphery and having current conducting means connected to its radially inward from its periphery, and the improvement comprises reducing the thickness of the plate-like member in an annular region thereof adjacent its periphery, in order to impart substantially increased resistance to the radial flow of electric and thermal energy in said annular region, as compared to said resistance in an inner region adjacent the connection of said conducting means. In addition to the reduction in thickness, perforations, preferably in a circular array, may be provided in or near said annular region in order further to increase said resistance. The result is to cause a substantial heating of the supporting member near its periphery and a corresponding diminution in the sharp temperature drop that would otherwise be present in the end region of the cathode Wires near their attachment to the support member. At the same time, the increased resistance to thermal flow reduces the loss of heat from the cathode by conduction.

According to a further feature preferably used in combination with the feature just described, the plate-like support member is provided with a dished shape so as to present an annular portion that projects into the tubular mesh cathode, and closely spaced radially inward from the latter. This annular portion is made reflective on its outer surface, and is preferably somewhat conical so as to reflect a substantial part of the heat radiated from the midsection of the tubular cathode, toward the end section of the cathode adjacent the supporting member.

By using a combination of the indicated features it has been found possible to reduce considerably the thermionically-ineffective end regions that are normally found present therein, with the advantages earlier indicated.

FIGURE 1 is a simplified view, generally in axial cross sectionfof a cathode assembly according to a first embodiment of the invention;

FIGURE 2 is a similar view illustrating one of the two end supporting members of the cathode assembly, constructed according to another embodiment;

FIGURE 3 is a similar to FIGURE 2 but shows a further embodiment of an end supporting member;

FIGURE 4 is again similar to FIGURE 2 and shows another embodiment;

FIGURE 5 is similar to FIGURES 2-4 and shows an end supporting member constructed according to a preferred embodiment of the invention, which includes a heat reflecting surface;

FIGURE 6 is a plan view of FIGURE 5; and

FIGURE 7 is a view similar to FIGURES 2-5 showing a modified construction of an end supporting member of a general type somewhat similar to that of FIG- URE 5.

The cathode structure shown in FIGURE 1 includes a tubular mesh 1 consisting of two series of helical wires coiled in opposite directions and preferably joined together at their cross-over points so as to build a comparatively rigid and self-supporting structure. The tubular mesh 1 is supported between an upper disc 2 and a lower disc 3 which have the respective ends of the wires 1 soldered to their mutually facing surfaces. The upper disc 2 is centrally supported by a rod 4 having its upper end joined as with solder to the center of the under surface of disc 2, the rod 4 extending freely through a central opening formed in the lower disc 3 so as to be electrically insulated therefrom. The lower disc 3 is supported by a plurality of rods 5, e.g. three in number, which have their upper ends joined to the under surface of disc 3 in an intermediate region thereof.

The wires 1 of the cathode structure may be made from any suitable material having suitable high melting or decomposition temperature and having electron-emissive properties when heated, e.g. thoriated tungsten. The supporting discs 2 and 3 as well as the rods 4 and 5 may be made from any suitable material having appropriate electric conductivity and high-temperature resistance, for instance molybdenum. It will be understood that the connector rods 4 and 5 at their lower ends, not shown, are connected to respective cathode connector terminals of the electron discharge tube of which the cathode structure described forms a part. Such cathode terminals would be led out through suitable seals, in the conventional way, of the evacuated envelope of the tube for connection to external circuits, including both a heating circuit connected to a DC voltage source, and a high frequency A-C circuit. In operation, heating current from the D-C source will flow by way of the connector rods 4 and 5 and the discs 2 and 3, through the wires 1 of the tubular cathode mesh. Due to the high electric resistance of these wires, the wires are heated to a high temperature, e.g. about 1700 C., and thereupon emit electrons by thermionic effect.

The type of operation just summarized is conventional, and has been found in practice to be accompanied by the following difliculty. Since the cathode wires 1 are heated to a very high temperature whereas the support discs 2 and 3 remain quite cool, a sharp sudden temperature drop must occur near the junctions of the wires with the discs. This sharply concentrated temperature gradient generates high heat losses through radiation and conduction from the end sections of the wires 1, so that these end sections in steady-state operation assume a temperature considerably lower than they should, and become inoperant to produce thermionic emission. Inactive regions are therefore present at each axial end of the tubular cathode mesh 1, and the general performance of the tube would be diminished with respect to its rated performance, if the axial length of the cathode and consequently the tube as a whole, were not increased over that theoretically required.

According to this invention, the diificulty is overcome by so forming one or each of the end support discs 2 and 3 as to increase its resistance to the flow of thermal and electric energy in a radial direction, within a peripheral region of the disc. In this way, the peripheral region of the disc becomes somewhat heated so that part of the afore-mentioned temperature gradient will now occur in the said region of the disc, correspondingly reducing the temperature drop that will be present in the adjacent end section of the wire mesh 1, and thereby eliminating partly or completely the above-mentioned inactive region.

In the embodiment of FIGURE 1, the above result is achieved by giving each of the end support discs 2 and 3 a profile that tapers radially outward from an intermediate part of the disc to its peripheral edge, as clearly shown in the drawing. Because of the reduction in cross section area of the disc in its peripheral region, its electrical resistance and the ohmic heat generated in this region are increased and the desired effect is attained. The tapered profile in the peripheral region of the support disc, at the same time as it causes generation of heat by ohm efiect, also results in a reduction of the thermal conductivity so that the disc will be less able to transfer heat radially inward through conduction from the end sections of the wires 1. This further contributes to reducing the objectionable cooling of the end sections of the cathode wires.

While it is not essential that the tapered region extends radially as far as the periphery of the disc 2 to which the cathode wires 1 are attached, this is preferably the case and it is in any event important to avoid a construction in which the supporting disc 2 presents a peripheral region of enlarged thickness and/or area, such as a flanged rim, radially outward of the points of attachment of the cathode wires. Such a peripheral enlargement would result in radiation heat losses from the supporting disc 2, and would thereby cancel some or all of the advantages of the invention.

The further figures of the drawing illustrate some modified forms of a cathode end support disc according to the invention. While the figures more especially illustrate the upper support disc to the cathode structure of FIGURE 1, it will be understood that the lower discs of such structure may be and preferably is given a corresponding shape.

In FIGURE 2 the cathode support disc is shaped with a thickened central section or boss 6 and an upper surface that tapers radially outward as in FIGURE 1, but the under side of the discs is shown generally flat and there is provided a downwardly projecting flange at the peripheral rim of the disc. The flange 7 serves for attachment of the ends of the cathode filament wires 1 (not here shown), and further it act as a stiffening means increasing the rigidity of the supporting disc which might otherwise be adversely affected by the taper. The rim flange 7 does not extend outward beyond the points of attachment of the cathode wires so that it will not serve as a radiation cooling fin, as explained above.

In FIGURE 3 the cathode support disc is made up from a plurality of, herein three, circular plates of stepped diameter joined in concentric relation as with zirconium solder throughout their mating surfaces, so as to provide the desired reduction in profile section in the radial direction.

In order that the thickness taper in the embodiment so far described shall reach its best efficiency by increasing the resistance to the radial flow of electricity and heat in the peripheral annular region of the supporting disc or plate, the taper must be sufficiently great to cancel the entire gradual incerasc in flow section area that is present in the radially outward direction along the disc due to its increasing radius. Specifically, referring for example to the FIGURE 2 embodiment, it will be readily understood that in order for this condition to be satisfied, it is necessary that the thickness ratio E/e of the central boss thickness to the peripheral rim thickness exceeds the diameter ratio D/o of the central boss diameter to the peripheral rim diameter. According to the invention, said thickness ratio may preferably be more than twice as large as said diameter ratio.

In some cases a sufiiciently large reduction in thickness may not be easily achievable without adversely affecting the mechanical strength of the support member. In such cases the desired effect may be enhanced by providing perforations in an annular peripheral region of the member which serve to reduce further the flow section area and thereby further increase the thermal and electrical resistance. Thus, in FIGURE 4 which illustrates this feature the cathode support disc is a circular plate 9 of outwardly tapered thickness, with a circular array of holes 10 just inward of the tapered region. The holes or perforations such as 10 may be of any suitable size, shape and number.

According to a feature of the invention, additional means may be assocaited with one or each of the end support discs of the cathode structure for further opposing the objectionable cooling of the adjacent end section of the cathode wires 1 and contributing to the elimination of the thermionically inactive end regions of the cathode wires. The additional means comprises an annular reflecting surface associated with the end support disc and directed to receive heat radiated from the hot mid-section of the tubular mesh cathode and reflect that heat back on to the end section.

Thus, in the embodiment illustrated in FIGURES 5 and 6, an end support member according to the invention comprises a unitary stamping 12 made from a plate of suitable metal, e.g. tantalum, press-formed to the dished configuration shown. Thus the member 12 is seen to include a flat, depressed central wall 13 surrounded by an upward-outward tapering frustoconical wall 14 surrounded in turn by a flat outward flange of short radial extent. Wall 14 and the surrounding flange are of reduced thickness, as shown. An array of perforations such as 15 are formed in the depressed bottom wall 13 near its periphery. The

' dimensioning is such that the extremities of the cathode wires 1 can be suitably attached to the peripheral flange of the member, and the frustoconical annular wall surface 14 then lies radially inward of the tubular mesh of said wires 1. The said outer surface 14 is made reflective as by bufling. Thus in the assembled condition of the cathode support member 12, the reduced thickness and the perforations 15 serve to increase the resistance of the flow path for the heating current so that the peripheral region of the member is ohmically heated, thereby reducing the temperature gradient (or the sharpness of the temperature drop) at the end region of the cathode wires, so that the equilibrium temperature assumed by said end region is heightened, and the thermionically inactive section to wire otherwise present at said end region is reduced. At the same time, the reflective annular surface 14 reflects back some of the heat radiated from the midsections of the cathode wires on to said end section, further contributing to the same effect. It will be noted that the reflective surface 14 in this modification serves at the same time to oppose heat radiation from the member 12 so that the desirable heating occurring in the peripheral region of said member is not interfered with. Further in this modification the dished profile contour of the end member 12 increases the length of the conductive heat leakage path from the cathode wires towards the centrally located supporting and connecting rod or rods such as 4 to 5 (FIGURE 1). Lastly, the form of construction just disclosed is very simple and economical to fabricate in a single stamping step.

FIGURE 7 shows another desirable embodiment of the invention which may be regarded as a variation of the embodiment of FIGURES 5 and 6. The cathode end support member is here provided in the form of dished plate of tantalium or other low-conductivity, high-melting metal having a relatively deep depressed central area 16 surrounded by an annular wall 18 which in this case is shown generally cylindrical rather than frustoconical. The annular wall 18 is in turn surrounded by an annular flange or shoulder 17 of short radial extent. The annular wall 18 is made to have a reduced thickness as compared to the main central wall 16, as by removing an annular layer of metal from the outer side of said wall 18, on a lathe or otherwise. Such a machining step can simultaneously serve to provide the outer shoulder or rim section 17 referred to above. If desired the machining step just described may he carried out so as to impart a slightly frustoconical downwardly-tapered form to the outer surface of wall 18. The outer surface is made reflective as by bufling.

In this embodiment the ends of the cathode wires 1 are attached to the surface of shoulder 17 as by welding. The reduced thickness of the annular wall sections 18 provides a path section of increased resistance to the flow of heating current through the cathode end member, whereby the peripheral region of the member is somewhat heated, correspondingly reducing the temperature drop in the adjacent end region of the cathode wires 1 and diminishing the thermionically inactive parts normally present. At the same time, the reflective outer surface of the wall 18 reflect heats on to said end regions of the wires, enhancing the preceding elfect.

It will be understood that the reflecting surfaces used in the invention as disclosed with reference to FIGURES 5, 6 and 7, are additional to and distinct from the heat baflles that are frequently provided in cathode structures for the purpose of reducing radiation heat losses from the cathode. The reflecting surfaces of the invention as disclosed are associated with the end support members of the cathode and are formed for the specific function of abstracting part of the radiated heat from the mid-section of the tubular cathode and redirecting it at the end sections for the purpose indicated. If desired, conventional antiradiant baffles and similar devices of the prior art may be used in a cathode structure according to the invention in addition to the means disclosed herein.

A cathode structure embodying the improvements of this invention can be made substantially shorter in axial extent than a conventional cathode structure, other factors remaining equal. This is a result of the elimination or reduction of the thermionically inactive regions at the ends of the cathode as earlier explained. Because the length of these inactive regions in conventional cathode structures is substantially the same regardless of the total axial extent of the cathode, the advantages achieved by the invention are especially appreciable in the case of relatively short cathodes. The invention will therefore be seen to be of particular interest in the field of the higher radio frequencies and ultra-high frequencies since in that field of applications it is desirable to reduce the length (while increasing the diameter) of the tube electrodes to a minimum in order to reduce the tube impedance and increase the resonant frequency of the tube as far as possible. Suppression of the thermionically inactive end regions of the cathode represents a net gain in this connection since in the past those regions have contributed to the objectionable internal capacity of the tube without adding to the tube performance.

In one practical embodiment the invention was applied to a television power tube having a 25 kilowatt useful power rating in continuous operation, of a type produced by the assignees under the designation TH491. In such tube the cathode mesh wires have a free length of 16 mm. and a diameter of 60 mm. In the operation of this type of tube when constructed in the conventional way, with cathode supporting discs of uniform thickness throughout their radius, when the cathode filaments were heated to their normal operating temperature of 1650 C., it was found that the cathode wires produced no substantial thermionic emission in end regions thereof about 2 mm. long at each extremity of the tubular mesh as measured in the axial direction. The temperature of the wires at their points of soldered attachment to the supporting discs was observed pyrometrically to be 1250 C. In the modified tube construction according to this invention, when the uniform-thickness cathode supporting members were replaced with members of the type hereinabove disclosed with reference to FIGURES 5 and 6 of the attached drawings, pyrometer measurements showed that the temperature at the extremities of the cathode wires had risen to 1500 C. under identical operating conditions. The thermionically inactive regions at each end of the tubular cathode were substantially less than 1 mm. long, i.e. were reduced to about one half their original length.

What I claim is:

1. An electron tube including a direct-heated cathode of tubular shape and at least one end supporting member in the general form of a transverse plate having one end of the tubular cathode attached to its periphery and means connecting said member to a source of electric heating current at a radially inner region thereof, wherein the improvement comprises:

a reduction in the thickness of said supporting member in an annular region thereof adjacent its periphery as compared to the thickness in an inner region adjacent said connecting means, said thickness reduc tion being sufl icient to produce a substantial degree of electrical heating of said member adjacent its periphery.

2. An electron tube as defined in claim 1, wherein said member includes an annular region which is perforate 3. An electron tube as defined in claim 1, wherein said cathode comprises a tubular wire mesh having one of said supporting members connected to each of its ends.

4. A direct-heated cathode assembly for an electron tube comprising:

a tubular mesh of wires having thermionic emissive properties;

at least one supporting member in the general form of a disc having one end of said tubular wire mesh connected to its periphery, and means connected to a radially inner region of said member and connectable to an electric source of cathode heating current; said supporting member comprising:

an annular portion coaxial with and radially inwardly spaced from said periphery and extending into the end part of the tubular mesh, said part having a reflective outward-directed surface;

said member being reduced in thickness in an annular region thereof adjacent its periphery as compared to the thickness in an inner region adjacent said connecting means, said thickness reduction being sufiicient to produce a substantial degree of electrical heating of said member by said source current adjacent its periphery.

7. The assembly defined in claim 5, wherein said annular wall section is generally frustoconical with a radius tapered in an axial direction away from said flange.

8. The assembly defined in claim 5, wherein said member comprises an integral stamping.

9. The assembly defined in claim 5, wherein said reflective surface is machined around said annular wall section.

10. The assembly defined in claim 5, wherein said member comprises tantalum.

References Cited UNITED STATES PATENTS Morton 313-341 X Ganswindt et a1 313341 FOREIGN PATENTS Great Britain. Great Britain. Great Britain. Great Britain.

JOHN W. HUCKERT, Primary Examiner A. 1. JAMES, Assistant Examiner US. Cl. X.R.