EP0350358A1 - Electron gun and electron tube including such an electron gun - Google Patents

Abstract

This cathode comprises a body (1) of non-emissive material of electrons having a non-emissive face (1A) substantially smooth and elements of emissive material (3) each having an emissive face (9), spaced from one another and fixed to the body (1), for example in recesses (2) with their emissive surface (9) in relief of a determined value (3S) relative to said non-emissive face (1A), so that a protective electrode can be placed between the projecting parts of these elements (3).

Description

The subject of the invention is an electron emitting cathode with which is associated at least one modulation or control grid for equipping an electronic tube of any type, in particular in the field of high frequencies. The invention also relates to any electronic tube comprising such a cathode.

In electron tubes of the traveling wave type or more generally in linear beam devices, the electron beams are emitted by a cathode and are controlled by at least one electrode and most often by a set of electrodes. particularly designed to produce and guide on determined trajectories this set of electrons.

It is necessary in all cases to produce electrodes whose configuration is such that the resulting equipotential lines in the vicinity of the cathode are as parallel as possible to the surface of the latter, both in blocking regime and in operating regime. This is what has led, with cathodes of large diameter in comparison with the distance from the cathode to the device for using the electron beam, to produce modulation electrodes comprising multiple passage openings and therefore taking the form of "grids".

Typically, a modulation grid with multiple passage openings uses a modulation voltage of a few hundred volts.

The energy required for the modulation of such an electrode is proportional to its capacity with respect to the cathode and to its positive voltage V² _ek with respect to the cathode. This shows the advantage of using low voltage electrodes, especially when the modulation frequencies are high.

However, a modulation grid placed opposite a surface of an electron-emitting cathode, and positive with respect to this surface, receives part of electronic transmission. There therefore occurs a "beam interception" which may not be annoying when the average density of the intercepted current remains low, which is the case for devices with medium or low power, in particular with a cathode with which is associated a simple wire rack

But with high power devices, it is necessary to eliminate as much as possible the interception of electrons by the control grid.

It has therefore been proposed to eliminate the electronic emission from the cathode in the zones which are situated opposite the modulation grid, by the deposition on the surface of the cathode of a layer of non-emissive material; an exemplary embodiment of such a solution is described in document US-A-4,459,323. However, it has become apparent in use that this layer itself becomes emissive, after a relatively short duration, even when it is separated of the cathode by an insulating layer, as a result of migration of emissive material from the cathode.

The main object of the invention is to provide a cathode whose design allows the use of a protective grid without contact with the cathode, therefore not likely to be contaminated by the emissive material of this cathode, and whose design at the same time facilitates the relative arrangement of this protective grid with respect to the cathode.

We know, moreover, that in certain circumstances the protective grid can be omitted; in this case, it is the position of the first modulation grid which must be established with precision with respect to the cathode.

A secondary object of the invention is to achieve a cathode of design such that it also allows the easy positioning, relative to this cathode, of the first modulation grid when the protection grid does not exist.

Since the protective grid, as well as the modulation grid (s), are non-emissive grids compared to the cathode, the expression "non-emissive grid" will be used in the following to denote both a protective grid electrically connected to the potential of the cathode or a modulation grid close to the cathode in the absence of a protective grid but not connected to the potential of the cathode.

A cathode according to the invention comprises a body of non-emissive material of electrons having a non-emissive face and elements of emissive material having an emissive surface, spaced from one another in a desired desired configuration and fixed to said body with their surface emissive in relief with respect to the non-emissive face of this body.

According to a preferred embodiment of the invention, the body of non-emissive material has a substantially smooth non-emissive face, recesses provided in this body open in this face and are spaced from each other according to a determined configuration; the elements in emissive material are introduced and immobilized respectively in said recesses, each element being immobilized in a corresponding recess so that its emitting surface is in relief relative to the non-emissive face of the body.

With a cathode produced in accordance with the invention, there is no special difficulty in placing a non-emissive electrode opposite the network of non-emissive zones which exists on the surface of the cathode body, between the emissive surfaces projecting from the elements in emissive matter.

In practice, the elements made of emissive material are, advantageously, for cost reasons, analogous to pellets of revolution and of a more or less concave shape on their emissive face oriented towards the interaction space. Furthermore, the surface of the part of revolution of these emissive pellets can advantageously be treated against emission by a vapor deposition of thick tungsten.

With such a cathode, it is possible to use a non-emissive grid having openings of substantially circular shape, in which the emissive faces of the pellets attached take place in the non-emissive body of the cathode. When this first grid is a protective grid, the immediately following modulation grid is placed with openings which are provided there having their geometric axes coincident with those of the axes of the first grid.

• - la figure 1 est une vue générale en perspective d'une cathode selon l'invention sur laquelle certains évide­ments sont représentés sans éléments émissifs et certains autres évidements sont munis chacun d'un élément émissif,
• - la figure 2 est une vue partielle en coupe selon II-II de la figure 1,
• - les figures 3 à 6 sont des vues partielles en coupe analogues à la figure 2 montrant des variantes de réa­lisation conformes à l'invention.

A description will now be given of a preferred embodiment of a cathode according to the invention and of several variants which may be made to it. Reference is made to the appended drawings in which:

• FIG. 1 is a general perspective view of a cathode according to the invention in which certain recesses are shown without emissive elements and certain other recesses are each provided with an emissive element,
• FIG. 2 is a partial sectional view along II-II of FIG. 1,
• - Figures 3 to 6 are partial sectional views similar to Figure 2 showing alternative embodiments according to the invention.

In the example illustrated by FIGS. 1 and 2, the cathode has a body 1 similar to a disc of reduced thickness, made of non-emissive material, for example made of boron nitride or aluminum, or silicon carbide or tungsten or pure tungsten or molybdenum, having a main face 1A smooth or at least substantially smooth. From this face 1A are produced by machining or otherwise recesses 2 which are in fact blind holes which do not pass through the body 1. These recesses 2 are spaced from one another and distributed in accordance with a configuration fixed ration suitable for the intended use of the cathode, for example in an electron gun.

Each recess 2 contains an emissive patch 3. The latter is for example made of porous tungsten impregnated with an emissive mixture such as a barium-calcium aluminate; It is fixed to the body 1 by brazing done using a molybdenum-ruthenium alloy. Each patch 3 has a thickness greater than the depth of the recess 2 which partially contains it so that this patch is protruding, as shown in FIG. 2, relative to the main face 1A of the body 1. Between the parts protruding 3S emissive pads 3 close to each other, there is an interval 4. The intervals 4 communicate with each other by forming a network.

In the example of Figure 1 the emissive pads 3 are immobilized in the corresponding recesses 2 by a brazing operation, known per se.

Figures 3 to 5 relate to variants in which the body 1 of the cathode is thicker and each recess 2 ′ is a hole which crosses the entire thickness of the body 1. Each hole 2 has two successive parts, part 2 ′ A with a larger diameter and a part 2′B with a smaller diameter, which reveals between the two, in the thickness of the body 1, an inner shoulder 5. As the part 2′B with smaller diameter is that which opens into the face 1A of the body 1, each shoulder 5 is turned opposite to the latter.

Each emissive element 3 ′ also has two parts 3′A, 3′B with different diameters, in correspondence with the diameters of parts 3A, 3B of the holes 3, with, consequently, an external shoulder 6 which is applied against the inner shoulder 5 after the introduction of the elements 3 ′ in the recesses 2 ′. This variant does not change the existence of the projecting part 3S described above.

On each emissive element 3 ′ the length of its part 3′B with smaller diameter, from its outer shoulder 6, is greater than the distance between the shoulder interior 5 and the face 1A so that this part 3′B projects from this face 1A.

In the variants of FIGS. 3 and 5, the emissive elements 3 ′ are immobilized in the recesses 2 ′ which contain them by a brazing bead 7. Preferably the part 3′A with a larger diameter has a length starting from the outer shoulder 6 such that the end face 8 of this part, opposite the projecting emissive face 9, is set back inside the corresponding part 2′A of the recess 2 ′, which makes it possible to produce the bead of brazing 7 inside the latter and on the end face 8.

In the variant of FIG. 4, the emissive elements 3 ′ are immobilized in the recesses 2 by springs R, formed for example in spherical caps, of a diameter chosen so that they can be introduced, by pushing, into the part 2'A with a larger diameter, up to the end face 8, and so that they produce a bracing effect in the opposite direction.

FIG. 1 also shows that the emissive face 9 of the emissive elements 3 can have a concave profile.

FIGS. 2 to 5 also show how a non-emissive grid can be associated with the cathode of the invention. Preferably such a grid has openings whose configuration is similar to that of the recesses 2, 2 ′. When the latter are holes, the grid openings are made at a diameter slightly greater than that of the emissive face 9 of each emissive element 3 (FIG. 2), for example equal to the diameter of the larger part 2'A diameter of the recess 3 ′ (Figures 3 to 5).

When it is a protective grid, drawn in small thickness and designated by the reference numeral 10 in Figures 2 to 4, it is placed in the intervals 4 between the projecting parts 3S; there is therefore an easy relationship to determine between the size of the projecting parts 3S and the thickness of the protective grid; at minimum the importance of the projecting parts 3S is equal to the thickness of this grid. The latter is electrically connected, in a known manner, not shown, to the body 1 of the cathode.

FIGS. 3 and 4 also show a modulation grid 11 mounted in the "shadow" of the protective grid.

FIG. 5 relates to the case where there is no protective grid; the modulation grid 11 is mounted outside the area of the projecting parts 3S.

Figure 6 relates to an alternative embodiment which shows that the arrangement of the protective grid 10 'can have a thickness greater than the importance of the projecting parts 3S of the emissive elements. But in this case, each opening of the protective grid 10 ′ which corresponds to an emissive element 3 ′ is flared, widening, at 10′B, from the plane in which the projecting emissive face 9 is substantially this emissive element 3 ′. Before this flared area 10B ′, from the face of the protective grid 10 ′ looking at the body 1, the opening has a cylindrical area 10A ′. In this case, it is by considering the length in axial direction of this cylindrical zone 10A ′ that the relationship with the importance of each projecting part 3S is determined. In this variant, the openings of the modulation grid 11 are established in relation to the largest dimension of the flared areas 10B ′.

The means for holding the non-emissive grids in place are known and conventional and have not been shown. Of course, the emissive elements 3, 3 ′ are electrically connected to each other, for example by their rear end faces 8, to equalize their potential.

In general, the body 1 of the cathode is made of non-emissive material but conductive of electricity. The invention does not exclude the use of a body made of non-conductive material. In this case, the 3.3 ′ emissive elements are electrically connected cally, also by their rear faces 8, for example.

In an electron gun or in another device where it is used, the cathode of the invention is associated, as is known, with a device for heating the emissive elements 3.3 ′ to the required temperature.

Claims (10)

1. electron gun comprising one or more grids and a cathode, characterized in that said cathode comprises a body (1) of non-emissive electron material (metallic or dielectric) having a non-emissive face (1a), elements made of emissive material (3,3 ′) mechanically attached to said body, each emissive element having an emissive surface (9), said emissive elements being spaces from one another according to a desired configuration determined so that the set of emitted beams are suitable particularly to a linear beam tube (called "o" type), and in that said emissive elements are fixed to said body (1) with their emissive surface (9) in relief of a determined value (3s) relative to said non-emissive face (1a) of the body, so that said value is sufficient to allow said emissive elements to exceed a metal grid (10) placed in the vicinity but not in met the surface of said body. 2. electron gun according to claim 1 characterized in that said body (1) has recesses (2, 2 ′) opening in the rear face of the cathode spaces from each other according to said desired configuration and said emissive elements (3.3 ′) are introduced and immobilized respectively in said recesses (2.2 ′), each emissive element (3.3 ′) being immobilized in a corresponding recess so that its emissive surface (9) is opposite a hole in said grid (10) to penetrate said grid through said holes. 3. An electron gun according to claim 2 characterized in that the recesses (2) are blind holes. 4. electron gun according to claim 2 characterized in that the recesses (2) are holes passing through the body (10) and comprising a part (2′a) of larger transverse dimension and a part (2′b) of smaller transverse dimension opening in the non-emissive face (1a), these two parts revealing an internal shoulder (5), however, while the emissive elements (3 ′) also have two parts (3′a, 3′b) with different transverse dimensions showing an external shoulder (6), these two shoulders (5, 6) being applied against each other. 5. An electron gun according to claim 4 characterized in that each emissive element (3 ′) is terminated opposite its emissive surface (9) by an end face (8) which is recessed inside the corresponding part (2′a) of the recess (2 ′). 6. electron gun according to claim 5 characterized in that each emissive element (3 ′) is immobilized in the recess (2 ′) corresponding by a brazing bead (7) inside said part (2′a ) and on the recessed end face (8). 7. electron gun according to claim 5 characterized in that each emissive element (3 ′) is immobilized in the recess (2 ′) corresponding by a spring (r) in a spherical cap supported inside said part (2 ′ A) against the recessed end face. 8. An electron gun according to any one of claims 1 to 7 characterized in that said body (1) is made of tungsten. 9. electron gun according to any one of claims 1 to 5 and 7 characterized in that said body (1) is made of a material chosen from an aluminum nitride, a silicon nitride, silicon carbide, tungsten carbide. 10. Electronic tube characterized in that it comprises an electron gun according to any one of claims 1 to 9.

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