EP0350358B1 - Electron gun and electron tube including such an electron gun - Google Patents

Description

The subject of the invention is an electron gun comprising 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 certain trajectories this set of electrons.

In all cases, it is necessary 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 thereof, 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 having 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.

par rapport à la cathode. On voit par là l'intérêt de l'utilisation d'électrodes à basse tension, surtout lorsque les fréquences de modulation sont élevées.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 V $\genfrac{}{}{0ex}{}{\text{2}}{\text{ek}}$

relative 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 is therefore 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 from 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.

In US-A-2 883 576 it is proposed to deposit on an non-emissive support an alternation of non-contiguous emissive and non-emissive strips. This patent discloses the features of the preamble of claim 1.

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.

The present invention relates to an electron gun as claimed in claim 1.

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 pads can advantageously be treated against emission by a vapor deposition of thick tungsten.

With such a cathode, one can use a non-emissive grid having openings of substantially circular shape, in which take place the emissive faces of the pellets added 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 provided therein 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 can 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 or aluminum nitride, or made of 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 determined appropriate to the intended use of the cathode in an electron gun.

Each recess 2 contains an emissive pellet 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 pellet 3 has a thickness greater than the depth of the recess 2 which partially contains it so that this pellet is projecting, as well 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, one part 2′A with a larger diameter and a part 2′B with a smaller diameter, which shows 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 the parts 2′A, 2′B of the holes 2 ′, with, consequently, an external shoulder 6 which s 'applies 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 a 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 soldering 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. 2 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. Such a grid has openings whose configuration is similar to that of the recesses 2, 2 ′. When the latter are holes, the openings of the grid 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, in 10B ′, from the plane in which the projecting emissive face 9 of this is substantially 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 non-emissive grids in place are known and conventional and have not been shown. Of course, the emissive elements 3, 3 ′ are electrically interconnected, for example by their rear end faces 8, for the equalization of 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 emitting elements 3.3 ′ are electrically connected, also by their rear faces 8, for example.

In an electron gun, 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 (14)

1. Elektronenstrahlsystem mit einem oder mehreren Gittern (10, 11) und einer Kathode, die einen Körper (1) aus nicht elektronenemissionsfähigem Material mit voneinander beabstandeten elektronenemissionsfähigen Elementen (3, 3′) aufweist, wobei jedes Element (3, 3′) eine bezüglich des nicht emissionsfähigen Körpers (1) vorspringende emissionsfähige Oberfläche (9) aufweist, dadurch gekennzeichnet, daß die emissionsfähigen Elemente (3, 3′) in Ausnehmungen (2, 2′) des Körpers (1) eingesetzt und festgelegt sind, wobei das oder die Gitter (10, 11) ohne Kontakt mit dem Körper (1) Öffnungen von ähnlicher Konfiguration wie diejenige der Ausnehmungen (2, 2′) haben, um die von den emissionsfähigen Elementen (3, 3′) emittierten Elektronen durchzulassen.
2. Elektronenstrahlsystem gemäß Anspruch 1, dadurch gekennzeichnet, daß die Ausnehmungen (2) Blindlöcher sind.
3. Elektronenstrahlsystem gemäß einem der Ansprüche 1 oder 2, dadurch gekennzeichnet, daß die Ausnehmungen (2′) durch den Körper hindurchgehen und einen ersten Teil (2′A) von größerer Querabmessung sowie einen an der Hauptfläche (1A) des Körpers (1) mündenden zweiten Teil (2′B) von kleinerer Querabmessung aufweisen, wobei die beiden Teile eine Innenschulter (5) entstehen lassen, wobei die emissionsfähigen Elemente ebenfalls einen ersten Teil (3′A) von größerer Querabmessung und einen zweiten Teil (3B′) von kleinerer Querabmessung haben, wobei diese beiden Teile eine Außenschulter (6) entstehen lassen, die in Berührung mit der Innenschulter (5) der Ausnehmung steht.
4. Elektronenstrahlsystem gemäß Anspruch 3, dadurch gekennzeichnet, daß das Gitter oder eines der mehreren Gitter ein Modulationsgitter (11) ist, das Öffnungen von gleichen Abmessungen wie die Querabmessungen des ersten Teils der Ausnehmung (2′A) hat.
5. Elektronenstrahlsystem gemäß einem der Ansprüche 3 oder 4, dadurch gekennzeichnet, daß der erste Teil (3′A) des emissionsfähigen Elements gegenüber dem ersten Teil der Ausnehmung (2′A) zurückspringt.
6. Elektronenstrahlsystem gemäß Anspruch 5, dadurch gekennzeichnet, daß das emissionsfähige Element (3′) in der Ausnehmung durch eine im Inneren des ersten Teils der Ausnehmung und auf dem ersten Teil des emissionsfähigen Elements angeordnete Lötraupe (7) festgelegt ist.
7. Elektronenstrahlsystem gemäß Anspruch 5, dadurch gekennzeichnet, daß das emissionsfähige Element (3′) in der Ausnehmung durch eine kugelkalottenförmige Feder (R) festgelegt ist, die im Inneren des ersten Teils der Ausnehmung angeordnet und gegen den ersten Teil des emissionsfähigen Elements angedrückt ist.
8. Elektronenstrahlsystem gemäß einem der Ansprüche 1 bis 7, dadurch gekennzeichnet, daß der Körper (1) aus elektrisch leitendem Material besteht.
9. Elektronenstrahlsystem gemäß Anspruch 8, dadurch gekennzeichnet, daß das Material Wolfram ist.
10. Elektronenstrahlsystem gemäß einem der Ansprüche 1 bis 5 oder 7, dadurch gekennzeichnet, daß der Körper aus einem Material besteht, das aus Aluminiumnitrid, Siliciumnitrid, Siliciumcarbid und Wolframcarbid ausgewählt ist.
11. Elektronenstrahlsystem nach einem der Ansprüche 1 bis 10, dadurch gekennzeichnet, daß das Gitter oder eines der mehreren Gitter ein Schutzgitter (10) ist, wobei die Höhe der vorspringenden Teile (3S) wenigstens gleich der Dicke des Gitters ist.
12. Elektronenstrahlsystem nach einem der Ansprüche 1 bis 10, dadurch gekennzeichnet, daß das Gitter (10′) oder eines der mehreren Gitter ein Schutzgitter ist, dessen Öffnungen jeweils mit einer ein emissionsfähiges Element (3′) umgebenden zylindrischen Zone (10A′) geformt ist, auf die eine sich erweiternde Zone (10B′) folgt, wobei die Höhe der emissionsfähigen Elemente (3′) mindestens gleich der axialen Lange der zylindrischen Zone (10A′) ist.
13. Elektronenstrahlsystem gemäß einem der Ansprüche 1 bis 10, dadurch gekennzeichnet, daß das Gitter (11) oder eines der mehreren Gitter ein Modulationsgitter ist, das außerhalb der vorspringenden Teile (3a) montiert ist.
14. Elektronenstrahlröhre, dadurch gekennzeichnet, daß sie ein Elektronenstrahlsystem gemäß einem der Ansprüche 1 bis 13 enthält.

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