Ring-shaped cathode for vacuum tube

Abstract

The present invention relates to an annular cathode for a vacuum tube, which comprises: - a cylindrical central support (7) whose axis is the axis of the cathode; - an outer peripheral electron emitter (6) with an annular cross-section extending above the outer periphery of the cathode, the axis of the electron emitter (6) being the axis of the cathode; and, - a folded skirt (4) which is fixed to the central support (7) at its inner end and is fixed to a plurality of lugs (5) at its outer end; - each lug (5) is arranged in series with the folded skirt (4) and is fixed to the folded skirt (4) and to the inner surface of the electron emitter (6).

Description

Technical Field

[0001] The present invention relates to a ring-shaped cathode for a vacuum tube or electron tube. The present invention relates to the development of high-performance thermoelectric / thermionic electron guns (such as gyrotrons). A gyrotron is a power vacuum tube (oscillator) that generates microwave frequency waves. Background Art

[0002] A major drawback of using a cathode electron emitter is its contact with the elements that support it.

[0003] The electron emitter must be held in a specific position, but at the same time must be thermally insulated from the rest of the cathode, including the support. Additionally, the electron emitter must have a uniform temperature across its entire surface.

[0004] The contact of the emitter with the structure that supports it is thus very important because it creates several regions exhibiting temperature discontinuities, which are also reflected by asymmetric thermal deformations. This non-uniformity and these deformations affect the quality of the beam emitted by the electron gun.

[0005] It has been known to use a single element with an expansion direction opposite to that of the main cathode, and to limit the flexible fixing elements (tilted or conical fixing elements) that connect the electron emitter to the support of the cathode, as described for example in document US3631290A.

[0006] Such an embodiment compensates for the longitudinal expansion of the electron emitter without compensating for the radial and orbital expansion, which means that the electron emitter is deformed by anisotropic thermal expansion. These expansions are unbalanced and there is non-uniformity in the thermal expansion. At the same time, the presence of discrete contact points between the electron emitter and the conventional connection creates a discontinuity in the heat flux and affects the thermal uniformity of the emitter.

[0007] The main difficulties in designing the cathode involve the alignment of the electron emitter and the thermal insulation between the electron emitter and the other parts of the cathode, especially with respect to the elements that support the emitter.

[0008] The electron emitter must be placed in a specific position, and thus there are elements dedicated to its support, hereinafter referred to as supports.

[0009] With respect to the alignment of the emitter, the existing cathodes exhibit the following main drawbacks.

[0010] The supports are sensitive to deformations caused by the thermal expansion of the material. These deformations impair the alignment of the supports, and the mechanical position of the supports at ambient temperature may change after heating. Generally, the mechanical position of the supports at ambient temperature may not correspond to their position when the electron emitter heats up. Therefore, the spatial position of the electron emitter can change before, during, and after heating.

[0011] In terms of thermal uniformity, existing cathodes exhibit the following main drawbacks.

[0012] The support must be in contact with the emitter at least partially. This contact is mechanical and thus thermal. The support element in contact with the electron emitter allows a heat flux that creates a temperature difference in the contact area. This temperature difference induces a temperature non-uniformity along the surface of the emitter. This temperature non-uniformity induces an energy non-uniformity of the emitted electrons, which is reflected by a low beam quality.

[0013] Figure 1 Schematically showing a cross-sectional view and a plan view of a cylindrical cathode known from the prior art, the cylindrical cathode comprising a flexible fixing element 1 (inclined or tapered fixing element) that connects an electron emitter 2 with a concave electron emission surface to a support 3 of the cathode.

[0014] The electron emitter 2 expands in the positive direction along the axial direction of the cathode, while the flexible fixing element 1 has a projection of thermal expansion in the same axial direction but in the opposite negative direction. The longitudinal expansions of the two structures are actually balanced. However, the radial and orbital expansions are unbalanced and the heat flux at the interface between the electron emitter 2 and the tapered fixing element 1 can be large. The presence of discrete contact points between the electron emitter 2 and the tapered fixing element 1 induces heat flux discontinuities and affects the thermal uniformity of the electron emitter 2. Summary of the Invention

[0015] An object of the present invention is to overcome the above problems.

[0016] According to one aspect of the present invention, there is provided an annular cathode for a vacuum tube, comprising:

[0017] - a cylindrical central support having an axis that is the axis of the cathode;

[0018] - an outer peripheral electron emitter having an annular cross-section extending above the outer periphery of the cathode, the axis of the outer peripheral electron emitter being the axis of the cathode; and

[0019] - a folded skirt that is fixed to the central support at its inner end and is fixed to a plurality of lugs at its outer end;

[0020] - each lug is arranged in series with the folded skirt and is fixed to the folded skirt and to the inner surface of the electron emitter.

[0021] The present invention improves the alignment and thermal uniformity of the cathode electron emitter under thermal operating conditions.

[0022] According to one embodiment, the folded skirt is a single piece.

[0023] The use of a single-piece skirt makes it possible to limit the number of welds.

[0024] As a variant, the folded skirt comprises a plurality of concentric tubular cylinders with a circular cross-section, two successive tubular cylinders being alternately connected by rings at one end and at the other end of the tubular cylinder.

[0025] The use of such a folded skirt is more easily feasible than a single-piece skirt.

[0026] For example, the cylindrical central support has a circular cross-section.

[0027] In one embodiment, the annular cathode comprises a fixed support for the lugs provided between the electron emitter and the lugs.

[0028] The presence of such a support simplifies the mounting of the lugs.

[0029] For example, the lugs are U-shaped.

[0030] Thus, this makes it possible to create a connection in a desired direction with a simplified mounting.

[0031] According to one embodiment, the lugs are evenly distributed angularly.

[0032] Thus, the thermal uniformity is enhanced because the thermal discontinuities are symmetrically distributed in the track direction, enhancing the thermal uniformity.

[0033] According to one embodiment, the ratio of the volume to the surface area of the lugs and / or of the tubular cylinders of the folded skirt is less than 0.06 mm.

[0034] In one embodiment, the lugs have a volume-to-surface area ratio of 0.05 mm.

[0035] According to one embodiment, the tubular cylinders of the folded skirt have a volume-to-surface area ratio of 0.025 mm.

[0036] Thus, a good trade-off is obtained between good heat resistance (thinness) and good heat exchange by radiation (good surface area), and allows good isotropic expansion. Description of the Drawings

[0037] The invention will be better understood on the basis of the study of several embodiments described as non-limiting examples and illustrated by the drawings, in which:

[0038] Figure 1 A cylindrical cathode according to the prior art is schematically shown in plan view and in sectional view;

[0039] Figure 2Schematically show a ring-shaped cathode according to one aspect of the present invention in sectional view and plan view;

[0040] Figure 3 Schematically show a comparison between a ring-shaped cathode of the prior art and a ring-shaped cathode according to one aspect of the present invention;

[0041] Figure 4 Schematically show an annular cathode according to one aspect of the present invention in plan view;

[0042] Figure 5 Schematically show a ring-shaped cathode according to one aspect of the present invention in sectional view; and

[0043] Figure 6 Schematically show an example of a lug and a folded skirt tubular cylinder of a ring-shaped cathode according to one aspect of the present invention.

[0044] In all the drawings, elements with the same reference numerals are similar. Detailed Description

[0045] As Figure 2 shown, the proposed invention is based on two coupled mechanical fixing elements that cooperate to maintain the concentricity and alignment of all components of the cathode when the cathode is in thermal operation. The two fixing elements are a folded skirt 4 and a plurality of lugs 5 that are serially arranged between a cylindrical central support 7 and an outer peripheral electron emitter 6. The axis of the cylindrical central support 7 is the axis of the cathode, and the outer peripheral electron emitter 6 has an annular cross-section that extends over the outer periphery of the cathode, and the axis of this annular cross-section is the axis of the cathode.

[0046] The folded skirt 4 makes it possible to compensate for axial and radial deformations of the geometry of the cathode. The folded skirt 4 includes a sleeve or concentric tubular cylinder 4a that implements a metered flexible fixing element with opposite thermal expansion vectors to neutralize thermal expansion.

[0047] The plurality of lugs 5 makes it possible to neutralize orbital and radial deformations of the cathode. The lugs 5 are radial supports that connect the outer peripheral electron emitter 6 to the folded skirt 4.

[0048] Axial neutralization can be obtained by adjusting the height (dimension in the axial direction) of the lugs. These lugs implement a flexible fixing element that neutralizes the radial expansion of the material by an isotropic reaction acting in the circumferential direction. Once the system has been sized to utilize the desired conditions, the opposite expansions within the symmetric deformations cause the electron emitter to be aligned under these conditions.

[0049] These two fixing elements are arranged in series. The folding skirt 4 is fixed to the cylindrical central support 7 of the annular cathode and is fixed to the lug 5, and the lug 5 is fixed to the inner surface of the peripheral electron emitter 6.

[0050] The tubular cylinder 4a of the folding skirt 4 expands in opposite longitudinal and radial directions, and at the same time, the lug 5 expands in opposite orbital and radial directions. The presence of the tubular cylinder 4a causes the expansion of the electron emitter 6 to be balanced in three axial, radial, and orbital directions. As a consequence of the symmetry of the expansion forces distributed over the sleeve itself, the tubular cylinder 4a of the folding skirt 4 expands symmetrically in both positive and negative directions in the axial direction of the annular cathode. The temperature discontinuity and mechanical expansion difference along the tubular cylinder 4a of the folding skirt 4 are greatly restricted.

[0051] By using multiple tubular cylinders 4a, the heat flux from the electron emitter 6 to the support is reduced, which ensures a long path for heat to radiate through the surface, as reflected by the enhanced thermal resistance (instead of using a single fixing element, in which case the heat would follow a shorter path with less surface area). During the heating of the structure, mechanical deformation acts in concert with the lug 5. The lug 5 expands symmetrically in the orbital and radial directions to produce isotropic thermal deformation, which maintains the concentricity between the peripheral electron emitter 6 and the rest of the cathode during the heating of the cathode. The result of symmetric deformation in all directions on the three axes makes it possible to obtain perfect concentricity and alignment when the cathode is heated.

[0052] Through the reduced-size contact points between the fixing elements of the electron emitter and the alternating star-shaped configuration of the sleeve, limited heat conduction between the peripheral electron emitter 6 and the fixing elements is ensured. The fixing elements are made of appropriate materials (tungsten, molybdenum, and molybdenum-rhenium alloys) and are designed by adhering to constraints on their shape factor (reduced volume compared to surface area) in order to limit heat conduction. As a direct consequence, thermal uniformity is ensured by: multiple contact points that produce low-amplitude thermal discontinuities, each contact point being placed in the nearby space. The result is negligible thermal non-uniformity of the electron emitter 6.

[0053] Due to the alternation of the contact points (instead of a single fixing element, in which case heat conducts directly from the emitter 6 to the support 7), there is a partial interruption in the heat flux from the electron emitter 6 to the support 7. At each contact point between the lug 5 and the electron emitter 6, there are smaller temperature and expansion discontinuities due to the multiple lugs 5 (instead of larger discontinuities in the case of a smaller number of fixing elements).

[0054] The structure of the fixing element can be inverted, and the number of tubular cylinders 4a of the folding skirt 4 and / or the number of lugs 5 can be adjusted as a function of the size of the cathode in order to have a better correlation of the dimensions of the central support 7 and of the electron emitter 6.

[0055] The tubular cylinders 4a of the folding skirt 4 expand in opposite longitudinal and radial directions, and at the same time, the star-shaped lugs 5 expand in opposite orbital and radial directions. The result of the symmetric deformation in all directions along the three axes causes the electron emitter 6 to thermally deform concentrically and align with the rest of the cathode structure. The limited heat conduction between the electron emitter 6 and the fixing element is ensured by the alternation of reduced-size contact points between the electron emitter 6 and the lugs 5, where the heat conduction is given by the properties of the materials and the length ratio of the fixing element.

[0056] Figure 3 A comparison is schematically shown between an annular cathode of the prior art and an annular cathode according to an aspect of the present invention.

[0057] Known annular cathodes may expand with anisotropic deformations when they heat up. The alignment and concentricity of the electron emitter 6 with respect to the other elements of the cathode may be impaired during heating. Some existing solutions attempt to compensate for the longitudinal expansion of the electron emitter 6 in the axial direction. However, the radial and orbital expansions are unbalanced, and as a result, there is non-uniformity of thermal movement, as shown in the left-hand part of Figure 3 At the same time, the presence of discrete contact points between the electron emitter 6 and the conventional connection creates heat flux discontinuities and affects the thermal uniformity of the electron emitter 6. The heat conduction of the support pads 8 (necessary for placing the electron emitter in the desired zone) creates non-uniformity of the temperature of the support 7 and the electron emitter 6 with respect to the contribution increased by the radiation effect.

[0058] The present invention makes it possible to improve the thermal uniformity of the electron emitter 6 in addition to compensating for thermal expansion. The reason lies in the geometry of the lugs 5: they are thin and flexible fixing elements. More fixing elements give a more uniform heat flux in more contact points, but the thinness of these fixing elements ensures extremely low heat conduction. In order to demonstrate how the proposed design makes it possible to solve the practical drawbacks, in Figure 3 an example of a known solution is compared with the proposed invention.

[0059] Figure 4 and Figure 5 show schematically, in plan view and cross-section, an annular cathode according to an aspect of the present invention.

[0060] The annular cathode for a vacuum tube comprises:

[0061] - A tubular cylindrical central support 7 with a circular cross-section, the axis of the central support 7 being the axis of the cathode;

[0062] - An outer peripheral electron emitter 6 with an annular cross-section extending over the outer periphery of the cathode, the axis of the outer peripheral electron emitter 6 being the axis of the cathode; and

[0063] - A folded skirt 4, fixed to the central support at its inner end and fixed to a plurality of lugs 5 at its outer end;

[0064] - Each lug 5 is arranged in series with the folded skirt 4 and is fixed to the folded skirt 4 and to the inner surface of the electron emitter 6.

[0065] The folded skirt 4 includes a plurality of concentric tubular cylinders 4a with a circular cross-section, and two consecutive tubular cylinders 4a are alternately connected by rings 4b at one end and at the other end of the tubular cylinder.

[0066] The electron emitter 6 has an outer surface for emitting electrons outward.

[0067] The cylindrical central support 7 is tubular with a circular cross-section, and the lug 5 is U-shaped.

[0068] A fixing support 9 for the lug 5 is provided between the electron emitter 6 and the lug 5. The support 9 is provided with a position such as a slit, which is designed to slidably receive the end of the lug 5, and the lug 5 can then be fixed by simple soldering. This simplifies the fixing of the lug 5 to the electron emitter 6.

[0069] Figure 6 Examples of the lug and the tubular cylinder are schematically shown.

[0070] The volume-to-surface area ratio of the lug and / or the tubular cylinder of the folded skirt can be less than 0.06 mm.

[0071] For example, the lug 5 has a volume-to-surface area ratio of 0.05 mm, and the tubular cylinder 4a of the folded skirt 4 has a volume-to-surface area ratio of 0.025 mm.

Claims

1. Ring-shaped cathode for a vacuum tube, which comprises: - a cylindrical central support (7) whose axis is the axis of the cathode; - an outer peripheral electron emitter (6) with an annular cross-section extending above the outer periphery of the cathode, the axis of the electron emitter (6) being the axis of the cathode; and - a folded skirt (4) which is fixed to the central support (7) at its inner end and to a plurality of lugs (5) at its outer end; - each lug (5) is arranged in series with the folded skirt (4) and is fixed to the folded skirt (4) and to the inner surface of the electron emitter (6).

2. The ring-shaped cathode according to claim 1, wherein the folded skirt (4) is a single piece.

3. The ring-shaped cathode according to claim 1, wherein the folded skirt (4) comprises a plurality of concentric tubular cylinders (4a) with a circular cross-section, and two consecutive tubular cylinders (4a) are alternately connected by rings (4b) at one end and the other end of the tubular cylinder.

4. The ring-shaped cathode according to any one of the preceding claims, wherein the cylindrical central support (7) is tubular with a circular cross-section.

5. The ring-shaped cathode according to any one of claims 1 to 3, which comprises a fixing support (9) for the lugs (5) arranged between the electron emitter (6) and the lugs (5).

6. The ring-shaped cathode according to any one of claims 1 to 3, wherein the lugs (5) are U-shaped.

7. The ring-shaped cathode according to any one of claims 1 to 3, wherein the lugs (5) are evenly distributed angularly.

8. The ring-shaped cathode according to claim 3, wherein the ratio of the volume to the surface area of the lugs (5) and / or the tubular cylinders (4a) of the folded skirt (4) is less than 0.06 mm.

9. The ring-shaped cathode according to claim 7, wherein the lugs (5) have a volume-to-surface area ratio of 0.05 mm.

10. The ring-shaped cathode according to claim 8, wherein the tubular cylinders (4a) of the folded skirt (4) have a volume-to-surface area ratio of 0.025 mm.

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