GB2297190A - Electron tubes with graphite coating to reduce multipactor dishcarge and diamond layer to conduct away heat - Google Patents

Abstract

In an electron tube such as a travelling wave tube, a ceramic member 1 having a surface exposed to electrons during operation of the tube bears a layer of diamond 3 and a top thin layer of graphite 4. The graphite tends to reduce multipactor discharge from the surface of the component hence allowing the tube to operate at relatively high power levels. An electrically conductive member 7 for the transmission of high frequency radiation may pass through the member 1 and carry a diamond layer for the conduction of heat from the interior of the tube. The transmission line is coaxial and tapered with a diamond coating.

Description

Electron Tubes This invention relates to electron tubes and more particularly, but not exclusively, to those devices in which electrons and high frequency energy are caused to interact to amplify or generate a high frequency signal.

In the phenomenon known as multipactor discharge, electrons or other charged particles incident on a material cause secondary electrons to be emitted. This effect often occurs under conditions where r.f. fields are present as any free electrons tend to be accelerated and hence have high energies. In some types of device, multipactor effects are required and play a part in the operation of the tube. However, in others multipactor discharge is undesirable as it leads to damage within the device and a reduction in operating efficiency. In some circumstances, this may be sufficiently severe as to cause the tube to cease operating.

Multipactor discharge may occur, for example, at windows for the transmission of microwave radiation, such as those used in coaxial transmission lines in which a conductive member is extensive through the window.

Microwave transmissive windows are used in many types of device, such as, for example, travelling wave tubes, klystrons and magnetrons and are typically employed to permit microwave radiation to be transmitted or applied to the device whilst maintaining its vacuum integrity.

According to a first aspect of the invention, there is provided an electron tube comprising a ceramic or ceramic composite member having a surface exposed to electrons during operation and having a layer of diamond at the surface and there being a graphite layer on the diamond whereby multipactor discharge is reduced compared to a tube in which the graphite layer is absent.

The member may be formed solely from a ceramic material or may be a composite of ceramic and another material. For example, it could be formed from a mixture of ceramic and diamond bound together in a matrix. In the latter case, the layer of diamond may be incorporated in the ceramic composite material instead of being separately laid down on the surface.

The graphite layer tends to inhibit the generation of secondary electrons. In many devices, it is necessary that the member is substantially electrically insulating. In such circumstances, the graphite layer must be thin to present a high electrical resistance. In one preferred embodiment of the invention, the diamond and the graphite layers are deposited on the surface of the ceramic or ceramic composite member using chemical vapour deposition techniques. The conditions in which artificial diamond is produced on a substrate may be controlled so as to select the amount of graphite which is also formed. The deposition process may be carried out relatively slowly using atomic hydrogen to scour the surface on which the diamond is laid down to produce a pure "white" diamond layer in which there is little or no graphite present.By increasing the rate at which deposition occurs, more graphite is formed. Thus if the deposition process is carried out more quickly, then a "grey" diamond is produced in which some graphite is also present. The most impure form of diamond, known as "black" diamond, includes considerable amounts of graphite. To implement the present invention using chemical vapour depositions initial stages of the deposition process may be relatively slow to produce pure diamond. When a sufficient thickness has been laid down for required purposes, such as to provide a thermal conduction path, the process speed is increased, or the conditions changed in some other way, to produce the graphite layer. The process may be controlled to produce a gradual change in the purity of the diamond, resulting in an undefined boundary between the diamond layer and the graphite layer.It is also possible using this process to produce a very thin graphite layer or to produce a sharper boundary between the diamond and the graphite layer.

In one preferred embodiment the graphite layer has a thickness of approximately 1OO . Advantageously the graphite thickness is in the range 5oA to 25oA.

The invention is particularly applicable to devices such as travelling wave tubes, klystrons and magnetrons but may be applied to any device in which multipactor discharge could cause a loss in performance.

It is preferred that the diamond and graphite layers are substantially continuous over the whole surface of the ceramic or ceramic composite. However, it may be acceptable to cover only those parts of the surface which are particularly liable to generate secondary electrons.

The invention may be applied to any surfaces in a device liable to cause multipactor discharge, and is particularly advantageously employed where the member is a window through which high frequency radiation is arranged to be transmitted. In this case, the layer of diamond conducts heat away from the interior of the tube, allowing high power levels to be sustained an#d the presence of the graphite layer enables this advantage from the use of diamond to be more fully utilised.

According to a second aspect of the invention, there is provided a window for the transmission of microwave radiation comprising a ceramic substrate and a layer of diamond on the substrate.

Diamond is a particularly good thermal conductor, having a thermal conductivity which is approximately six times greater than that of copper, and it is also electrically insulating. By including a layer of diamond on the substrate, heat generated during the transmission of microwave radiation through the ceramic substrate may be rapidly dispersed, being directed, for example, to a heat sink region where it may be removed from the device in which the window is incorporated. Thus, a window in accordance with the invention may be particularly advantageously used in high power devices. The window may, for example, form part of an output at which an amplified high frequency signal is removed from a travelling wave tube.

The diamond may be present on a face of the window through which the microwave energy is transmitted, such that it extends in a plane substantially normal to the direction of the radiation. The diamond layer does not appreciably impair the transmission of the microwave radiation through the ceramic substrate but allows excess heat to be conducted away. The thermal properties of the ceramic substrate with its layer of diamond may be further enhanced by also including diamond on a surface in a plane substantially parallel to a direction in which microwave radiation is transmitted through a face of a substrate. This maximises the area over which the thermally conductive material may be deposited and may also increase the surface area over which heat is conducted to a heat sink via a surrounding wall or frame.However, it may be advantageous in some applications only to include diamond on a surface, or surfaces substantially parallel to the direction in which microwave radiation passes through substrate.

The second aspect of the invention is applicable to substrates of different configurations. In one embodiment, the substrate has two major faces and peripheral edge surface extensive between them, for example, the substrate may comprise a planar disc.

Diamond may be deposited on one, two or all faces of the disc, as required. In another embodiment, the substrate is a hollow cylinder having two cylindrical coaxial faces, such as the type of window known as a "lighthouse" window. In a third embodiment, the substrate may be a hollow block having substantially rectangular orthogonal faces. Other window configurations may also benefit from use of this aspect of the invention.

In one preferred embodiment of the invention, an electrically conductive member is extensive in an aperture through the substrate. The member may be included in a coaxial transmission line, being the central conductor, for example, of such a line. A surrounding wall around the window may then form the outer conductor of the coaxial transmission line.

Microwave radiation transmitted along the transmission line would tend to cause substantial heating effects at the ceramic window and could lead to distortion, for example, with a loss of vacuum. By including diamond on the window surface heat can be conducted away from the interior of the arrangement, allowing relatively high power signals to be transmitted without damage to the device. Preferably, the electrically conductive member is coated with diamond. The high thermal conductivity of the diamond on the conductive member in combination with the coating on the window enhances the thermal characteristics of the arrangement.

According to a feature of the invention, an electron beam tube includes a window in accordance with the second aspect of the invention. The invention is particularly applicable to travelling wave tubes but may also be advantageously used in other devices.

According to a third aspect of the invention, an electrical transmission arrangement comprises an electrically conductive member having a transverse cross-sectional area which decreases along its length to present a change in impedance and which is coated with a layer of diamond.

Tapered transmission lines give a gradual change in impedance and hence allow good matching to be achieved. For example, typically, in a travelling wave tube having a helix slow wave structure, the output coaxial transmission line, via which the amplified high frequency signal is taken, has an impedance of the order of 100Q at the end of the helix in the slow wave structure whereas at the output of the travelling wave tube, the impedance is typically required to be 50so. The cross-sectional area of the conductive member may be gradually increased from the helix towards the output to give a change in impedance to improve matching. However, tapering the transmission line could lead to excessive heating, possibly of sufficient severity to cause the device to stop operating.By coating a part of a transmission line with a layer of diamond in accordance with this second aspect of the invention, heat may be rapidly transferred away from the interior of the tube, allowing tapering of the transmission line to be used with advantage.

Advantageously, the electrically conductive member is part of a coaxial transmission line and in some embodiments of the invention, the conductive member is extensive through a ceramic window and the ceramic window may also include a layer of diamond on its surface to enhance the thermal characteristics of the device.

Some ways in which the invention may be performed are now described by way of example with reference to the accompanying drawings in which: Figure 1 schematically illustrates part of an electron tube in accordance with the first aspect of the invention; Figure 2 is a schematic longitudinal sectional view of part of a travelling wave tube in accordance with the second aspect of the invention; Figure 3 illustrates in greater detail part of the travelling wave tube shown in Figure 2; and Figures 4 and 5 illustrate alternative windows in accordance with the invention.

With reference to Figure 1, a travelling wave tube includes a ceramic window 1 forming part of the vacuum envelope and being surrounded by a wall 2. The surface of the ceramic window 1 is coated with a layer of diamond 3 and has a top layer of graphite 4. In this arrangement, the layer of diamond and the graphite layer are formed during the same process by chemical vapour deposition, giving a gradual change from the pure diamond at the surface of the ceramic member 1 to the almost pure graphite layer. In other arrangements, the diamond layer 3 and the graphite layer 4 have a more defined boundary between them. The graphite layer 4 has a thickness of the order of iooA.

During operation of the tube, high energy electrons impinge on the window 1. In absence of the graphite layer, this would tend to lead to secondary electrons being emitted.

However, the graphite layer reduces such multipactor discharge and hence allow the tube to operate at high power levels.

In other arrangements, the window is formed from a ceramic composite consisting of a combination of ceramic material and diamond.

With reference to Figures 2 and 3, a travelling wave tube includes a slow wave structure consisting of a helix 5 surrounded by an outer metal envelope 6 and having an output coaxial transmission line 7. The slow wave structure is under vacuum and a ceramic window 8 is sealed by a gas tight seal to a surrounding wall 9 at the output. The window 8 is typically of alumina or beryllia. The central conductor 10 of the coaxial transmission line passes through an aperture in the ceramic window 8 and the outer conductor of the transmission line is constituted by the surrounding wall 9. The face 11 of the ceramic window 8 at which microwave radiation is incident from the interior of the slow wave structure is coated with a thin layer of diamond 12.

The window 8 is a planar disc and the layer of diamond 12 on its interior surface is substantially uniformly distributed over the entire inner face of the window 8. The central conductor 10 has a transverse sectional area which decreases in steps from the window towards the interior of the tube and the beginning of the helix so as to give a gradual change in impedance. The surface of the conductor 10 within the vacuum envelope is coated with a layer 13 of diamond which joins the layer 12 at the window 8. The diamond layer 13 extends from the last turn of the helix 5 to the window 8. In some embodiments the diamond may also coat the outer surface of the helix 5.

Heat generated during operation of the travelling wave tube is conducted via the diamond layers 12 and 13 to the outer wall 9 and hence from the interior of the tube.

In other embodiments, diamond is also included on the outer surface of the window 8 where it joins the wall 9. Diamond may also be located on the outer face 14 of the window 8 and the interior of the aperture through which the conductor 10 passes.

The input window via which the signal to be amplified is transmitted into the slow wave structure may also be coated with diamond similarly to the output window 8.

Figure 4 schematically illustrates a hollow cylindrical ceramic window 15 in which microwave radiation is transmitted from the interior of the window 16 after conduction along a line 17 and passes through the window in a radial direction 18. In this arrangement, the inner surface 19 of the cylinder 16 is coated with a thin layer of diamond 20. In other embodiments, the outer surface 21 and the edge surface 22 and 23 are also coated with diamond.

A hollow block window shown in Figure 5 has six ceramic faces which are substantially rectangular. In this embodiment, radiation is arranged to pass through two of the faces 24 and 25 and these are coated on their outer surfaces with a diamond layer 26 and 27 which conducts heat from the face to the surrounding metal walls to which the window is fixed.

Claims (34)

1. An electron tube comprising a ceramic or ceramic composite member having a surface exposed to electrons during operation and having a layer of diamond at the surface and there being a graphite layer on the diamond whereby multipactor discharge is reduced compared to a tube in which the graphite layer is absent.

2. A tube as claimed in claim 1 wherein the ceramic composite includes ceramic material combined with diamond.

3. A tube as claimed in claim 2 wherein the layer of diamond is present on the surface of ceramic composite member.

4. A tube as claimed in claim 1, 2 or 3 wherein the surface is within a vacuum envelope.

5. A tube as claimed in any preceding claim wherein the diamond and graphite layers are substantially continuous over the whole surface.

6. A tube as claimed in any preceding claim wherein there is a gradual change from diamond to graphite through the thickness of the two layers.

7. A tube as claimed in any preceding claim wherein the member is a window through which high frequency radiation is arranged to be transmitted.

8. A tube as claimed in any preceding claim wherein the layer of diamond and the graphite layer are deposited on the surface using a chemical vapour deposition technique.

9. A tube as claimed in any preceding claim wherein the graphite layer has a thickness in the range of 50 to 250A

10. A travelling wave tube arrangement comprising an electron tube as claimed in any preceding claim.

11. An electron tube substantially as illustrated in and described with reference to the accompanying drawings.

12. A window for the transmission of microwave radiation comprising a ceramic substrate and a layer of diamond on the substrate.

13. A window as claimed in claim 12 wherein the ceramic substrate includes a face through which microwave radiation is arranged to be transmitted and wherein diamond is present on the face.

14. A window as claimed in claim 12 or 13 wherein the ceramic substrate includes a surface in a plane substantially parallel to a direction in which microwave radiation is transmitted through a face of the substrate and wherein diamond is present on the surface.

15. A window as claimed in claim 12, 13 or 14 wherein the substrate has two major faces and a peripheral edge surface extensive between them.

16. A window as claimed in claim 12, 13 or 14 wherein the substrate is a hollow cylinder having two cylindrical coaxial faces.

17. A window as claimed in claim 12, 13 or 14 wherein the substrate is a hollow block having substantially rectangular orthogonal faces.

18. A window as claimed in any one of claims 12 to 17 wherein the substrate is sealed to a surrounding wall by a gas-tight seal.

19. A window as claimed in any one of claims 12 to 18 wherein an electrically conductive member is extensive through an aperture through the substrate.

20. A window as claimed in claim 19 wherein the member is included in coaxial transmission line.

21. A window as claimed in claim 19 or 20 wherein the member is coated with diamond.

22. A window as claimed in claim 19, 20 or 21 wherein the member has a transverse crosssectional area which decreases along part of its length.

23. An electron beam tube comprising a window as claimed in any one of claims 12 to 22.

24. A tube as claimed in claim 23 when dependent on any one of claims 19 to 22 wherein an amplified high frequency output signal is transmitted via the window.

25. An electrical transmission arrangement comprising an electrically conductive member having a transverse cross-sectional area which decreases along its length to present a change in impedance and which is coated with a layer of diamond.

26. An arrangement as claimed in claim 25 wherein the member is part of a coaxial transmission line.

27. An arrangement as claimed in claim 25 or 26 wherein the member is extensive through a ceramic window.

28. An arrangement as claimed in claim 27 wherein the ceramic window has a layer of diamond present on its surface.

29. An arrangement as claimed in claim 27 wherein the window is as claimed in any one of claims 1 to 7.

30. An arrangement as claimed in any one of claims 25 to 29 wherein the diameter of the member changes in steps.

31. An electron beam tube including an arrangement as claimed in any one of claims 25 to 30.

32. A window substantially as illustrated in and described with reference to the accompanying drawings.

33. A transmission line arrangement substantially as illustrated in and described with reference to Figure 2 or 3 of the accompanying drawings.

34. A travelling wave tube substantially as illustrated in and described with reference to the accompanying drawings.