GB2278012A

GB2278012A - Linear electron beam tube with rf chokes

Info

Publication number: GB2278012A
Application number: GB9309660A
Authority: GB
Inventors: Alan Hugh Pickering
Original assignee: EEV Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1993-05-11
Filing date: 1993-05-11
Publication date: 1994-11-16
Anticipated expiration: 2013-05-11
Also published as: GB9309660D0; GB2278012B

Abstract

A linear electron beam tube, such as an inductive output tetrode device, includes an input cavity 6 formed from an inner body portion 12 and an outer body portion 13 and electrical insulator 14. Co-extensive cylindrical flanges 18 and 23, and 19 and 24 define r.f. chokes and have curved edges 25 and 26 and 27 to reduce electrical stresses between them and hence give good voltage hold-off. The construction enables high voltage parts of the arrangement to be insulated from the low voltage outer body portion whilst presenting a low leakage path for r.f. energy within the cavity 6. The resiliently deformable insulator 14 enables good electrical breakdown characteristics to be achieved under adverse mechanical stresses on the tube. <IMAGE>

Description

Electron Beam Tubes This invention relates to electron beam tubes and more particularly to input resonator cavities of such tubes at which high frequency energy is applied.

The present invention is particularly applicable to inductive output tetrode devices (hereinafter referred to as "lOT's") such as those referred to by the trade name Klystrode (Registered Trade Mark, Varian Associates Inc.) An IOT device includes an electron gun arranged to produce a linear electron beam and an input resonant cavity at which an r.f. signal to be amplified is applied to produce modulation of the beam at a grid of the electron gun. The resultant interaction between the r.f. energy and the electron beam causes amplification of the high frequency signal which is then extracted from an output resonant cavity.

During operation of the tube, electrodes of the electron gun must be operated at relatively high voltages, of the order of tens of kilovolts, and this may cause problems, especially as the input cavity may form an external part of the IOT and therefore be handled during normal usage of the device.

The present invention arose from an attempt to provide an improved IOT input cavity arrangement but is also applicable to other types of linear electron beam devices having input resonant cavities.

According to the invention, there is provided a linear electron beam tube comprising: an input cavity which is substantially cylindrical about a longitudinal axis and arranged to receive, in use, a high frequency signal to be amplified; an electron gun arranged to produce an electron beam in a substantially longitudinal direction; and an output cavity from which the amplified high frequency signal is extracted; wherein the input cavity substantially surrounds the electron gun and comprises an inner body portion electrically connected to part of the electron gun and an outer body portion electrically insulated from the inner body portion, the inner body portion being maintained at a relatively high voltage compared to that of the outer body portion1 and wherein the inner and outer body portions have respective parts which are coextensive to present a choke impedance to high frequency energy within the input cavity and wherein an edge of one or more of the parts terminating in a region where I a part of the other body portion is extensive is curved.

By "high voltage" it is meant of the order of tens of kilovolts.

A linear electron beam tube employing the invention enables electrodes which operate at relatively high voltages to be located such that they are not readily accessible during normal operation of the tube. In addition, the arrangement of the respective parts of the inner and outer body portions enables the two portions to be separated to achieve desired electrical isolation between them whilst permiting the input cavity to be such that there is low r.f. leakage from it, thereby affording efficient operation whilst achieving low electrical stresses between the two body portions.

Use of the invention, enables increased voltage hold-off to be obtained between the co-extensive parts. The curved edge of the part or parts reduces electrical stresses compared to an arrangement in which no such curvature is employed when electrical field lines tend to be concentrated at the end of a part.

Thus, by employing the invention, greater design freedom is offered in selecting spacing between the co-extensive parts. This may result in a more efficient choke impedance being feasible and may also lead to a more compact arrangement.

Only one of the parts may have a curved edge but it would generally be desirable for both or all parts to have curved edges where these terminate in regions of high electrical field.

In one preferred embodiment of the invention, the parts, including their edges, are substantially planar and the edge is curved out of the plane. The parts may be flat plates or may be planar and curved, that is, cylindrical. In the latter case, the edge is the end of the cylinder and may be curved inwardly or outwardly depending on the particular arrangement. In an alternative embodiment the curved edge is a solid rim of, say, circular cross-section similar to a beading along the end of the part.

For example, it may be a region of increased thickness around the inner circumference of a flat annular plate.

Preferably, the edge is curved such that its end is substantially adjacent a region of the part remote from the edge. The edge may be curved sufficiently so that its end actually touches the surface of the part or may be spaced a little way from it.

It a preferred embodiment, the edge is curved with a substantially constant radius of curvature. However, the edge could be folded over to present a more oval crosssection.

In another embodiment of the invention, the said respective parts are extensive in planes substantially transverse to the longitudinal axis. In one particular embodiment, one of the body portions includes two parts extensive in a substantially transverse direction and the other includes one part located between them. In such an arrangement, advantageously, the two outermost parts have edges which curve in a direction away from the said one part. The said one part may be located closer to one of the two parts than the other. In this case, it is preferred that it has an edge which is curved away from the part which is closer to it. In a particularly advantageous embodiment of the invention, the parts are annular plates, giving a cylindrically symmetrical arrangement.

In another embodiment of the invention, the parts are axially extensive flanges which are substantially co-extensive in an axial direction. Preferably, the flanges are substantially cylindrical.

In many arrangements, electrically insulating material is advantageously included between the said co-extensive parts. This enables good voltage hold-off to be achieved and may also improve mechanical stability of the arrangement.

Advantageously, at least one of the said parts is at least partially embedded in the electrically insulating material. In some arrangements it may be desirable to wholly encase the parts in the material for optimum breakdown characteristics.

Preferably, the inner and outer body portions include two pairs of coextensive respective parts. Such an arrangement minimizes r.f. losses in the region between the inner and outer body portions. Although the input cavity could alternatively comprise only one pair of said parts1 this would tend to result in an r.f.

leakage path being present between other portions of the cavity.

It is preferred that the inner body portion comprises two sections which are electrically separate from one another. Again, this facilitates manufacture and assembly and advantageously also permits different voltages to be applied to different parts of the electron gun via the inner body portion. In one preferred embodiment of the invention, the inner body portion is electrically connected to a cathode and a grid of the electron gun. Where two sections are included1 one of them may be physically . .

and electrically connected to the cathode and the other to the grid.

Advantageously, electrically insulating material located between the coextensive parts is generally cylindrical in form. This permits insulation to be distributed in a symmetrical manner around the longitudinal axis of the tube. Where two pairs of respective parts are included in the tube, the electrically insulating material may be present as two separate rings, for example, being interposed between different pairs.

Alternatively, the electrically insulating material is a unitary member which is extensive between both pairs of parts.

Advantageously, the inner and outer body portions are physically joined together by the electrically insulating material which may, for example, be moulded into a particular shape.

Preferably, the outer body portion is at ground potential.

Some ways in which the invention may be performed are now described by way of example with the reference to the accompanying drawings in which: Figure 1 is a schematic sectional view of an IOT in accordance with the present invention, some parts of which have been omitted for the sake of clarity; Figure 2 schematically illustrates another IOT in accordance with the invention; and Figures 3 and 4 schematically illustrate parts of respective different lOT's.

, . .

With reference to Figure 1, an IOT comprises an electron gun 1 which includes a cathode 2 and grid 3 arranged to produce an electron beam along the longitudinal axis X-X of the arrangement. The IOT includes drift tubes 4 and 5 via which the electron beam passes before being collected by a collector (not shown). A cylindrical input resonant cavity 6 is arranged coaxially about the electron gun 1 and includes an input coupling 7 at which an r.f. signal to be amplified is applied. An output cavity 8 surrounds the drift tubes 4 and 5 and includes a coupling loop 9 via which an amplified r.f. signal is extracted and coupled into a secondary output cavity 10 and an output coupling 11.

During operation of this device, the cathode 2 and grid 3 are maintained at potentials of the order of 30kV, the grid 3 being maintained at a dc bias voltage at about 100 volts less than the cathode potential. The input high frequency signal applied at 7 results in an r.f. voltage of a few hundred volts being produced between the cathode 2 and the grid 3.

The input cavity 6 is defined by an inner body portion 12 and an outer body portion 13 with a substantially cylindrical moulded insulating member 14 between them. The member 14 is of a resiliently deformable material and in this case is of silicone rubber. The inner body portion 12 is electrically insulated from the outer body portion 13 by the intervening dielectric material 14. The outer body portion 13 is maintained at substantially ground potential, thus facilitating safe handling of device, whilst the inner body portion 12 is maintained at much higher voltages.

The outer body portion consists of two annular plates 15 and 16 arranged parallel to one another and transverse to the longitudinal axis X-X with a cylindrical outer wall 17 defining the outer extent of the cavity 6. The inner part of the outer body portion 13 includes two cylindrical flanges 18 and 19 extending outwardly from the cavity volume and arranged cylindrically about the axis X-X.

The inner body portion 12 comprises two sections. The first section 20 is mechanically and electrically connected to the cathode 2 and the second section 21 is mechanically and electrically connected to the grid 3. In the embodiment shown, a ceramic cylinder 22 is located between the sections 20 and 21.

The inner body portion 12 also includes cylindrical flanges 23 and 24 which extend outwardly away from the input cavity 6 and are arranged coaxially about the axis X-X and within the flanges 18 and 19 of the outer body portion 13. The two pairs of flanges 18 and 23, and 19 and 24 are arranged to extend substantially parallel to one another and are substantially co-extensive in the axial direction. They define r.f.

choke impedances and are substantially wholly embedded within the insulating material 14.

Two of the co-extensive flanges 18 and 23 are extensive from the input cavity 6 over approximately the same axial distance and are substantially parallel to one another. The two flanges 18 and 23 have edges 25 and 26 which are curved away from each other such that each end of the flange is shielded from the other body . .

portion by the axially extensive part of that flange. The parts of flanges 18 and 23 which are fixed to the outer and inner body portions defined by plates 15 and 20 are arranged to join in a smooth curve to reduce electrical stresses between the two body portions.

The other pair of co-extensive cylindrical flanges 19 and 24 are similarly curved where they join plates 16 and 21. The inner flange 24 extends in axial direction for approximately half the distance of the outer flange 19. Thus, the inner flange 24 terminates in a region where it is co-extensive with the outer flange 19 whereas the outer flange 19 terminates at a location remote from the inner flange 24. The inner flange 24 has a curved edge 27 which is curved away from the outer flange 19.

In this arrangement of co-extensive parts the end 28 of the outer flange 19 is not curved out of the plane of the flange 19.

A power lead 29 is routed via an aperture in the flange 19 and through the dielectric material 14 to supply the grid 3 with the appropriate bias voltage whilst maintaining the electrical insulation of the exterior body portion 137 the connection being made via the lead 29 to the section 21.

Another IOT is shown in Figure 2 and is similar in many aspects to the Figure 1 arrangement. The input cavity includes parts which are extensive in a substantially transverse direction to the longitudinal axis and which are interleaved to provide the required de isolation between the inner body portion 30 and the outer body portion 31 and providing an r.f. choke The inner body portion 30 comprises two annular plates 32 and 33. One of the plates 32 is connected electrically to the cathode and is interleaved between two annular plates 34 and 35 forming part of the outer body portion 31. The annular plate 33 is connected to the electrode gun grid and is interleaved with annular plates 36 and 37 which between them define an annular channel into which the plate 33 is extensive. The regions between the interleaved transverse parts are occupied by resiliently deformable electrically insulating material 38 which is of silicone rubber.

The outer edges of the annular plates 32 and 33 terminate in the annular channels and in the regions of the transverse plates of the outer body portion 31.

The edges of the plates 32 and 33 are curved out of the plane of the plates so as to present a smooth surface, the ends of the edges touching the surfaces of the plates 32 and 33.

The annular plates 34, 35 and 36, 37 of the outer body portion are curved outwardly away from the interleaved part of the inner body portion but do not touch the surfaces of the plates.

Figure 3 illustrates schematically part of another IOT in accordance with the invention and is a longitudinal section showing some components only of the IOT.

The components are cylindrically symmetrical about the axis X-X and only half is shown.

The input cavity 39 of the IOT includes an outer body portion defined by plates 40 and 41 in combination with an outer cylindrical wall (not shown). A metal cylinder 42 is mounted on one of the plates 40 via a plurality of screws 43, one of which is shown, around its circumference. Two annular plates 44 and 45 are fixed at each end of the cylinder 42 and extend radially inwardly from it. The IOT includes a grid electrode 46 which is mounted on an annular plate 47 which surrounds it and is extensive between the plates 44 and 45 forming part of the outer body portion of the input cavity 39. The plates 44, 45 and 47 together define an r.f. choke impedance Which prevents loss of r.f. energy from the cavity 39. The plates 44 and 45 have inner edges which are curved outwardly away from the plate 47 located between them. The interleaved plate 47 is located in a plane which is spaced a distance a from the lower plate 45 in an axial direction and a larger distance b from the upper plate 45.

However, the curvature of its end is such that the nearest point of the interleaved plate 47 from the upper plate 45 is also a in the axial direction. The region between plates 44, 45 and 47 includes electrically insulating material 48 to improve voltage hold-off between the plates and hence permit relatively large potential differences to be applied between the inner body portion and the outer body portion.

The plate 41 defining the outer body portion is also electrically connected to two annular plates 49 and 50 and a generally cylindrical outer member 51 to which they are mounted. A central plate 52 of the inner body portion is connected to the cathode (not shown) of the IOT electron gun. The spacing of the inner body portion 52 relative to the plates 49 and 50 is arranged in a similar manner to that of plates 44, 45 and 47.

In this arrangernent, the inner body portion of the input cavity 39 is essentially defined by the plates 47 and 52 which themselves are also the parts of the inner body portion which are co-extensive with corresponding parts of the outer body portion.

Figure 4 illustrates an arrangement similar to that shown in Figure 3. However, in this arrangement, the plates 53 and 54 forming part of the inner body portion are spaced equidistantly between adjacent plates 55, 56, 57 and 58 of the outer body portion. The radial inner edges of the outer body portion plates and the outer edge of the inner body plates are defined by beading of substantially circular cross-section to give the required curved edge in accordance with the invention.

Claims

1. A linear electron beam tube comprising: an input cavity which is substantially cylindrical about a longitudinal axis and arranged to receive, in use, a high frequency signal to be amplified; an electron gun arranged to produce an electron beam in a substantially longitudinal direction; and an output cavity from which the amplified high frequency signal is extracted; wherein the input cavity substantially surrounds the electron gun and comprises an inner body portion electrically connected to part of the electron gun and an outer body portion electrically insulated from the inner body portion, the inner body portion being maintained at a relatively high voltage compared to that of the outer body portion, and wherein the inner and outer body portions have respective parts which are coextensive to present a choke impedance to high frequency energy within the input cavity and wherein an edge of one or more of the parts terminating in a region where a part of the other body portion is extensive is curved.

2. A tube as claimed in claim 1 wherein the parts substantially planar and the edge is curved out of the plane.

3. A tube as claimed in claim 2 wherein the edge is curved to an extent such that its end is substantially adjacent a region of the part remote from the edge.

+. A tube as claimed in claim 2 or 3 wherein the edge has a substantially constant radius of curvature.

5. A tube as claimed in any preceding claim wherein the said respective parts are extensive in planes substantially transverse to the longitudinal axis.

6. A tube as claimed in claim 5 wherein one of the body portions includes two parts extensive in a substantially transverse direction and the other of the body portions includes one part extensive in substantially transverse direction and interleaved between the two parts of the other portion.

7. A tube as claimed in claim 6 wherein the said two parts have edges which curve in a direction away from the said one part.

8. A tube as claimed in claim 7 wherein the said one part is located nearer one of the said two parts than the other and has an edge which is curved away from the closer part.

9. A tube as claimed in claim 5, 6, 7 or 8 wherein the parts are annular plates.

10. A tube as claimed in claim 1, 2, 3 or 4 wherein the said parts are axially extensive flanges which are substantially co-extensive in an axial direction.

11. A tube as claimed in claim 10 wherein the flanges are substantially cylindrical.

12. A tube as claimed in any preceding claim wherein the inner and outer body portions include two pairs of co-extensive respective parts.

13. A tube as claimed in any preceding claim and including electrically insulating material located between the said co-extensive parts

14. A tube as claimed in claim 13 wherein the resiliently deformable electrically insulating material is silicone rubber.

15. A tube as claimed in claim 13 or 14 wherein the electrically insulating material is moulded into the required configuration.

16. A tube as claimed in claim 13, 14 or 15 wherein at least one of the said parts is at least partially embedded in the electrically insulating material.

17. A tube as claimed in any of claims 13 to 16 when dependent on claim 12 wherein the insulating material is in the form of a single member which is extensive , . .

between both pairs of parts.

18. A tube as claimed in any of claims wherein the electrically insulating material is generally cylindrical in form.

19. An arrangement as claimed in any of claims 13 to 18 wherein the inner and outer body portions are physically joined together by the electrically insulating material.

20. A tube as claimed in any preceding claim wherein the inner body portion comprises two sections which are electrically separate from one another.

21. A tube as claimed in any preceding claim wherein the inner body portion is electrically connected to a cathode and a grid of the electron gun.

22. A tube as claimed in any preceding claim wherein the outer body portion is at ground potential.

23. A tube as claimed in any preceding claim wherein electrical connection is provided between the outside of the input cavity and the inner body portion through the insulating material.

24. A linear electron beam tube substantially as illustrated and described with reference to Figure 1, 2 , 3 or 4 of the accompanying drawings.