Cavity arrangements

Abstract: In an output resonant cavity 7 used with a linear electron beam tube such as an IOT, magnetic material 17 and 23 is mounted on walls 12 and 13 of the cavity and defines annular channels within which coils 21, 22 and 27 are located to provide focusing of an electron beam travelling along the axis X-X of a tube located in the apertures 15 and 16. An inner rim 28 locates the electron beam tube in relation to the cavity.

Description: This invention relates to cavity arrangements and more particularly to arrangements for use with linear electron beam tubes. A linear electron beam tube is a device, such as a klystron or an inductive output tube (IOT), in which an electron beam is generated to travel along a linear path and interact with high frequency energy at resonant cavities. In an IOT, for example, an annular input resonant cavity surrounds an electron gun and is arranged such that high frequency energy coupled into the cavity generates a modulating signal between the cathode and grid of the electron gun, hence density modulating the electron beam. After transmission along a drift tube, an amplified high frequency signal is coupled from an output resonant cavity located further along the electron beam path. GB-A-946 020 describes a travelling wave tube in which pole pieces and permanent magnets are mounted on the outer surface of the walls of a resonant cavity. FR-E- 73213 also concerns a travelling wave tube in which waveguides are used to apply and extract a signal from the tube. According to the present invention there is provided a cavity arrangement, for use with a linear electron beam tube having a vacuum envelope, comprising an external resonant cavity located outside the vacuum envelope, the cavity having two walls with apertures therein through which in use the tube is extensive, and characterised by magnetic focusing means including electromagnetic coil means or permanent magnetic material mounted on the outer surface of at least one of the two walls. Magnetic focusing means is used with linear beam tubes to counteract the repulsion of electrons in the beam due to space charge effects and hence prevent impact of electrons on the parts of the tube surrounding the electron beam path. By mounting electromagnetic coil means or permanent magnetic material on the outer surface of the resonant cavity wall or walls, a more compact overall arrangement is provided compared to a conventional arrangement in which the magnetic focusing means is provided by completely separate items. The integration of the electromagnetic coil means or permanent magnetic material with the cavity also facilitates assembly of the complete device as these components may be accurately aligned relative to one another and securely fixed together prior to combining them with the electron beam tube. Also, the number of components in the complete device is reduced, improving ease of handling and installation. The electromagnetic coil means or permanent magnetic material may be mounted on the surface of the wall by a suitable glue or other fixing mechanism. The electromagnetic coil means or permanent magnetic material is located outside the vacuum envelope of the electron beam tube and is separately mounted from the tube, giving good accessibility for servicing and the like. The electromagnetic coil means or permanent magnetic material may be carried on one wall only but in one embodiment of the invention electromagnetic coil means or permanent magnetic material is carried on the outer surfaces of both cavity walls. Preferably, the magnetic focusing means includes electromagnetic coil means. The coil means may be wound on a magnetic material or may the formed separately and positioned adjacent a magnetic material. In other alternative embodiments of the invention, the magnetic focusing means includes permanent magnetic material. Where coil means is included, preferably, the magnetic material comprises a member having a first portion parallel to the wall surface and attached thereto and a second portion substantially normal to the surface. The magnetic member may also include a third portion substantially parallel to the surface and spaced therefrom to define an annular channel between it and the surface, and preferably coil means is located in the channel. In one embodiment of the invention, the magnetic member comprises a cylindrical wall having at an inwardly extensive flange which is secured to the cavity wall and at its other end an outwardly extensive flange. Advantageously, the electromagnetic coil means or permanent magnetic material includes an inner projection extensive inside the aperture diameter which in use may be engaged with the electron beam tube to accurately locate it relative to the cavity and the focusing means. According to a feature of the invention, a tube assembly comprises a cavity arrangement in accordance with the invention and a linear electron beam tube, which advantageously is an inductive output tube. One way in which the invention may be performed is now described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic representation of an IOT assembly including a cavity arrangement in accordance with the invention; and Figure 2 shows schematically and in greater detail part of the assembly illustrated in Figure 1. With reference to Figure 1, an IOT includes an electron gun 1 comprising a cathode 2 and grid 3 about which is located an annular resonant cavity 4 having a coupling loop 5 via which a high frequency signal may be coupled into the cavity 4. The electron beam produced by the gun 1 is arranged to travel along the longitudinal axis X-X of the tube through a drift region to a drift tube gap 6 which is surrounded by a output resonant cavity 7. An amplified high frequency output signal is coupled from cavity 7 into a secondary output cavity 8 via coupling means 9 and thence via coupling means 10 externally from the arrangement. A collector 11 is arranged to receive electrons of the beam after they have passed through the interaction region 6. With reference to Figure 2, the output cavity 7 comprises two walls 12 and 13 substantially normal to the axis X-X and a wall 14 which joins them. The transverse walls 12 and 13 have apertures 15 and 16 respectively therethrough through which in use the tube is extensive. A magnetic material 17 is located on the outer surface of cavity wall 12 and includes a cylindrical wall portion 18 extending parallel to the axis X-X and having an inwardly extensive portion 19 substantially parallel to the outer surface of the wall 12 and mounted thereon. It also includes an outwardly extensive flange 20 spaced from the wall 12 and substantially parallel thereto. Coils 21 and 22 are located in the annular channel defined by the flange 20, cylinder 18 and wall 12. A second member 23 of a magnetic material is fixed to the other transverse wall 13 and also includes cylindrical wall portion 24 and inner and outer flanges 25 and 26. A coil 27 is located in the channel defined by the magnetic material 23 and the wall 13. The member 24 includes an inwardly projecting rim 28 extending inside the outer diameter of the aperture 16 and defining a ledge which locates the electron beam tube in the complete assembly relative to the output cavity 7 and the focusing means carried by it, the tube having a reduced diameter which engages with the rim 28. The cavities 7 and 8 are external cavities located outside the vacuum envelope. The electromagnetic coil means or permanent magnetic material is also positioned outside the vacuum envelope.

Claims: A cavity arrangement, for use with a linear electron beam tube having a vacuum envelope, comprising: an external resonant cavity (7) located outside the vacuum envelope, the cavity (7) having two walls (12, 13) with apertures (15, 16) therein through which in use the tube is extensive; and characterised by magnetic focusing means including electromagnetic coil means (21,22,27) or permanent magnetic material mounted on the outer surface of at least one of the two walls (12, 13). An arrangement as claimed in claim 1 wherein the magnetic focusing means includes electromagnetic coil means (21, 22, 27). An arrangement as claimed in claim 2 wherein the coil means (21, 22, 27) is wound on a magnetic material. An arrangement as claimed in claim 1, 2 or 3 wherein the magnetic material comprises a member having a first portion (19) substantially parallel to the outer surface (12) and attached thereto and second portion (18) substantially normal to the outer surface (12). An arrangement as claimed in claim 4 wherein the member includes a third portion (20) substantially parallel to the outer surface (12) and spaced therefrom to define an annular channel between it and the outer surface (12). An arrangement as claimed in claim 5 wherein coil means (21, 22) is located in the annular channel. An arrangement as claimed in any preceding claim wherein the magnetic material is arranged around an aperture (16) in one of the walls (13) and includes a projection (28) projecting inside the diameter of the aperture (16). An arrangement as claimed in claim 7 wherein the projection (28) in use is engagable with the tube to locate it in the aperture relative to the cavity and focusing means An arrangement as claimed in any preceding claim wherein the resonant cavity (7) is an output cavity and includes coupling means (9) for coupling energy therefrom. A tube assembly comprising a cavity arrangement as claimed in any preceding claim and a linear electron beam tube. An assembly as claimed in claim 10 wherein the tube is an inductive output tube.

Inventors: BARDELL STEVEN, BARDELL, STEVEN
Applicants: MARCONI APPLIED TECHN LTD, MARCONI APPLIED TECHNOLOGIES LIMITED
Application year: 1995
Main classification: H01J23
IPC codes: H01J25, H01J23