GB2302206A

GB2302206A - Coupled cavity travelling wave tubes

Info

Publication number: GB2302206A
Application number: GB8315692A
Authority: GB
Inventors: David Harris Cottey
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1983-06-08
Filing date: 1983-06-08
Publication date: 1997-01-08
Anticipated expiration: 2003-06-08
Also published as: GB8315692D0; IT8467583A0; DE3421037C1; GB2302206B

Abstract

The invention provides a coupled cavity travelling wave tube in particular for operating at millimetre wavelengths which is formed of a plurality of longitudinally extending individual laminae 7 each having a thickness of between 25 and 50 microns and having a profile in plan which is appropriate to its position within the finished tube.

Description

Improvements in or relating to Coupled Cavity Travelling Wave Tubes This invention relates to coupled cavity travelling wave tubes and in particular, though not exclusively, to millimetre coupled cavity travelling wave tubes, that is to say travelling wave tubes for operation at a frequency in the region of 35GHz and up to and beyond 95GHz.

A typical coupled cavity travelling wave tube as at present known is illustrated in Figures 1 and 2 of the accompanying drawings of which, Figure 1 is a longitudinal section through part of the slow wave structure of the tube, and Figure 2 shows the slow wave structure in crosssection along the line A...A of Figure 1.

Referring to Figures 1 and 2, the slow wave structure consists of a series of cylindrical cavities 1. In each of the walls 2 separating one of the cavities 1 from another is an arcuate coupling slot 3, 4 of which the slots 3 in alternate ones of the walls 2 are staggered relative to the slots 4 in the remaining ones of the walls 2. Coupling slots 3, 4 provide for radio frequency (R.F.) coupling between the cavities 1.

In the centre of each of the walls 2 is provided an axially aligned drift tube or ferrule 5 through which, in operation, the electron beam passes down the length of the slow wave structure from an electron gun (not shown) to a collector (not shown).

The dimensions and positions of the drift tubes 5 influence both bandwidth and efficiency.

The method of construction normally employed to construct the slow wave structure illustrated in Figures 1 and 2 is as follows.

The slow wave structure is made up of sections each of which comprises a wall 2, a coupling slot 3 or 4, a drift tube 5 and a short length of the cylindrical wall 6 of the slow wave structure. Each section is blanked out and then machined to achieve the final dimensions.

The complete structure is then built up by stacking one section upon the next together with brazing wire or foil suitably placed in between.

The whole assembly is then jigged for alignment and furnace brazed.

Whilst such methods of construction are very satisfactory for coupled cavity travelling wave tubes for operation below millimetre frequencies, it is believed that if applied to millimetre coupled cavity travelling wave tubes difficulties would be experienced.

It will be appreciated that in a millimetre coupled cavity travelling wave tube, the overall dimensions of each cavity in the slow wave structure are very small and in each tube typically three slow wave structures may be required each containing up to thirty cavities.

It is estimated that for consistent performance, particularly for a tube operating in the region of 95GHz, dimensional and assembly tolerances of the order of 0.0001" (one ten thousandth of an inch or in other words,.00254 mm) are required.

Tolerances of this order cannot readily be achieved by machining with conventional machines under normal workshop conditions. To carry out the machining of the individual sections as hereinbefore described in the quantities which would be required for normal production, would require high precision lathes and control equipment, of the type used for the diamond turning of optical components, operated in a closely controlled environment.

Associated with the problems of manufacture outlined above are difficulties relating to inspection, measurement and handling.

One object of the present invention is to provide an improved method of manufacturing a coupled cavity travelling wave tube, and in particular the slow wave structure thereof, in which one or more of the above difficulties is reduced or avoided.

According to this invention a method of constructing a coupled cavity travelling wave tube is provided wherein at least the slow wave structure thereof is formed of a plurality of longitudinally extending laminae each lamina having a profile corresponding to that required in a longitudinal section through the finished tube appropriate to the position of that lamina within the finished tube.

Each individual lamina is preferably formed by a photo-etching or electro-forming technique and preferably the individual laminae are jigged for alignment during assembly and bonded one to another by diffusion bonding at high temperature.

Preferably each lamina is of copper and typically each lamina is from 25 to 50 microns thick.

Particularly where each individual lamina has a thickness as mentioned above the cross section of the cavities and the beam holes in the finished tube may have a good approximation to the conventional circular cross section.

Preferably drift tubes or ferrules projecting into each cavity if required are formed as part of the laminating process. As required also waveguide transitions may also be formed in the laminating process.

In order to avoid having to rely upon the bonded stack of laminae being vacuum tight the laminated structure may be enclosed by a separately formed outer sleeve having a solid wall whose function is to maintain a vacuum within the finished tube against any leakage which may be permitted due to the laminated construction.

The invention is illustrated in and further described with reference to Figures 3 and 4 of the accompanying drawings of which Figure 3 is a view in the direction of the arrow x of Figure 4.

Referring to Figures 3 and 4 the slow wave structure of the travelling wave tube is formed by stacking a plurality of individual longitudinally extending laminae 7. The laminae 7 are of course not represented to scale. Each individual lamina is of copper, 50 microns thick, and formed individually by a photo-etching technique from a photographic master being a profile corresponding to that required of the individual lamina having regard to its eventual position within the stack. Each individual lamina 7 is provided to extend longitudinally parallel to the tube axis over the entire length of the slow wave structure.

Thus one lamina will encompass parts of all of the cavity sections. By suitably varying the profiles from lamina to lamina not only the cavities 8 (in this case of rectangular cross section) but also the required coupling holes 9 and drift tubes 10 (again in this example of rectangular cross section) may be formed as part of the laminating process.

Once the individual lamina are stacked and jigged for alignment, bonding one to another is achieved by means of diffusion bonding at high temperature.

In order to ensure that the finished tube is vacuum tight the laminated tube is enclosed within an outer sleeve (not shown in Figures 3 and 4) formed with a solid wall.

Whilst in the example described the cavities 8 and drift tubes 10 (and of course the coupling holes passing through the cavity end walls) are of rectangular construction, by suitably adjusting the profile from one lamina to the next it is possible to achieve a good approximation to a structure which is circular in cross section. To this end it is advantageous to provide each individual lamina (or at least those which contribute to the shaping of the cavities 8 or the drift tubes 10) to be as thin as possible. Also, although not illustrated, it is possible to form desired waveguide transitions by the laminating process.

Claims

1. A method of constructing a coupled cavity travelling wave tube wherein at least the slow wave structure thereof is formed of a plurality of longitudinally extending laminae each lamina having a profile corresponding to that required in a longitudinal section through the finished tube appropriate to the position of that lamina within the finished tube.

2. A method as claimed in claim 1 and wherein each individual lamina is formed by a photo-etching technique.

3. A method as claimed in claim 1 and wherein each individual lamina is formed by an electro-forming technique.

4. A method as claimed in any of the above claims and wherein the individual lamina are jigged for alignment during assembly and bonded one to another by diffusion bonding at high temperature.

5. A method as claimed in any of the above claims and wherein each lamina is of copper.

6. A method as claimed in any of the above claims and wherein each lamina is from 25 to 50 microns thick.

7. A method as claimed in any of the above claims and wherein drift tubes or ferrules projecting into each cavity if required are formed as part of the laminating process.

8. A method as claimed in any of the above claims and wherein waveguide transitions are also formed in the laminating process.

9. A method as claimed in any of the above claims and wherein the laminated structure is enclosed by a separately formed outer sleeve having a solid wall whose function is to maintain a vacuum within the finished tube against any leakage which may be permitted due to the laminated construction.

10. A method of constructing a coupled cavity travelling wave tube substantially as herein described with reference to Figures 3 and 4 of the accompanying drawing.

11. A coupled cavity travelling wave tube constructed by a method as claimed in any of the above claims.

12. A tube as claimed in claim 11 and wherein the cavities thereof and any drift tubes or ferrules projecting into each cavity are circular in cross section.

13. A tube as claimed in claim 12 or 13 and wherein said tube is a millimetre coupled cavity travelling wave tube.