US3195006A

US3195006A - Travelling wave tube output coupling

Info

Publication number: US3195006A
Application number: US20983A
Authority: US
Inventors: John W Sullivan; William L Rorden
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1960-04-08
Filing date: 1960-04-08
Publication date: 1965-07-13
Anticipated expiration: 1982-07-13

Description

y 1965 J. w. SULLIVAN ETAL 3,195,006

TRAVELLING WAVE TUBE OUTPUT COUPLING Filed April 8, 1960 WW Q MW mum INVENTORS 71 2 zfz'amzd arden L/oi?) 7}: 314227267:

United States Patent 3,ll95,uti TRAVELLING WAVE TUBE GUTPUT CGUPLIN'G John W. Sullivan, Sunnyvale, and William L. Rorden, Pain Alto, Calif., assi nors to Varian Associates, Palo Alto, Calili, a corporation of California Filed Apr. 3, N60, Ser. No. 20,983 6 Claims. (3. 3l53.5)

The present invention relates in general to electron dis charge devices and more particularly to travelling wave tubes, especially to travelling wave tubes with periodic magnetic focusing wherein the means which provides the magnetic field closely surrounds the travelling wave tube.

In conventional travelling wave tubes it is desirable to provide a magnetic field longitudinally of the slow wave structure of the travelling wave tube to keep the electron beam in alignment with the wave guiding structure whereby the electron beam can be kept close enough to the structure for good interaction with the radio frequency electric fields associated therewith and yet a minimum number of electrons will be lost in striking the structure. In previous travelling Wave tubes this magnetic field has been provided either by means of a solenoid or by means of a number of short permanent magnets which provide periodic focusing along the length of the interaction region of the travelling Wave tube.

While it is desirable to provide the focusing magnetic field along the entire length of the wave guiding structure, means must be provided for couplin a signal to and from the wave guiding structure without interfering with the magnetic field. Furthermore, in most cases, especially when using periodic focusing, it is desirable to have the travelling wave tube fit as closely as possible within the focusing magnet assembly.

Some previous travelling wave tubes have been provided with a coupled cavities, wave guides, or other means connected to the slow Wave structure of the tube and axially arranged with respect to the tube envelope to couple the signal from the slow wave structure out the end of the focusing magnet.

In other tubes the coupling means has been directed radially out through the magnet. In the latter case, this coupling means must be connected to the tube itself after the tube has been inserted in the magnet, and this is diflicult if the travelling wave tube is to fit as closely as possible Within the focusing magnet.

The principal object of the present invention is to provide a rugged, efficient, easily assembled travelling wave tube.

One feature of the present invention is a provision of a metal and ceramic travelling wave tube wherein the outside diameter of the interaction and collector regions of the tube are substantially entirely of uniform diameter, thereby permitting insertion of the travelling wave tube within a focusing magnet closely spaced therefrom.

Another feature of the present invention is a provision of a metal and ceramic travelling wave tube wherein a strip line is connected to the slow wave structure of the tube, passes along on the inside Wall of the tube envelope,

and passes radially through a ceramic segment of the tube envelope and a hollow cylindrical collector is provided adjacent the strip line shielding the ceramic portion of the tube envelope from electron bombardment and provided with a cut-out section adjacent to the strip line.

Another feature of the present invention is the provision of a metal and ceramic travelling wave tube of the last aforementioned feature wherein a movable short is provided between the travelling wave tube collector and ground whereby the output match of the travelling wave tube can be adjusted when the travelling wave tube is inserted in its focusing magnet.

e Other features and advantages of the present invention will become more apparent upon perusal of the following specification taken in connection with the accompanying drawing wherein:

FIG. 1 is a longitudinal cross-section view of a travelin wave tube utilizing features of the present invention,

FIG. 2 is a cross-section view of a portion of the structure shown in FIG. 1, taken along line 22 in the direction of the arrows, and,

FIG. 3 is an enlarged cross-section view of a portion of the structure shown in FIG. 1, taken along line 3-3 in the direction of the arrows.

Referring now to the drawing, the travelling Wave tube 11 includes a central wave guiding portion 12, provided at one end with a cathode assembly 13, and at the other end with a collector assembly 14. The central wave guiding portion 12 includes a tubular metallic envelope 15 as of, for example, stainless steel, which at the cathode end of the tube is connected to a tubular envelope input extension 15 as of, for example, Monel of the same inside and outside diameter as the envelope 15. The envelope input extension 16 is joined as by brazing to an annular anode 17 as of, for example, steel. A hollow cylindrical envelope output extension 18 of wave permeable material such as alumina ceramic is joined on one end to the collector end of the envelope 15 by an annular flange 19 such as Kovar, and on the other end to a tubular collector support 21 such as Monel by an annular flange 22 such as Kovar. The outside diameters of the envelope 15, the envelope extensions 16 and 18, the annular flanges l9 and 22, and the collector support 21 are all substantially the same.

Axially supported within the wave guiding portion 12 by means of three sapphire rods 23 is the tube slow wave structure 24 shown as a ring and bar type of contra-Wound helix as of, for example, molybdenum, and further described in US. Patent No. 2,836,758 to Chodorow.

The slow wave structure 24 is positioned within the tubular envelope 15 by applying pressure directed radially inwardly longitudinally of the envelope 15 at three equally spaced points about the circumference thereof. The sapphire rods 23 with the slow wave structure attached thereto are slidably inserted within the portions of the envelope which are expanded due to the pressure applied thereto. The pressure is then relaxed and the slow wave structure 24 supported on the sapphire rods 23 is tightly held within the envelope 15.

An input coaxial line 25 through the envelope input extension 16 provides means for applying R.F. waves to the slow wave structure 24 and is provided with an annular window 26 held in a metallic cup within an annular flange 27 on the external end thereof.

At the collector end of the tube the slow wave structure 24- is terminated adjacent the annular flange memher 1% on which is supported the output extension 18. The output coupling for the slow wave structure 24 consists of a short length of strip line 28 such as platinum which is positioned along the inside wall of the output extension 18 with one end directly connected to the slow wave structure 24 and the other to an output connector pin 29 such as Kovar which passes radially through the output extension 18 and is vacuum sealed in a recess 32. on the outside surface of the output extension 18 by a brazing washer 31 as of, for example, Kovar. The pin 29 does not extend radially beyond the outside surface of the other elements of the tube in the interaction and collector regions.

To minimize various undesirable effects resulting from mismatches at the coupling connections, the slow wave structure 24 is terminated for a short distance by means of a drift tube 33 as of, for example, stainless steel, and the surface of the sapphire rods 23, adjacent the drift tube 33, is provided wit-h a coating 34 of a dissipative material such as aquadag.

The cathode assembly 113 includes an oxide coated cathode button 35, supported on the open end of a cup 36, as of, for example, nickel within which is contained the cathode heat-er 37. The cup 36 is supported within a tubular focus electrode 38 by means of a tapered annular flange 39 fixed to the inside surface of the electrode 38 and a plurality of wires 41 fixed to the flange 39 and the cup 36. The forward end of the focus electrode 38 is provided with an inwardly directed annular .portion 42 which surrounds the periphery of the cathode button 35. An insulator ring 43 such as ceramic positioned on the external surface of the focus electrode 38 supports a cup shaped grid support 44, the bottom of which comprises a mesh control grid 45 such as mesh tungsten spaced from the front emission surface of the cathode button 35. An apertured cup-shaped auxiliary focus electrode 46 is supported on the grid support 44 providing a foe-using member between the control grid 45 and the anode 17 better to focus the electron beam.

The focus electrode 42 is supported at its end opposite'from the cathode button 35 on an annular flange member 47, which is in turn connected to the fiat surface of a disc shaped insulator 48 such as ceramic constituting the end of the tube. The insulator 48 is supported in a vacuum tight manner by means of several metallic flanges 49 within one end of an elongated hollow cylindrical insulator 50 such as ceramic which surrounds the cathode button 35 and its support members. This cylindrical insulator 50 is supported on its other end on an annular flange 51 such as Kovar, which is connected to a ring 52 which is, in turn, held, as by brazing, within an annular flange projecting from the anode 17 whereby the entire cathode assembly 13 is supported from the anode 17 and axially aligned with the rest of the travelling wave tube 11. The heater and electrode leads 53 for the cathode assembly 13 are vacuum sealed within apertures in the disc insulator 48.

The collector assembly 14 includes a hollow cylindrical collector 54 such as copper, connected as by brazing on to the collector support 21. The collector 54 is provided with a hollow cylindrical extension 55 which projects axially within the output extension 13, thereby preventing bombardment of the output extension 18 by electrons from the electron beam, and permitting the focusing structure to end as short a distance as possible after the end of the slow wave structure 24. The collector is provided with a cut-out slot 56 in the region where the strip line 28 passes along the inner surface of the output extension 18. i

A pinch-off tube 57 for evacuating the tube closes off the end of the collector 54 furthermost from the cathode. An internally threaded hollow cylindrical cooling member 58 provided with radially outwardly extending cooling fins 59 is adapted to be screwed on threads on the end of the collector 54- after the tube has been inserted into its magnet whereby the cooling member 58,

and thus the collector 54, can be cooled by an air stream directed over the cooling fins 59. A cross-bar 61 as of, for example, copper, is positioned across the bore of the collector 63 to collect and deflect particles travelling -down the tube thereby preventing bombardment and possible melting of the pinch-ofi? tube 57.

The magnet assembly 62 for the travelling Wave tube 11 includes an inner tubular member 63 such as stainless steel into which the central wave guiding portion 12 of the travelling wave tube is inserted. A plurality of annular pole pieces 64 such as steel, each provided the axially directed portions adjacent the inner surface thereof, slideably surround the inner tubular member 63, and are spaced along the length thereof by means of annular permanent magnets 65 as of, for example, Indox 1. Annular spacers 66 as of, for example, brass, with radially outwardly extending projections 67 bear against the external periphery of the pole pieces 64 and the magnets 65 and support them axially within an outer tubular member '68, such as steel, which extends from the anode member 17 to the end of the output extension 18 when the tube is positioned within the magnet assembly. The cathode and collector ends of the outer tubular member 68 are screwed to rings 76a and 7tib respectively such as steel which hold the spacers 66, the magnets 65 and the pole pieces 64 therebetween. The ring 7% and the outer tubular member 68 adjacent the oathode assembly 13 are provided with a recessed notched portion 69 into which the annular flange 27 on the end of the input coaxial line 25 is inserted when the tube is positioned within the magnet assembly. 1

The magnet assembly 62 is also provided with a hollow cylindrical outer conductor 71 lined with an insulating material 72 such as Teflon for use with an output coaxial connector. The outer conductor 71 will be radially ali-gned with the output connector pin 29 when the input coaxialtline 25 is positioned in the recessed notched portion 69 of the magnet assembly 62.

The center conductor '73 of an external coaxial line, not shown, then connects to the output connector pin 29. When the travelling wave tube 11 is inserted Within the magnet assembly 62, the outer conductor 71 for the output coaxial line is connected to the metallic envelope of the tube by means of a flexible contact spring 74 positioned between the travelling wave tube 11 and the magnet assembly 62.

An internally threaded adapter ring 75, as of, for example, brass, projects outwardly from the end pole piece 64 of the magnet assembly 62 adjacent the collector assembly 13. When the travelling wave tube 11 has been inserted within the magnet assembly 62, an externally threaded hollow cylindrical coupling nut 76 such as brass and a flexible contact spring '77 such as beryllium-copper contained in an annular recess on the interior surface thereof is threadably engaged in the adapter ring 75, the contact spring 77 providing a movable short between the collector and ground. The tube support structure and thus the tube envelope are grounded as is customary in the art. By proper adjustment of this movable short when the tube is first operated, the length from the end of the collector extension 55 to ground can be made a quarter wavelength long at the operating frequency, thereby making the end of the collector extension 55 a high impedance point so that the output match will not be degraded by capacity between the strip line 28 and the collector extension 55.

Once the travelling wave tube 11 has been inserted in the magnet assembly 62 end covers '78 and '79 are placed around the cathode assembly 13 and the collector assembly 14 respectively.

, The end cover 7 8 includes a hollow cylindrical member 81 the same size as the outer tubular member 68 of the magnet assembly 62, one end of the member 81 being fastened by screws to the ring a of the magnet assembly 62, and the other end closed by a cup shaped insulator 82 such as Teflon which slideably fits within the member 81 to insulate it from the cathode assembly 13. A plug connector 83 fits in the end of the insulator 82 for the electric leads for the cathode assembly 13.

The end cover '79 includes a hollow cylindrical member 84 one end of which is connected by screws to the ring 70b of the magnet assembly 62. The open end of the member 34 is provided with a ring 85, the aperture through which is provided With a screen 86 and is closed halfway across by a baffle plate 87.

To cool the tube forced air is directed through the open half of the screen 86. A portion of this air is converted into a flow across the cooling fins 59 and is exhausted through holes in the end cover 79. The rest of the air travels down the length of the tube through the open space provided by the projections 67 of the spacers 66 and a) cools the cathode assembly 13 before it is exhausted through holes in the end cover 7 8.

Since many changes could be made in the above construction and many apparently widely diiferent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a travelling wave tube a tubular metallic envelope disposed about and thereby defining an elongated longitudinal axis along which an electron beam travels, said envelope being provided with means for directing electromagnetic wave energy therethrough, a tubular extension on the end of said envelope, said extension being made of material which is permeable to said electromagnetic wave energy, said tubular extension being disposed around at least a portion of the axial extent of said elongated longitudinal axis and forming a part of the vacuum envelope of said travelling wave tube in conjunction with said tubular metallic envelope and coupling means for coupling electromagnetic wave energy from the means for directing the electromagnetic wave energy through the envelope through said electromagnetic wave permeable tubular extension, the outside diameter of said tubular extension and said coupling means being no greater than the outside diameter of said envelope.

2. The travelling wave tube of claim 1 wherein said means for directing said electromagnetic wave energy through said envelope includes a slow wave structure and said coupling means includes a metallic conducting member projecting through said extension and a metallic connector connecting said slow wave structure to said conducting member.

3. The travelling Wave tube of claim 1 including tubular, electron beam collector means connected to said tubular extension and provided with an axially projecting portion positioned within said tubular extension thereby shielding said tubular beam extension from bombardment by electrons from the electron beam.

4. The travelling wave tube of claim 3 wherein said metallic envelope is grounded and including means forming a movable short between said collector means and ground whereby the axially projecting portion of said collector adjacent said coupling means can be made a high impedance point.

5. A travelling wave tube comprising in combination, a surrounding metallic envelope, means to project an electron beam along a given path within said envelope, transmission means mounted within said envelope and extending along said path for transmitting electromagnetic wave energy for interaction with said beam, said envelope including an extension at the end of said transmission means opposite from the means for projecting the electron beam, said extension surrounding said electron beam path, said extension being made of a material which is permeable to said electromagnetic wave energy, collector means for collecting the electron beam at the end of the transmission means adjacent said extension which is permeable to said electromagnetic wave energy, said collector means provided with an axially projecting portion positioned within said extension thereby shielding said extension from bornbardment by electrons from the beam, and coupling means for coupling said electromagnetic Wave energy from said transmission means through said wave permeable extension of said envelope.

6. The travelling wave tube of claim 5 wherein said metallic envelope is grounded and said coupling means includes a metallic conducting member projecting through said extension and a metallic connector connecting said transmission means to said conducting member and including means forming a movable short between said collector means and ground whereby the axially projecting portion of said collector adjacent said coupling means can be made a high impedance point.

References Cited by the Examiner UNITED STATES PATENTS 2,845,690 8/58 Harrison 29-25 2,869,217 1/59 Saunders 2925 2,899,594 8/59 Johnson 315-3.5 2,922,067 1/60 Van Dien 3153.5 2,945,154 7/60 Bittman et al. 315-3.5 2,984,762 5/61 Haas 315-393 3,050,656 8/62 Mayer 3153.5'

GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, ROBERT SEGAL, Examiners.