US3070721A

US3070721A - Electron tube

Info

Publication number: US3070721A
Application number: US847260A
Authority: US
Inventors: Wilbur V Ferry; Richard M Helley; Ormsby P Taylor
Original assignee: Eitel Mccullough Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1959-10-19
Filing date: 1959-10-19
Publication date: 1962-12-25
Anticipated expiration: 1979-12-25

Description

Dec. 25, 1962 w. v. FERRY ET AL ELECTRON TUBE Filed Oct. 19, 1959 s R w S w n R M an WE R IFMP. my WAS B M mmm Y B United States Patent 3,076,721 ELEtZTRUN TUBE Wilbur V. Ferry, San Mateo, Richard M. Helley, San Jose, and Ormshy P. Tayior, San Qarios, Califi, assignors to Eitel-Mctlullough, Inc, San Carlos, Calii, a

corporation of California Filed Get. 19, 1959, Ser. No. 847,26!)

9 Claims. (Cl. 3l3l07) This invention relates to electron tubes and more particularly to electron tubes which are subjected to a high inverse voltage between the anode and cathode.

In general, the flow of electrons in an electron tube is from the cathode to the anode because the cathode is heated to errit electrons and the anode, being at a Positive potential with respect to the cathode, attracts the electrons. If the anode is at the same potential as, or at a negative potential with respect to, the cathode, there should be no electron-flow within the tube. But in highpower tubes where there is a large flow of high-energy etectrons from the cathode to the anode, the anode can be heated to electron-emission temperature by the kinetic energy of the bombarding electrons forming free electrons which can flow in the reverse direction when the voltage is reversed. A cooler is usually added to the anode of high-power tubes so that the anode temperature and the ambient tube-temperature are maintained at a relatively low value.

Cathodes in most electron tubes are indirectly heated and generally have an electron-'emissive coating of bari urn or strontium oxide so that the cathodes will emit more electrons at lower temperatures. This emissive coating is relatively volatile at high temperatures and it will eventually evaporate from the cathode. The emissive coating will then condense on the anode because it is the coolest object in the tube and it is in the line of sight from the cathode. This coating on the anode enhances the ability of the anode to emit electrons, especially if highi inverse voltage is applied between the cathode and ano e.

Since electron tubes are permanently sealed to form the vacuum envelope, this emitting film that has coated the anode cannot be removed from the anode before the anode is made to emit electrons by the coating. The anode should be made of a material that will destroy the electron-emitting properties of the coating after it condenses on the anode. Barium and strontium oxide, when they corre in contact with gold, lose their emitting properties. Since gold is structurally weak, has a high density, and is very expensive, a tube with a gold anode would be very heavy and very costly. A more practical approach would be to make the anode of a less expensive and stronger metal and plate it with gold. The anode should also be made of a metal which is a good heat conductor and a good electrical conductor. Most metals which are good heat and electrical conductors will diffuse into any gold that is plated thereon.

It is an object of the present invention to provide a high voltage tube that can withstand a high inverse voltage.

It is another object of the present invention to provide an anode which is non-emissive.

It is another object of the present invention to provide an electron tube with an anode having a plating of gold.

It is yet another object of the present invention to provide an improved high-power rectifier tube or a clipperdiode.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of the invention. It is to be understood that the invention-is not limited to the disclosed species, as variant embodiments thereof are contemplated and may be adapted within the scope of the claims.

Referring to the drawings:

FIGURE 1 is an electron tube in cross section.

FIGURE 2 is an enlarged sectional view of a portion of the tubes anode enclosed by circle 2 in FIGURE 1.

Referring to the drawings in more detail:

FIGURE 1 shows a high-power rectifier having an external anode 11 with a cooler 12. The anode 11 is cupshaped and an indirectly heated cathode 14 is disposed within the cup-shaped anode 11. Cathode 14 has a cupshaped exterior emitting area and is heated by a heater (not shown) disposed internally within the cathode. The The cathode cup 14 is mounted coaxially on a heat barrier 16. Heat barrier 16 is a short, relatively thin Inetallic tubular member. Heat barrier 16 is mounted coaxially on one end of a metallic tubular cathode-support 18, which in turn is supported by an axially disposed rod 20 with the aid of ametallic dish 21. The filament is supported within the cathode 14 by having one end connected to a lead 22 and the other end connected to the cathode support 18 (not shown). The anode 11, the cathode 14, and the heating filament are supported on a glass insulating header 24. The anode 11 is mounted on header 24 through a tubular glass insulator 26 which has metallic sealing flanges 28 and 30 fused at each end. The glass header 24 has a metallic sealing flange 32 disposed around its periphery. Flanges 30 and 32 are arcwelded together and flange 28 is brazed to the tubular sides of the anode cup 11. A plurality of electrical leadthroughs 34 are fused through the glass header 24. The rod 20 which supports the cathode has one end fixed in the header 24 and one of the lead-throughs 34 is connected electrically to the rod 20. A metallic cup-shaped structure 36 has its open end brazed to the flange 32 on the header 2.4 with the rod 20 protruding through an axial aperture 38 in the bottom of the cup. The rod 20 is brazed to the cup-shaped structure 36 at the aperture 38, rigidly supporting the rod 20 within the tube. The lead 22 for the filament passes through an aperture 40 in the dish 21 and also through an aperture 42 in the structure 36 and is connected to one of the lead-throughs 34. Insulated bushings 44 are applied between aperture 40 and lead 22 and between aperture 42 and lead 22. A suitable getter 46 is connected across two lead-throughs 34.

In operation the high-power rectifier has an alternating high potential applied between the anode and cathode. When the potential is in the proper phase, electrons will flow from the cathode to the anode. When the potential reverses, the electron flow should terminate, since the anode should be relatively cool and should not be at electron-emitting temperature. As stated before, in most tubes which have an indirectly heated cathode as shown, the cathode emitting-surface has an oxide coating and is generally either barium oxide or strontium oxide. These oxides are known to evaporate at a relatively low rate from the hot cathode and condense on near-by cooler surfaces. The anode, being the coolest surface in the tube, will naturally be coated with these oxides which condense thereon. Since oxide coatings are used on a cathode to irrprove emission, they will also improve emission of the anode, and one finds that the anode with the 0X- ide coating although it may be relatively cool, is at electron-emission temperature. But when either barium or strontium oxide condenses on gold, their emitting properties are destroyed. A simple method of solving the prob lem would be to gold-plate the anode so that the anode would not be altected by the oxide coating. But tubes which generate large amounts of power have an external anode for dissipating large amounts of heat. This heat must be conducted from the interior of the tube to the exterior. The anode must have good thermal conducting properties and must have structural strength since it is a part of the vacuum wall. Therefore, external anodes are made, preferably, of copper. But gold and copper, when in physical contact although solids, readily diffuse into each other and form an alloy. Gold and copper alloys will not destroy the erritting properties of the X- ides. Unfortunately, solid gold will diffuse readily into any solid metal that is a good heat conductor but will not appreciably diffuse with most solid metals which have poorer heat conducting properties than copper. Both solid iron and nickel have poorer heat conducting properties than solid copper, and solid gold diffuses into these solid metals at a much lower rate than into solid copper. There are other metals into which gold diffuses at a much lower rate than into solid copper. These other metals are more expensive and more difiicult to work with than are iron and nickel. In this embodiment of the invention only an anode having an intermediate layer of either iron or nickel will be described since these were the only metals actually used by the applicants in their experiments. Also one must bear in mind that the diffusion rate between copper and any metal which is coated thereon to form the intermediate layer must, of course, be low so that the copper does not diffuse through this layer and into the gold.

Referring to FIGURE 2, there is shown a section of the anode 11 with a portion of the cooler 12. The inside surface of the anode 11, the surface which faces the cathode and on which the barium or strontium oxides may condense, has a layer 48 of iron and a layer 50 of gold is over the iron. Also, instead of iron, nickel can be used to make the layer 48. A suitable thickness for layer 48 of either iron or nickel is about .0005 of an inch and suitable thickness for the layer 50 of gold is .00035 of an inch.

The invention produces an improved external anode tube that will dissipate large amounts of power and will withstand high peak voltages applied between its anode and cathode. The anode is made of good thermal conducting material to enable the tube to operate with a low anode temperature. The tube shown in the drawing has an anode which will dissipate 1,000 watts and will withstand a peak inverse voltage of 25 kilovolts. A tube according to this invention has a useful life of over 1,000 hours.

We claim:

1. An electron tube comprising an envelope containing a cathode and another electrode, a layer of metal disposed on said other electrode, a layer of gold disposed on said layer of metal, said layer of metal having a lower diffusion rate with gold than the diffusion rate of the base material of said other electrode with gold.

2. An electron tube comprising an envelope containing an anode and a cathode, a layer of metal chosen from the group consisting of iron and nickel disposed on said anode, and a layer of gold disposed on said layer of metal.

3. An electron tube of claim 2 in which said anode is made of copper.

4. An electron tube comprising an envelope containing a cathode and another electrode, said other electrode having a surface facing said cathode, a layer of metal chosen from the group consisting of iron and nickel disposed on said surface of said other electrode, and a layer of gold disposed on said layer of metal.

5. An electron tube of claim 4 in which said other electrode is made of copper.

6. An electron tube comprising an envelope containing an anode and a cathode, said anode having a surface facing said cathode, a layer of iron disposed on said surface of said anode, and a layer of gold disposed On said layer of iron.

7. An electron tube of claim 6 in which said anode is made of copper.

8. An electron tube comprising an envelope contain ing an anode and a cathode, said anode having a surface facing said cathode, a layer of nickel disposed on said surface of said anode, and a layer of gold disposed on said layer of nickel.

9. An electron tube of claim 8 in which said anode is made of copper.

References Cited in the file of this patent UNITED STATES PATENTS 2,198,327 Badringa et al Apr. 23, 1940 2,361,203 Holdaway Oct. 24, 1944 2,503,949 Jensen et al Apr. 11, 1950 2,858,466 Sternglass et a1. Oct. 28, 1958 FOREIGN PATENTS 500,589 Great Britain Feb. 13, 1939