US2956192A

US2956192A - Gettering electron gun

Info

Publication number: US2956192A
Application number: US784690A
Authority: US
Inventors: Heil Oskar; Lowell A Noble; Anthony S Wolfson
Original assignee: Eitel Mccullough Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1959-01-02
Filing date: 1959-01-02
Publication date: 1960-10-11
Anticipated expiration: 1977-10-11

Description

Oct. 11, 1960 o. HElL ETAL GETTERING ELECTRON cum 2 Sheets-Sheet 1 Filed Jan. 2, 1959 IN VEN TORS OSKAR HEIL LOWELL A. NOBLE ANTHONY s. WOLFSON BY Yu F W W44 32% ATTORNEYS Oct. 11, 1960 o. HEIL 'ET AL 2,956,192

GETTERING ELECTRON GUN Filed Jan. 2, 1959 I 2 Sheets-Sheet 2 FIG. 4

F i G 5 tNVENTORS OSKAR HEIL LOWELL A. NOBLE ANTHONY 5. WOLFSON BY 41. F W

WWI 9% ATTORNEYS United States Patent C) GE'ITERING ELECTRON GUN Filed wan. 2,1959, Ser. No. 784,690

4 Claims. 01. 313-82) This invention relates generally to an electron gun and more particularly to an electron gun adapted to perform a gettering action.

In many vacuum tubes, especially larger tubes, the gas pressure in the tube may rise due to the release of gas atoms trapped in the tube structure and due to leakages which may occur. Further, gases in the tube are not completely gettered during the evacuation process. It is desirable in tubes to be able to continuously monitor the gas pressure in the tube.

It is a general object of the present invention to provide an electron gun which serves to maintain or improve the vacuum in an electron tube with which it is associated.

It is another object of the present invention to provide an electron gun which pumps ions formed by the electron beam into a selected portion of the electron tube with which it is associated. The ions are neutralized and gettered in this portion of the tube.

It is a further object of the present invention to provide an electron gun which utilizes the ions formed by the beam and focused by the gun to sputter gettering material which, in turn, serves to getter the neutralized ions.

Briefly, an electron gun according to the present invention comprises a cathode assembly including an emitting surface, a focusing electrode, and an anode, in which a hole is provided in the emitting surface through which the ions formed by the electron beam and attracted by the negative potential of the emitting surface are focused by the gun. A gettering means is disposed behind the emitting surface and may include a target 'on which the ions impinge, and suitable gettering material.

Other objects of the invention will become more clearly apparent from the following description when taken in conjunction with the accompanying drawing.

Referring the the drawing:

Figure 1 is an elevational view, partly in section, of the lower portion of a beam tube including an electron gun in accordance with the invention;

Figure 2 is a sectional elevational view showing an electron gun in accordance with the invention;

Figure 3 is a plan view of the filament assembly;

Figure 4 is an elevational View, in section, of the cathode assembly of an electron gun in accordance with another embodiment of the invention; and

Figure 5 is a fragmentary elevational view, in section, of the lower portion of an electron gun in accordance with another embodiment of the invention.

In Figure 1, the electron gun 11 of a beam tube is illustrated. Generally, a beam tube comprises an elongated cylindrical envelope having an electron gun 11 at one end of the envelope, and a collector electrode (not shown) at the other end. An electron beam is projected by the gun to the collector and passes through an interaction means where the beam interacts with the electromagnetic fields. The end 12 of the interaction means adjacent the gun may serve as the main anode of the gun.

As shown in Figure 2, the electron gun 11 includes a cathode assembly having an oxide coated emitter dish 13 which may be prepared for emission in a Well known manner. The emitter dish 13 includes a central opening v 14 through which ions formed in the electron beam and attracted by the dish may pass, as will be presently described.

The cathode is indirectly heated by a filamentary heater 16. A cylindrical focus electrode 17 surrounds the dish 13 and shapes the electron beam. An apertured modulat-- ing anode 18 is interposed between the main anode 12 andthe cathode 13 and serves to control the current in the electron beam.

The cathode assembly is carried on a metal base 19.. Thus, emitter dish 13 is carried on a cylindrical cathode support 21 which has its lower end secured to the base 19 by the screws 22. 'A member 23 is secured to the base and receives the screws 22. A second cylindrical support 24 is mounted within and concentric with the support 21. The support 24 carries a plurality of heat shields designated generally by the reference numeral 26. The shields 26 each include a central opening 25. The openings in the various shields are aligned with one another and with the opening 14. The cylinder also carries at its upper end a cylindrical member 27 which surrounds the heater 16. Thus, it is seen that the combination of the lower surface of the dish 13, shields 26 and

cylindrical members

21 and 27 form a double walled oven-like enclosure which encloses the filamentary heater 16.

The filamentary heater 16 may be a four-section heater, each section being formed of zig-zagged filamentary material supported at its two ends by posts which extend upwardly from the base 19. Thus, central posts 31 engage the ring 32 to which is secured the inner end of each of the heater sections 16. The outer posts 33 engage the outer end of each of the sections. As shownin Figure 1, the posts 31 are in direct electrical contact with the base 19; a pair of the posts 33 is supported on a ring 36, while another pair of the posts 33 is supported on an axially spaced ring 37. The

rings

36 and 37 are insulated one from the other by the insulating spacers 38 associated with the screws 39. Leads are secured to the

rings

36 and 37. One of such leads is shown at 41. The leads are connected to lead-in posts 42, one of which is shown. It is seen that the sections may be connected in two series of connected pairs in parallel with one another, or

all four sections may be connected in parallel with each other to thereby accommodate different voltage sources for a given amount of heating. The various sections are sector shaped and fit together like the pieces of a pie having a central opening aligned with the opening 14 of;- the cathode dish 13.

The base 19 also serves to carry ceramic cylinders 46;

and 47. The ceramic cylinders are preferably of high,v

refractory material such as alumina and are metallizecL, at both ends by suitable metallizing procedures such as.

by molybdenum manganese powder sintering; The lower end of the cylinder 46 is suitably sealed to the base.1S!-.- by sealing assembly designated generally by the reference numeral 48. The sealing assembly 48 includes a fil'Sl'.;

sealing ring 49 which surrounds the end of ceramic cylinder 46 and has one end brazed to the base 19, and itsw to the opposite side of the flange of ring 51 from the ceramic cylinder 46 to add strength to the seal.

'A lead-in ring 53 is brazed between the ceramic cylin=-. ders 46 and 47 and surrounds the cathode assembly. The

ring 53 provides an electrical terminal for and supports: the cylindrical focusing electrode 17, previously described; 7

One end of the focusing electrode 17 may be mounted on the ring 53 by means of spaced screws, for example.

The upper end of the ceramic cylinder 47 carries the modulating anode 18 in a terminal ring 54 which provides means for making connection to the anode l8. A suitable seal assembly 56 is provided which is substantially similar to the seal assembly 48 heretofore described. Similarly, a ceramic cylinder 63 is sealed to the opposite side of ring 54 by means of a seal assembly 57. The other end of the ceramic cylinder 63 extends upwardly and is sealed to the anode end 12 (Figure 1) of the R.-F. interaction means by a seal assembly 65. V

The modulating anode 18 includes a rounded recessed portion 66 adjacent to the upper end of the focusing electrode 17. The upper end of the focusing electrode 17 is rounded as shown at 67. The opposed surfaces of the anode and focusing electrode present rounded curved surfaces to one another to reduce the likelihood of arcing.

The base 19 carries mounting lugs 71 which may be brazed thereto. Lead-through terminals, previously referred to, are provided for making electrical connection to the post 33 previously described. These connections may be of the type shown in Figure 2, which include a lead-in post 42. Sealing rings 76 and 77 have one end sandwiched between and brazed to the ceramic ring 78 and the ceramic backing rings 79 and 81. The other end of the ring 76 is suitably secured to the base 19 as, for example, by brazing. The other end of the ring 77 is suitably secured to a flange 82 in the post 42 as, for example by brazing.

When the electron beam formed by the gun is projected along the axis of the interaction means, the electrons thereof may strike gas molecules and will tend to form ionized atoms and molecules. The ionized atoms and molecules will have a positive charge and are thus called positive ions. The positive ions tend to be attracted to the center of the beam by the negative electrons. The action of the electrons in the beam is to form a trough or gutter which traps the ions within the beam.

The ions move along the beam toward the emitter dish due to the negative potential of the emitter dish. When they enter the electron gun, the action of the electric fields of the gun tends to focus the ions to a very small diameter spot at the center of the emitter dish. In the present invention, the emitter dish is provided with a central opening 14 through which the sharply focused ion beam thus formed passes.

The beam passes on through the aligned axial openings formed in the heater and heat shields and strikes the target 91 which is supported on a post 92 disposed on the axis of the tube. The post is supported on a terminal 93 of a lead-in assembly of the type previously described. The heat shields 26 may be coated with or made of getter material such as magnesium, barium, zirconium or titanium. Additional gettering discs 96 and 97 may be carried by the cylindrical support 24 and spaced along the axis of the tube. The gettering discs include openings 98 which are aligned with the

openings

14 and 25. The inner surface of the cylindrical support 24 may also be coated with getter material. Thus, the cathode structure including particularly the cylindrical support, gettering discs 96 and 97 and heat shields 26 form an enclosure into which the ions are directed. A large portion of the inner surface of the enclosure carries gettering material to provide an effective gettering action.

It is believed that the ions which are accelerated by the cathode assembly and which strike the target form activated particles such as disassociated gas atoms, metastable gas atoms, metastable gas molecules and active free radicals which are easily absorbed by the gettering surfaces. It may be that certain of the ions which impinge upon the target are gettered or trapped'at or in the target. However, it is believed that a majority of the ions form activated particles which are effectively absorbed by the gettering surfaces of the enclosure.

The target 91 may be used to indicate gas pressure in the tube. The number of positive ions, ionized gas atoms and molecules formed by the electron beam is a direct function of gas pressure in the tube. Consequently, the number of ions striking the target is a function of the gas pressure. By suitable electrical connection to the target, the number of ions can be recorded as an electrical current. Hence, the gas pressure in the tube may be continuously monitored.

The complete electron gun assembly acts as an ion pump, the pumping speed of which varies inversely with the gas pressure within the tube. Thus, if the pressure in the tube increases, more ions are formed by the electron beam which ions are pumped into the cathode structure and gettered.

Referring to Figure 4, another cathode assembly is illustrated. Like reference numerals refer to like parts. In the embodiment illustrated, a getter 106 which consists of titanium wire wound on a tungsten wire mandrel is supported between one of the inner posts 31 and one of the outer posts 33. When the getter 106 is fired, that is, heated to a high temperature by applying a voltage between the base 19 and the post 33 through one of the lead-in connections to induce a current flow through the tungsten wire mandrel, the titanium will tend to be evaporated. The titanium vapor thus produced then condenses on the adjacent surfaces, for example, the heat shields 26 and the inner surface of the member 24 to thereby form gettering surfaces which serve to getter activated particles and ions which are in the cathode enclosure described above. The getter 106 may be so constructed that it may be fired several times, thus a fresh surface of gettering material may be provided as needed or desired.

Referring to Figure 5, another embodiment of the invention is shown. The lower inner portion of the cathode structure is illustrated. An axial post 111 supports a conical member 112 made of gettering material such as titanium. Thus, ions formed by the beam and accelerated into the interior of the anode structure will strike the member 112. As previously described, the ions travel with considerable velocity. Thus, on striking the member 112, they will serve to sputter gettering material from the member onto the inner surfaces of the cathode structure. Such sputtering of material by ion bombardment is well known in the gas tube art, and according to the subject invention is used to provide gettering surfaces which are continuously replenished in accordance with need since the sputtering of material will increase in direct proportion to the gas present.

Thus, it is seen that there is provided an improved electron gun. The electron gun serves to continuously pump any gases present in the tube into a portion of the tube where they may be gettered. This is accomplished by the electrons of the beam striking gas molecules and forming ions which are attracted by andfocused into the cathode structure which forms an enclosure having the inner surfaces coated with gettering material. The ions pumped into the structure form activated particles which are gettered by material disposed on the surfaces of the structure. Means may be provided in conjunction with the electron gun for continuously monitoring the gas pressure within the vacuum envelope. Further, means may be provided in the electron gun structure for continuously renewing the supply of gettering material on the inner surfaces of the cathode structure.

We claim:

1. In a beam type electron tube, an electron gun serving to project a beam of electrons, said gun comprising a base, a cathode assembly including an emitter, a cylindrical member mounted on said base and serving to support said emitter, a filamentary heater disposed to heat said emitter, said heater being mounted on posts supported by said base, an inner cylindrical support serving to support a plurality of heat shields for enclosing said filamentary heater between said emitter and said heat shields whereby an oven-like enclosure is formed which surrounds said filamentary heater to efficiently heat the emitter, said inner cylindrical support and heat shields forming an enclosure, axial openings for-med in said emitter and shields through which pass ions formed by the beam and attracted by the potential on the emitter, and getter material disposed on certain of the inner surfaces of said last named enclosure.

2. In a beam type electron tube, an electron gun serving to project a beam of electrons, said gun comprising a base, a cathode assembly including an emitter, a cylindrical member mounted on said base and serving to support said emitter, a filamentary heater disposed to heat said emitter, said heater mounted on posts supported by said base, an inner cylindrical support serving to support a plurality of heat shields for enclosing said filamentary heater between said emitter and said heat shields whereby an oven-like enclosure is formed which surrounds said filamentary heater to efficiently heat the emitter, said inner cylindrical support and heater shields forming a second enclosure, axial openings formed in said emitter and shields through which pass ions formed by the beam and attracted by the potential on the emitter, getter material disposed on certain of the inner surfaces of said second enclosure, and a target supported on said base disposed to be bombarded by the ions.

3. In a beam type electron tube, an electron gun serving to project a beam of electrons, a cathode assembly including an emitter, a cylindrical member serving to support said emitter, said emitter including a central opening through which ions formed by the electron beam and attracted by the potential on the emitter pass into said cylindrical member, and a cone-shape target made of gettering material disposed within said cylindrical member axially of said central opening with the apex of said cone-shape target facing said central opening whereby the ions strike said target at an angle less than for sputtering gettering material therefrom onto the inner surface of said cylindrical member for renewing the fresh film of gettering material on said inner surface.

4. In a beam type electron tube, an electron gun serving to project a beam of electrons, a cathode assembly including an emitter, a cylindrical member serving to support said emitter, said emitter including a central opening through which pass ions formed by the electron beam and attracted by the emitter potential, a plurality of metallic shields mounted transversely to the axis of said cylindrical member and spaced from each other within the cylindrical member, each of said shields including a central opening coaxial with the opening through said emitter and through which ions may pass, the inner surface of said second cylindrical member and the surfaces of said shields being coated with gettering material.

References Cited in the file of this patent UNITED STATES PATENTS Dorgelo Apr. 3, 1951 Rogers June 2, 1959