US3631280A

US3631280A - Ionic vacuum pump incorporating an ion trap

Info

Publication number: US3631280A
Application number: US864064A
Authority: US
Inventors: Nathan D Levin; Andreas Niewold; Stephen D Sparks
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1969-10-06
Filing date: 1969-10-06
Publication date: 1971-12-28
Anticipated expiration: 1988-12-28

Abstract

An ionic vacuum pump, which is continuously operated to maintain a vacuum within an image tube, is located at the end of a cylindrical tubulation connected to the envelope of the tube. An ion trap comprises a cylindrical cathode sleeve in contact with the walls of the tubulation and a rod-shaped anode electrode, which is connected to the anode of the pump and extends from the pump into the envelope of the tube where it connects to one of the tube electrodes, which is in turn connected to a 2.5 kv. DC source. The radial electric field between the anode and cathode of the ion trap drives positive ions escaping from the pump toward the cathode of the trap where they are collected and thus prevented from entering the tube.

Description

United States Patent [72] Inventors NathanD.Levln 8/1963 Wehner 3,505,554 4/1970 Vekshinsky etal Primary Examiner-Roy Lake Assistant Examiner-Lawrence .I. Dahl AMorneysStanley 2. Cole and Gerald L. Moore ABSTRACT: An ionic vacuum pump, which is continuously operated to maintain a vacuum within an image tube, is located at the end of a cylindrical tubulation connected to the envelope of the tube. An ion trap comprises a cylindrical cathode sleeve in contact with the walls of the tubulation and a rod-shaped anode electrode, which is connected to the anode of the pump and extends from the pump into the envelope of the tube where it connects to one of the tube electrodes, which is in turn connected to a 2.5 kv. DC source. The radial electric field between the anode and cathode of the ion trap drives positive ions escaping from the pump toward the cathode of the trap where they are collected and thus prevented from entering the tube.

Iplpl.

PATENTEI] HEB28 I57] FIG. 3

FIG.4

BY %M-U MM ATTORNEY IONIC VACUUM PUMP INCORPORATING AN ION TRAP DESCRIPTION OF THE PRIOR ART Small ionic vacuum pumps have frequently been incorporated as an appendage on the envelopes of image intensifier and electron discharge tubes. These pumps are operated more or less continuously to maintain a low pressure within the envelope despite the release during operation of minor amounts of gas adsorbed on the internal surfaces of the tube.

The ionic vacuum pumps which are commonly used for this purpose have the unfortunate disadvantage of releasing some stray ions into the tube where they may cause difficulties of operation such as spurious signals or deleterious bombardment of electrodes. In order to avoid these problems, the prior art tubes have frequently been operated only when the appendage pump was shut down. However, this mode of operation entails a disadvantage in that pumping cannot be accomplished during the periods when outgassing of the tube parts is highest, viz, during normal operation. Furthermore, it has been necessary to make some special provision for operation of the pump only during the periods when the tube is not operating. Generally rather complex electronic circuitry is required to insure that the tube and pump are not operated simultaneously.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an ionic vacuum pump incorporating ion trap means to prevent migration of ions out of the pumping region and into the tube being evacuated.

Another object of the present invention is the provision of a cathode for such a trap comprising a cylindrical sleeve dimensioned to fit securely in contact with the walls of the tubulation connecting the pump and tube to be evacuated.

Another object is similar to the preceding object, wherein an anode for the trap is formed by a metallic conductor passing through the tubulation concentrically therewith and connecting to the anode of the vacuum pump, whereby a single source of voltage supplies the anodes of both the trap and the pump.

Another object of the present invention is the provision of a cathode for the trap comprising a vacuum-deposited metallic coating on the walls of the tubulation.

Another object of the present invention is similar to the preceding objects, wherein the cathode comprises a hollow cylindrical conductive segment forming a portion of the tubulation.

The above and other objects and advantages of the present invention will become apparent upon reading the following description and studying the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of an image intensifier tube incorporating an ionic vacuum pump and ion trap according to the present invention;

FIG. 2 is a longitudinal sectional view of the portion of the structure of FIG. I delineated by the line 2-2 and turned 90 counterclockwise;

FIG. 3 is a sectional view of a portion of the structure of FIG. 2 showing an alternative construction of the ion trap cathode; and

FIG. 4 is a sectional view of a portion of the structure of FIG. 2 showing yet another alternative construction of the ion trap cathode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To describe one application of the invention, FIG. 1 illustrates an image intensifier tube I having an evacuated envelope structure made of glass for example, including a central cylindrical portion 2 and an outwardly domed faceplate portion 3 through which photon images to be intensified pass to a scintillator. A photocathode in contact with the scintillator produces an image formed by emitted electrons which are accelerated to a fluorescent screen via other electrodes disposed inside intensifier tube 1. The opposite end of tube includes a constricted neck portion 4 connected to the envelope via an outwardly domed portion 5. A magnetically confined getter ion appendage pump 6 is sealed onto the domed portion 5 for evacuating the tube after sealing during processing.

Turning to FIG. 2, the ionic vacuum pump and ion trap are shown. Since the details of the ionic vacuum pump, which is operated to continuously maintain a high vacuum within the envelope, are not particularly important in describing the subject invention, the pump has been shown somewhat schematically. For the details of such a pump, reference may be made to US. Pat. application Ser. No. 859,004 filed Sept. l8, I969, entitled Anode Structure for a Magnetically Confined Glow Discharge Getter Ion Pump," by Nathan D. Levin, and assigned to the assignee of the present application. The ionic vacuum pump 6 is located at the end of a glass tubulation 7 which communicates the pump to the interior of the envelope of tube 1. Although the tubulation is conveniently made circular in cross section, plainly other shapes such as rectangular could be used. Since the image intensifier tube typically utilizes an acceleration voltage in the range of 2 to 5 kv., pump 6 has been designed to utilize the same voltage in order to simplify power supply requirements. Accordingly, a metallic conductor 8, for example of nickel, connects the anode 9 pump 6 to an electrode (not shown) within the image tube which electrode is at the 2 to 5 kv. voltage level.

A lower pole piece 10 and an upper pole piece 11 may be made of kovar or other material of high magnetic permeability. Kovar is preferred even though more permeable materials exist because of its well-known ability to form reliable joints with glass. Pole pieces 10 and 11 serve to direct the magnetic field produced by an annular magnet 12 into the region of the pump 6 wherein anode 9 is located. As shown, conductor 8 is provided with an insulative shield 8a over the portion of the conductor which extends adjacent to and through the lower pole piece.

Pole pieces 10 and Il may be plated with an active getter material such as titanium or tantalum over their portions opposite anode 9 so that pole pieces 10 and II, which are grounded, form getter cathodes. Otherwise, separate cathode plates (not shown) may be located opposite the open ends of anode 9 and in contact with pole pieces 10 and 11 as is well known. In operation, a magnetically confined glow discharge is produced within the pump 6 such that molecules of gas entering the cathode-anode region of pump 6 through a plurality of apertures 13 are ionized. The ions so formed are accelerated toward the pump cathodes in spiral paths. The impact of the ions sputters the active getter metal onto the surfaces of the pump where gas molecules are then gettered and buried as is well known. The ions are principally collected upon the surfaces of the cathode-forming coatings or separate cathodes on pole pieces 10 and 11. During this glow discharge, a few ions may escape through apertures 13 and migrate towards the interior of tube 1, where their presence could cause damage to the tube or degradation of its per= formance.

In accordance with the present invention these ions are collected and thus eliminated by an ion trap formed by the metallic conductor 8 and a cylindrical sleeve I4 of metal, for example stainless steel. Sleeve 14 is connected to lower pole piece 10 which is grounded as is upper pole piece I1. As already noted, conductor 8 carries a high voltage, for instance, 2 to 5 kv. Thus, conductor 8, which has a portion centered within sleeve 14, provides a radial electric field within the region of space bounded by the sleeve. This sleeve forms the cathode and conductor 8 the anode of a diode type ion trap. In operation, the radial electric field produced within this ion trap causes positive ions formed within the pump and escaping into tubulation 7 to be collected on cathode-forming sleeve 14, thus minimizing the escape of such ions into the body of tube 1. Although sleeve I4 has been shown as having dimples which engage the inner surface-of tubulation 7 to frictionally support sleeve 14 therein, other means of supporting sleeve 14 could be used, such as, for example, low vapor pressure resinous adhesives. Furthermore a cathode electrode for the ion trap could have other shapes such as a flat plate or a sleeve rectangular in cross section. However, the circular cylinder shape fits conveniently within a cylindrical tubulation and is simple to construct.

In FIG. 3 an alternative means of forming sleeve 14 comprises a deposited layer of metal l5.on the inner surface of tubulation 7. Layer 15 forms a cathode for the ion trap in the same way as sleeve 14 and would be grounded to lower pole piece in the same way as shown in FIG. 2.

In FIG. 4 a cathode for the ion trap is formed by a metallic cylinder portion or segment 16 which is of the same diameter as the remaining glass portions of tubulation 7 and which is vacuum tightly joined thereto. For example, segment 16 could be a cylinder of kovar. In this case grounding could be accomplished by an external connection. Similarly, although not shown, a suitable cathode for the ion trap could be formed by merely providing a portion of tubulation 7 of a conductivetype glass while remaining portions of the tubulation would be nonconducting.

What is claimed is:

1. In a vacuum system comprising: a chamber to be evacuated; a tubulation in gas flow communication with said chamber through a wall thereof; an ionic vacuum pump in gas communication with said chamber through said tubulation, said pump having a pump cathode and a pump anode; the improvement comprising an ion trap comprising a trap cathode mounted within said tubulation between said pump and chamber, a trap anode mounted in spaced relation to said cathode electrode, said pump cathode being electrically connected to said trap cathode, said pump anode being electrically connected to said trap anode, and means to energize said cathodes and anodes to produce a potential difference therebetween, whereby ions produced within said ionic pump and moving through said tubulation are collected.

2. The apparatus according to claim 1, wherein saidcathode electrode defines an aperture therethrough and wherein said anode electrode is supported within said aperture.

3. The apparatus according to claim 2 wherein said cathode comprises a conductive cylinder supported concentrically within said tubulation and wherein said anode comprises a conductive rod supported concentrically within said cathode.

4. The apparatus according to claim 2 wherein said cathode electrode comprises a conductive coating deposited on the wall of said tubulation.

5. The apparatus according to claim 2 wherein said tubulation includes an axial portion thereof formed of a conductive material, said axial portion comprising said cathode electrode.

Claims

2. The apparatus according to claim 1, wherein said cathode electrode defines an aperture therethrough and wherein said anode electrode is supported within said aperture.