US3713754A

US3713754A - Vacuum pumps

Info

Publication number: US3713754A
Application number: US00089427A
Authority: US
Inventors: L Laurenson; L Holland
Original assignee: British Oxigen Ltd
Current assignee: BOC Group Ltd
Priority date: 1969-11-14
Filing date: 1970-11-13
Publication date: 1973-01-30
Anticipated expiration: 1990-01-30
Also published as: DE2056155A1; NL7016786A; FR2069508A5; GB1333388A

Abstract

In a vacuum pump of the getter-ion type, particularly one using Penning discharges, the problem of ''''argon instability'''' is at least partially solved by making at least one of the cathodic surfaces of two different materials. One of the materials may be titanium or like reactive metal, while the other material should also be refractory and of considerably higher atomic mass.

Description

United States Patent Laurenson et' al.

1 51 Jan. 30, 1973 VACUUM PUMPS [56] References Cited Inventors: Laurence Laurenson, Horsham; UNITED STATES PATENTS Lesl'e Arthur wand, 3,112,863 12/1963 Brubakeretal ..417 49 both of England 3,018,944 l/l962 Zaphiropoulous ..417 49 Assignee: The British Oxygen Company i i London, England Primary Examiner-William L. Freeh Assistant Examiner.l0hn T. Winburn 1970 Attorney-Dennison, Dennison, Townshend & Appl. No.: 89,427 Meserole 57 AB TRACT Foreign Application Priority Data I S In a vacuum pump of the getter-ion type, particularly Nov. 14, I969 Great Bntam ..55,998/69 one using Penning discharges, the problem of argon instability is at least partially solved by making at u.s.c1. ..417/49 ease ene'ef the eeehedle Surfaces 0f different 1m. c1... ............:::III...............:.::..F04b 37/02 meterieeone efthe materials may be titanium er like F d of Search "417/48 49 1 313/338 reactive metal, while the other material should also be refractory and of considerably higher atomic mass. Y

4 Claims, 4 Drawing Figures .IAI IO VACUUM, PUMPS This invention relates to vacuum pumps, and particularly to the type known as Penning pumps. These are getter-ion pumps which trap gas by sorption processes by using a large number of Penning discharges.

When a Penning pump is used to remove inert gases, such as argon, from evacuated spaces the phenomenon of argon instability arises. In this, large amounts of argon are released intermittently from the electrodes, causing a rapid and large fluctuation in the pressure of the system. These pressure variations are normally unacceptable and therefore steps have to be taken against them.

The present invention aims at providing an improved Penning pump in which the risk of argon instability is largely reduced or eliminated. 4

Accordingly the present invention provides a Penning pump which is as claimed in the appended claims.

The present invention will now be described by way of example with reference to the accompanying drawing, in which:

FIG. 1 is a diagrammatic side view of a Penning pump of the present invention with one cathode removed;

FIG. 2 is a side view, displaced by 90, of the pump shown in FIG. 1',

FIG. 3 is a section along the line llllll in FIG. 1 of the respective cathode, and

FIG. 4 is a diagrammatic cross-section, drawn to a larger scale than that of FIG. 3, of an alternative form of cathode.

A conventional Penning pump employs two cathodes of titanium bracketing a multi-cellular anode. This form is known to develop pumping instabilities when sorbing pure inert gases. This instability can be removed by substituting for one of the cathodes a cathode made of tantalum. This considerably increases the cost of the pump, and also, because tantalum is less reactive than titanium, the speed of the pump for pumping active gas is appreciably lower than that using two cathodes of titanium. The pump of the present invention is able to pump active gases and yet is not subject to argon instability to the same extent as a Penning pump using two titanium cathodes.

A pump of the present invention has its essential components shown in FIGS. 1 and 2. It includes an anode 2 positioned between two cathodes 4. The anode 2 is made up of an array of cylindrical cells 6 of stainless steel which are joined together to produce a honeycomb arrangement, as can be most clearly seen from FIG. 1. The cathodes 4 are made primarily of titanium, but at least one of the cathodes has part of its surface area 8 made of tantalum. As shown in FIG. 1, the tantalum may be in theform of a strip of metal, but it could alternatively be applied to the surface of the cathode 4 as a series of discs or other separate pieces of tantalum.

For clarity, the means are omitted by which the electrodes are connected to their associated electrical circuitry.

The manner in which the tantalum is secured to the cathodes does not form part of the present invention, but as a feature of the invention the tantalum may be detachable so that it can be conveniently replaced or alternatively removed altogether when it is desired to use the pump as a known Penning pump having two wholly-titanium cathodes, with the consequent risk of argon instability.

FIG. 3 of the accompanying drawing illustrates diagrammatically how the strip of tantalum 8 is positioned in front of the respective major face of the titanium cathode 4.

In known Penning pumps the opposed major faces of the cathodes 4 may be grooved, as illustrated in FIG. 4. This has been found to increase the effectiveness of the pump for pumping inert gases, in that the ions thereof tend to get buried in the bottoms of the grooves by the deposition of sputtered electrode material. Experiments have shown that a pump is far more stable when pumping argon if tantalum strips are placed in front of plain cathodes. This improvement is even more marked when such strips are placed in front of slotted-face cathodes.

The manner in which the present invention works is not'completely clear, but it would seem that the most likely explanation is given by Jepsens energetic neutral theory. Briefly, this theory suggests that as the energized ions of a gas such as argon impinges on a material which is as heavy as tantalum, the ions tend to bounce back from the tantalum without burying themselves below its surface. During this reflection phase the ion is discharged so that the cathode now produces a supply of neutral species still having a high energy. These electrically neutral particles are then able to embed themselves in the anode, becoming covered with further sputtered material. It will be appreciated that some inert gas will still be taken up on the cathode, but the amount embedded in a tantalum cathode will be appreciably lower than that in a titanium cathode operated under the same conditions. This means that not all the pumping of the inert gas has been transferred to the anode, but only that the concentration of argon embedded or buried in the cathode has been reduced to what appears to be a stable level.

One of the features of this invention is that'only one of the cathodes need have its respective surface formed partly of tantalum or like material. Symmetrical cathodes can of course be used, but it is felt that these do not offer sufficient additional advantages over the asymmetrical arrangement to merit the additional cost of the tantalum or like metal. It will then seem that if a pump of the present invention used one cathode made wholly of titanium, then the pump would still be argon stable. A tentative explanation for the non-appearance of this phenomenon in a pump of the present invention is that although the inert gas is not pumped stably at the titanium cathode, any reemission of argon from the titanium cathode can immediately be taken up by reionization and transference into the tantalum cathode/cellular anode pumping combination. Because the reemitted argon is so quickly absorbed by this pumping combination elsewhere in the pump, then to an observer viewing a vacuum gauge connected to the pump, the pressure remains constant in the pump and the system being evacuated.

The applicants put forward the above explanation only tentatively, and do not wish to be committed to a particular theory of operation.

An alternative material for the main part of the cathode can be zirconium, which is sufficiently reactive and refractory.

The cathode materials dissimilar to titanium are chosen from a group consisting of tantalum, tungsten, molybdenum, zirconium, niobium and rhenium.

We claim:

l. A getter-ion pump of the Penning type, including a pair of laterally spaced parallel imperforate cathodes, a

multi-cellular anode positioned between the cathodes and spaced therefrom, at least one cathode having a major surface facing the anode, being constituted and arranged to provide a plurality of slots therein, said major surface having fixed thereon a separate portion constituted of a refractory metal dissimilar to the metal of said major surface and chosen from a group consist-

Claims

1. A getter-ion pump of the Penning type, including a pair of laterally spaced parallel imperforate cathodes, a multi-cellular anode positioned between the cathodes and spaced therefrom, at least one cathode having a major surface facing the anode, being constituted and arranged to provide a plurality of slots therein, said major surface having fixed thereon a separate portion constituted of a refractory metal dissimilar to the metal of said major surface and chosen from a group consisting of tantalum, molybdenum, zirconium, niobium and rhenium.

2. A pump as claimed in claim 1 in which the metal comprising said major surface is constructed and arranged to provide a plurality of grooves therein.

3. A pump as claimed in claim 1 in which a portion of the slots is spanned by a covering strip of said dissimilar metal.