US3338506A

US3338506A - Vacuum pump apparatus

Info

Publication number: US3338506A
Application number: US437537A
Authority: US
Inventors: John C Helmer
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1965-03-05
Filing date: 1965-03-05
Publication date: 1967-08-29
Anticipated expiration: 1984-08-29
Also published as: US3343780A; GB1127796A

Description

g 29, 1967 .1. c. HELMER VACUUM PUMP APPARATUS Filed Marc h 1965 INVENTOR. JOHN C. HELMER United States Patent 3,338,506 VACUUM PUMP APPARATUS.

John C. Helmet, Menlo Park, Calif, assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Mar. 5, 1965, Ser. No. 437,537 Claims. (Cl. 23069) ABSTRACT OF THE DISCLOSURE An ion-getter vacuum pump. A titanium-wrapped filament electrode in the pump envelope is electrically heated causing both thermionic emission of electrons and sublimation of the titanium. A second electrode is used to cause the emitted electrons to travel long paths Within the envelope to increase the probability of collision with gas molecules and the production of ions of gas. The ions deposit on the walls of the envelope where they are gettered or buried by condensing titanium.

The present invention relates to vacuum pump apparatus and particularly to such apparatus employing sublimation and ionization techniques.

Pumping by thermally evaporating or subliming getter materials onto the internal walls of a housing, suitablyforepumped, has been well known for many years. Gas molecules coming into contact therewith combine chemically or physically with the condensed material and are removed from the gaseous state so as to reduce the pressure. Using pumping apparatus of this type, extremely high pumping speeds have been achieved, in excess of 3500 liters/see, for the active gases such as 0 N CO, and CO which can be scaled up as the need arises.

Sublimation pumping apparatus suffer from the drawback that they are ineffective in pumping the inactive gases, such as the noble gases and methane gas. For this reason sublimation pumping apparatus are frequently used in combination with some means which is capable of pumping the inactive gases, such as a diffusion pump or, where extremely low pressure in a contaminant free fashion are desired, a device which employs an ionizing discharge. Typical examples of apparatus which employ both sublimation and ionization techniques (hereinafter referred to as combination vacuum pumps) are described in a note entitled Evapor-Ion Pump, by R. G. Herb, R. H. Davis, A. S. Divatia and D. Saxon, Physical Review, second series, vol. 89, No. 4, p. 897, Feb. 13, 1953 and inan article entitled Orbitron Vacuum Pump, by R. A. Douglas, J. Zabritski and R. G. Herb, Review of Scientific Instruments, vol. 36, pp. 1-6, January 1965.

These prior art combination vacuum pumps are characterized by the provision of a sublimation means including a source of getter material adapted to sublime upon heating, onto the interior walls of the pump, and an ionization means including a source of electrons, which source may also 'serve to heat the getter material by electron bombardment, and a means for elongating the path of the electrons between the source and electrode serving as a collector to increase the probability of ionizing collisions.

It is the object of the present invention to provide an improved vacuum pump apparatus employing both sublimation and ionization techniques in which the need for a separate source of ionizing electrons is eliminated.

Briefly stated, in accordance with one teaching of the present invention there is disclosed a combination vacuum pump in which the need for a separate electrode which acts as a source of electrons is eliminated comprising a dual functioning sublimation means which acts both as a source of electrons and getter material, in combination with a means for elongating the path of the electrons.

One feature of the present invention is the provision in a combination vacuum pump of a dual functioning sublimation means, which acts both as a source of ionizing electrons and getter material, in combination with a means for elongating the path of electrons.

Another feature of the present invention is the provision of a combination vacuum pump of the above type wherein the means for elongating the path of the electrons comprises an electrode, for example, a grid electrode.

These and other objects and features of the present invention and a further understanding may be had by referring to the following description and claims taken in conjunction with the following drawing in which:

FIG. 1 is a diagrammatic view of one embodiment of the present invention;

FIG. 2 is a diagrammatic view of another embodiment of the present invention;

FIG. 3 is a diagrammatic view of still another embodiment of the present invention; and,

FIG. 4 is an enlarged fragmentary view of the sublimation means utilized in the present invention.

Referring now to FIG. 1, there is disclosed a vacuum pump incorporating the novel features of the present invention comprising: an evacuable envelope 11 adapted to contain gas molecules; an electron emissive sublimation means 12, including a source of getter material 13, adapted to be heated so as to cause electron emission and sublimation of getter material therefrom; a surface 14 for receiving getter material and collecting positive ions; and, a means 15 for elongating the path traveled by the electrons between emission and collection so as to increase the probability of ionizing collisions between electrons and gas molecules.

Envelope 11 has a cylindrical side wall 16 with a rotationally symmetric interior wall forming the surface 14, and apertured top 17 and bottom 18 Walls, all made of gas impervious material, for example, stainless steel. Bottom wall 18 is apertured for receiving, in vacuum tight manner, high voltage 19 and high current 20 feedthroughs. The envelope 11 is adapted to be connected in vacuum tight manner to a structure to be evacuated (not shown) through the intermediary of a high conductance passage 21 and vacuum flange 22 welded thereto. During operation, the envelope 11 is maintained at a potential equal to or greater than that of the sublimation means 12 so that electrons emitted therefrom will not :be attracted to surface 14.

Axially positioned within envelope 11 is the electron emissive sublimation means 12 supported on high current feedthrough 20. The means 12 is electrically isolated from the envelope 11 and is described with more particularity in US. application Ser. No. 392,417, filed Aug. 27, 1964 and assigned to the same assignee as the present invention. As best seen in FIG. 4, it includes a core 23, typically tungsten, and an overlay made up of a plurality of getter material wires 13, typically titanium, and a single wire 24 of a refractory material such as molybdenum, helically twisted tightly about the core 23 so that the overlay is in direct thermal contact with the core 23 throughout its length. Core 23 is preferably located on the axis of symmetry of the surface 14. It is believed that this overlay constitutes the source of electrons. During operation, the sublimation means 12 is essentially at ground potential. A power supply (not shown) of, the example,

5-6 volts is connected to the sublimation means 12 and current passed therethrough, typically 40-45 amperes, which is high enough to cause sublimation of the getter material. In addition, it has been found that at sublimation temperatures the sublimation means itself is a copious source of electrons enough to support an ionizing discharge in combination with the means 15. By way of example, in a typical embodiment at sublimation temperatures of about 1650 C. a temperature limited electron emission of about 170 milliamps. is obtained.

To obtain easy starting and low base pressure one may cool the walls of the envelope 11, for example, by the provision of a cooling conduit 25 brazed to the wall 16 through which a coolant fluid such as water may be circulated, to dissipate heat from the sublimation means 12.

Surface 14 acts as a collector for getter material sublimed from the means 12 and for gas atoms or molecules ionized by impact, the ions being impelled by electric fields into contact with the sublimed getter material which removes them by absorption, adsorption, or entrapment.

The means 15 may comprise a grid cage of relatively small surface area spaced from and coaxially surrounding the sublimation means 12, supported on high voltage feedthroughs 19 and insulated from the envelope 11. During operation, the grid cage is maintained positive with respect to sublimation means 12 and envelope 11 high enough to draw saturation electron emission from the means 12, but low enough that ionization probability remains high, typically 5004000 volts. The electrons emitted by the sublimation means 12 are caused to oscillate back and forth within envelope 11 greatly lengthening the path traveled by the electrons emitted by the means 12. In their flight back and forth, the electrons will gain kinetic energy and may collide with a neutral gas molecule. If the electron has gained sufiicient energy it will ionize the neutral molecule and produce a free electron. Positive ions formed between the grid 15 and surface 14 will be driven toward the surface 14, where they may be absorbed by getter material deposited thereon, as well as covered over by subsequently sublimed titanium. The embodiment of FIG. 1 is particularly advantageous for ion pumping, since the region of maximum ion bombardment coincides with the region of maximum deposition of getter material, Some ions created between the grid 15 and means 12 may be driven back towards the means 12 and therefore may not be pumped. This loss is minimized by making the radius of the grid a small fraction of the radius of the chamber, for example, a grid of one inch radius in an envelope with an inside radius of three inches.

FIG. 2 discloses still another embodiment of the present invention similar to the embodiment of FIG. 1 but modified in that the sublimation means 12 and grid cage 15 now enlarged, are disposed in side-by-side rather than coaxial relationship with each other. In this embodiment it is believed that there is a higher probability that ions formed within the grid cage will reach the collector surface 14.

FIG. 3 discloses an alternate embodiment of the present invention in which a large portion of the electrons emitted by the sublimation means 12 are orbited about an axial rod 26 of small surface area in a symmetric electrostatic field between surface 14 and axial rod 26. The potential of surface 14 may be equal to or greater than the potential of the grounded sublimation means 12 while the axial rod 26 is held at a high positive potential by a power supply means (not shown), typically -10kv. The sublimation means 12 is disposed parallel to the axis of surface 14, being spaced between the rod 26 and surface 14 so that adequate electron emission and long electron paths may be achieved. The electrons so orbited serve to ionize gas molecules which are driven to the collector surface 14 where they may be absorbed and subsequently covered over by the sublimed getter material. This embodiment has the advantage that extremely long electron paths between emission and collection may be obtained if the diameter of the rod 26 is sufiiciently small and voltage is sufficiently high. This embodiment is claimed in copending application Ser. No. 437,536, of Wesley H. Hayward, filed Mar. 5, 1965, and assigned to the same assignee as the present invention.

Since many changes can be made in the above construction and many apparently widely different 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 accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A vacuum pump apparatus comprising:

(a) an evacuable envelope adapted to contain gas molecules;

(b) electron emissive sublimation means disposed within said envelope and including a source of getter material, said means adapted to be heated so as to cause electron emission and sublimation of getter material therefrom;

(c) a surface disposed Within said envelope for receiving getter material and collecting positive ions; and

(d) means for elongating the path of said electrons between emission and collection so as to increase the probability of ionizing collisions between said electrons and said gas molecules, said means for elongating comprising a wire mesh cage enclosing a volume of space within said envelope.

2. The apparatus according to claim 1 wherein said wire mesh cage is spaced from said sublimation means and said surface and adapted to be maintained at a potential positive with respect to said surface.

3. The apparatus according to claim 2 including means for cooling said envelope in intimate contact therewith.

4. The apparatus according to claim 1 wherein said envelope has a rotational symmetric interior wall defining said surface, said sublimation means lies on the axis of symmetry of said wall and said wire mesh cage is spaced from said sublimation means and adapted to be maintained at a potential positive with respect to said surface.

5. The apparatus according to claim 4 wherein said wire mesh cage is coaxially disposed about said sublimation means.

References Cited UNITED STATES PATENTS 2,131,897 10/1938 Malter 230-69 2,888,189 5/1959 Herb 23069 2,925,504 2/1960 Cloud et al. 313-7 2,979,254 4/ 1961 Reich 230-69 2,988,657 6/1961 Klopfer 313-7 3,244,969 4/1966 Herb et al. 32433 ROBERT M. WALKER, Primary Examiner,

Claims

1. A VACUUM PUMP APPARATUS COMPRISING: (A) AN EVACUABLE ENVELOPE ADAPTED TO CONTAIN GAS MOLECULES; (B) ELECTRON EMISSIVE SUBLIMATION MEANS DISPOSED WITHIN SAID ENVELOPE AND INCLUDING A SOURCE OF GETTER MATERIAL, SAID MEANS ADAPTED TO BE HEATED SO AS TO CAUSE ELECTRON EMISSION AND SUBLIMATION OF GETTER MATERIAL THEREFROM; (C) A SURFACE DISPOSED WITHIN SAID ENVELOPE FOR RECEIVING GETTER MATERIAL AND COLLECTING POSITIVE IONS; AND (D) MEANS FOR ELONGATING THE PATH OF SAID ELECTRONS BETWEEN EMISSION AND COLLECTION SO AS TO INCREASE THE PROBABILITY OF IONIZING COLLISIONS BETWEEN SAID ELECTRONS AND SAID GAS MOLECULES, SAID MEANS FOR ELONGATING COMPRISING A WIRE MESH CAGE ENCLOSING A VOLUME OF SPACE WITHIN SAID ENVELOPE.