US3377499A

US3377499A - Relatively large consumable pelletized getter source element for sublimation type getter vacuum pumps

Info

Publication number: US3377499A
Application number: US552374A
Authority: US
Inventors: William A Lloyd
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1966-05-16
Filing date: 1966-05-16
Publication date: 1968-04-09
Anticipated expiration: 1985-04-09

Description

April 9, E968 W. A. LLOYD 3,377,499

RELATIVELY LARGE CONSUMABLE PELLETIZED GETTER SOURCE ELEMENT FOR SUBLIMATION TYPE GETTER VACUUM PUMPS Filed May 16, 1966 FIGA L L 'LS PuLsE i 32 f TIMER L STEPP'NG l v INVENTOR. FlG wl LLIAM A. LLoYn -E GENERATOR MOTOR l `large consumable rod of titanium is advanced for sub- United States Patent Olltce ABSTRACT 0F THE DISCLOSURE A sublimat-ion type getter vacuum pump is disclosed. The pump includes a hot zone into which a relatively liming titanium from the rod onto collecting surfaces within fthe pump envelope. The sublimed getter material serves to getter gas within the pump envelope and to, thus, reduce the pressure within the pump and structures in gas communication with lthe pump. The titanium rod is formed by an aggregate of titanium pellets for reducing the thermal conductivity of the rod such that the region of the rod which extends into the hot zone and from which getter material is sublimed is relatively Well defined such that getter material is not sublimed from relatively large regions of the rodremoved from the immediate area of the rod which is within the hot zone. In one embodiment, an electron gun bombards the end of the pelletized rod for heating same to sublimation temperature. In another embodiment, a thermal radiator is heated by electron :bombardment and the titanium rod is heated by thermal radiation from the radiator.

The present invention relates in general to getter source elements for sublimation getter vacuum pumps and, more particularly, to an improved getter element formed by an aggregation of pellets of getter material, whereby sublimation of the getter material is facilitated and more precisely controlled. Such improved getter source elements are especially useful in, but not limited to use in, large capacity sublimation getter pumps wherein a zone of the element is heated to sublimation temperature by means of thermal radiation or by means of direct electron bombardment.4

Heretofore, solid non pelletized titanium rods have :been employed in sublimation getter pumps. The problem with the solid titanium rod is that it conducts heat away from the desired sublimation zone, usually the inner end of the rod, thus, requiring more heat for sublimation than desired. In add-ition the sublimation zone tends to extend over a larger lengthwise region of the rod than desired such that sublimation is obtained in region-s where it can cause buildup and aking of sublimed getter material. Oftentimes these buildups and/or flakes of getter material serve to shortout the sublimator.

In the present invention the source of getter material, which is to .be sublimed, is formed of an aggregation of getter pellets. The pellets serve to increase the area available for sublimation and to reduce the thermal conduction away from the sublimation zone, thereby increasing the sublimation eiciency and more precisely controlling the zone of sublimation. s

3,377,499 Patented Apr. 9, 1968 The principal object of the present invention is the provision of an improved source of getter material and improved sublimation vacuum pumps using same.

One feature of the present invention is the provision of a getter source element for sublimation vacuum pumps wherein the element is formed -by an aggregation of getter pellets, whereby sublimation efficiency is increased and the zone of sublimation more precisely delined than obtained from Ia similar unpelletized source element.

Another feature of the present invention is the same as the preceding wherein the pellets are contained in an elongated tube.

Another feature of the present invention is the same as any one or more of the preceding features including an electron gun for bombarding either the source element or a thermal radiator adjacent the source element to heat a Zone of the element above its sublimation temperature for subliming getter material from the source.

Other features and advantages of the present invention will ybecome apparent upon a perusal of the following specication taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram, partly in block diagram form, of a vacuum pump system employing a getter source element of the present invention;

FIG. 2 is a schematic diagram of an alternative sublimation pump dispenser portion, delineated by line 2 2 of FIG. l, having an electron bombarded getter source element employing features of the present invention;

FIG. 3 is a sectional view of the structure of FIG. 2 taken along line 3 3 in the direction of the arrows;

FIG. 4 is an enlarged view, partly in section, of a portion of the structure of FIG. 1 delineated by Iline FIG. 5 is a longitudinally foreshortened view of the structure of FIG. 4 before being subjected to sublimation; and

FIG. 6 is an alternative embodiment to the structure of FIG. 4.

Referring now to FIG. 1 there is shown a portion of a vacuum chamber 1 such as, for example, a space simulation chamber 40 feet in diameter and of 50,000 cubic feet volume which is to be evacuated to a pressure on the order of 1040 to l0-11 torr or less. A vacuum pump assembly 2 is connected into the vacuum chamber 1 via an elbow connection 3, as of in diameter, communicating with Ithe chamber 1 through an exhaust port 4. Liquid nitrogen cooled chevron type batlies S are disposed across the exhaust port 4 and also line the interior surfaces of the elbow 3 for providing surface lm getter pumping regions, more fully described below.

The vacuum pump assembly 2 comprises a conventional high capacity getter ion pump structure 6 of the type described and claimed in U.S. Patent 2,993,638 issued July 25, 1961, and assigned to the same assignee as the present invention. Brieily, the getter ion pump 6y includes a plurality lof rectangular pumping chambers 7 communicating with the elbow 3. Each pumping chamber 7 includes a multiple cold cathode magnetically confined glow discharge anode array 8 ldisposed between a pair of cold cathode plates 9. The plates 9 are made of a getter material such as titanium. C-shaped magnets 11 are disposed around each of the pumping chambers 7 for providing a magnetic field which threads through the anode cells. The getter ion pump 6 provides in excess of 2,000 liters/second of pumping speed down t-o pressures of 10-10 to 10-11 torr.

A sublimation getter vacuum pump 12 is coaxially disposed of the getter ion pumpv 6 and is carried from a flange assembly 13 closing off the lower end of the elbow 3. Briefly, the sublimation pump 12 comprises an open ended tubular thermal radiator 14- heated to operating temperature in excess of 2,000o C. Iby electron bombardment with 6 kv. electron obtained from a pair of filamentary thermionic emitters 15 disposed around the outside of the tubular radiator 14. A cylindrical radiation shield 16 surrounds the emitters 15.

The tubular thermal radiator 14 defines a sublimation Zone 17 within its interior which operates at sublimation temperat-ures of about 2,000 C. A rod of getter material 18 is coaxially disposed of the thermal radiator 14. The upper end of the getter rod is advanced into the sublimation zone 17 from the bottom via a closed loop drive chain 19 linked to the rod 18. The drive chain 17 is driven via a drive sprocket 21 actuated by a rotary feedthrough, not shown.

The sublimed getter material etnses through the open end of the tubular radiator 14 in a cone about 90 to 120 wide and is collected on the surfaces of the liquid nitrogen cooled battles 5 which face the sublimation zone. The deposited getter ilm serves to getter (pump) chemically active gaseous constitutents of the atmosphere inside the chamber 1 which flow or diffuse into the elbow 3. The getter ion pump 6 serves to pump the non chemically active gases as Well as the chemically active gases. The pumping speed of the sublimation pump 12, for chemically active gases such as nitrogen, is about liters/second per square inch of getter film which is deposited on liquid nitrogen cooled surfaces and about 15 liters/second per square inch for getter film deposited on room temperature surfaces. The sublimation pump provides a pumping speed in excess of 120,000 liters/second when the pressure is suticiently high such that the getter film is used for gettering as rapidly as it is deposited. The getter rod 18 is about 1% in diameter and 26 long and contains about 2,000 grams of titanium which is sublimed at a maximum rate of 1.3 grams per hour.

The thermal radiator 14 forms the anode for the thermionic emitters 15. A high voltage supply 22, as of 6 kv., is connected to the thermal radiator 14. A filament supply 24 is connected across the thermionic emitters 15 for controlling the heating current to the filamentary thermionic emitters 15. The filament supply voltage is set to maintain the temperature of the thermal radiator 14 and thus the sublimation zone 17 at some predetermined temperature such as 2,000 C. `Either one of the -lamentary emitters 15 is sufficient to heat the radiator 14 to its operating temperature.

The rate at which getter material is sublimed is determined by the rate at which the getter rod 18 is advanced into the sublimation zone 17. A stepping motor 25, disposed outside of the vacuum envelope, serves to advance the rod 18 through the intermediary of a mechanical drive mechanism, which includes a shaft 26, and a wobble stick rotary feedthrough, not shown. The rotary feedthrough drives the chain 19 through the rotation of the drive sprocket 21. Each step of the stepping motor advances the rod by about 0.002. A pulse timer and generator 27 supplies the drive pulses to the stepping motor 25 at some predetermined rate.

Referring now to FIGS. 2 and 3 there is shown an alternative sublimation dispensing head portion of the sublimation pump 12. In this case the rod 18 is advanced into a sublimation zone 17 wherein the end of the getter rod 18 is heated to sublimation temperature by direct electron bombardment. The rod 18 is operated at anode potential as of +6 kv. relative to an array of axially directed ilamentary thermionic emitters 28 arranged around the end of the rod 18. Strip-like shield members 29, operating at cathode potential, are disposed to block a line-of-sight path between the filaments 23 and the rod 18 to prevent sublimed getter material from hitting the hot filament.

The shields do not serve to shield the filaments from the anode voltage applied to the rod 18 and, thus, electrons emitted from the filaments 28 bombard the rod 18 at anode potential, as of 6 kv. The end of the rod I 18 which is bombarded is thereby heated to sublimation temperatures of around 2,000 C. The rod 18 is advanced into the sublimation zone 17 as described above with regard to FIG. 1.

Referring now to FIG. 4, the getter rod 18 of the present invention is formed by an aggregation of pellets 31 of getter material which may be conveniently contained Within a relatively thin walled tubular sheath 32 as of titanium having a wall thickness of 0.010. The pellets 31 may be spherical, cubic, flaked, or other shapes to provide increased surface area to facilitate sublimation and to reduce thermal conduction down the length of the composite getter rod 18. By reducing thermal conduction along the rod 18, the subliming region of the rod 18 is more narrowly defined to prevent unwanted sublimation from parts of the rod 18 below the desired sublimation zone 17, and loss of thermal energy.

For high pumping capacity sublimation pumps 12 of the type herein involved, the rod 18 should have a transverse cross sectional area exceeding 300 circular mils and a length in excess of 4 inches. `In one example of the present invention, the rod 18 was cylindrical as shown in FIG. 4 having a transverse cross sectioned area of 1,375 circular mils and a length of 26 inches and contained 2,000 grams of getter material.

The getter pellets may be of any convenient size or shape to provide increased surface area as compared to an unpelletized rod 18. In one example the pellets were cubic and about 0.125" on a side. While the sheath 32 is desired for containing the pellets 31 it may Ibe eliminated by presintering the pellets 31 in the desired rod shape. In the sublimation zone 17, the rod 18, due to sublimation of the getter material, takes on the cone shape at the end as shown in FIG. 4. The pellets 31 do not fall out of the rod because they are sntered together in the sublimation zone 17 due to the intense heat in the sublimation zone 17. Prior to sublimation the pellets 31 may be held in the sheath 32 by gravity or if the rod 18 is to be used in a non vertical structure the pellets may be presintered, as described above, or retained within the sheath 32 by end cap members 33, as 0f 0.010" thick titanium as shown in FIG. 5. When the end cap 33 is introduced into the sublimation zone 17, it is sublimed and the pellets 31, below the cap 33, are sntered together thereby preventing their escape from the sheath 32.

In some embodiments of the sublimator pump 12 it may be desired to have the rod 18 heated from the inside or from both the inside and outside. In such a case the rod 18 would take a tubular form as shown in FIG. 6. In this case the pellets 31 are either presintered into the tubular form or contained within a tubular sheath 34, as of titanium having a wall thickness of 0.010".

Since many changes could 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 accom panying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A consumable source element of getter material to be sublimed in a sublimation getter vacuum pump by being yby advanced into a sublimation zone including, an elongated element of getter material formed by an aggregation of pellets of getter material, said element having a transverse cross sectional area in excess of 300 circular mils and a length in excess of 4 inches, whereby sublimation from an end of the element is facilitated and the end of the sublimation zone more precisely dened.

2. The apparatus of claim 1 wherein said element in cludes a tubular sheath containing said getter pellets there Within.

3. The apparatus of claim made of titanium.

4. The apparatus of claim 2 including, means forming caps lfor capping pellets therewithin.

1 wherein said pellets are CII said sheath at its ends to retain said 1U References Cited UNITED STATES PATENTS JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, Assistant Examiner.