US3367564A - Sublimation getter pump employing a consumable getter source element heated by radiation - Google Patents

Description

Feb. 6, 1968 w. A. LLOYD 3,367,564

SUBLIMATION GETTER PUMP EMPLOYING A CONSUMABLE GETTER SOURCE ELEMENT HEATED BY RADIATION Filed May 16, 1966 5 Sheets-Sheet l.

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23 av. ac. CURRENT ,NVENTOR SUPPLY SENSOR BYWILLI MA.LLOYD Et -H RNEY Feb. 6, 1968 w. A. LLOYD SUBLIMATION GETTER PUMP EMPLOYING A CONSUMABLE (BETTER SOURCE ELEMENT HEATED BY RADIATION Filed May 16, 1966 5 Sheets-Sheet 2 FlG.4

INVENTOR. ,WIL IAM A.LLOYD Call ORNEY 3,367,564 GETTER Feb. 6, 1968 w. A. LLOYD SUBLIMATION GETTER PUMP EMPLOYING A CONSUMABLE SOURCE ELEMENT HEATED BY RADIATION Filed May 16, 1966 5 Sheets-Sheet 3 FIG.5

INVENTOR BY WILL A A. LLOYD WJG'K NEY United States Patent 3,367,564 SUBLIMATION GETTER PUMP EMPLOYING A CONSUMABLE GETTER SOURCE ELEMENT HEATED BY RADIATION William A. Lloyd, San Jose, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed May 16, 1966, Ser. No. 550,383

Claims. (Cl. 230-69) simulator chambers and vacuum metallurgial refining systems.

Heretofore, high speed, high capacity sublimation vacuum pumps have been used wherein a consumable getter source was heated to sublimation temperatures by direct electron bombardment from a plurality of shielded filamentary emitters positioned around the getter source element, typically a rod of titanium. The problems with such an arrangement are that the emitter shields, typically operating at or near cathode potential, are relatively cool compared to the operating temperature of the sublimation region of the consumable rod. Therefore, the sublimed getter material tends to condense on the emitter shields and other elements of the relatively open structure producing 'a substantial build up of getter material. This build up is wasteful of getter material which flakes off in use and oftentimes produces undesired arcs with resultant failure of the sublimator. Moreover, the consumable rod forms the anode for the bombarding electron guns. The rod is not always equally heated over its exposed bombarded surface area. As a consequence the end of the rod, forming the anode, takes different random time varying contours which are dependent upon its history of heating and sublimation. These different time varying anode contours serve to effectively time vary the perveance of the various electron guns in different ways. As a result it is not possible to precisely control the sublimaton rate by controlling the current or voltage supplied to the sublimator or by controlling the rate of advance of the rod in any practical way.

In the present invention, the consumable getter element is advanced into a high temperature zone which is heated to getter sublimation temperatures by means of a thermal radiator. Thethermal radiator is heated by electron bombardment from one or more electron emitters which are shielded from the getter source element by means of the radiator disposed therebetween. The advantages of this arrangement, over the prior electron bombarded sublimator, are that the radiator, since it operates above sublimation temperatures, does not collect I the sublimed material, thereby obtaining more efficient use of the sublimed getter material and avoiding undesired flaking of getter material in the sublimator. In addition, since the parameters of the electron gun or guns remain fixed, the radiator may be easily held at predetermined temperature and the sublimation rate precisely controlled by controlling the rate at which the getter source element is advanced into the sublimation zone.

The principal object of the present invention is the provision of an improved getter sublimation pump.

One feature of the present invention is the provision of a thermal radiator, heated above getter sublimation temperature by electron bombardment and disposed adjacent the getter source element to be sublimed, for defining a 3,367,564 Patented Feb. 6, 1968 sublimation temperature zone into which the getter source element is advanced for sublimation, whereby the radiator which may be used to shield the electron gun does not collect sublimed getter material.

Another feature of the present invention is the same as the preceding feature wherein the radiator surrounds the getter source element and is disposed intermediate the source of bombarding electrons and the getter source element.

Another feature of the present invention is the provision of means for automatically advancing the source of getter material into the sublimation Zone at a controlled rate.

Another feature of the present invention is the same as the preceding feature wherein the getter source material is advanced into the sublimation zone at a rate controlled in response to a measure of the gas pressure in the system being pumped.

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

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

FIG. 2 is an enlarged longitudinal sectional view of a portion of the structure of FIG. 1 delineated by line 2-2.

FIG. 3 is a transverse view, partly broken away, of the structure of FIG. 2 taken along line 3-3 in the direction of the arrows.

FIG. 4 is a schematic longitudinal sectional view of a similar region to that of FIG. 2 depicting an alternative embodiment of the present invention.

FIG. 5 is an enlarged longitudinal sectional view of a portion of the structure of FIG. 1 delineated by line 5-5, and

FIG. 6 is transverse sectional view of the structure of FIG. 5 taken along line 6-6 in the direction of the arrows.

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 10- to 10 torr or less. A vacuum pump assembly 2 is connected into the vacuum chamber 1 via an elbow connection 3, as of 60 in diameter, communicating with the chamber 1 through an exhaust port 4. Liquid nitrogen cooled chevron type baffles 5 are disposed across the exhaust port 4 and also line the interior surfaces of the elbow 3 for providing surface film getter pumping regions, more fully described below.

The vacuum pump assembly 2 comprises a convention- 211 high capacity getter ion pump structure 6 of the type described and claimed in US. Patent 2,993,638, issued July 25, 1961, and assigned to the same assignee as the present invention. Briefly, the getter ion pump 6 includes a plurality of 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 disposed 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 2000 liters/ second of pumping speed down to pressures of 10* to 10- torr.

A sublimation getter vacuum pump 12 is coaxially disposed of the getter ion pump 6 and is carried from a flange assembly 13 closing off the lower end of the elbow 3. The structure of the sublimation pump 12 will be more fully described below with regard to FIGS. 2-6. Briefly, the sublimation pump 12 comprises an open ended tubular thermal radiator 14 heated to operating temperature in excess of 2000 C. by electron bombardment with 6 kv. electrons 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 temperatures of about 2000 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 19 is driven via a drive sprocket 21 actuated by a rotary feed through, not shown in FIG. 1.

The sublimed getter material effuses 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 baffles which face the sublimation zone. The deposited getter film serves to getter (pump) chemically active gaseous constituents 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 45 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 sufficiently high such that the getter film is used for getter-ing as rapidly as it is deposited. The getter rod 18 is about 1 /8" in diameter and 26" long and contains about 2000 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., has its positive terminal connected to the thermal radiator 14. A current sensor 23 is connected in the anode circuit to ground to derive an output proportional to the anode current. The output of the current sensor is fed to a filament supply 24 which is connected across the thermionic emitters 15 for controlling the heating current to the filamentary thermionic emitters 15. The output of the current sensor 23 is adjusted to maintain the temperature of the thermal radiator 14 and thus the sublimation zone 17 at some predetermined temperature such as 2000 C. Either one of the filamentary emitters is sulficient to heat the radiator 14 to its operating temperature. Upon failure of either one of the emitters 15, the remaining emitter 15 takes over the heating function.

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 rotation of the drive sprocket 21. Each step of the stepping motor advances the rod by about 0.002". A pulser 27 supplies the drive pulses to the stepping motor 25 in response to an input derived from either a timer and sequencer 28 or a pulse rate controller 29. Switches 31 and 32 are provided to interconnect the pulser 27 to either one of these pulse rate control devices 28 or 29, respectively.

When switch 32 is closed the timer and sequencer automatically supplies command signals to the pulser 27 to cause the stepping motor to advance the rod 18 at some predetermined fixed rate such as, for example, at a rate to sublime 1.3 grams/hour. With each 0.002" advance,

the rod sublimes about 0.1 of a gram. Thus, for the above 1.3 g./hr. rate, the timer 28 commands pulses from the pulser 27 at the rate of 13 pulses per hour.

When the switch 31 is closed and switch 32 opened the rod 18 is automatically advanced at a rate proportional to the gas pressure within the system, thereby obtaining optimum use of the available getter material in the rod 18. A vacuum gauge 33 senses the pressure in the vacuum chamber 1 or elbow 3 via pressure sensor 34. The output of the vacuum gauge 33 is fed to the pulse rate control 29 for controlling the pulse rate output of the pulser 27 and thus the rate of advance of the rod 18 into the sublimation zone 17. The output of the pulse rate control is proportional to the gas pressure within the chamber 1 being evacuated. Thus, at relatively high pressures of 10- torr, the rod 18 is advanced at a maximum rate, whereas at lower pressures such as 10" torr the rod 18 is advanced at lower rates such as, for example, 0.002" per hour.

A counter 35 is coupled to the output of the stepping motor 25 for counting the number of steps through which the rod 18 has been advanced. Thus the output reading on the counter 35 is a measure of the amount of the rod 18 that remains unconsumed. At some predetermined number of counts, corresponding to only an hour or two of remaining getter material, the counter feeds an output to a warning alarm 36 to sound the alarm and warn the operator in suflicient time to replace the rod 18. In the event the operator takes no remedial action, the counter, at some greater number of counts corresponding to an exhausted supply of getter material, feeds an output to a shut down circuit 37 which deactivates the pulser 27 to prevent possible overrun damage to the sublimation pump 12.

Referring now to FIGS. 2 and 3 there is shown, in greater detail, the structure of the dispensing head portion of the sublimation pump 12. The tubular thermal radiator 14 is, for example, a 2% long by 1% inside diameter, V thick wall refractory metal material such as tantalum, molybdenum or tungsten. The tube 14 is flanged at its ends for strength. The radiator tube 14 is carried from a coaxially aligned tubular sleeve 41, as of 1.5" I.D., Via four axially directed support legs 42, as of tantalum ribbon 0.010" by A" cross section and 1 /2" long, which are spot welded to the radiator 14 and sleeve 41. A tantalum bearing sleeve 43, as of 1 I.D., is coaxially carried of the sleeve 41 for providing an upper sliding bearing support for the getter rod 18 which passes through the sleeve 43.

The pair of filamentary emitters 15, as of tungsten wire, surround the midsection of the tubular radiator 14 and are each supported from three axially directed separate support legs 44, as of 0.090" diameter tantalum wire. The support legs 44, which are connected to the filaments 15 intermediate the ends of the filaments 15, are supported at their base ends from ceramic insulator assemblies 45.

A double walled radiation shield assembly 46 is formed by a pair of thin walled radially spaced cylinders 47 and 48, as of 0.020 thick tantalum. The outer shield 48 is cup shaped with the bottom 49 of the cup 48 being apertured to accommodate the rod 18. The inner shield 47 is supported from the bottom 49 of the cup via support tabs 51 spot Welded to the cup 47 and shield 46. The radiation shields 46 serve to reflect heat from the emitters 15 and radiator 14 back to the radiator 14. The outer cup-shaped shield 48 is carried by its lip 52 from a cup-shaped liquid cooled jacket 53 as of thick walled copper. An annular coolant channel 54 is provided in the base of the cup-shaped jacket 53 for cooling. A pair of axially directed coolant pipes 55 connect to the channel 54. A disk shaped radiation shield 56, as of a double thickness of 0.020 thick sheet tantalum, closes off the upper end of the cup-shaped cooling jacket 43. The shield 56 is centrally apertured to accommoda te the tubular radiator 14. A tubular support 57, as of A1" thick wall 6" OD. stainless steel supports the jacket 53 from a demountable vacuum tight mounting flange assembly 58 (see FIGS. 5 and 6). The tubular radiator 14 is carried from the bottom side of the cupshaped cooling jacket 53 via three axially directed ceramic insulator assemblies 59 capable of holding off the 6 kv. applied between the radiator 14 and the grounded jacket 53 and emitters 15. One of the terminal emitter support legs 44 is connected to the grounded jacket 53 and the other terminal leg 44 is insulated from the jacket 53.

The upper end of the getter rod 18, where it enters the sublimation zone 17, takes on a cone shape in use due to sublimation of the rod 18. The sublimed getter material effuses out through the open end of the radiator 14 into a cone pattern about 90 to 120 in width. In some applications of the sublimator 12, it is desirable to shape the effusion cone pattern to reduce effusion along the longitudinal axis of the sublimator 12. In these cases a director assembly 61, such as, for example, a spiral filament of 0.060" diameter tantalum wire wound into a cone shape with 0.080 center to center spacing of the wire, is provided over the end of the radiator 14 to direct the effusing getter material away from the longitudinal ax1s.

Referring now to FIG. 4 there is shown an alternative embodiment of the thermal radiator 14' wherein the thermionic emitters 15' are located inside the tubular thermal radiator 14' for bombarding the radiator from the inside. The getter material, which is to be sublimed, is formed into a relatively thick walled tube 18'. The sublimation zone 17' is located in the region surrounding the radiator 14. The tubular rod of getter material 18 enters the sublimation zone 17' from one end and the sublimed getter material effuses away from the zone 17. This design has the advantages of directing the sublimed material away from the longitudinal axis of the sublimator and of reducing the thickness of getter rod in the sublimation zone to facilitate sublimation. A double radiation shield 56' closes off the end of the tubular radiator 14' for reducing unwanted thermal radiation.

Referring now to FIGS. 5 and 6 there is shown the support and actuator structure for the sublimator of the present invention. The getter rod 18 is supported at its lower end from a carriage 65 via a high voltage insulator 66. The carriage 65 includes an upper flange 67 which is notched at 68 to ride axially along a pair of guide rods 69 as of diameter stainless steel rods supported at their ends from the side walls of the tubular support 57 via support arms 71.

The drive chain 19 is pinned to the carriage 65and is driven via the drive sprocket 21 which is turned by a rotary feedthrough 72. An idler sprocket 73 is supported at the upper end of the sublimator 12 from an axle connected to the support tube 57. The pair of fluid coolant tubes 55, which connect to the fluid cooled jacket 53, enter the tubular support 57 at the lower end and pass axially thereof to the jacket 53. A high voltage anode lead 75 is connected at its upper end to the radiator 14 via bearing sleeve 43 and is held away from the tubular support 57 via stand off insulators 76. The lower end of the lead 75 is connected to the high voltage power supply 22 via high voltage feedthrough assembly 77. A similar lead and feedthrough assembly 78 provides the operating voltage and current for the filamentary emitters 15. The lower end of the tubular support 57 is closed off by a cover plate 79.

The sublimator pump 12 of the present invention requires about 5 kw. of power and sublimes up to 1.3 grams of titanium per hour and provides an operating life in excess of 5000 hours. It has the advantage over prior electron bombarded titanium rod devices in that the rod is operated at the same potential as nearby elements such as the radiator and its sleeves and bearing surfaces such that any unwanted accumulation of sublimed getter material will not bridge between elements at different potentials to produce a failure of the device. Also the radiator 14 shields the emitters 15 and other elements from sublimed getter material whereby wasteful and unwanted accumulations of getter materials are not collected on the sublimator structure.

In a preferred embodiment of the present invention the getter rod 18 is formed of a relatively thin walled tube as of 0.010" thick wall titanium filled with titanium pellets. The pellets may be spherical, cubic, 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 and loss of thermal energy. The pellet-filled rod forms the subject matter of and is claimed in copending US. application 552,374, filed May 16, 1966 and assigned to the same assignee as the present invention.

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 accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A sublimation vacuum pump apparatus of the type having a source of getter material which is sublimed onto interior surfaces of a vacuum system for gettering and thus pumping gases within the system to be evacuated including, means for forming a thermal radiator disposed adjacent the source of getter material for producing a sublimation zone having a temperature in operation above the sublimation temperature of the getter material for subliming the getter material in said zone, and means for directing a stream of electrons onto said radiator means for heating said radiator to its operating temperature.

2. The apparatus of claim 1 including, means for advancing the getter material into said sublimation Zone as the getter material is sublimed.

3. The apparatus of claim 2 wherein said electron stream directing means directs the electron stream against a side of said radiator means remote from that portion of said sublimation zone into which the getter material is advanced for sublimation, thereby shielding the source of electrons from the sublimed getter material.

4. The apparatus of claim 3 wherein said radiator means is disposed around the outside of that portion of said sublimation zone into which the getter material is advanced.

5. The apparatus of claim 4 wherein said radiator means is a tube.

6. The apparatus of claim 4 including, means forming a fluid cooled jacket surrounding said radiator means to shield certain of the interior surfaces of the vacuum system from heat radiated outwardly from said radiator means.

7. The apparatus of claim 2 wherein said getter advancing means automaticaly advances the getter material into said sublimation zone at a controlled rate.

8. The apparatus of claim 7 including, means for sensing the gas pressure Within the vacuum system and for controlling, in response to the gas pressure being sensed, the rate at which the getter material is automatically advanced into the sublimation zone.

9. The apparatus of claim 5 wherein said sublimation zone is disposed inside said radiator tube, wherein the getter material is advanced into said radiator tube from one end thereof with sublimed getter material etfusing out the other end of said radiator tube, and means disposed over the effusion end of said radiator tube for directing the eifusing getter material in directions away from the longitudinal axis of said radiator tube.

10. The apparatus of claim 2 including in combination, means forming a chamber to be evacuated by the sublimation pump apparatus, means forming Wall portions within said chamber means having surfaces for collecting a surface film of the sublimed getter material for gettering gas coming in conact therewith, means for cooling said film collecting wall portions to at least liquid nitrogen temperature, and means for ionizing gas Within said chamber means and for bombarding getter material with the ionized gas for pumping the ionized gas.

References Cited UNITED STATES PATENTS 3,244,969 4/1966 Herb et al 32433 3,313,474 4/1967 Hamilton 230-69 ROBERT M. WALKER, Primary Examiner.

Claims (1)

1. A SUBLIMATION VACUUM PUMP APPARATUS OF THE TYPE HAVING A SOURCE OF GETTER MATERIAL WHICH IS SUBLIMED ONTO INTERIOR SURFACES OF A VACUUM SYSTEM FOR GETTERING AND THUS PUMPING GASES WITHIN THE SYSTEM TO BE EVACUATED INCLUDING, MEANS FOR FORMING A THERMAL RADIATOR DISPOSED ADJACENT THE SOURCE OF GETTER MATERIAL FOR PRODUCING A SUBLIMATION ZONE HAVING A TEMPERATURE IN OPERATION ABOVE THE SUBLIMATION TEMPERATURE OF THE GETTER MATERIAL FOR SUBLIMING THE GETTER MATERIAL IN SAID ZONE, AND MEANS FOR DIRECTING A STREAM OF ELECTRONS ONTO SAID RADIATOR MEANS FOR HEATING SAID RADIATOR TO ITS OPERATING TEMPERATURE.

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