US3214245A

US3214245A - Reactive metal diffusion pump

Info

Publication number: US3214245A
Application number: US233652A
Authority: US
Inventors: Jr Philip H Peters
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1962-10-29
Filing date: 1962-10-29
Publication date: 1965-10-26
Anticipated expiration: 1982-10-26

Description

Oct. 26, 1965 p E JR 3,214,245

REACTIVE METAL DIFFUSION PUMP Filed 001'. 29, 1962 Fig. 2.

2 2| HYDROGEN 9. D. It C) V) [H zzoxvasn TEMPERATURE 39 Fig. 3.

42 Inventor:

Philip H. Peter's Jrt,

dc'. by 117 zzig t IS an EXHAUST United States Patent 3,214,245 REACTWE METAL DIFFUSION PUMP Philip H. Peters, J12, Greenwich, N .Y., assignor to General Electric Company, a corporation of New York Filed Oct. 29, 1962, Ser. No. 233,652 Claims. ((11. 23--252) This invention relates to a pump device of relatively simple construction which produces a high vacuum by the reactive absorption of some gases and the diffusion of others. When coupled to a sealed vacuum chamber which acts as a source of various gases, the pumping device draws gases therefrom at an intake port, absorbs some of these gases, and delivers others at an exhaust port.

There are various problems associated with the production of high vacuums. Some problems are associated with vacuum conditions in general .and others relate to the particular type of pump considered. The highest vacuums presently obtainable with commercially available equipment are about 10" mm. of mercury. If this is produced in a region which was originally at standard atmospheric pressure 760 mm. of mercury, the change in conditions of the gases present is approximately fifteen orders of magnitude relative to the final condition. This change in conditions is so great that it has become accepted that at least two pumping devices, preferably three or more, operating in series are necessary for high vacuums. These systems can be considered to produce a succession of vacuum chambers within vacuum chambers wherein each pumping device operates over a pressure range within which it is effective. A representative system would include a good mechanical rotary pump producing a moderate vacuum of about 10* mm. of mercury. An oil diffusion pump would operate with its exhaust port in this moderate vacuum to produce a high vacuum of typically 10- mm. of mercury. Finally, an ultrahigh vacuum would be produced by an ion pump.

The most common high vacuum pumps are oil vapor diffusion pumps. This type pump utilizes a jet of vaporized fluid to entrain gases diffusing from a vacuum chamber. By its nature, this type of pump requires partial evacuation to produce a gas density at which it can begin to operate efiiciently. Refinements such as vapor traps utilizing liquid nitrogen and fiuid purification apparatus permit these pumps to produce vacuums in the order of 10- mm. of mercury. The limit is generally determined by the vapor pressure of the pumping fluid. The most serious problem encountered with these pumps is that some oil vapor inevitably reaches the vacuum chamber by back-diffusion or surface migration and requires difficult cleaning and baking procedures as regular maintenance. Designs of these pumps for greater efficiency and higher vacuums are generally characterized by substantially greater complexity and bulk.

A type of ultrahigh vacuum pump is the ion pump which relies on the volume ionization of residual gas by a strong static electric field in the presence of a static magnetic field to remove the gas and produce very low pressures. Such a pump is susceptible to contamination by silicones and hydrocarbons and pumped gas is often re-evolved when the pump is used again. This often requires careful use and maintenance. Normal bakeout procedures are ineffective in removing contaminants and an ion pump can only be restored to its original activity by replacement of the active electrodes. Use

33,214,245 Patented Oct. 26, 1965 of such a pump in the 10 and 10" mm. range drastically shortens electrode life and the pumping speed in this pressure range is not rapid enough to be of practical interest. Generally, it is necessary to produce a vacuum of 10 mm. of mercury before these pumps can be safely used. Furthermore, an ion pump requires a strong magnetic field. This magnetic field 'is frequently undesirable since it cannot be easily or completely isolated. Also, the apparatus for producing the magnetic fields is bulky and can require substantial power.

Gettering has long provided a useful pumping mechanism and a getter is a standard part in a vacuum tube. Pumping by gettering involves the absorption of gases from the chamber to be evacuated in the material of which the getter is composed. Often, this absorption is produced by a chemical reaction with the gettering element. On the other hand, the pumped gas may merely dissolve or be buried in the gettering material. Since the constitution of the atmosphere is complex, gettering action is complex and there is no known element or compound which can absorb all the gases of the atmosphere. Since the atmosphere also contains the inert gases, it is impossible to remove all gases by chemical reaction. However, there are several known materials which are effective getters for many of the gases which constitute the atmosphere of which one is titanium. Gettering elements generally have substantial variations in their gettering properties as a function of temperature. For example, titanium operates well in the range of 950- 1000 C. for most gases such as oxygen and nitrogen. At this temperature, titanium exhibits very active surface effects whereby gases rapidly react with the metal and the reaction products formed on the surface are continually drawn below the surface of the metal so that there is always a clean gettering surface presented to the gases. The process is so eflicient for oxygen that it is not practicable to measure the rate at which oxygen is absorbed.

On the other hand, a clean titanium surface will absorb large quantities of hydrogen near room temperature. Its gettering property for hydrogen is so great that it is diflicult to keep titanium free of this gas. Unfortunately, titanium does not act as a getter for hydrogen at elevtaed temperatures In fact, titanium desorbs hydrogen when heated. In addition to this, it can produce hydrogen by dissociating hydrocarbons and water vapor. This behavior compromises its overall efliciency for gettertype pumping.

Accordingly, it is an object of this invention to provide a high vacuum pump which is not significantly dependent upon a specific pressure range at its exhaust port for efficient pumping.

It is a further object of the invention to provide a high vacuum pump which can efficiently pump 'down from an initial low vacuum such as 1 mm. of mercury to a high vacuum such as 10' mm. of mercury.

It is another object of the invention to provide a high vacuum pump which can pump down from a high pressure such as atmospheric pressure in suitable circumstances such as a hydrogen filled vacuum chamber.

It is another object of the invention to provide a high vacuum pump which is clean and does not introduce contaminants into the vacuum chamber such as oil from a diffusion pump.

It is another object of the invention to provide a vacuum pump which is easily maintained by replacing simple parts.

It is another object of the invention to provide a high vacuum pump which during reuse does not desorb gases that were pumped during prior operation.

It is another object of the invention to provide a vacuum pump which maintains the vacuum integrity of the vacuum chamber when its operation is discontinue of if there is a pump failure.

It is another object of the invention to provide a high vacuum pump device which is smaller and lightereven substantially smaller than the vacuum chamber.

It is another object of the invention to provide a high vacuum pumping device which is not primarily dependent upon a high voltage source and/or a strong magnetic field for its operation.

It is another object of the invention to provide a vacuum pump that is characterized by relatively simple construction and does not require large associated equipment such as motors so that simple, portable configurations are easily produced.

It is another object of the invention to provide a high vacuum pump device of large capacity which has no moving parts and is therefore free of noise, vibration and frictional wear.

It is another object of the invention to provide a pump device which selectively exhausts specific gases.

Briefly stated, in accordance with certain aspects of a preferred embodiment of the invention, a high vacuum pump is provided by a structure which utilizes gettering action together with diffusion to pump gases. A titanium gettering element is arranged to provide a surface for absorbing the major constituent gases in a chamber to be evacuated. This gettering element is fabricated so as to act as a sieve for those gases which it does not absorb and is arranged to form a wall between the vacuum chamber and other regions of the pump. Provision is made for protecting the exhaust side of the gettering element and for removal of non-inert gases which are not absorbed by the gettering element by providing a housing on the exhaust side of the getter sieve element in which there is an exhaust port formed by a sheet of palladium. The palladium acts as a selective sieve passing hydrogen to the exhaust but protects the titanium from external atmospheric gases. Conversely, the titanium sieve protects the inner palladium surface from contaminant gases.

These and other objects and features of the present invention will become apparent from the accompanying detailed description and drawings in which:

FIGURE 1 is a digrammatic illustration of the essential parts of the invention and their interrelations.

FIGURE 2 is a graph illustrating the absorption rate of hydrogen and oxygen by titanium as a function of temperature.

FIGURE 3 illustrates a preferred embodiment of the invention.

FIGURE 4 is a modification of the FIGURE 3 embodiment which incorporates ionization for enhancing gas removal.

Referring now to the drawing, FIGURE 1 illustrates in semidiagrammatic form the novel high vacuum pump invention. The intake port of the pump faces a vacuum chamber in which there are generally a number of constituent gases. Depending upon the conditions, the occurrence and proportion of the gases can take different forms. However, since there is generally some leakage from the atmosphere into the chamber of all the atmospheric gases and pumping down from a standard atmosphere is common, the common atmospheric gases are represented and their presence in decreasing order is as follows: nitrogen, N oxygen, 0 argon, Ar; water, H O; carbon dioxide, CO neon, Ne; helium, He; methane, CH krypton, Kr; nitrous oxide, N 0; hydrogen, H ozone, 0 carbon monoxide CO; xenon, Xe; sulphur dioxide, S0 Generally, there are no other gases with partial pressures in air of more than 10- mm. of mercury.

In accordance with the preferred embodiments of the invention, these gases are removed by three mechanisms: gettering such as reaction of oxygen through oxidation of gettering material; diffusion such as removal of hydrogen through metallic sieve members; and burial of inert gases through ionization and the application of electromagnetic forces or other means. The gettering element illustrated in FIGURE 1 is a thin sheet of titanium 11, which is mounted in an opening of a housing 12. The titanium element is heated by a heater coil 13 which maintains the getter at a temperature of 1000 C. at which it is highly reactive. The element 11 has two characteristics in addition to gettering: it forms a wall which separates the vacuum chamber from the region beyond the gettering element and it is formed in such a manner that hydrogen will diffuse therethrough during pump operation. Accordingly, the titanium element is a getter sieve wall. A diffusion element is provided by a sheet of palladium 14 which is mounted over a :second opening in the housing 12 and heated by a second heater coil 15 which maintains the palladium at a temperature for efficient diffusion. Gases including the inert gases can also be removed from the vacuum chambers through ionization due to applied (by means of grid 9) electrostatic (or magnetic) forces. A suitable electrostatic field is established when a high potential is applied to grid 9. A solenoidal magnetic field can also be applied to increase the collision cross-section for ionization.

The operation of the FIGURE 1 pump is as follows: Most of the gases are removed from the vacuum chamber 10 by gettering action at the intake port. The titanium getter sieve operating .at 1000 C. reacts with oxygen, nitrogen, carbon dioxide, nitrous oxide, ozone, carbon monoxide, and sulphur dioxide. Also, water and methane are decomposed into their constituent elements at the surface of the titanium. Accordingly, all the gases of the atmosphere except hydrogen and the inert gases are removed by chemical reaction. The hydrogen permeates through the heated titanium and is removed from housing 12 (and thus the vacuum chamber) by diffusion through the palladium sieve 14 which operates at 600 C. and forms the exhaust port. The hydrogen diffuses through the palladium in an ionized form and rapidly oxidizes to water on the exhaust side. As a result, the partial pressure of hydrogen in the housing 12 (and hence the chamber) is many orders of magnitude less than hydrogen at the exhaust port which if it is atmosphere is approximately 3-10' mm. of mercury. The inert gases are ionized by the potential of the grid 9 and are removed by the applied fields which cause gases including the inert gases to bombard the hot titanium. Because of the crystal structure change caused by heating titanium, these gases are more deeply and permanently buried than they would be on cold titanium in conventional ion pumps.

The getter sieve wall 11 in FIGURE 1 is simply a thin sheet of titanium. The choice of the thickness of this titanium sieve requires consideration of both the diffusion function and the gettering function. Since titanium at elevated temperatures tends to draw reaction products from the surface into its interior, the gettering action can be considered a volume eifect. The gettering rate is a function of the surface area and the effective lifetime of the device is a function of the sieve volume. Because of the strength of titanium, there is ample latitude for the selection of getter thickness on the basis of the desired diffusion rate.

FIGURE 2 is a graphical illustration of the absorption characteirstics of titanium as a function of temperature. As indicated by curve 21, titanium dissolves hydrogen at a high rate below a temperature of 300 C. For increasing temperature, the rate of absorption decreases. When titanium has initially absorbed substantial quantities of hydrogen at room temperature, as is normally the case, a subsequent increase in temperature not only reduces the absorption rate, but, more important, the capacity for retaining the dissolved gas is drastically reduced and the titanium will actually evolve large quantities of previously dissolved hydrogen gas.

The irreversible absorption characteristic of titanium for oxygen as a function of temperature is illustrated in FIGURE 2 at 22. Titanium is only negligibly reactive with oxygen at room temperature but has exceptionally high reaction rates at temperatures much above 850 C. The behavior for other gases of the atmosphere (except the inert gases) is similar to that for oxygen.

FIGURE 3 illustrates a preferred embodiment of the invention in a practical configuration. A closure flange 31, having a central opening for the pump intake port, provides means for attaching the pump to a chamber to be evacuated. A titanium getter-sieve wall is in the form of a cylinder 33 which is welded to a mating cylindrical supporting section 34 of Inconel or a ceramic that is sealed to the closure flange 31. The titanium getter-sieve 33 is open at the intake port and closed at its other end so that all gases from the vacuum chamber which are not absorbed by gettering action and which diffuse to the exhaust port must pass through the titanium. Accordingly, only hydrogen passes through the cylinder 33. Wound about the titanium getter-sieve wall 33 is a bifilar heater wire 38 which maintains the titanium at the desired opearting temperature. On the heater wire 38 is an electrical insulating coating of alumina 36. A demountable pump housing is formed by a

casing

35, 39 which is sealed to the closure flange 31 and separated from the heater 38 by heater insulator 37. An exhaust port is formed by openings in the housing in which palladium diffusion sieves 42 in a thimble-like configuration for heat efiiciency are inserted and sealed.

The vacuum pump of FIGURE 3 operates in the following manner. Electrical power is applied to heater 38 which produces heat that raises the temperature of the titanium getter-sieve wall 33 to a temperature of approximately 1000 C. and heats the palladium diffusion thimbles 42 to a temperature of approximately 600 C. Under these conditions, the titanium getter-sieve reactively absorbs the gases in the chamber to be evacuated with the exception of hydrogen and the inert gases. As the titanium is heated, it actually desorbs large quantities of hydrogen and may produce additional hydrogen gas from the vapors in the chamber to be evacuated by decomposition. However, the hydrogen will diffuse through the titanium wall inside the housing 35 and from the housing it will pass through the palladium thimbles 42 to the surrounding atmosphere.

Inert gas pumping can be produced by a process of evaporative burial. A portion of the titanium surface is heated for a short period of time to a temperature at which the metal evaporates and subsequently settles on cooler surfaces of the pump which are exposed to the chamber being evacuated. As the titanium metal vapor travels to a cooler surface it entraps atoms of inert gases and buries such atoms on this cooler surface. Reheating to the same evaporation temperature does not cause reevolution of the buried atoms but merely overlays more metal on the already captured gas.

In the FIGURE 3 embodiment, the titanium gettersieve 33 is made in the form of a cylinder in order to maximize the elfective surface area, to provide a relatively large volume of titanium for chemical reaction and to produce a thermally eflicient arrangement. The larger the surface area is, the faster the gettering rate is. The heat losses are due mainly to thermal radiation and these can be kept very low with suitable heat shielding. Therefore this pump can be built to operate with only a few watts of heater power per square inch of getter surface. It is desirable to arrange the heater to uniformly heat the titanium cylinder 33 to avoid delayed hydrogen evolution from slowly heated titanium.

The reactive metal diifusion pump of FIGURE 3 is characterized by a conveniently compact construction of small size and light weight. It is only necessary to provide a connection for the intake port of the pump to the vacuum chamber while maintaining the vacuum sealed integrity of the chamber. Hardware such as the

housing

35, 39 can be of any conventional form known to those skilled in the art. Stainless steel is a convenient material which combines structural strength with other advantageous characteristics such as low absorption of gases and low diffusion rates for gases, particularly the inert gases. The heater wire 38 should be made with metalceramic seals to the casing 35 to gain similar advantages. The coupling flange 31 of the pump device can be any demountable high vacuum sealing device. Of course, for a permanent installation the pump device can be permanently sealed to the vacuum chamber or fabricated as an integral part of the chamber with or without a separate closure member for access to the chamber as required.

In the preferred mode of pumping down from atmospheric pressure, the vacuum chamber is first flushed to remove the inert gases originally present. Because of its lightness, hydrogen is particularly effective in displacing inert gases. To insure a long operating life for the pump, when pumping air gas, it is usually desirable to pump the air to a pressure near 10* mm. of mercury by convenient pumping means before the reactive metal diffusion pump is turned on. This initial pumping can be done by conventional mechanical pumping or sorption pumping.

Alternative forms of construction of the novel reactive metal diffusion pump can be employed. For example, in the embodiment of FIGURE 3, the palladium diffusion thimbles 42 can be replaced by other hydrogen removal means. One such means can take the form of powdered titanium which is separated from the exhaust side of the titanium getter sieve and arranged so that its temperature is below 300 C. where it is an effective getter for hydrogen. In this embodiment the pump would have a finite capacity for removing hydrogen since the powder would act simply as a reservoir. It could be activated periodically by heating the powder and driving hydrogen back through the titanium sieve into a vacuum chamber.

Other getter materials which have the requisite mechanical strength to provide a wall and can efficiently diffuse one or more non-inert gases which are not gettered are contemplated as a getter sieve.

However, the pump arrangements illustrated in FIG- URES 1 and 3 utilizing titanium and palladium have special, advantageous features. The most important of these is that the two metals, as arranged, operate to protect each other. Palladium dilfusion sieves are prone to being clogged by various materials especially at low gas pressures but the titanium wall isolates the palladium from all gases but hydrogen on the intake side. Similarly, the palladium protects the exhaust side of the titanium from external gases which it might getter rapidly such as oxygen. Furthermore, the titanium and palladium members together with the housing form a sealed chamber within which a heating element such as heating wire 38 is protected during pump operation from all gases except hydrogen.

These preferred embodiments of the invention have additional advantages. Because the palladium sieve can efficiently operate in air at standard atmospheric pressure, the reactive metal diffusion pump can maintain an ultrahigh vacuum directly at its intake port while its exhaust port is directly in air. Associated with this property is the adaptability of the pump whereby auxiliary apparatus such as flushing systems or other pumps can be connected to the chamber to be evacuated in parallel rather than in series with the reactive metal diffusion pump so that the auxiliary apparatus is accessible and can be easily removed.

Furthermore, the vacuum integrity of the system is unusual. Since the primary gas removal mechanisms, chemical reaction and diffusion, are effectively irreversible in this particular apparatus, neither during shutdown or a following start-up will the pump desorb gases previously pumped or admit gas directly from the exhaust port. Also, the pump has fail-safe characteristics in that a mechanical failure of either the titanium or palladium wall leaves the other to protect at least temporarily the evacuated chamber from the atmosphere.

FIGURE 4 illustrates a modification of the FIGURE 3 embodiment in which the gas removal is enhanced by means of ionization. The construction of the reactive metal pumping components is substantially the same as in FIGURE 3 and they are accordingly represented by the same reference characters. However, the heater 38 is formed as a regular helix (rather than bifilar) so that a magnetic field B is created as indicated inside the titanium cylinder 33 a poor conductor) by a D.-C. heater current which causes gas ions and electrons to move in spiral paths. An electrode 49 is provided along the axis of the titanium cylinder 33 and is positively biased by a voltage source 48 to provide the gas ionization. As a result, the gases are more rapidly drawn to the surface of the titanium getter, hydrocarbons are cracked and inert gases are buried in the titanium.

While particular embodiments of the invention have been shown and described, it is not intended that the invention be limited to such disclosure, but that changes and modifications obvious to those skilled in the art can be made and incorporated within the scope of the claims. For example, any suitable temperature control means may be employed where the materials selected require, for efficiency, specific operating temperatures above or below the ambient temperature.

What is claimed is:

1. A reactive metal diffusion pump comprising:

(a) a housing adapted to form a vacuum tight seal with a vacuum chamber;

(b) a permeable, reactive, getter-sieve wall, vacuum sealed over an opening between said housing and said vacuum chamber, said getter-sieve wall being comprised of a getter material which getters some gases in said chamber and being fabricated to provide diffusion of other gases;

(c) means to continuously remove said other gases from said housing; and

(d) said means to continuously remove said other gases, and said permeable getter-sieve wall being mounted in said housing in such a manner that all of said other gases must pass through said getter wall.

2. A reactive metal diffusion pump comprising:

(a) a closure member adapted to form a seal with a vacuum chamber opening,

(b) a permeable, reactive, getter-sieve wall, vacuum sealed to an opening in said closure member to form a partition for said chamber, said getter-sieve wall' being comprised of a material which is a getter for some gases in said chamber and through which other gases diffuse;

(c) a housing mounted over the exhaust side of said diaphragm; and

(d) means to continuously remove said other gases from said housing.

3. A reactive metal diffusion pump comprising:

(a) a housing;

(b) a permeable, reactive, getter-sieve wall arranged over an opening in said housing to form a pump intake port, said getter-sieve wall being comprised of a material which acts as a getter to absorb the major gases to be pumped and said getter-sieve wall having sufficient thinness to permit diffusion therethrough of other gases not absorbed;

(c) a selective diffusion sieve mounted over a second opening in said housing to form a pump exhaust port, said diffusion sieve being comprised of a material through which only said other gases diffuse; and

((1) said getter wall and said sieve being mounted so that all gases pumped by said diffusion sieve must pass through said permeable getter Wall.

4. A reactive metal diffusion pump comprising:

(a) a housing;

(b) a permeable, reactive, getter-sieve wall mounted over an opening in said housing to form a pump intake port, said getter-sieve wall being comprised of titanium which acts as a getter to absorb the major gases to be pumped and said getter-sieve wall having sufficient thinness to permit diffusion therethrough of other gases not absorbed;

(c) a selective diffusion element mounted over a second opening in said housing to form a pump exhaust port, said diffusion element being comprised of a material through which only said other gases diffuse; and

(d) said getter wall and said sieve being mounted so that all gases pumped by said diffusion sieve must pass through said permeable getter wall.

5. A reactive metal diffusion pump comprising:

(a) a housing;

(b) a getter-sieve wall mounted over an opening in said housing to form a pump intake port, said gettersieve wall being comprised of titanium which acts as a getter to absorb the major gases to be pumped and said getter-sieve wall having sufficient thinness to permit hydrogen diffusion therethrough;

(c) a selective diffusion element mounted over a second opening in said housing to form a pump exhaust port, said diffusion element being comprised of palladium through which only hydrogen diffuses; and

(d) heating means for maintaining said titanium gettersieve at an efficient gettering temperature and said palladium element at a temperature for efficient diffusion of hydrogen.

6. The reactive metal diffusion pump of claim 5 further comprising:

(e) ionization means to ionize the gases to be pumped.

7. A reactive metal diffusion pump comprising:

(a) a housing;

(b) a titanium cylinder in said housing having one end open to form a pump intake port through an opening in said housing; the titanium cylinder being fabricated to permit hydrogen diffusion therethrough;

(c) a palladium thimble element having its open end forming an exhaust port through a second opening in said housing, the palladium thimble being fabricated to permit hydrogen diffusion therethrough; and

(d) heating means for maintaining said titanium gettersieve at an efficient gettering temperature and said palladium element at a temperature for eflicient diffusion of hydrogen.

8. The reactive metal diffusion pump of claim 7 further comrising:

(e) said heater including a section of helically wound wire arranged to produce an axial magnetic field in said titanium cylinder; and

(f) an electode in said titanium cylinder for ionizing gases to be pumped.

9 10 9. The reactive metal diffusion pump of claim 7 References Cited by the Examiner UNITED STATES PATENTS (e) said heating means 15 arranged to heat a portion of said titanium cylinder so titanium can be evap- 31037685 6/62 Mlnemn 230 69 orated in such a manner that inert gases can be en- 5 3,085,739 4/63 Ames et 230 69 trapped upon cooler surfaces of said cylinder. 10. The reactive metal diffusion pump of claim 7 FOREIGN PATENTS wherein: 358,086 9/31 Great Britain.

(e) said heating means maintains the titanium at a temperature at which the titanium has a face cen- 10 MORRIS O. WOLK, Primary Examiner.

tered cubic structure whereby hydrogen is freely diffused therethrough JAMES H. TAYMAN, IR., Exammer.

Claims

1. A REACTIVE METAL DIFFUSION PUMP COMPRISING: (A) A HOUSING ADAPTED TO FORM A VACUUM TIGHT SEAL WXXXX VACUUM CHAMBER; (B) A PERMEABLE, REACTIVE, GETTER-SIEVE WALL, VACUUM SEALED OVER AN OPENING BETWEEN SAID HOUSING AND SAID VACUUM CHAMBER, SAID GETTER-SIEVE WALL BEING COMPRISED OF A GETTER MATERIAL WHICH GETTERS SOME GASES IN SAID CHAMBER AND BEING FABRICATED TO PROVIDE DIFFUSION OF OTHER GASES; (C) MEANS TO CONTINOUSLY REMOVE SAID OTHER GASES FROM SAID HOUSING; AND (D) SAID MEANS TO CONTINOUSLY REMOVE SAID OTHER GASES, AND SAID PERMEABLE GETTER-SEIVE WALL BEING MOUNTED IN SAID HOUSING IN SUCH A MANNER THAT ALL OF SAID OTHER GASES MUST PASS THROUGH SAID GETTER WALL.