US3383032A - Vacuum pumping method and apparatus - Google Patents

Description

y 968 N. MILLERON 3,383,032

VACUUM PUMPING METHOD AND APPARATUS Filed Jan. 51, 1967 INVENTOR, NORMAN MILLERON BY m 0 WW,

ATTORNEY United States Patent 3,383,032 VACUUM PUMPING METHOD AND APPARATUS Norman Milleron, Berkeley, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Jan. 31, 1967, Ser. No. 613,053 Claims. (Cl. 230-69) ABSTRACT OF THE DISCLOSURE A high vacuum apparatus has a beam of electrons impinged on a continuously moving target. The target comprises a conductive material having a vapor pressure of less than about 10- torr.

The present invention relates to a method and apparatus for high vacuum evacuation, and more particularly to a method and apparatus for secondarily enhancing vacuum conditions to achieve very low pressures, as low as or lower than 10- torr, wherein a low energy electron beam is directed to impinge on a target of electrically conductive adsor-ber material having a low vapor pressure. The impinging electrons increase the gas adsorption afiinity of the target material and enhance adsorption of residual gaseous material from the vacuum chamber. Although the present method is discussed regarding adsorption of gaseous materials, that term is intended to be synonomous with chemisorption and to possibly include absorption insofar as the modus operandi is presently understood and as further discussed below.

In the prior art, common evacuation means, for example diffusion pumps, may achieve vacuums as low as 10* torr. However, in applications such as controlled thermonuclear energy generation, more perfect vacuums are required. For example, substantially sustained vacuums of 10- to 10 and even higher vacuums are commonly desirable. A variety of secondary evacuation means have been employed to reach such high vacuums beyond the capability of diffusion pumps. Examples of such pumping means include active getter materials disposed in communication with a chamber to be evacuated. The pumping power of getter pumps alone is determined by the adsorptive capacity of the getter surface. Cryogenic pumping has been employed by itself as a secondary high vacuum pumping means or in combination with an active getter. A cryogenically cooled surface presented to a chamber to be evacuated removes energy from impinging gaseous particles, enhancing the sticking probability and causes adsorption or condensation of the particles and thus prevents their re-entry into the chamber. A possibly more advanced approach is the employment of a Penning type discharge functioning as an ion-getter pump. In such equipment, a cathodic material is vaporized to impinge upon an anodic surface with the vaporization region in communication with the chamber to be evacuated. Gaseous particles are caused to be adsorbed on the anodic surface by interaction with the vaporized cathodic material. Each of these prior art exemplary evacuation methods has undesirable characteristics. The simple getter pumps provide pumping rates and capacities inherently limited by the rate of production of fresh getter surface and other general vacuum characteristics. Cryogenic pumping may require considerable time for the adsorptive surfaces to attain the proper temperature and sufiicient cooling must be provided to maintain the adsorptive surface during operation. Although the Penning discharge type ion-getters are very effective, they have several serious limitations. For example, a minimum pressure is necessary to initiate the discharge phenomenon from cold cathodes. Their intrinsic speed decreases to zero with decreasing pressure 3,383,032 Patented May 14, 1968 and there is at least a risk of vaporized cathodic material entering the working area of the vacuum chamber. All of the above pumping methods require periodic shutdown to purge adbosr-bed gaseous particles from the adsorptive surfaces.

The method and apparatus of the present invention provides a novel and unexepectedly effective means of increasing and prolonging getter adsorption 'by subjecting the getter material to electron impingement to achieve high vacuum conditions. In particular, a low energy electron beam is directed to impinge on a getter material in communication with a chamber to be evacuated. The method is capable of constant or intermittent operation with evacuation commencing immediately on activation and continuing for extended periods of operation. Power requirements are low. The apparatus of the present invention may be provided as a simple and economical structure. Further, the pumping capacity of the present method is virtually limitless and is also capable of accommodating surge pumping loads since continuous regeneration is provided during vacuum operation.

Accordingly, it is an object of the present invention to provide a method and apparatus for achieving high vacuum conditions. More particularly, it is an object to provide secondary pumping apparatus for evacuation of a priorly evacuated chamber to high vacuum conditions. It is a further object to provide a vacuum pumping method and apparatus wherein the pumping capacity is not limited by the inherent material characteristics of the apparatus. It is a further object to provide an adsorptive vacuum pumping method ,and apparatus wherein the adsorptive pumping means may be regenerated and maintained during evacuation operation. A still further object is to provide a getter high vacuum pumping method having a long continuous operating life and low power requirements.

Other objects and features of advantage will be apparent in the following description and accompanying drawings in which the manner of operation and construction are set forth:

FIGURE 1 is an illustration of a preferred embodiment of the present invention.

FIGURE 2 is an illustration of an alternative embodiment of the present invention.

The enhancement feature of the method of the present invention comprises essentially directing a low energy beam of electrons having an energy of in the range of approximately 5-100 ev. to impinge on at least a portion of a surface of an adsorptive target of electrically conductive material maintained in communication with the chamber to be evacuated. The adsorptive target may take the form of a liquifiable metal or solid material, respectively, in the embodiments set forth hereinafter.

Referring now to FIGURE 1, a preferred embodiment of the apparatus of the present invention comprises an evacuable housing 11 defining a chamber 15, provided with a large port opening 12, for example in a sidewall of housing 11. Flange means 13 surrounding port 12 may be joined with a similar flanged port in a vessel 14 to be evacuated. Thereby, chamber 15 is in free direct line communication with the interior of vessel 14 to be evacuated so as to provide maximum pumping speed. A tubular member 16 is hermetically joined to a wall portion of housing 11 defining a port 17. Preferably, tubular member 16 is disposed to extend angularly upward from its juncture with port 17 for purposes set forth hereinafter. There is provided in the lower portion of chamber 11 a receptacle 18 in the form of a rimmed structure providing an open depression in its upper surface 19 suitable for supporting and maintaining a liquifiable target material 21. Receptacle 18 is disposed within chamber 15 to maintain a substantial portion of the open surface 22 of liquifiable target 21 in the line-of-sight path defined by tube 16. As noted hereinafter, receptacle 18 is preferably arranged for continuous or intermittent replacement of the material 21. An electron source means 23, e.g., an electron beam gun adapted for connection to a suitable power circuit (not shown) is hermetically sealed to an end of tube 16 distal port 17. Electron source 23 is disposed to emit, propel and direct electrons along path 24 to impinge on surface 22 of liquifiable target 21. Electron source 23 is preferably of a type providing a controlled emission of a low energy beam of electrons, e.g., of about 1()0 electron volts. An example of such an electron gun 23 may be one having an electron emitter filament (not shown) and an electrostatically biased member (not shown) therearound for focusing and accelerating a beam of electrons from the emitter.

Provision for supporting receptacle 18 and draining liquifiable material therefrom may take the form of a conduit tube 26 preferably sealed to a portion of the receptacle floor 27 defining receptacle outlet port 23. Outlet tube 26 extends downward from receptacle 18, hermetically penetrates lower wall 29 of vessel 11 with an end thereof distal receptacle 18 and hermetically secured to an inlet port of desorber 31. A liquid metal pump 32 may be coupled into outlet tube 26 between receptacle i8 and desorber 31 to control or propel liquid metal flow from receptacle 18. Desorber 31 may comprise a hermetic shell 34 provided with electrically resistant heating means (not shown) and desorber evacuation means 36 for desorbing gas from and thereby purifying said liquid metal. Liquid metal cooling means 37 is disposed to hermetically receive liquid metal from desorber 31 and may be equipped with radiating fins (not shown) through which water or another suitable coolant is circulated, etc. Cooled liquid metal is returned to receptacle 18 by conduit means 33, having a first end secured to an outlet port of cooling means 37 to receive cooled purified liquid metal therefrom. Tube 38 hermetically penetrates wall 29 of vessel 11 and has a second end secured to the receptacle floor 27 defining receptacle inlet port 39. Liquid metal pump 41 may be provided in conduit 38 to propel or control liquid metal flow from cooling means 37 to liquid metal receptacle 18. Receptacle 18 and inlet and outlet tubes 26 and 38 may be of a metal suitable for high vacuum operation, e.g., stainless steel.

Liquifiable metal target 21 is preferably of a metal which exists as a liquid under temperatures readily attainable in a vacuum system, preferably less than 100 C. Suitable metals include bismuth (Bi), lead (Pb), tin (Sn),

or indium (In), gallium (Ga) and alloys comprising these metals. It is further desirable that the metal target material have a suitably low vapor pressure, e.g., l0 torr or less, at a temperature under which the metal exists as a liquid to permit circulation of the metal and yet prevent vaporizing metal from contaminating the high vacuum to be attained. A metal alloy comprising equal parts by weight of bismuth, lead, tin and indium, exhibiting a melting point of approximately 588 C., has been found to be admirably suitable. At a liquifying temperature of approximately 60 C., e.g., in said receptacle 18, the above metal alloy has a vapor pressure of less than approximately 10- torr. Further, the above alloy readily permits degassing or removal of adsorbed gaseous particles by, for example, heating the gas-laden liquid metal to a temperature of approximately 300-400 C., e.g., in desorber 31, at a vacuum pressure of 10* torr.

In operation, vacuum chamber 14 and accordingly chamber 11 are initially evacuated by primary evacuation means, e.g., a mechanical pump and/or an ion diffusion pump. An initial vacuum of less than 10- torr is desirable so that low energy electrons may travel substantially unimpeded along path 24 to impinge on target surface 22. Preferably, vessels 11 and 14 are initially evacuated to achieve a constant equilibrium pressure allowing free molecular flow of gas particles therein. Free molecular flow is herein defined by the average mean free path of gaseous particles within vessels 11 and 14 being approximately equal to or greater than the characteristic internal dimensions of vessel 11. By such an initial evacuation, the optimum pumping characteristics of the subject high vacuum evacuation method may be employed to greatest advantage.

Liquifiable metal target material 21 is heated above its melting point, e.g., the hereinabove preferred alloy is heated to 6070 C. Such heating may be accomplished by imparted energy of electrons originating from electron source 23, or circulation of the liquifiable metal 21 may be commenced in contact with the heating means of desorber 31.

Electron source 23 is activated to propel and direct a beam of electrons along path 24 to impinge surface 22 of liquid metal target 21. An energy of 5-100 electron volts is imparted to the electron beam traveling along path 24- by means of source 23. An electron beam of this energy range has the surprising and novel efiect of enhancing or increasing the chemical affinity or adsorption capability of target 21 for residual gas particles impinging thereon. Within the above energy range, it is anticipated that one or more limited ranges will provide optimum adsorption enhancement for a given residual gas or combination of gases. See Table I below for a specific example.

For most efiicient operation of electron source 23, the frequency of electron impingement on target surface 22 is maintained at a substantially greater value than the impingement frequency of the residual gas molecules on target surface 22. Such a relationship has been found desirable, and is apparently based on the target affinity for residual gases being enhanced by one electron impinging thereon for each gaseous particle to be adsorbed on target surface 22. For example, under an initial equilibrium pressure sufiiciently low to permit substantially mean free path molecular flow of residual gases in chambers 11 and 14, the residual gas will exhibit an impingement frequency 11 on target surface 22. Electron impingement frequency w is preferably maintained at a greater value than 1 Further, electron source 23 is preferably disposed and operated to propel and direct a beam of electrons to impnge target surface 22 with a grazing or oblique incidence. Such an oblique electron impingement incidence has been found to further enhance (with respect to normal electron impingement incidence) the gas adsorption afiinity of target 21. This phenomenon may possibly be explained in that a portion of normally impinging electrons may penetrate target surface 22 into the center of target 21 where they are ineffective to promote adsorption of gaseous particles on surface 22.

In accordance with the above preferred steps of the present method, an initial equilibrium pressure within chambers 11 and 14 is reduced to a new equilibrium pressure P The lower limit for the new equilibrium pressure P is limited for practical purposes by the vapor pressure of the liquid target material. For example, with the above preferred liquid alloy target material having a vapor pressure of less than 10* torr, a high vacuum equilibrium pressure P of 10 and as low as approximately 10 torr may be attained.

While a steady base pressure may be obtained often, because of leaks, production of gas in vessel 14, or due to gas particles released from the walls and interior surfaces of vessels 11 and 14, continued pumping by the present electron beam vacuum pump may be required. During sustained pumping, the liquid target material, if static, may reach adsorption equilibrium at Which point absorbed gas is released at a rate equivalent to the adsorption of additional gas particles upon the target. Adsorption equilibrium will be reached first at target :material surface 22. During low volume pumping operations, normal convective and diffusive motion within the liquid.

target metal can maintain an uppermost metal surface 22 in which adsorption equilibrium has not been reached. However, in larger volume apparatus or long duration pumping, the liquid target metal is preferably continuously withdrawn in small quantities by gravity flow or by pump 32 into desorber 31. The withdrawn gas-laden target metal is heated by heater means 34 to a temperature of approximately 300-400 C. and degassed under a vacuum pressure of, e.g., torr. The degassed liquid metal then is passed through cooling means 37 to cool it to approximately 60-70" C., and continuously returned by pump 41 to receptacle 18 in a degassed, purified state capable of adsorbing additional gas particles. Gas particles removed from the liquid metal in desorber 31 may be evacuated by desorber vacuum pump 36.

Typical and exemplary operating characteristics for the above preferred embodiment are set forth in Table I below.

TABLE I Typical operating characteristics Initial evacuated pressure P typically 1 10-= torr in small chambers, i.e., l,000 liters Typical residual gas-CO, H H O, CO and hydrocarbons Ratio of projected vacuum chamber area to the area of target surface 225100 Liquid metal target mass 10 g./cm. on the target surface, plus an additional amount as necessary for circulation Electron beam energyabout 5-100 ev.

A second preferred embodiment utilizing a strip form solid target is described with reference to FIGURE 2, wherein components similar to those in the first embodiment of FIGURE 1 are designated with prime numbers, e.g., vessels 11 and 14 and electron source 23. The discussion above regarding those components appearing as prime numbered members in FIGURE 2 also pertain to the second embodiment. In addition, degassing chamber 42 is preferably disposed to have a common wall 29' with chamber 11. Target 43 is preferably a flexible continuous, elongated fiat strip of low vapor pressure material and comprising electrically conductive metal at least on the strip surface 44 which is to be presented to receive i-mpinging electrons traveling along path 24'. For example, target strip 43 may be of stainless steel, tantalum, zirconium, copper or even a low volatile non-metallic material having, e.g., a zinc sulfide or other metallic coating along the target strip length on surface 44. Further, target strip 43 or at least surface 44 thereof is porous or irregular to provide increased surface area for adsorption of gaseous particles. In the present embodiment, target strip 43 substantially hermetically penetrates common chamber wall 29 through first and second ports 46 and 47. Ports 46 and 47, defined by the material of wall 29', preferably present a high impedance to gas fiow therethrough. For example, the cross sectional dimensions of ports 46 and 47 are of dimensions just larger than the maximum dimensions of target strip 43. To increase gas flow impedance, the path length through ports 46 and 47, connecting chambers 11' and 42, are of substantial length, e.g., at least ten times the cross sectional width of port 46 or 47. To permit each portion of target strip surface 44 to continuously rotate between chambers 11 and 42, strip 43 is movably supported, e.g., on rotatable supports 48, 49, 51 and 52 and adapted to be rotatably driven by motor means 53, e.g., a friction-drive electrical motor. Preferably, rotatable supports 48 and 49 are disposed in chamber 11 to maintain a portion of target strip surface 44 in parallel alignment and at a selected angle, e.g., 30, with respect to the path 24 of electrons impinging thereon. Degassing chamber 42 is provided with degassing means 45 to receive strip surface 44 from chamber 11 in a gas-laden condition and to reconstitute surface 44 to its original gas adsorbing capacity. For example, where target strip surface 44 is of porous copper, electron beam 'vapor deposition means 45 may be employed to deposit fresh copper particles over the gas-laden surface 44 and thus restore its gas-adsorbing capacity. Alternatively, degassing means 45 may be heating means or a high energy electron beam source, e.g., of 20 kev. or greater, disposed to impinge thermal energy or electron flow onto surface 44 to desorb gas particles from strip surface 44. Further, if the degassing means is a heater or electron beam source, degassing vacuum means, e.g., a diffusion pump (not shown), may be connected to remove desorbed gas particles from degassing chamber 42. Where target strip 43 is heated above its gas-adsorbing range, e.g., by heating or high energy electron beam means, strip 43 may be cooled before returning to chamber 11 by either natural thermal radiation or by cooling means (not shown) in degassing chamber 42.

In operation, prior evacuation and impinging of elec trons on target strip surface 44 are carried out similarly as in the above-described first embodiment. Also similarly, target surface 44 may reach adsorption equilibrium during high volume or extended pumping operation. To maintain high gas adsorption capacity, motor means 53 is operated to continuously rotate target strip surface 44 between chambers 11 and 42 to continuously present a highly adsorptive surface to impinging residual gas particles and electrons in chamber 11'. Where vapor deposition is employed to add new adsorptive material over gas-laden surface 44, the vapor deposition source is preferably maintained in non-line-of-sight communication with the paths through ports 46 and 47. Since the vapor deposition particles will travel substantially along straight line paths, and since there will be substantially no excess gaseous particles desorbed from surface 44 in chamber 42, the tolerance of cross sectional dimensions of ports 46 and 47 may be relaxed with respect to the dimensions of strip 44. Relaxing of the port tolerances may be desirable to permit passage of target strip 43 therethrough after vapor deposition of additional material onto target strip surface 44.

Although the present invention has been describe-d with particular reference to two preferred embodiments, it is obvious that many variations of method and apparatus are possible within the scope of the present invention. For example, although particular geometrics are described for the vacuum chambers and other components of the two preferred embodiments, it is obvious that the method of the present invention may be practiced with many variations and rearrangements of the apparatus geometry. Further, although the target materials particularly described are all electrically conductive metals, it is anticipated that the invention will be equally operable employing a target of, e.g., ionically conductive materials such as glass, mica, germanium, etc.

Accordingly, the scope of the present invention is not to be limited by the above description or figures, but only by the terms of the hereinafter appended claims.

What is claimed is:

1. In a high vacuum evacuation method utilizing a conductive material adsorptive surface in communication with a vacuum vessel, the steps comprising evacuating said vessel to a level of below about 10- mm. Hg, and then directing a beam of electrons having an energy in the range of about 5-100 ev. to impinge on said surface of the target of a conductive material having a vapor pressure of less than about 10- torr to increase the gas adsorption afiinity of said target and thereby enhance adsorption of residual gaseous materials from said vessel.

2. The method of claim 1, further defined by selecting said target to be an electrically conductive metal existing as a liquid at approximately C.

3. The method of claim 2, further defined by selecting said metal to be a metal alloy comprising an admixture of equal parts by weight of bismuth, lead, tin and indium.

4. The method of claim 2, further defined by heating said metal target to maintain its temperature below the target metal melting temperature, continuously and hermetically transferring a portion of said target metal from said chamber into a secondary vacuum chamber, activating heating means to heat said metal portion to approximately 300-400 C. to reduce its gas afiinity and drive olf adsorbent gas operating cooling means to cool said degassed metal below its melting point, and hermetically and continuously pumping said substantially gas-free metal portion back into said vacuum chamber.

5. The method of claim 4, further defined by pre-evacuating said vacuum chamber sufficiently to permit free molecular flow state of the gaseous material to be evacuated from said vacuum chamber.

6. The method of claim 5, wherein said beam of electrons is directed to impinge said target surface at a grazing angle to the plane of said surface to further increase the gas adsorption affinity of said target material.

7. Apparatus for achieving high vacuum evacuation of a first vacuum chamber having at least one wall, the combination comprising a target of a conductive material having a vapor pressure of less than approximately l torr, said target disposed to have at least a portion of one surface thereof in free communication with the interior of said vacuum chamber and an electron source means disposed to originate and direct an electron beam of the order of 5-100 ev. to impinge on at least a portion of said target surface.

8. The apparatus of claim 7, further defined in that said target material is an electrically conductive metal employable in a liquid state, open receptacle means disposed to support said liquid metal, degassing means to remove adsorbed gas from said liquid target material, receptacle outlet and inlet means disposed to carry said liquid metal from said receptacle to said degassing means and back to said receptacle, and pumping means disposed to circulate said liquid metal.

9. The apparatus of claim 8, wherein said liquid metal is an admixture of equal parts of bismuth, lead, tin and indium maintained at a temperature of approximately C. or more, said receptacle defines first and second ports beneath the liquid level, said degassing means is a second vacuum chamber having at least one wall external said vacuum chamber to be evacuated and comprising liquid containment means adapted to be heated and disposed below the liquid level in said receptacle, and standard evacuation means, said receptacle outlet means is a tube hermetically penetrating said first and second vacuum chamber walls and communicating between said first receptacle port and said liquid containment means, said pumping means is a liquid metal pump having a pump outlet and pump inlet disposed to receive liquid metal from said liquid containment means, and said receptacle inlet means is a tube hermetically penetrating said first and second vacuum chamber Walls and communicating between said pump outlet and said second receptacle port.

10. The apparatus of claim 7, further defined in that said target material is a continuous, elongated, flexible strip of metal of which at least said surface is porous, a second vacuum chamber having at least one wall, and standard evacuating means is disposed external said first vacuum chamber, said target strip substantially hermetically penetrates said first and second vacuum chamber walls to have a portion of said target surface disposed in said second vacuum chamber, heater means disposed in said second chamber to heat and de-gas said portion of target surface therein, and support and motor means to propel said target strip along a fixed path between said first and second vacuum chambers.

References Cited UNITED STATES PATENTS 2,858,972 11/1958 Gurewitsch 23069 2,897,036 7/1959 Gale et al. 230-69 2,925,504 2/1960 Cloud et al. 3l3-7 2,973,134 2/1961 Von Ardenne 230-69 3,074,621 1/1963 Lorenz et al. 230-69 3,116,764 1/1964 Jepsen et al. 230-69 ROBERT M. WALKER, Primary Examiner.

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