Abstract:
Disclosed herein is an ion pump construction having constructional components to avoid etch through failure of the vacuum housing and, additionally, constructional features to reduce the latent period within which the ion pumping function is initiated.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to ion pumps and is more particularly concerned with an ion pump construction in which gas leakage failure due to the phenomenon of ion etching through the vacuum housing component thereof is prevented and in which the latent period prior to the onset of ion pumping is accelerated. 
     Ion pumps are well known devices utilized for purposes of creating, maintaining and/or gauging vacuums in various technical systems. In general, an ion pump comprises: a vacuum housing composed of a non-ferromagnetic metal, usually a stainless steel or monel, and equipped with gas inlet means; a magnet system external of the housing for providing a magnetic field therewithin, said field defining a magnetic axis traversing the housing; an internal anode positioned substantially centrally within the housing aligned with the magnetic field and an internal cathode comprising opposed reactive sheet metal elements spaced from opposite ends of said anode and being interposed between the anode and the housing, said reactive elements overlying the magnetic axis. In use, usually after mechanically pumping the system to an at least somewhat reduced pressure, a high voltage is impressed across the anode and cathode, thereby to excite and ionize the gas admitted into the housing. Due to the influence of the magnetic field, the resulting positive ions are focussed as a core stream surrounding the magnetic axis of the field and flow to the negatively charged reactive sheet metal cathode elements to impinge upon, react with and thus become implanted as solid products of reaction therein. Generally, the cathode and housing are in electrical communication with one another and both are held at ground potential. Titanium is often the reactive metal employed for the construction of the reactive sheet metal cathode elements. Exemplary of such ion pump constructions is that disclosed in U.S. Pat. No. 4,460,317, to Robert H. Kern and Wilfred P. Levesque, entitled ION PUMP, issued Jul. 17, 1984, the entire disclosure of which is incorporated herein by reference. 
     One of the problems associated with ion pumps of the prior art is that, over time, the highly focussed ions contacting the reactive sheet metal cathode elements, particularly ionized noble gases concentrated as a core stream along the magnetic axis, ultimately etch holes through one or both said elements whereupon at least a portion of the streaming ions thereafter flow through the so holed reactive cathode elements to impinge upon and ultimately etch holes through the overlying walls of the vacuum housing. This last, of course, results in gas leakage through the vacuum housing and catastrophic failure of the pump. The problem is particularly acute where the gas to be pumped comprises one or more of the relatively readily ionized noble gas species, in other words, helium, neon, argon, krypton, radon or xenon. Noble gas ions tend to be highly reactive with respect to those metal compositions conventionally employed as materials of construction for the vacuum housing component. The predominant gas pumped in the many known ion pump applications is normal atmosphere, which, of course, contains such noble gas species. Accordingly, the foregoing mode of depletion and then subsequent failure of the housing represents a problem in the ion pump art. 
     Another operational problem in ion pumps of the prior art resides in the inherent occurrence of an initial, relatively lengthy, latent non-pumping period after impressing the high voltage across the anode and cathode components. This latent period is due to the fact that a certain amount of time is required to allow the high potential electric field across anode and cathode to experience initial ionization of a sufficient population of gaseous atoms and molecules to bring about the onset of a discharge in the feed gas and to thereby allow the pumping sputtering function to begin. This latent period is variable due to pressure and varying times to initial ionization. Where the application of the pump is in a system which is used repetitively throughout the work day, such as in a mass spectrometer of an analytical laboratory, these initial latent non-pumping periods can constitute troublesome delays, from seconds to periods approaching an hour. 
     In accordance with the present invention, the foregoing problems associated with ion pumps of the prior art have either been entirely eliminated or, at the least, substantially ameliorated. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided an ion pump comprising a vacuum housing; a magnet system external of said housing for providing a magnetic field therewithin, said magnetic field defining a magnetic axis traversing the housing; an internal anode aligned within said magnetic field and an internal cathode having opposed reactive sheet metal elements spaced from the respective ends of said anode and being interposed between said anode and said housing, said reactive cathode elements being disposed over said magnetic axis. The improvement of the invention comprises a pair of protective mask components, each composed of a material inert with respect to the ion sputtering indigenous to ionized noble gases. These mask components are interposed between the respective ends of said anode and the opposed sides of said housing overlying said reactive sheet metal cathode elements, said protective mask components being located so as to intercept the concentrated core ion stream focussed along the central magnetic axis. In another aspect of the invention there is located within the housing at least one component comprising an alpha particle emitting substance. In an additional embodiment, at least one of the foregoing opposed protective mask components is conformed of an inert material comprising an alpha particle emitting substance. In yet another embodiment of the invention, said pair of inert protective mask components are each composed of an electrically conductive material, are each conformed with a conically shaped face and are affixed to the opposed reactive sheet metal cathode elements with the conical faces thereof protruding inwardly from the inner surfaces of said reactive sheet metal cathode elements, the apices of said conical faces facing the anode and being substantially coincident with the magnetic axis. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 hereof is a schematic, diagrammatic, sectional side view of an ion pump in accordance with the invention, showing various preferred embodiments thereof. 
     FIG. 2 is a schematic, diagrammatic, sectional end view of the ion pump of FIG. 1 taken through line  2 - 2 ′ thereof, the plane defined by said line  2 - 2 ′ being coincident with the magnetic axis MA-MA′. 
     FIG. 3 is a schematic, diagrammatic, sectional bottom view of a portion of the ion pump of FIGS. 1 and 2, taken through lines  3 - 3 ′ thereof. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 through 3, wherein like reference numerals refer to like structures, the ion pump of the invention comprises a vacuum housing  10 , an external magnet system  30  for providing a magnetic field within said housing  10 , said magnetic field defining a magnetic axis MA-MA′ traversing said housing, an internal anode  40  disposed within said magnetic field and an internal cathode  50  comprising a pair of opposed reactive sheet metal cathode elements  51 ,  51 ′, said elements being spaced from opposite ends of said anode  40  and being disposed across the magnetic axis MA-MA′. 
     The vacuum housing  10  comprises a tubular housing  11  composed of a non-ferromagnetic material such as a monel or stainless steel alloy and back closure  12 . The back closure  12  comprises a forwardly extending cup-shaped member  13  whose outer rim  13 ′ is continuously welded to the interior surface of the back edge of tubular housing  11  and, coaxial with said member  13 , a rearwardly extending central cup-shaped member  14  whose outer rim  14 ′ is continuously welded to said cup-shaped member  13 . The rearwardly facing central cup-shaped member  14  comprises an aperture  15  through which there is sealingly mounted a high voltage feed through means  41  comprising a ceramic insulator jacket  42  and a high voltage conductor core  43  which extends forwardly beyond the front edge of said insulator jacket  42 . A cup-shaped sputter shield  44  having a central aperture  45  of somewhat larger diameter than the ceramic insulator jacket  42  of feed through means  41  is welded at its outer rim  44 ′ to the frontal surface of the back closure  12 , the aperture  45  thereof overlying and being coaxially spaced from the front end of said ceramic insulator jacket  42 . Said sputter shield  44  mitigates against deposition of ionization sputter products onto the surface of the insulator jacket  42 . Such deposition of sputter products on the insulator jacket  42 , particularly on the back portion thereof, can eventually lead to electrical leakage therethrough. Additionally, the vacuum housing  10  also comprises a gas inlet means  131  which, in the specific embodiment shown, comprises an axial inlet  141  opening into the front end of the tubular housing  11 . Obviously, however, the gas inlet means  131  into the vacuum housing  10  can comprise many alternative arrangements, such as one or more tubular inlets entering vacuum housing  10  through the sides of tubular housing  11 . Of course, where such alternative gas inlet means are employed, the front end of the tubular housing  11  will be suitably sealed, such as by means of a front closure continuously welded about the front end thereof. 
     Lying exterior of the vacuum housing  10  and secured thereto is a magnet system  30  which provides a magnetic field within said housing, said magnetic field defining a magnetic axis MA-MA′ running transversely across the interior thereof. In the particular embodiment of the invention shown said magnet system  30  comprises a pair of identical opposed permanent magnets  31 ,  31 ′ which reside in an opposed pair of recesses  16 ,  16 ′ formed in the tubular housing  11  of vacuum housing  10  and in which respective recesses  16 ,  16 ′ said identical cylindrical permanent magnet  31 ,  31 ′ are secured by means of a circumscribing yoke  32 . 
     Disposed within the vacuum housing  10  and affixed to the inner end of the conductor core  43  of the high voltage feed through means  41  is a generally tubular metallic anode  40 , said tubular anode being located within the magnetic field and being coaxially oriented about the magnetic axis MA-MA′. 
     Also contained within the vacuum housing  10  is a generally U-shaped cathode  50 , said cathode  50  comprising a base leg  52  and a pair of side members defined by opposed reactive sheet metal cathode elements  51 ,  51 ′. The base leg  52  is welded to the frontal surface of sputter shield  44 , thereby to establish a common electrical ground with the vacuum housing  10 . The reactive sheet metal cathode elements  51 ,  51 ′ extend forwardly from the base leg  52  and are spaced from the respective ends of said anode  40 , said reactive sheet metal cathode elements  51 , 51 ′ lying substantially adjacent the respective interior surfaces of the tubular housing  11  underlying the opposed magnets  31 ,  31 ′ and, additionally, being disposed over the magnetic axis MA-MA′. 
     In accordance with the invention, there is interposed between said anode  40  and the respective opposed interior surfaces of the tubular housing  11  underlying the opposed magnets  31 ,  31 ′ a pair of opposed disk conical or button-shaped protective mask components  60 ,  60 ′, each said mask component intercepting the magnetic axis MA-MA′. Each said mask component  60 ,  60 ′ is composed of a ceramic or metallic material which is inert with respect to the ion bombardment within the vacuum housing  10 . Preferably, the protective mask components  60 ,  60 ′ are each composed of an electrically conductive material and even more preferably are composed of an inert metal, such as tungsten. It is the principal purpose of the inert protective mask components  60 ,  60 ′ to protect the vacuum housing  10  from contact with the concentrated gaseous ions streaming along the magnetic axis MA-MA′ and to thereby prevent ion etching of said housing. Accordingly, the diameters of the mask components  60 ,  60 ′ should be sufficient to achieve this purpose and appropriate diameters for any given pump construction can be readily selected based upon knowledge of the cross sectional dimension of the concentrated ion stream flowing along said magnetic axis MA-Ma′. Upon contact with said inert mask components, the ions are deflected therefrom, thereby to ultimately come into contact with and become implanted in those portions of the reactive sheet metal cathode elements  51 ,  51 ′ which lie outside the magnetic axis MA-MA′. Those ions which do not come into contact with the reactive sheet metal cathode elements  51 ,  51 ′ during their first pass through the pump are simply recycled therethrough and are, therefore, eventually implanted into said reactive elements. Accordingly, while the protective inert mask components  60 ,  60 ′ may be affixed to the ion pump construction in any manner and position suitable for intercepting and deflecting the ions streaming towards the vacuum housing  10  along the magnetic axis MA-MA′, such as by fixation thereof directly to the interior surface of the tubular housing  11 , it is preferred that said mask components be affixed to the facing surfaces of the opposed reactive sheet metal cathode elements  51 ,  51 ′ underlying the tubular housing  11 , as shown. In this way, both the tubular housing  11  and the cathode elements  51 ,  51 ′ are protected from ion etching therethrough by the concentrated ion stream flowing along the magnetic axis MA-MA′ and, as will be explained in detail hereinafter, an additional benefit relating to the reduction of the latent non-pumping period can also be achieved. It is yet further preferred that at least one, and preferably both, inert protective mask components  60 ,  60 ′ be composed of an inert material which contains an alpha particle emitting substance therein. Thoriated tungsten, containing between about 0.5 and about 4% by weight thoria is a preferred example of such an inert material containing an alpha particle emitting substance. Such thoriated tungsten compositions are commercially available in the form of cylindrical welding rods which can be readily machined into one or both protective mask components  60 ,  60 ′ of the invention. When the inert material utilized for at least one of the protective mask components  60 ,  60 ′ comprises such an alpha particle emitting substance, the low level alpha particle radiation emitted therefrom acts to independently and rapidly initiate ionization of the gas admitted into the vacuum housing  10 , thereby shortening the aforedescribed latent non-pumping period. Of course, if desired, said ionization initiation benefit can also be achieved by installing a separate alpha particle emitting component, distinct and physically divorced from either or both of the protective mask components  60 ,  60 ′, within the vacuum housing  10 . However, the addition of an added alpha particle emitting component, separate and distinct from one or both protective mask components  60 ,  60 ′ of the invention, obviously entails additional complexity of construction and design and additional fabrication time and expense. In another preferred embodiment of the invention, the protective disks  60 ,  60 ′ are: (a) each composed of a conductive material; (b) opposed faces  61 ,  61 ′ thereof are conically shaped and (c) said disks are mounted to the facing surfaces of the opposed reactive sheet metal cathode elements  51 ,  51 ′ such as to protrude slightly inwardly therefrom with the apices  63 ,  63 ′ of their opposed conical faces  61 ,  61 ′ being substantially coincident with the magnetic axis MA-MA′. By meeting this last-mentioned combination of constructional criteria it will be seen that the path length between each of the apices  63 ,  63 ′ and its respective end of anode  40 , being slightly less than the path lengths between the facing surface of each of the reactive sheet metal cathode elements  51 ,  51 ′ and its respective end of the anode  40 , gives rise to a beneficial field emission contribution whereby the time to initiation of ion pumping of the gas admitted to vacuum housing may be further reduced by the advent of electrons resulting from field emission. Utilizing thoriated tungsten disks  60 ,  60 ′ in combination with the preferred conical geometry thereof and the fixation of said disks to the facing surfaces of the reactive sheet metal cathode elements  51 ,  51 ′, with the apices  63 ,  63 ′ being coincident with the magnetic axis MA-MA′ ion pumping has been consistently initiated within seconds after the high voltage potential has been impressed across the anode and cathode. 
     Ancillary equipment to service the ion pump of the invention includes a high voltage power supply  70  equipped with suitable circuitry to establish a high potential field between the anode  40  and the cathode  50 . As is conventional, the high voltage power supply  70  is supplied with a low voltage input from an external source, not shown. Optionally, the low voltage current draw of the high voltage power supply  70  is monitored by means of a microammeter  80  which is desirably provided with a logarithmic scale whereby, as is conventional in the ion pump art, a direct pressure reading within the vacuum housing  10  may be had. 
     While the foregoing description demonstrates certain embodiments of the invention and techniques for implementation and use thereof, it should be recognized and understood that said description is not to be construed as limiting of the invention because many obvious changes, modifications and variations may be made therein without departing from the essential scope and spirit thereof. Accordingly, it is intended that the invention is to be limited only by the scope of the appended claims.