US2925214A - Ionic vacuum pump - Google Patents

Description

Feb. 16, 1960 su w -rsd ETAL 2,925,214

IONIC VACUUM PUMP Filed March 27, 19 56 [r7 ven tors Anette/e M. Guhewitsch, Herbert G. Po//oc/r, o y )4! pal-M 777 air A ttorney IONIC VACUUM PUMP Application March 27, 1956, Serial No. 574,309 Claims. (Cl. 230-69) Thisinvention relates to ionic vacuum pumps and, more particularly, to such pumps employing a novel electrode structure and to a method of fonning the same which results in maximum pumping efficiency.

In the copending application, Serial No. 350,964, of Willem F. Westendorp and Anatole M. Gurewitsch, filed April 24, 1953, now Patent No. 2,755,014, there is disclosed an ionic pump which is capable of performing in an improved manner both the function of producing high vacua and of maintaining high vacua in sealed-off systems. Such a pump comprises a vacuum-tight envelope which is adapted to be connected to a system to be evacuated. Located within and coaxially with the envelope is a ring electrode energized at a positive potential relative to cold cathode discs attached to the envelope. External means are provided for producing a magnetic -field directed perpendicular to the plane of the anode so that electrons accelerated by the applied voltage are constrained to move axially of the device over relatively long spiral paths. Gas molecules are ionized by electron bombardment and the resulting positively charged ions are driven into and absorbed by the cold cathodes. In this manner the inert gases which cannot be chemically gettered are effectively removed from the system at high pumping speeds at very low pressures.

In the prior ionic pump, the cold cathode elements were required to be machined from gas absorbent materials and very carefully outgassed by conventional techniques which are time-consuming, expensive, and critical to efficiency of the device. Further, the pump had to have relatively large dimensions to accommodate discs having a sufiicient surface area.

A principal object of the present invention is to provide a novel ionic pump construction which is more com.- pact, less complicated, less expensive to produce, and which results in improved pumping characteristics.

A further object of this invention is to provide a method for the construction of a cold cathode electrode for ion pumps which electrode has optimum gas absorbent characteristics.

According to the illustrated embodiment of this in-- vention, there is provided in an ionic vacuum pump of the type which comprises an evacuable envelope, an arcuate anode, and means for producing a magnetic fieldi directed axially to the anode, a gas absorbent electrodewhich is formed by evaporation of a metal after a forepump vacuum has been produced.

The features of the invention desired to be protected herein are pointed out with particularity in the appended claims, and the invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection. with the'acco-mpanyingdrawing, in which:

Fig. 1 is a cross-section view of one embodiment of the ionic pump of this invention;

Fig. 2 is an elevational view, partially broken away, of the ionic vacuum pump of Fig. 1;

Fig. 3 is a detailed view of an anode structure for the pump of Fig. 1;

Fig. Sis a graph of pressure within a closed system' as a function of time.

Referring now to the drawing, in which like numerals represent like elements among the several figures, there is shown in Figs. 1 and 2 an ionic pump according to this invention which comprises an evacuable envelope 1 of non-magnetic material, such as glass, a ring shapedpositive electrode 2 supported approximately in the center and coaxially of substantially planar surfaces 3 and 4 of the envelope. Electrode 2 is supported by means ofconductive rods 5 and 6 extending through glass-tometal seals in the envelope and providingterminals 7 and 8 for connection to external circuitry. Softiron pole pieces 9 and 10 and permanent magnet 11 provide a magnetic field axially ofelectrode 2. The terminals provide electrical communication through the insulating envelope but may be omitted where a metal such as stainless steel is employed instead of glass or ceramic. Agas absorbing metal 13 supported by-theelectrode 2 and evaporable at a temperature lower than the fusion temperature of the ring electrode is the material from which the cold cathode is to be formed. Abulbous portion 14 of the envelope 1 and ashield 15 are provided to obscure a portion of the inner surface of the envelope from the evaporable material to assure electrical insulation of theanode 2.

To form the cathode the pump is connected to the system to be evacuated by means of tubulation 16 and after evacuation by a fore pump (not shown) to a pressure below 1 micron. The ring electrode is then heated by passing a current supplied by evaporation source ES until a temperature is reached at which the gasabsorbent metal 13 is evaporated. The evaporated metal will condense onthe inner surface of the envelope and form a thin layer 17 of gas absorbing material which serves as the cathode for the ionic pump. Contact between the evaporated coating and theterminal 12 may be assured by apreliminary deposit 18 of gold or other metal of low surface resistivity preformed on the inner surface of the envelope. After formation of the cathode 17 the evaporation source circuit is interrupted and the pump is operated by applying a high voltage from. the pumping source PS between the ring electrode and the cathode.

Electrons in proximity to the ring electrode are accelerated along spiral paths under the constraint of the magnetic field, which electrons bombard gas molecules diffusing into the pump volume, ionizing them. The ion ized gas molecules are in turn accelerated toward the cathode, driven into the surface, and effectively re, moved from the system under evacuation.

A cathode formed in this manner after a partial evacuation of the pump will have a clean uncontaminated surface and greatly increases the efliciency of the pump over designs in which the cathodes comprise pre-formed discs inserted in the pump while exposed to atmospheric pressure. The ions produce sputtering of the cathode and the redeposit of the sputtered material further aids in the gas removal process. Such increased efficiency may be taken advantage of by miniaturization of the pump components while still retaining satisfactory pumping characteristics. The gasabsorbent metal 13 may advantageously be chosen from titanium, zirconium, barium, magnesium, aluminum and alloys of these metals; however, other metals may be used to form the evaporated cathode 17 as suited to special purposes and the selective pumping of certain gases.

In applications where the ionic vacuum pump is employed with a sealed-off system, a portion of gas absorb- Pa'tented Feb. 16,1960? taran ula! 1; may be evapor after the p p n operation has been completed to form an additional deposit upon the envelope 1. This assures that gas absorbed by the previous cathode surface will not be re-evolved duri g p r od wh n e pump is o n ope ti nu ther, to increase the pumping capacity of this device, suc-v cessive e ap ra io s f gas a sorbent material m y b made during a continuous pumping operation assuring the maintenance ofan eflicient gas absorbent cathode.

Fig. 3 illustrates a preferred type of anode for the pump of Fig. 1 wherein an arcuate portion of theelectrode 2 comprises one or moreintertwined resistance wires 19 of a metal having a high melting point such as tantalum. These wires are coated or covered by acoil 20 of a lower melting point m ta a n d g s ab p h ra teristics, such as titanium. Upon heating the anode by passing a current through the resistance wires, the gas ab-. sorbent metal melts and is retained on the electrode structit e during the subsequent evaporation by its surface tension at interstices among the twisted wires. Other metals than titanium may be employed advantageously either in the form of a coil as shown or a fused coating for support andretention during the evaporation process.

Fig. 4 illustrates a further type of anode structure in which theanode 2 comprises a hollow tube M containingagas absorbing metal 22 such as barium. The tube may be ground away forming lateral surface areas as at 24 7 through which the material permeates and is evaporated upon heating. Induction heating as illustrated schematically by rthe opposedcoils 25 and 26 may be employed as an alternative to resistance heating, in which case the anode may be continuous and supported by a single membenz'l'.

In Fig. is plotted the pressure increase in a sealed-off system including the ionic pump of this invention. Thedashed curve 28 illustrates pressure build up with time for .a pump having preformed machined cold cathode discs of titanium. The solid curve 29 illustrates the similar. data obtained for a pump having an evaporated titanium cathode formed according to this invention. In each case, the-system was pumped to a given pressure, a measured quantity of helium introduced, and the pumps again operated to reduce the pressure to a given level. The pump was then stopped and the pressure increase investigated as a function of time. It is apparent that the method of forming the gas absorbent cathode [by evaporation after partial evacuation of the pump results in a many-fold increase in the retention of pumped gas.

While this invention has been described by reference to particular embodiments thereof, it will be understood that numerous changes can be made by those skilled in the art without actually departing from the invention, and

it is the aim of the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An ionic vacuum pump which comprises an evacuable envelope defining a space adapted to contain gas molecules, a first electrode located within said envelope, at gas absorbent metal supported by said first electrode,

means to evaporate said metal upon a surface exposed to the interior of said envelope to form a second electrode, ineans for connecting said first and second electrodes to a source of electrical potential to accelerate charged partic'les therebetween todrive gas molecules ionized by electron bombardment into said gas absorbent metal of said secondelectrode, and means to produce a magnetic field directed axially of said first electrode to increase the path length traversed by said electrons.

:2. ionic-vacuum pump which comprises an evacuable -envelope defining a space adapted to contain gas V first and second electrodes to a source of electrical po-- 4 molecules, an arcuate'first electrode located within said envelope, a gas absorbent metal chosen from the group consisting of titanium, zirconium, barium, magnesium, aluminum, and alloys thereofisupported by said first electrode, means to evaporate said metal upon a surface exposed to the interior of said envelope to form a second electrode, means for connecting said first and second electrodes to a source of electrical potential to accelerate charged particles therebetween to drive gas molecules ionized by electron bombardment into said gas absorbent metalof'said second electrode, and means to produce a magnetic field directed axially of said first electrode to increase the path length traversed by said electrons.

3. An ionic vacuum pump which comprises an evacuable envelope defining a space adapted to contain gas molecules, an arcuate first electrode located within said envelope, a gas absorbent metal supported by and evaporable at a temperature lower than the fusion temperature of said first electrode, resistance heating means including said first electrode to evaporate said metal upon a surface exposed to the interior of said envelope to form a second electrode, means for connecting said first and second electrodes to a source of electrical potential to accelerate charged particles therebetween to drive gas molecules ionized by electron bombardment into said gas absorbent metal of said second electrode, and means to produce a,

magnetic field directed axially of said first electrode to increase the path length traversed by said electrons.

4. An ionic vacuum pump which comprises an evacua able envelope defining a space adapted to contain gas molecules, an arcuate first electrode located within said envelope, a gas absorbent metal supported by and.

evaporable at a temperature lower than the fusion tern; perature of said first electr de, induction heating means: to evaporate said metal upon a surface exposed to the.- interior of said envelope tov form a second electrode, means for connecting said first and second electrodes to. a source of electrical potential to, accelerate charged particles therebetween to drive gas molecules ionized by electron bombardment into said gas absorbent metal of said second electrode, and meansto produce, a mag;- netic field directed axially of said first electrode. to. in-. crease the path length traversed by said electrons.

5. An ionic vacuum pump which comprises. an evacuable envelope defining a space adapted to contain gas,

molecules, an arcuate electrode located within said en.- veiope including a plurality of intertwined tantalum wires, a titanium filament coiled about said wires, an elongated element including conductormeans extending through said envelope, said arcuate electrode being amxed at each endto said elongated element to evaporate said filament on a surface exposed to, the interior of said envelope toform a second electrode when said.

conducting-means is connected to a source of currentto heat said arcuate electrode, means for connecting said tential to accelerate charged particles therebetween to. drive gas molecules ionized by electron bombardment into said gas absorbent material of said second electrode, and means to producea magnetic field directed axially of said first electrode to increasethe path length traversed. by said electrons.

References Cited inthe file of this patent

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