US3371854A

US3371854A - High capacity orbiting electron vacuum pump

Info

Publication number: US3371854A
Application number: US568170A
Authority: US
Inventors: Raymond G Herb
Original assignee: Wisconsin Alumni Research Foundation
Current assignee: Wisconsin Alumni Research Foundation
Priority date: 1966-07-27
Filing date: 1966-07-27
Publication date: 1968-03-05
Anticipated expiration: 1985-03-05

Description

R. G. HERB v Filed July 27, 1966 HIGH CAPACITY ORBITING ELECTRON VACUUM PUMP March 5, 1968 United States Patent O 3,371,854 HIGH CAPACITY GRIBITING ELECTRON VACUUM I'UMP Raymond G. Herb, Madison, Wis., assignor to Wisconsin Alumni Research Foundation, Madison, Wis., a corporation of Wisconsin Filed July 27, 1966, Ser. No. 568,170 22 Claims. (Cl. 23d-69) This invention relates to orbiting electron vacuum pumps of the general type disclosed and claimed in the Herb and Pauly U.S. Patent No. 3,244,969, patented Apr. 5, 1966. The invention also is applicable to orbiting electron pumps of the type disclosed and claimed in the copending application of Raymond G. Herb and Theodore E. Pauly, Ser. No. 447,678, filed Apr. 13, 1965. In certain aspects, the present invention is an improvement over the invention disclosed and claimed in the co-pending applicati-on of Joseph C. Maliakal on Orbitron Vacuum Pump With Getter Vaporization by Resistance Heating, Ser. No. 558,383, tiled June 17, 1966.

In the orbiting electron vacuum pumps of the prior Herb and Pauly disclosures, electrons are caused to travel in spiral orbits between `a central electrode, normally in the form of a cylindrical wire or rod, and an outer or boundary electrode, generally in the form of a cylindrical wall or cage. The orbiting of the electrons is achieved without the use of a magnetic field. A positive Voltage is employed between the central electrode and the boundary electrode to produce an electric eld therebetween which normally has cylindrical symmetry. The electrons are introduced into the space between the central and boundary electrodes in auch a manner that the electrons are given an initial angular momentum about the central electrode. Preferably, the electrons .are introduced by means of thermionic filament structures of the types disclosed and claimed in the above-mentioned Herb and Pauly patent and co-pending application.

In such orbiting electron pumps, the space within the boundary electrode is part of or connected to a vacuum system. A roughing pump or forepump is employed for initially bringing about a partial evacuation of the vacuum system. The orbiting electron pump is then employed to improve the vacuum so as to reduce the pressure in the vacuum system to an extremely low level.

In the -orbiting electron pump, some of the obriting electrons -collide with gas molecules so as to produce positively charged gas ions, These ions are propelled outwardly by the electric lield to the negatively charged boundary electrode, where they tend to stick and, thus, are effectively removed 'from the vacuum system. The orbiting of the electrons greatly increases the mean free path of the electrons, so as to increase very greatly the probability that any particular electron will cause ionization of a gas molecule. Thus, a high degree of ionization is produced with a low electron current.

p In the orbiting electron pump, it is preferred to provide a getter material, such as titanium, for example, on the boundary electrode to provide greatly improved sticking of the ions and gas molecules to the boundary electrode. Furthermore, it is preferred to provide a source within the pump lfrom which the getter material may be continuously evaporated, so that the evaporated getter material will continuously condense on the boundary electrode. The freshly condensed getter material is especially receptive to the ions and gas molecules. Moreover, the continuous condensation of the getter material buries the gas ions and molecules so that they will be permanently retained on the boundary electrode. The continuous eva-poration and condensation of the getter material greatly increases the gettering efficiency of the orbiting electron pump.

lCC

One object of the present invention is to provide a new and improved orbiting electron vacuum pump capable of producing signicantly greater ionization of the gas molecules, so that the ion pumping rate is greatly increased. This increase in the ion pumping rate is especially important for removing the noble gases, such as helium, argon, and neon, for example, from the vacuum system. Unlike the more active gases, such as oxygen, nitrogen and hydrogen, the noble -gases are not removed to any great extent by chemical or physical getteringaction, but must be removed primarily by ion pumping, in which the ions 4of these gases are driven into the gettering surfaces on the boundary electrodeA and .are buried by freshly deposited getter material.

A further object is to provide a new and improved orbiting electron vacuum pump having means whereby an electron cloud of -greatly increased density may be maintained between the central and boundary electrodes, so that much greater ionization of the gas molecules may be achieved. In this way, the ion pumping rate may be greatly increased. In the orbiting electron pump, the multitude. of orbiting electrons form a cloud of electrons which generates a space charge effect between the central and boundary electrodes. In accordance with the present invention, it has been found that the magnitude of the space charge, and hence the density of the electron cloud, is limited by the electrical capacitance of the electrode system.

Thus it is a further object of the present invention to provide a new and improved orbiting electron vacuum' pump in which the electrical capacitance between the central and boundary electrodes is greatly increased, so that la much larger number lof electrons may be maintained in orbits between the electrodes.

A further object is to provide such a new and improved orbiting electron vacuum pump in which a cylinder of large diameter is employed as the central electrode, rather than a Wire or rod of small diameter, as employed heretofore, whereby the ratio between the diameters of the boundary kand central electrodes is greatly reduced. In this way, the electrical capacitance between the electrodes is greatly increased.

The above-mentioned co-pending application of Joseph C. Maliakal, on Orbitron Vacuum Pump With Getter Vaporization by Resistance Heating discloses orbiting electron vacuum pumps in which getter material, preferably titanium, is vaporized by causing electrical 'currents to pass along a plurality of longitudinal rods or wires disposed adjacent the boundary electrode, but electrically insulated therefrom. The getter material is preferably in the form of titanium wire wrapped around the longitudinal rods.

Another object of the present invention is to provide a new and improved orbiting electron vacuum pump in which the getter material is vaporized by resistance heating in a new and improved manner, by causing electrical currents to pass along one or more lhelical or .annular members of a generally cylindrical cage, made of getter material and disposed between the central and boundary electrodes.

Further objects and advantages of the present invention will appear from the following description, taken with the accompanying drawings, in which:

FIG. 1 is a diagrammatic longitudinal section of an orbiting electron vacuum pump to be described as an illustrative embodiment of the present invention.

FIG. 2 is a fragmentary elevational View showing details of one of the thermionic filaments of the pump, the view being taken generally as indicated by `line 2-2 in FIG. 1.

FIG. 3 is a horizontal section,

taken generally along the line 3-3 in FIG. l.

FIG. 4 is a fragmentary longitudinal section showing a modified getter vaporizing cage for the pump of FIG. 1.

It will be recognized by those familiar with the prior Herb and Pauly disclosures that FIG. 1 illustrates an orbiting electron vacuum pump which is basically of the type disclosed and claimed in the co-pending Herb and Pauly application, Ser. No. 447,678, filed Apr. 13, 1965. The orbiting electron pump 10 of the present invention may also be of the basic type disclosed and claimed in the Herb and Pauly Patent No. 3,244,969, patented Apr. 5, 1966.

It will be seen that the vacuum pump 10 comprises a central or inner electrode 12 which is disposed axially within an outer or boundary electrode 14. In this case, the boundary electrode 14 is in the form of a hollow metal cylinder, which also serves as a casing fo-r the pump.

10. However, in some instances, the boundary electrode may be separate and distinct from the casing of the pump. Further details of the central electrode 12 will be given presently.

The space within the casing 14 is connected with the interior of a vacuum system 16 which is to be evacuated. In this case, the lower end of the casing 14 is connected to the vacuum system 16. Such system normally includes a mechanical roughing or forepump which is employed to produce an initial vacuum in the system. The orbiting electron pump 10 is then employed to improve the vacuurn so that the pressure in the system will be reduced to an extremely low level. In this case, the lower end of the casing 14 is formed with a ange 18 which is bolted or otherwise secured to a flange 20 on the vacuum system. The lower end of the casing 14 is open and is in full communication with the interior of the vacuum sys` tem 16.

As shown, the upper end of the casing 14 is closed by an end plate 22 which is bolted, welded, `or otherwise secured to a flange 24 on the casing 14. The central electrode 12 is supported by a rod or wire 26 which is carried out of the casing by a feedthrough insulator 28 sealed into a suitable opening in the end plate 22.

During normal operation, a high positive voltage is impressed between the central electrode 12 and the casing 14. Thus, a direct current power supply 30 is connected between the rod 26 and ground. The casing 14 is also connected to ground. Any desired voltage may be employed, but a typical voltage range would be from 5000 to 7500 volts.

The high positive voltage between the central electrode 12 and the boundary electrode 14 produces an electrical field therebetween which is cylindrically symmetrical. Electrons are introduced into this field with initial angular momentum about the central electrode 12, in such `a manner that the electrons will travel in spiral orbits around the central electrode. Eventually, virtually all of the electrons will be captured by the central electrode 12, but many of the electrons will travel around and along the central electrode through many spiral orbits, before being captured. Thus, the mean free path of the electrons may be many times the size of the pump.

Various arrangements may be employed to introduce the electrons into the pump. The illustrated pump 10 employs the basic arrangement disclosed and claimed in the eopending Herb and Pauly application Ser. No. 447,678, filed Apr. 13, 1965, as mentioned above. If desired, the electrons may be introduced as disclosed and claimed in the Herb and Pauly Patent No. 3,244,969, patented Apr. 5, 1966, as mentioned above.

To introduce the electrons, the illustrated pump 10 comprises one or more cathodes in the form of thermionic filaments 32 which are positioned between the central electrode 12 and the boundary electrode 14. Each filament 32 is substantially parallel to the central electrode 12 and is disposed toward one end thereof. Each filament 32 is adapted to be heated so that it will emit electrons. Thus, the opposite ends of each filament 32 are connected and supported by wires or leads 34 and 36 which are brought out of the casing by feed-through insulators 38 and 40 sealed into suitable openings in the end plate 22. In each case, the

leads

34 and 36 are connected to a source of electrical power, which is illustrated as the secondary winding 42 of a power transformer 44. The primary winding 46 of the transformer 44 may be connected to power lines 48 and 50, adapted to supply alternating current at 117 volts, or some other suitable voltage.

The filaments 32 are preferably biased to a relatively small positive voltage, compared with the positive voltage on the central electrode 12. In this case, the biasing voltage is supplied by the power supply 30, which has a low voltage terminal 52, connected to a center tap on the secondary winding 42 of the transformer 44. The biasing voltage may be widely varied, but may typically be in the neighborhood of 1000 volts.

To provide high efficiency in the orbiting of the electrons, the thermionie filaments 32 are shown as being of the construction disclosed and claimed in the copending Herb and Pauly application, Ser. No. 447,678, filed Apr. 13, 1965. Thus, each filament 32 is in the form of a thin ribbon or strip which is folded or bent into a U shape. Thus, each filament 32 has a pair of closely spaced parallel

elongated legs

60 and 62. In this case, the upper ends of the

legs

60 and 62 are welded or otherwise secured to the

leads

34 and 36, while the lower ends of the

legs

60 and 62 are integrally joined by a connecting portion 64 which is bent or folded into a semicylindrical shape.

Each filament 32 is preferably positioned so that the

legs

60 and 62 are directed edgewise toward the central electrode 12.

The ribbon filament 32 is preferably made of a metal or alloy having a high melting point, so that the filament may be heated to a temperature at which electrons will be copiously emitted from the filament. Thus, the ribbon filament may be made of iridium, platinum, tungsten, or the like. The filament may be thoriated to increase the emission of electrons.

Most of the electrons are emitted from the flat outer sur-faces of the

legs

60 and 62. These electrons are given considerable initial angular momentum about the central electrode 12 and thus are introduced into spiral orbits around the central electrode.

In accordance with the present invention, the inner or central electrode 12 preferably comprises a cylinder 68 of relatively large diameter, rather than a slender rod or wire, as employed heretofore. In the illustrated construction, the cylinder 68 is hollow and preferably is in the form of a thin walled cylindrical metal tube. Discs 70 and 72 are preferably mounted in the ends of the cylin drical tube 68. The upper disc 70 is apertured to receive the support rod 26 which is welded or otherwise secured to the disc. Similarly, the lower disc 72 is apertured to receive a supporting rod 74 which is welded or otherwise secured to the disc 72. An insulating support is provided for the rod 74. As shown, the rod 74 is welded or other- Wise secured to a plate 76 which is supported by a plurality of insulating rods or pillars 78. It will be seen that the pillars 78 are connected between the plate 76 and a disc 80. A plurality of arms 82 extend between the disc 80 and the casing 14 to form a supporting spider. The arms 82 provide a secure support for the lower end of the central electrode 12, while affording a minimum of resistance to the flow of gases between the vacuum system 16 and the casing 14.

To improve the orbiting of the electrons, a terminating electrode 84 is preferably provided around the supporting rod 74. As shown, the terminating electrode 84 is in the form of a cylindrical metal sleeve which is mounted on the disc 80 and extends upwardly therefrom, part way toward the enlarged cylinder 68. The terminating sleeve 84 is spaced outwardly from the supporting rod 74. It

will be noted that the sleeve 84 is grounded to the outer casing 14 through the disc 80 and the arms 82. The electric field between the terminating sleeve 84 and the supporting rod 74 promotes the reflection of the spiralling electrons at the lower end of the enlarged cylinder 68.

Similarly, a terminating electrode 86 is preferably provided around the upper supporting rod 26. The illustrated electrode 86 is in the form of a cylindrical metal sleeve which projects downwardly from the metal mounting disc 88. In this case, the disc 88 is connected to the end plate 22`by metal pillars 90, so that the terminating sleeve 86 is maintained as ground potential.

The provision of the enlarged central cylinder 68 greatly increases the capacitance between the central cylinder and the outer casing 14, so that a greatly increased space charge can be established and maintained therebetween. In this connection, it will be helpful to consider the following expression, which gives the capacltance of a cylindrical condenser:

C farads/crn.

In this expression, as applied to the orbiting electron vacuum pump of the present invention, r2 is the internal radius of the outer or boundary electrode, while r1 is the external radius of the inner or central electrode. In the prior orbiting electron pumps, the ratio of r2 to r1 has been l about 50` on the average, so that the logarithm of the ratio has been about 1.7. In accordance with the present invention, the radius of the inner or central electrode may be greatly increased, so as to decrease the ratio of r2 to r1 to about 3. The logarithm of the ratio is then about .48. Thus, the charge holding capacitance of the pump, per unit length, is increased by the ratio of 1.7 to .48, or by a factor of about 3.5. Thus, the density of the electron cloud between the inner and outer electrodes may be increased by a factor of about 3.5, so that much greater ion pumping will be produced. Moreover, with the construction of the present invention, the length of the pump may be increased substantially, because the enlarged central electrode is much stiffer than heretofore, and thus is capable of withstanding much greater electrostatic forces, due to the high voltage between the central electrode and the outer casing. The illustrated central cylinder 68 is supported at both ends and thus may be made quite long without becoming so flexible as to create a hazard that the electrostatic attraction due to the high voltage will cause the central cylinder to flex outwardly against the outer casing. The hazard of short circuits due to fiexing of the central rod or wire has been a limiting factor in prior orbiting electron vacuum pumps.

The illustrated orbiting electron 'pump 10 employs a plurality of filaments 32 which are spaced at equal angular intervals around the central electrode 12. However, a single filament would be sufficient for efficient operation of the pump. `lt is preferred to use a plurality of filaments, for the sake of increased emission of electrons. If desired, the filaments may be operated individually, or in any desired combination.

The filaments 32 are preferably biased to a positive potential which differs substantially from the potential which would exist in the pump at the location of the filaments, in the absence of the filaments. The positive bias prevents any substantial number of electrons from reaching the grounded container.

The orbiting electron vacuum pum'p 10 is preferably provided with means for vaporizing getter material within the pump, so that the getter material will condense on the boundary electrode or casing 14. The getter material is preferably titanium, but other suitable getter materials may be employed, as will be known to those skilled in the art. The getter material on the boundary electrode 14 combines chemically and physically with gas molecules, particularly those of the active gases, and thus effec- 6 tively removes the gas molecules from the vacuum system. In addition, the getter material retains the ions which are driven outwardly to the boundary electrode 14 by the electric field. The gas molecules and ions are buried by the continued condensation of getter material on the boundary electrode.

The getter material is preferably vaporized by electrical resistance heating of a generally cylindrical cage 100, generally in accordance with the invention disclosed and claimed in the co-pending Maliakal application, identified above. The cage is made at least in part of a getter material and is spaced inwardly from the casing 14. In accordance with the present invention, the cage 100 is preferably in the form of .a plurality of helical or annular convolutions or coils which are concentrically disposed between the central electrode or anode 12 and the boundary electrode or casing 14. In the embodiment shown in FIGS. 1-3, the cage 100 comprises a helical coil 101 having several turns or convolutions. Preferably, the coil 101 is made of titanium wire. The coil 101 is arranged so that the entire coil, or various portions thereof, may be heated by electrical resistance heating. Thus, as shown to best advantage in FIGS. l and 3, the coil 101 is preferably provided with a pair of end leads 102 and 103, connected to the ends of the coil, as well as a plurality of taps or intermediate leads 104, connected to various intermediate points along the coil 101. Eight such taps 104 are shown in the construction illustrated in FIGS. 1 and 3, but it will be understood that any desired number of taps may be employed. The end leads 102 and 103 and intermediate leads 104 may `be in the form of rods or wires which are welded or otherwise secured to the coil 101. Such rods are preferably made of tungsten, tantalum, or some other metal having a high melting point so that the rods will be able to withstand the high temperatures developed by the resistance heating of the coil 101. The leads or rods 102-104 are brought out of the casing 14 by feedthrough insulators 105, sealed into suitable openings in the end plate 22.

It will be understood that the rods 102-104 serves as supports for the coil 101. Thus, the coil 101 is supported at numerous points spaced along its length. Additional support and reinforcement for the coil 101 is preferably provided by a plurality of insulators 107 which are secured to the coil. The illustrated insulators 107 are in the form of insulating rods, made of ceramic or other suitable material. Four such rods 107 are shown in the construction of FIGS. l and 3. The rods 107 extend parallel to the axis of the coil 101 and are spaced at angular intervals around the periphery of the coil. The insulating rods 107 are preferably secured to the turns or convolutions of the coil 101. This may be done by wrapping tie wires 108 around the rods 107 and the convolutions of the coil 101.

The insulating rods 107 impart rigidity and strength to the coil 101. In this way, the coil 101 will be able to withstand the effects of heat and vibration, as well as the forces which are imposed upon the coil due to the high voltage between the coil and the central electrode or anode 12.

A power supply 109 is preferably provided to Cause electrical currents to flow along the cage or coil 101, or various portions thereof. As shown, the power supply 109 comprises a transformer 110 having primary and

secondary windings

111 and 112. By means of suitable switching connections 113, the entire coil 101, or various portions thereof, may be connected across the secondary winding 112. As shown, the switching connections or leads 113 are arranged to connect the ends of the secondary winding 112 to one end lead 102 and one of the taps 104 on the coil 101. However, the switching connections 113 may be changed, if desired, to connect the secondary winding 112 to the various portions of the coil 101 between the taps 104 and the end leads 102 and 103. The various portions of the coil 101 may be connected either in series or in parallel across the secondary winding 112. In this way, any desired portion of the coil 101, or the entire coil, may lbe heated, so that the getter material will be vaporized therefrom. Initially, when the pump is first put into operation, it may be desirable to heat the entire coil 101, or a large portion thereof, to provide a high rate of getter vaporization, so that the getter pumping rate will be high. After the pressure has been reduced to the working level, the rate of getter vaporization may be reduced to a level which will be sufficient to maintain the desired vacuum. Thus, the switching connections may be changed so that only one or more small portions of the coil 101 will be heated.

The power supply 109 also preferably comprises a cur rent regulating device, to vary the current which is supplied to the coil 101. As shown in FIG. l, the regulating device takes the form of a variable transformer 114 which is connected between the primary winding 111 of the transformer 110 and the power lines 48 and 50. It will be seen that the end terminals of the variable transformer 114 are connected directly to the power lines 48 and 50. The primary winding 111 may be connected between one end of the transformer 114 and a variable tap 120 thereon. An ammeter 122 may be connected in series with the primary winding 111 to measure the current supplied to the transformer 110.

As disclosed and claimed in the co-pending Maliakal application, identified above, the getter vapo-rizing cage 100 is also preferably employed to provide a measurement of the vacuum in the pump. The cage 100 is prefera-bly maintained at approximately the same potential as the casing or boundary electrode 14. As a result, positive ions are attracted to the cage 100. In the ar-rangement of FIG. 1, the resulting ion current is measured by connecting a sensitive microammeter 124 between the cage 100 and ground. As the vacuum is improved by the pump, the number of gas molecules available for ionization is decreased, with the result that the ion current decreases. Thus, the ion current registered by the meter 124 gives an indication of the residual pressure in the vacuum system.

Shield plates 125 may be provided on the various leads extending through the end plate 22, to prevent the getter material from condensing on the feed-through

insulators

28, 38, 40 and 105. In the absence of such shielding, the condensed getter material would tend to short-circuit the insulators. The plates 125 provide a shadowing effect so that the insulators behind the plates are kept free of the condensed getter material. Various other shielding arrangements may be p-rovided if desired.

As previously indicated, the ribbon filaments 32, as employed in the orbiting electron pump of FIGS. 1 and 2, are of the type disclosed and claimed in the copending Herb and Pauly application Ser. No. 447,678. If desired, the ribbon filaments may be replaced with any of the filament structures disclosed in the Herb and Pauly Patent No. 3,244,969. By way of example, FIG. 4 illustrates a modified construction in which the ribbon filaments are replaced with one or more filaments 132 of one of the types disclosed and claimed in the Herb and Pauly patent. The illustrated filament 132 is in the form of a straight length of wire which is substantially parallel to the axis of the central electrode 12 and is disposed toward one end thereof. The opposite ends of the filament 132 are connected to and supported by wires or leads 134 and 136, which correspond to the

wires

34 and 36 of FIG. 1.

To improve the efficiency with which electrons are introduced into orbits around the central electrode 12, the wire 136, constituting one of the supports for the filament 132, is preferably arranged so that a portion 138 thereof acts as a shield or field modifying electrode. The shield portion 138 is substantially parallel to the filament 132 and is interposed between the filament and the central electrode 12. Preferably, the filament 132 and the shield electrode 138 are in the same radial plane, which also includes the axis of the central electrode 12. However, the position of the filament 132 may depart to a considerable extent from the radial plane of the shield electrode 138 while still providing highly efiicient orbiting of the electrons. The shield electrode 136 largely prevents direct radial movement of the electrons between the filament 132 and the central electrode 12. In addition, the shield electrode 138 modifies the electric field in the neighborhood of the filament 132 so as to increase the initial angular momentum of the electrons emitted by the filament. While the radial alignment of the filament 132 and the shield electrode 133 is believed to produce the best conditions for the orbiting of electrons, it has been found that greater emission of electrons from the filament may be obtained if the position of the filament departs from radial alignment. Thus, some departure from radial alignment maybe desirable in some cases.

In the modified constructon of FIG. 4, the helical cage is replaced with a cage 200 comprising a plurality of I annular convolutions or rings 202. As before, the cage 200 is concentrically disposed between the central electrode or anode 12 and the outer electrode or casing 14. The rings 202 are made at least in part of a getter material. It is preferred to make the rings 202 of titanium.

The cage 200 is preferably arranged so that all or some of the rings 202 may be heated by electrical resistance heating. Thus, the arrangement is such that electrical currents may be caused to pass along some or all of the rings 202. For initial pumping at a high rate, all or most of the rings may be heated, so as to vaporize a large amount of titanium. For pumping on a maintenance basis, the connections may be changed so that only a few rings are heated. In this way, closer control may be exercised over the amount of getter material which is vaporized. Moreover, the supply of getter material may be conserved so that the effective life of the pump will be increased.

Thus, the rings 202 are preferably arranged in a plurality of groups, each of which comprises one or more rings. Each group of rings is provided with a pair of supporting rods or leads 208 and 210, which are brought out of the casing 14 by feed-through insulators 212 and 214, sealed into the appropriate openings in the end plate 22. The

lrods

208 and 210 extend generally parallel to the axis of the rings 202 and are welded or otherwise secured to the rings at `diametrically opposite points. The

rods

208 and 210 for the various groups of rings are preferably staggered at angular intervals around the rings.

The

rods

208 and 210 provide numerous supports for the cage 200. If desired, the longitudinal insulating rods 107 may be tied to the various rings of the cage, in the manner illustrated in FIG. l.

The modified pump of FIG. 4 may utilize the same power supply 109` as described in connection with FIG. l. The switching connections 113 may be arranged to supply current to any of the

leads

208 and 210. Thus, any of the various groups of rings 202 may be connected across the'secondary windings 111 of the transformer 110. The various groups of rings may be connected either in series or in parallel.

Both the helical coil 101 of FIG. l and the annular convolutions or rings 202 of FIG. 4 have the advantage that the vaporization of the getter material is distributed uniformly around the periphery of the getter vaporizing cage. The uniform peripheral Vapo-rization 'of the getter material -results in a uniform peripheral distribution of the condensed getter material on the 'inside of the casing 14. Such uniform peripheral distribution lis desirable, `because the peripheral distribution of the positive io-ns is also uniform.

The longitudinal distribution of the greater material may be changed from time 'to time by heating the various portions of 'the getter vaporizing cage. In this way, the lgetter vaporization may be closely controlled at -a reduced rate to conserve the getter material. All of the various portions of the -cage may be used from time to time, so that the gradual attrition of getter material from the cage will be substantially uniform along the l entire cage. In this way, the maximum service life will be obtained from the greater vaporizing cage.

It may be helpful to review the operation of the orbiting electron vacuum pump as shown in FIGS. 1 and 2. An initial vacuum is established in the vacuum system 16 and the interior of the casing 14 by the use of roughing pumps or forepumps. The orbiting electron pump is then energized so as to improve the vacuum by bringing about a great lreduction in the residual pressure in Ithe vacuum system. The ribbon filaments 32 are heated to `incandescence so as to emit electrons. Due to the construction and locat-ion of the ribbon filaments 32. the electrons are given an initial angular momentum about the central electrode 12. As a result, a large portion of electrons go into spiral orbits around the central electrode 12. Efiicient orbiting of the electrons is achieved, even though the central electrode 12 is of much greater diameter than employed heretofore. The efiicient orbiting of the electrons is due to a number of factions, including the provision of the relatively small rod 26 to support the large central electrode 12. When the central electrode is supported in this manner, the distribution of the electric field around the end of the central electrode, adjacent the ribbon filaments 32. is similar to that which would exist around the free end of a cylindrical electrode supported at only one end. This field distribution is conducive to the efficient orbiting of the electrons. The terminating sleeve 86 around the supporting rod 26 is also conducive to the efiicient orbiting of the electrons.

At `the opposite end of the large central electrode 12, the orbiting electrons are efficiently reflected by the electric field distribution which results from the provision of the small supporting rod 74, with its terminating sleeve 84. Here again, the distribution of the electric field is similar tc that which would exist at the free end of a cylinder supported at only one end.

The orbiting electrons have an extremely long mean `free path, and thus are able to achieve a high degree of ionization vin the residual gases within the pump. Moreover, fdue to the large diameter of the central electrode 12. lthe electrical capacitance between the inner and

outer electrodes

12 and 14 is greatly increased, so that a much larger number of electrons may be maintained in orbits around the central electrode. The electrical capacitance acts as a limiting factor which restricts the space charge which can be maintained between the inner and

outer electrodes

12 and 14. The increase in the number of orbiting electrons brings about a corresponding increase in the ionization of the gas molecules, so that the ion pumping speed of the pump is greatly increased. This is an extremely important factor in achieving a high pumping speed for argon and other noble gases, which are not susceptible to gettering by chemical action, but are susceptible to ion pumping.

The positive ions generated by collision of the orbiting electrons with gas molecules are propelled outwardly against the casing 14 by the electric field. The ions tend to stick to the outer casing so as to be removed from the vacuum system.

The vaporization of the titanium from the heated cage 100 produces a film of condensed titanium on the inside of the outer casing 14. This layer of getter material provides a direct gettering action upon the molecules of the more active gases, and also provides better sticking or retention of the ions driven to the outer casing by the electric field. Preferably, the titanium is continuously vaporized from the cage 100 and thus is continuously condensed on the outer casing 14. The freshly condensed titanium buries the ions and gas molecules so that they will be securely retained on the outer casing. In this way, the gas molecules and ions are effectively removed from the vacuum system.

The entire getter vaporizing cage 100, or various portions thereof, are heated by causing an electrical current 10 to pass therethrough. By changing the switching connections 113, the current may be `supplied to any of the numerous taps 104 on the helical coil 101 of the cage 100. The current may be regulated by adjusting the position of the tap 120 on the variable transformer 114. The temperature of the cage is kept below the melting point of titanium, but high enough to cause substantial vaporization of the titanium by sublimation, directly between the solid and vapor states.

When the orbiting electron pump 10` is first put into operation, and the pressure in the vacuum system is still relatively high, a high rate of getter vaporization is normally employed, so` as to achieve a high rate of getter pumping. Thus, the switching connections 113 may be arranged initially so that the entire coil 101, or a large portion thereof, is heated. The high rate of getter vaporization causes a rapid reduction in the pressure.

At lower pressures, the rate of getter vaporization may normally be reduced. At such lower pressures, ion pumping becomes a more important factor in reducing the pressure. Thus, the switching connections 113 may be changed so that only a small portion of the coil 101 is heated at any particular time. All of the various portions of the coil 161 may be used from time to time to distribute the attrition of getter material along the entire coil. The reduction in the rate of getter vaporization conserves the supply of getter material so that the effective life of the pump is increased.

Similarly, in the modified construction of FIG. 4, the switching connections 113 may be changed from time to time to cause heating of various portions of the cage 200. The rate of getter Vaporization may be varied in accordance with the need for getter pumping.

Some of the positively charged gas ions are attracted to the getter cage, which is at ground potential and thus is negatively charged relative to the central electrode 12. The impact of these ions upon the getter cage produces sputtering of the getter material. The resulting vapor of the getter -material is condensed on the inside of the casing 14.

The positive ions which are attracted to the getter cage produce an ion current to groun-d through the meter 124. This current is a measure of the pressure within the pump, because the amount of ionization produced in the pump is decreased as the pressure decreases. Thus, the meter 124 produces a continuous indication of the pressure in the vacuum system.

Because of the large diameter of the central electrode 12, the pump may be made especially long before any short-circuit hazard is encountered. The large diameter central electrode is especially stiff and strong and thus is able to resist the electrostatic forces due to the high voltage between the central electrode and the outer casing. Such electrostatic forces ltend to attract the central electrode toward the outer casing. In prior orbiting electron vacuum pumps with small diameter central electrodes, the

flexibility of the central electrode was a limiting factor,

because the central electrode would tend to flex outwardly until a short circuit was produced between the central electrode and the outer casing.

Due to the strength and rigidity of the large diameter central electrode, the orbiting electron pumps of the present invention may be mounted in any position, horizontally as well as vertically. The large diameter central electrode is so stiff that the fiexing effect of gravity is insignicant.

The large diameter of the inner or central electrode 12 is illustrated in FIG. l, and also in FIG. 4, in which the centr-al electrode has an even larger diameter than in FIG. 1. The provision of the large diameter central electrode greatly increases the electrical capacitance between the central electrode and the outer or boundary electrode 14, so that fa much larger number of electrons can be kept in orbits around the central electrode. The diameter of the central electrode should be of the same order of magnitude as the diameter of the boundary electrode. In the construction of FIG. 4, the ratio between the-diameter of the outer electrode 14 and the diameter of the central electrode 12 is three or less, which produces a great increase in the electrical capacitance and a corresponding increase in the ion pumpingy capacity of the pump.

The getter vaporizing cage of FIG. 1 comprises helical elements which encircle the inner electrode or anode. In the modilied construction of FIG. 4, the encircling elements are in the form of annular members or rings. In each case, the cage is subdivided into `a plurality of units which may be heated separately. The encircling elements are distributed among the -various units. When the encircling elements are heated, the vaporized getter material is projected outwardly in all directions so that it is deposited uniformly around the axis of the boundary electrode.

The encircling elements of the getter vaporizing cage may be made of solid material, such as wire, rod stock, or sheet material. In addition, the encircling elements may be made of mesh, screen, or expanded metal, formed into a honeycomb pattern or the like.

Various other modications, alternative constructions, and equivalents may be employed without departing from the true spirit and scope of the invention, as exempliied in the foregoing description and defined in the following claims.

I claim:

1. In an ion vacuum pump,

the combination comprising a hollow generally cylindrical boundary electrode,

means forming an enclosure for connecting the interior of said boundary electrode to a vacuum system to be evacuated,

an inner electrode disposed centrally in said boundary electrode,

means for impressing a positive voltage between said inner and boundary electrodes to produce a generally cylindrical eld therebetween,

and means for introducing electrons into said field between said inner and boundary electrodes with initial angular momentum so that the electrons will travel in spiral orbits around said inner electrode,

said inner electrode comprising a substantially cylindrical member of relatively large diameter,

at least one end of said cylindrical member having a generally axial supporting member of relatively small diameter connected thereto whereby the electric field at said end of said cylindrical member is favorable for eHicient orbiting of electrons.

2. A combination according to claim 1,

in which said means for introducing the electrons are disposed adjacent said one end of said cylindrical member.

3. A combination according to claim 1,

in which said supporting member constitutes the sole support for said one end of said cylindrical member.

4. A combination according to claim 1,

comprising a pair of generally axial supporting members of relatively small diameter connected to and lsaupporting the opposite ends of said cylindrical memer. 5. A combination according to claim 1, in which said cylindrical member is in the form of a hollow metal tube with at least one end wall thereon,

and in which said supporting member is in the form of a rod of relatively small diameter connected to said end wall.

6. A combination according to claim 1,

comprising a terminating sleeve mounted around and spaced outwardly from said supporting member, said terminating sleeve being spaced from said one end `of said cylindrical member and being of a substantially smaller diameter `than said cylindrical member.

7. A combination according to claim 1,

comprising a cage disposed between said inner and boundary electrodes and including members encircling said inner electrode,

said members being made at least in part of getter material,

land means for passing electrical heating currents along said encircling members to cause vaporization of said getter material therefrom.

8. A combination according to claim 7,

in which said cage is subdivided into a plurality of separate units, i

said encircling members being distributed among said units,

and means for separately energizing said units with electrical heating currents.

9. In an ion vacuum pump,

the combination comprising a hollow generally cylindrical boundary electrode,

a generally cylindrical inner electrode disposed centrally in -said boundary electrode,

means for impressing a positive voltage between said inner and boundary electrodes to produce a generally cylindrical electric eld therebetween,

and means including a thermionic filament for introducing electrons into said field between said inner and boundary electrodes with initial angular momentum so that the electrons will travel in spiral orbits around said inner electrode,

at least one end of said inner electrode Ahaving a `generally axial supporting member of substantially reduced diameter connected thereto whereby the distribution of the electric field in the neighborhood of said one end is favorable for efficient orbiting of electrons.

10. A combination according to claim 9,

in which said filament comprises a liat ribbon disposed generally edgewise toward said inner electrode.

11. A combination according to claim 9,

in which said filament comprises a wire generally parallel to the axis of said inner electrode,

and a shield wire disposed between said iilament wire and said inner electrode.

12. A combination according to claim 9,

in which said filament is disposed adjacent said one end of said inner electrode.

13. A combination according to claim 9,

including a pair of supporting members of substantially reduced diameter connected axially to the opposite ends of said inner electrode.

14. A combination according to claim 9,

including an annular terminating electrode disposed aroundand spaced outwardly from said supporting member.

15. In an ion vacuum pump,

the combination comprising a hollow generally cylindrical boundary electrode,

a generally cylindrical inner electrode disposed centrally in said boundary electrode,

means for impressing a positive voltage between said inner and boundary electrode-s to produce a generally cylindrical electric `field therebetween,

said inner electrode having a relatively large diameter to produce a high electrical capacitance between said inner and boundary electrodes so as to increase the number of electrons which can be maintained in orbits between said inner and boundary electrodes.

16u A combination according to claim 15,

in which the diameter of said inner electrode is so large that the ratio between the diameters of said boundary electrode and said inner electrode is three or less.

17. A combination according to claim 15,

in which said means for introducing electrons comprise a thermionic filament disposed generally parallel to said inner electrode and toward one end thereof.

v 18, In an ion getter vacuum pump,

the combination comprising an inner generally cylindrical electrode,

a hollow boundary electrode of larger diameter spaced outwardly from said inner electrode,

means for introducing electrons into said ield between said inner and boundary electrodes with initial angular momentum so that the electrons will travel in spiral orbits around said inner electrode,

a cage disposed between said inner and boundary electrodes,

said cage being made at least in part of getter material,

said cage comprising members encircling said inner electrode,

and means for passing electrical currents along said encircling members to heat said cage so as to cause vaporization of said getter material.

19. A combination according to claim 18,

in which said cage comprises a plurality of helical convolutions encircling said inner electrode.

20. A combination according to claim 18,

in which said cage comprises a plurality of annular members encircling said inner electrode.

21. A combination according to claim 18,

in which ysaid cage is subdivided into a plurality of separate units,

each of said units comprising at least one of said encircling members,

22. A combination according to claim 15,

in which the diameter of said inner electrode is less than but of the same order of magnitude as the diameter of said boundary electrode.

References Cited UNITED STATES PATENTS ROBERT M. WALKER, Primary Examiner.

Claims

1. IN AN ION VACUUM PUMP, THE COMBINATION COMPRISING A HOLLOW GENERALLY CYLINDRICAL BOUNDARY ELECTRODE, MEANS FORMING AN ENCLOSURE FOR CONNECTING THE INTERIOR OF SAID BOUNDARY ELECTRODE TO A VACUUM SYSTEM TO BE EVACUATED, AN INNER ELECTRODE DISPOSED CENTRALLY IN SAID BOUNDARY ELECTRODE, MEANS FOR IMPRESSING A POSITIVE VOLTAGE BETWEEN SAID INNER AND BOUNDARY ELECTRODES TO PRODUCE A GENERALLY CYLINDRICAL FIELD THEREBETWEEN, AND MEANS FOR INTRODUCING ELECTRONS INTO SAID FIELD BETWEEN SAID INNER AND BOUNDARY ELECTRODES WITH INITIAL ANGULAR MOMENTUM SO THAT THE ELECTRONS WILL TRAVEL IN SPIRAL ORBITS AROUND SAID INNER ELETRODE, SAID INNER ELECTRODE COMPRISING A SUBSTANTIALLY CYLINDRICAL MEMBER OF RELATIVELY LARGER DIAMETER, AT LEAST ONE END OF SAID CYLINDRICAL MEMBER HAVING A GENERALLY AXIAL SUPPORTING MEMBER OF RELATIVELY SMALL DIAMETER CONNECTED THERETO WHEREBY THE ELECTRIC FIELD AT SAID END OF SAID CYLINDRICAL MEMBER IS FAVORABLE FOR EFFICIENT ORBITING OF ELECTRONS.