US3495769A - Space charge controlled vacuum pump - Google Patents

Description

Feb. 17, 1970 R. HIRSCH 3,495,169

SPACE CHARGE CONTROLLED VACUUM PUMP Filed March 12, 19 68 v 2 Sheets-Sheet l Fla! 6A5 FLOW Q l 9 22 l I FJBCTQAZBEJT TL 5-IO) .V 26 n4 4 l I 3/0 I I 5 l2 25 I /5 Ila, 3 1 Z TOVACUUM PUMP INVENTOR.

BYM,MQ\WL ATTORNEYS Feb. 17, 1970 R. L. HIRSCH 3,495,759

SPACE CHARGE CONTROLLED VACUUM PUMP Filed March 12, 1968 r 2 Sheets-Sheet 2 F'ILAPI'IENT $UPPLY INVENTOE. ROBERT L HnescH,

T0 was Pump ATTORNEYS United States Patent O US. Cl. 230-69 5 Claims ABSTRACT OF THE DISCLOSURE The pump of this invention is of generally spherical geometry, having essentially two, hollow spherical shells for electrodes, one being concentrically mounted inside and spaced in relation to the other. A potential is applied to these electrodes in such polarity as to make the inner one positive with respect to the outer one. The inner electrode is open to the flow of gas therethrough and defines an inner volume centrally located with respect to both electrodes. A thermionic cathode is mounted in the space between the two electrodes and emits electrons which are directed radially inwardly toward the center of the aforesaid volume. By reason of the spherical geometry, electrons themselves and ions produced by collision thereof with neutral molecules form an electrical discharge composed of high-order magnitude electron and ion currents in the aforesaid volume as Well as electron currents in the interelectrode space between the two electrodes. One or the other of these electrodes is formed of titanium which will either sublime or sputter as a consequence of bombardment by either ions or electrons or both. The vaporized titanium is caused to deposit on the other electrode thereby providing a surface which can getter both ionized and neutral molecules. The pumping rate can be controlled by varying the magnitude of the circulatory electron current thereby correspondingly varying the rate of ionization, the greater the ionization, the greater being the rate of pumping.

BACKGROUND OF THE INVENTION Field of the invention This invention relates generally to vacuum pumps and more particularly to vacuum pumps which utilize circulatory electron currents to ionize the gas which is being evacuated, the ionized gas being collected by a gettering material which is constantly being restored so as to present a fresh, uncontaminated surface for providing maximum gettering ability.

Description of the prior art One prior art vacuum pump bearing some resemblance to the present invention employs a tungsten rod which supports titanium cylinders of larger diameter. This assembly is mounted coaxially within a cylindrical envelope, this envelope and the rod assembly serving as electrodes to which a potential is connected. The rod assembly is made positive with respect to the envelope. A cathode is disposed within the space between the two electrodes such that electrons emitted therefrom may be directed either radially inwardly toward the rod assembly or into orbiting paths thereabout, depending upon the direction in which the electrons are initially emitted. Eventually, all of the electrons are collected by the titanium cylinders which are thereby heated to sublima tion temperatures. This results in coating the walls of the envelope with titanium, this coating collecting the gases within the envelope.

The electrons in their transit to the titanium ionize a number of the gas molecules, the ions being thereby attracted toward and buried in the walls of the envelope, escape of the buried ions being prevented by the contin- 3,495,769 Patented Feb. 17, 1970 uously subliming titanium. Pumping speed is limited by the ionization efiiciency of the electron current, this being limited by the fact that all of the electrons have a definite terminus on the rod assembly, some of these electrons proceeding without orbiting, directly to the rod assembly and the remaining orbiting thereabout a number of times until they are collected. A second limitation in the use of this particular pump resides in the fact that the titanium on the rod assembly actually is consumed, there-by requiring dismantling of the pump and replacement of the rod assembly.

SUMMARY OF THE INVENTION In accordance with the broader aspects of this invention, there is provided a vacuum pump which includes positive and negative potential electrodes, one of the electrodes being mounted symmetrically inside the other. The positive electrode is hollow and defines a volume centrally located with respect to both electrodes. The volume is free of tangible structure, and the positive electrode is open to the flow of charged particles therethrough. Means are provided for app-lying a potential to the electrodes, for establishing an electric field therebetween, the inner electrode being positive with respect to the outer one. An electron emissive element is mounted in the interspace between the two electrodes and this element in combination with the electrodes produce a circulatory current composed of electrons which follow paths intersecting in the central region of the aforesaid volume. One of the electrodes is formed of a gettering material, such as titanium, which vaporizes upon being heated on upon being bombarded by changed particles. The circulatory electron current ionizes neutral gas in the interspace, the ions being It is an object of this invention to provide a vacuum pump in which the gas to be pumped is ionized and then collected, the ionization occurring in an electrostatically confined space charge.

It is another object of this invention to provide a vacuum pump in which the rate of pumping may be controlled by varying the magnitude of a circulatoryelectron current.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following descrip tion of an embodiment of the invention taken in com junction with the accompany drawings, wherein:

FIG. 1 is a cross-section of an embodiment of this invention with certain circuit connections being shown in diagrammatic form;

FIG. 2 is a side view of the innermost electrode of the embodiment of FIG. 1; and

FIG. 3 is a diagrammatic illustration of another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Involved in this invention is a nonmagnetic method for the confinement of ionized gases. Structures and theories nvolved in achieving this confinement are disclosed in arnsworth Patent No. 3,258,402, issued on June 28, v966. Generally speaking, the appartuses of this prior patnt as well as the structures disclosed herein utilize a pherical geometry in which two generally spherical elecrodes, one (the anode) having a positive potential aptlied thereto and the other (the cathode) a negative poential, are concentrically positioned with one inside the )ther. The inner electrode is permeable to the flow of ttomic particles, while the outer one is not and pref- :rably constitutes a hermetic envelope which may be evaciated. In operation, an electrical discharge, composed of L high-order magnitude electron current in the space en- 'eloped by the inner electrode, produces a radial poential distribution, which is, generally speaking a mininum at a radius adjacent to the center of the space and a naximum at the anode.

Developed at the radius of the potential minimum is a pherical, virtual cathode.

Referring now more particularly to FIGS. 1 and 2, here is illustrated a spherically shaped structure in which he outer member is a hollow, spherical cathode shell 1 vhich serves as a hermetically sealed envelope. Concenrically disposed inside the shell 1 is another spherical :lectrode 2 having a spherically shaped hollow cavity 3, 1 post assembly 4 serving to mount rigidly the electrode 2 in position. The post assembly 4 is composed of an elongated insulator 5 which is securely fastened to the electrode 2, as shown.

A ring assembly indicated generally by the numeral 5 mounts the post assembly 4 on the shell 1 in rigid, iermetically sealed relationship, the ring assembly tightly ;urrounding the insulator 5 and fitting into an opening in ;he shell 1. Extending coaxially through the insulator 5 s a rod-shaped conductor 7 having an enlargement 8 on :he inner end thereof intimately, conductively secured to :he electrode 2. The conductor 7 is connected at its outer :nd to the positive terminal of a suitable power supply represented by the battery 9.

The electrode 2 is shown also in FIG. 2 and in one form is fabricated of metal such as stainless steel or titanium to 1 spherical shape, as will be explained more fully later an, to have a concentric spherical cavity 3 therein. This electrode 2 is of open construction and has a plurality of :apered openings or passages, as indicated by the numeral 10. Other shapes for the passages 10 may be used without departing from the spirit and scope of this invention. Also, other structures for electrode 2, as will be explained later may be used.

The electrode 2 preferably is composed of two sets of arcuate, flat, metal stampings 11a and 11b suitably rigidly :onductively secured together by welding or the like. The first set of stampings 11a are welded along a common diameter of the sphere and correspond to intersecting annular discs. The second set of stampings 11b are welded together along a common diameter and also correspond to intersecting annular discs. The stampings 11a and 11b are also appropriately slotted to fit one to the other and welded where they intersect.

The axes of the various openings 10 lie along radii of the sphere mentioned hereinabove and intersect the center 12 of the cavity 3. This is also the center of the shell 1. The openings 10 are arranged in diametric pairs, each pair having a common axis which coincides wit-h the shell diameter. A thermionic filament assembly indicated by the numeral 13 is mounted on the shell 1 with filaments 14 and 15 supported thereby extending into the space between the electrodes 1 and 2 as shown. The assembly 13 includes suitable metal flanges and insulators so arranged as to mount rigidly the thermionic filaments 14 and 15 in position. Outer terminals 16 and 17 of the filaments 14 and 15 are adapted to be connected to a suitable source 18 of voltage for heating the filaments 14 and 15. Preferably, the filaments 14 and 15 are aligned on the axes of the openings 10 so as to minimize electron interception by the anode 2.

An intake assembly 19 is fitted to the shell 1 as shown and includes a short conduit portion 20 having a ring element 21 coaxially fitted therein and a baffle plate 22 mounted on but in spaced relation from the ring 21 so as to overlie the openings 23 in the latter.

Incoming gas flows through the opening 23 and around the plate 22 into the interior of the shell 1. Another conduit portion 24 is mounted on the shell 1 for the purpose of providing a connection to a so-called rough pump useful in reducing the pressure inside the shell 1 to a value in the vicinity of l0 torr.

In one embodiment of this invention, the inner electrode 2 is formed of titanium or of some other suitable gettering material having the same physical and chemical characteristics of titanium, while the shell 1 is formed of stainless steel or a suitable similar material.

In operation, the tubular portion 20 is connected to the vessel to be evacuated while the other conduit 24 is connected to a vacuum pump as explained previously capable of reducing the pressure inside the shell 1 to a value of about 10- torr. Gas molecules from the vessel to be pumped pass through the conduit portion 20 and into the shell 1, some of these gas molecules in neutral form being collected by the titanium of the element 2. However, it is possible for the exposed surface areas of the electrode 2 to soon become contaminated such that no further gas molecules will be collected. To overcome this problem, fresh, uncontaminated titanium surface is, in this invention, continuously being presented by applying a potential to the two electrodes 1 and 2, as shown. Electrons emitted by the filaments 14 and 15 will fio-w toward the positive electrode 2, and eventually be intercepted by the latter. This results in heating of the electrode 2 causing the titanium to sublime and to deposit on the inner surface of the shell 1. This electron current also ionizes the gas which is being pumped. The positive ions so formed in the space between the electrodes 1 and 2 are driven toward element 1 and become embedded therein. Subliming titanium from the electrode 2 getters and covers these ions, thereby preventing them from being re-emitted. Neutral molecules are also collected by the inner surface of the shell 1 inasmuch as the subliming titanium is being constantly deposited thereon providing a fresh titanium surface having maximum gettering efficiency. Such collected neutral molecules are also buried by subsequent titanium coatings. This process of gettering is continuous and results in reducing the pressure inside the shell 1 to, for example, 10 to 10* torr.

Of significance is the fact that the pumping rate is extremely high, this being due to the fact that the circulatory electron currents continuously maintained are of high order magnitude thereby resulting in a maximum degree of ionization of the background gas within the shell 1. The concept of this space current is explained in Farmworth Patent No. 3,258,402, but is briefly treated in the following. Initially, with the background pressure inside the shell 1 reduced to a value of about 1() torr, and a potential applied to the two electrodes 1 and 2, electrons from the filaments 14 and 15 are directed radially inwardly toward the electrode 2. A certain number of such electrons will follow precisely radial paths and will pass through the openings 10 toward the center 12. Initially, a certain number of these electrons will travel in a straight line onwardly through the diametrically opposite opening 10 and will approach negative shell 1 until it is turned back by the repelling force thereof. On its return trip, assuming that the electron will undergo no scattering, it will retrace its previous transit passing through the center 12 and continuing onwardly toward the negative shell 1 until it once more is turned backwardly.

As the number of these electrons increase, the interaction therebetween results in the development of a nega tive space charge potential well about the center 12 inside the cavity 3. This negative space charge well is in the form of a spherical sheath and the potential of this well 25 can approach that of shell 1. Thus, electrons, filling the interelectrode space between the electrodes 1 and 2, following radial paths indicated by the numeral 26 will approach the negative well 25 and be repelled thereby along essentially the same radial paths until they are again repelled by the potential on the negative shell 1. Thus, such electrons will repeat these trips a large number of times until they eventually collide with and ionize neutral gas molecules or are intercepted by the anode 2. However, as explained previously, this interception is a useful event inasmuch as the electrode 2 is heated resulting in the sublimation of titanium therefrom. The circulatory electron currents just described build to large values thereby resulting in high-order ionization probabilities of the background gas within the shell 1. Electrons in the space charge are not only derived from the filaments 14 and 15, but also from the ionization process. If greater pumping rates are desired, it is only necessary to increase the electron output from the filaments 14 and 15 by increasing the filament voltage 18.

In an alternative embodiment of this invention, the shell 1 may be made of titanium and the shell 2 of tantalum or stainless steel. As before, ions formed in the interelectrode space are driven toward and will impact the shell 1 with sufficient energy to cause sputtering of the titanium therefrom which deposits on the surfaces of the electrode 2. Since this process is continuous, the exposed titanium surface on the electrode 2 will undergo continuous restoration, thereby presenting a clean surface for gettering of the gas which is being pumped, Similarly, because of the sputtering of the titanium from the interior of the shell 1, this surface will be cleaned constantly and presents maximum collection efliciency. Ions created in the interelectrode space become embedded deeply in the shell 1 and neutral molecules are gettered by the electrode 2 and buried by the subsequent deposits of fresh titanium thereon.

A slightly different embodiment is shown in FIG. 3, wherein like numerals indicate like parts. A primary difference between this arrangement and that of FIG. 1 is that the inner electrode 2 is formed of a titanium screen instead of arcuate fins as in FIG. 2. The titanium sphere 2a is supported inside the shell 1 by means of a conductor 28 which connects to the electrode 2a. The power supply 9 is connected to the two electrodes 1 and 2a the same as in the case of FIG. 1. The shell 1 in this embodiment of FIG. 3 is preferably formed of titanium.

In operation, the circulatory electron currents traverse generally radial paths as previously explained, a certain number of the electrons being collected by the screen 211 causing the heating thereof and sublimation of titanium onto shell 1. This titanium coats the interior of the shell 1 the same as previously explained, resulting in the re moval of gas from the atmosphere inside shell 1. Also, ions buried in shell 1 are gettered and buried as described above.

Titanium will readily combine with hydrogen, nitrogen, oxygen, carbon monoxide, carbon dioxide, and other active gases. The invention produces large magnitude, circulatory electron currents by electrostatic techniques. These currents result in high ionization probabilit in the interelectrode space. Thus, the vacuum pump of this invention is capable of high pumping rates without the use of magnets utilized in many prior art devices. While titanium has been specified as the gettering material,

other materials such as barium may be used without departing from the spirit and scope of this invention.

While the phenomena of sputtering and subliming are used in the different embodiments of this invention, in both instances the gettering material is caused to vaporize and deposit on large surface electrodes within the pump, thereby resulting in surfaces being continuously renewed and restored so as to provide for maximum gettering efficiency.

While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

1. A vacuum pump comprising positive and negative potential electrode means, said negative electrode means being a hollow outer metallic member having a generally spherical shape, said positive electrode means being a hollow inner member having generally spherical inner and outer surfaces, said inner member having a multiplicity of openings for the passage of electrons therethrough, said outer member concentrically surrounding and spaced from said inner member, said inner member defining a volume centrally located with respect to both of said members, said volume being free of tangible structure, a source of gaseous particles connected to said outer member, means for applying potential to said positive and negative members for establishing an electric field therebetween, means for emitting electrons within the interspace between said members to ionize said gaseous particles and traverse paths intersecting in the central region of said volume, the electrons: bombarding said positive electrode means and ions bombarding said negative electrode means, one of said members being of a gettering material which vaporizes and coats the other member upon said bombardment, said potential being of a magnitude as will impart sufiicient energy to said ions to bury them deeply in said outer member.

2. The vacuum pump of claim 1 in which said outer member is a hermetic enclosure, said electron emitting means being an electrically heated filament mounted on said outer member, said outer member having an intake port connected to said gaseous source for evacuation thereof.

3. The vacuum pump of claim 2 in which said inner member is of metallic screen material.

4. The vacuum pump of claim 2 in which the openings in said inner member are tapered with the axes thereof intersecting the center of said volume, the widest portions of said openings being in the outer surface of said inner member, said openings in combination with said inner and outer members serving as lens which focuses said electrons toward said center.

5. The vacuum pump of claim 4 in which said gettering material is titanium.

References Cited UNITED STATES PATENTS 3,177,672 4/1965 Seelandt 230-69 XR 3,207,423 9/1965 Huber et al. 230-69 3,244,969 4/1966 Herb et al. 32433 3,338,507 8/ 1967 McKeever 230-69 3,339,106 8/1967 Redhead 315108 ROBERT M. WALKER, Primary Examiner

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