US3357634A - Orbiting electron vacuum device and anode-getter apparatus therefor - Google Patents

Description

Dec. 12, 1967 J. c. MALIAKAL I 3,357,634

ORBITING ELECTRON VACUUM DEVICE AND ANODE-GETTER APPARATUS THEREFOR Filed Feb. 28, 1966 United States Patent 3,357,634 ORBITING ELECTRON VACUUM DEVICE AND ANODE-GETTER APPARATUS THEREFOR Joseph C. Maliakal, Newton, Mass, assignor to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed Feb. 28, 1966, Ser. No. 530,629 Claims. (Cl. 230-69) ABSTRACT OF THE DISCLOSURE An orbiting electron device of the type described in US. Patent 3,244,969 with the addition of a cap over the getter slugs mounted on the anode rod, the cap being of a material having a lower vapor pressure than the getter slugs. In this arrangement the cap absorbs electrons in a manner to automatically limit sublimitation from the getter slugs at low pressure thus providing a desirable conservation of getter material.

This application is a continuation-in-part of SN. 496,- 645, filed Oct. 15, 1965 and now abandoned.

The present invention relates generally togetter ion vacuum pumps and particularly to orbiting electron vacuum pumps (orbitron pumps).

Orbitron pumps, developed at the University of Wisconsin, are well known in the art. These pumps provide a high speed, contaminant free, electrical and chemical pumping action which is used to advantage in various laboratory and industrial apparatus such as coaters, altitude simulators and furnaces. One problem which obtains in prior art pumps is that the getter material, usually titanium, streams out through the entrance of the pump in a manner analogous to oil backstreaming in diffusion pumps. If the titanium reaches the vacuum system being pumped, it may interfere with the experimental or industrial process in progress. Also, this stray titanium condenses on walls of the vacuum system and piping and provides extraneous chemical pumping which is not desirable because its rate is not predictable and because it is not reliable the gasses so pumped and the titanium itself will be desorbed when their temperatures rise.

It is possible to reducethis titanium backstreaming by providing a long or bent piping between the pump and vacuum system or by providing optically tight baffles atthe pump inlet. But these techniques reduce the effective speed which the pump exerts on the system.

It is also possible to confine the sources of titanium to the end of the pump away from the inlet. But this leads to poor coverage of titanium on the wall of the pump because of shadowing effects due to microscopic roughness of the wall. As pointed out in the paper of Arden, Herb, Limon and Maliakal, Orbitron Pump of 30-cm. Diameter, Journal of Vacuum. Science and Technology, 1: 54, 58, 59 (December 1964), a good distribution of titanium sources along the length of the pump is necessary for efficient chemical pumping.

It is therefore the object of the present invention to provide a way of reducing the titanium backstreaming from the pump without significant loss of pumping speed except when desired and as desired.

It is a further object of the invention to provide a pump with self-contained backstreaming reduction means avoiding the need for external backstreaming reduction accesso-ries.

I have discovered that the principal source of titanium backstreaming is the end cylinder of titanium and particularly the face of that cylinder facing the inlet. All other sources of backstreaming are negligible in comparison to H this. I eliminate this source of backstreaming by placing 3,357,634 Patented Dec. 12, 196'? a Washer of lower vapor pressure material (tantalum or molybdenum) over the troublesome cylinder face. The washer has substantially the same diameter as the initial diameter of the cylinder face. Operation of the pump can proceed in the normal manner. The washer has no adverse effect on the establishment of electric field necessary for pump operation. Pumping speed within the pump itself sulfers no significant reduction; the washer does not interfere with the useful flow of titanium vapors from the end cylinder.

Another and more important aspect of the operation of orbitron pumps is that once a pump gets into the low pressure range (e.g. on the order of 10- Torr), then the sublimation of titanium is not required in large amounts (compared to the amount required while operating at, say 10- Torr). It would be desirable to have a way of cutting back on the sublimation rate without reducing the electron emission from the pump filament. In my Work with the above-described washers, I have discovered that the above washer can be sized and operated at low pressure to absorb many electrons that would otherwise strike the titanium sources. Yet, at higher pressures, the electrons strike the titanium sources to provide faster sublimation rates. Thus, I provide a self contained means within the pump to prevent wasteful sublimation, thereby extending the life of the titanium sources. And I provide this at minimal additional cost, without the need for:

complex accessories such as pressure gauges, amplifiers, etc.

It is therefore another and principal object of this invention to provide a way of extending titanium source lifetime without significant loss of inert gas pumping speed at higher pressures.

It is a further object of the invention to provide a pump with a self-contained titanium source lifetime extending means avoiding the need for external titanium-sublimation-rate-control accessories.

It is a further object of the invention related to the foregoing objectives to provide a pump with means for extending titanium source lifetime without the need for reducing power input to the pump.

The invention accordingly comprises the improved getter ion pump incorporating the above-described backstreaming reduction and sublimation control means and being constructed and arranged to utilize such means to advantage.

The principal advantages of the invention flow from realization of the above objects, through a reliable and inexpensive structural addition. Additionally, regarding the backstreaming reduction feature, my invention makes practical the direct mating of the pump inlet to the exit of the vacuum chamber to be evacuated, with no intervening piping, or a minimum of such piping, as desired.

Other objects, features, and advantages will in part be obvious and will in part appear hereinafter from a reading of the following specific description of my invention according to my best known mode of utilization and incorporating and having reference to the accompanying drawings wherein:

FIG. 1 is a partly sectional view of the pump prior to operation;

FIG. 2 is a sectional view of a portion of the anode getter after prolonged operation of the pump; and

FIG. 3 is a sectional view of a portion of the anodegetter of a second embodiment of the invention prior to substantial erosion of the titanium.

Referring to FIG. 1, the orbitron pump 10 comp-rises a pump body 12 traced with cooling coils 14 and capped by a header flange 16 and having an inlet opening 18 with a flange 20. The inlet is secured to the exit flange 22 of a vacuum system 24 such as a bell jar coater. Contained within the pump are a straight anode rod 26 (tungsten, .06 inch diameter), with four slugs 28A-28D of titanium mounted thereon (commercially pure, inch diameter) and a filament 30. Omitted from the drawing are such accessories and details as additional filaments, filament shielding arrangements, reflection shield and termination tube, all of which can be as disclosed in the above-cited paper of Arden et al.

The four titanium slugs are distributed along the pump to minimize microscopic shadowing along the wall. In.

the embodiment shown in FIG. 1, the total length of the rod 26 is 11 and inches and it is arranged to terminate at a distance y from the inlet which is equal to the pump radius R (2 inches here). The topmost titanium. slug 28A is located a distance x from the end of the rod which is preferably one inch. Springs 32 hold the cylinders in place. Mounted over the cylinder 28A is a tantalum washer inch diameter, inch thick) having a center hole accommodating the center rod 26.

In operation, the filament 30 of the pump is heated to thermionically emit electrons which spiral up and down the length of the pump, the trajectory of the electrons being terminated when these electrons either (a) strike a gas molecule in the pump or (b) strike one of the getter slugs 28A-28D. Other events affecting the electrons are (c) return to filament 30 (d) striking rod 26, (e) striking wall 12; but these events are statistically small compared to (a) and (b). As operation of the pump continues, the titanium slugs 28A28D are heated by the electron bombardment and titanium vaporizes from the slugs and streams to the wall of the pump.

Referring to FIG. 2, the upper portion of the rod 26 is shown after an extended period of operation. The slugs 28A and 28B are considerably reduced in diameter due to vaporization. However, the portion of slug 28A just under the washer is sacrificed at a slower rate and forms a frusto-conical shape with a broad vertex angle (in excess of 90 degrees). The emission of titanium is indicated by arrows 50. This emission tends to be greatest in the radially outward and downward directions and least in the upward direction toward inlet 18. The vaporization of the tantalum washer is negligible and no titanium can escape directly up the axis of the pump from the inlet face of slug 28A because the washer blocks such exit. Additionally, the conical form of slug 28A tends to throw titanium from the sides of slug 28A primarily in a downward direction, away from inlet. Such titanium as is thrown from slugs 28A and 28B is partially halted by collision with gas molecules and by collision with upper radial streams of titanium in a manner analogous to operation of a diffusion pump. The titanium flux is densest immediately adjacent the slugs. Therefore, this diffusionpump effect which limits upward travel of titanium is strongest just where it is needed most. If titanium escaping from the top slug 28A were directed to the inlet 18, the diffusion-pump effect would not prevent backstreaming from this source, but the washer 34 and conical form of slug 28A substantially prevent backstreaming from this source in the first instance. Such backstreaming titanium vapors, as survive the above protection mechanisms, are almost entirely collected on the portion of wall 12 within the axial length 2 and this is desirable.

Referring to FIG. 3, a similar rod upper portion 26 is shown for a second and preferred embodiment of the invention. As in the first embodiment (FIGS. l-2) there are provided two titanium slugs 128A, 128B, near the rod end remote from the filament, held in place by a spring 32. A tantalum washer 134 is provided in the FIG. 3 embodiment. The difference is in the size of the washer. Typically, the washer 134 would have a length of 4 inch while the slugs 128A, 128B would have lengths of one inch and /1 inch, respectively. The cross-section diameter of the tantalum washer (in a plane normal to the anode rod) would be slightly less than the original diameter of the titanium slug (e.g. for a typical four-inch a pump, A inch washer diameter vs. 7 original slug diameter). Too large a tantalum washer absorbs too many electrons. Too small a tantalum absorbs too few electrons. There can, of course, be several different desirable shapes for the anode-getter arrangement tailored to different pumping cycles. For instance, an extremely long pumping cycle would be benefitted by a relatively large tantalum washer since this would insure that the titanium would last as long as the cycle and be available to pump away random bursts of active gas while maintaining a high steady speed for pumping inert gas. On the other hand, a relatively small wsaher. approaching the size of FIGS. l-2 embodiment is best for very rapid repetitive cycling where high active gas speed is needed for starting each cycle and where the repetitive cycling provides ample opportunity for changing the anode-getter.

The washer can have .a wide variety in composition and geometry. For instance, the material could be molybdenum, tantalum, tungsten. The washer geometry could be of slug form as in FIG. 3 or cup-shaped or hemispherical, the latter two forms providing a cost saving by lesser use of washer material. The principal desiderata are a material that has a much lower vapor pressure than the getter slugs, radiates heat effectively in a vacuum system, and is not melted or adversely degraded at temperatures on the order of 1500 C. under vacuum.

I have run several orbitron pumps with improved anodegetter arrangements of both species described above. The anti-backstreaming performance is best for the FIG. 2 arrangement and the titanium source life extension is best for the FIGS. 1-3 arrangement.

It is not entirely understood why the titanium life extension effect works as well as it does, but the mechanism is believed to be as follows. Electrons are emitted from the filament(s) 30 in all directions. Many of these electrons have suflicient components of angular momentum around the rod 26 and axial momentum towards the mouth of the pump, that they spiral around substantially the full length of the rod 26 until they are reflected back by the electrostatic field extending from the inlet end of the rod to the pump wall. Upon this reflection, I believe that there is a substantial shift of momentum wherein the axial component of momentum going away from the inlet is large compared to the original axial component coming toward the inlet and wherein the component of angular momentum is sharply decreased. Thus, the reflected electrons travel a much tighter spiral while coming back and are more likely to be intercepted by any central obstacle which is of larger diameter than rod 26 and/or closer to the plane of reflection than the slugs.

The above provides an explanation for the low pressure behavior of the invention, but of course the usefulness of the invention would be lessened if the washer absorbed too many electrons at high pressure where active gas sublimation is needed. Fortunately, at high pressure, the getter slugs get a larger share of the electrons and the mechanism of this is believed to be the greater scattering of electrons occurring at high pressure-more molecules of gas are available, hence more electron-molecule collisions.

The beneficial effects of the invention can be enhanced by the operators adjusting the bias of filament(s) 30. For instance, at low pressure, the operator might adjust the filament bias to zero (with respect to the grounded pump body) and at high pressure to 250 volts positive or higher. This can be understood in terms of my explanation above. The zero bias allows a loose initial electron spiral pathhence more ion pumping and less sublimation pumping is performed by these electrons. The 250 volt or higher bias used for starting at high pressures tightens the initial electron path and the reflected electrons have an even tighter spiral to favor electron collisions with the getter sources.

Whatever the actual model of operation is, the above ultimate efiects are repeatable and predictable. For instance, I have operated a six'inch pump at 500 watts power kilovolts, 50 milliamperes emission) and varying filament bias and I have observed that the washer 134 (FIG. 3) glows hot at zero filament bias and reduces the heat to comparatively dark at 250 volts. The slugs 128A, 128B (and 28C) are comparatively dark at Zero bias and glowing hot at 250 volt filament bias. The slug 28D glows hot in either case and this apparently is due to its close axial proximity to the filament.

With or without the aid of filament bias adjust, I find that with proper sizing of the refractory washer relative to its adjacent getter slugs near the inlet end of the anode rod, any orbitron pump can be made to exhibit 30-50% drop in sublimation rate in the pressure decade between 10*" Torr and 10* Torr and lower.

The relevant size consideration is area of the tantalum slug facing the heat sink provided by the cooled pump wall 12. In the above example, this area is about A; of the slug area pi by A1 length compared to 7 pi by 3 and length). However, the area can vary to as low as 1% (i.e. a washer) or as high as about 20%, or even higher, if the top of the cap, as well as the side of the cap, absorbs electrons and radiates heat to the pump Wall depending on the particular pump design. The relative dimensions of getter and refractory slug can be tailored to the pump use. The diameter of the refractory slug is preferably slightly less than the average slug diameter, as noted above.

I have found that this improved anode-getter arrangement (according to either of the above species) has no adverse effects on the electrostatic field which is necessary for operation of the pump. As in the prior art, the electric field extending from the top of anode 26 to the wall 12 of the pump form a virtual shield which reverses the axial motion of the electrons spirallng towards the pump inlet. I have also found that this improved anode-getter arrangement has the same or better mechanical stability compared to prior art anode-getter arrangements.

The general conditions for effectively combining the anode-getter with an orbitron pump may be expressed in terms of the dimensions x, y and z in FIG. 1. The length x should be between /2 inch and R where R is the pump radius. The length y should be such, so that in conjunction with x, the total z will be between R and 2R.

The above reflects the usual case wherein the anodegetter is positioned along the axis of a circular pump body 12. However, the invention is also applicable to pumps Where the body is non-circular and/or where the anode-getter is off-center. In those contexts, the term radius (R) as used herein refers to longest reach from the anode to the pump Wall in a plane perpendicular to the linearily arranged getter-anode.

Several other variations can be made from the preferred form of my invention without departing from the scope of the invention herein claimed. -It is not therefore intended that the above material shall be read as illustrative and not in a limiting sense.

What is claimed is:

1. An improved anode-getter apparatus for use in getter ion pumps of the orbiting electron type wherein electrons are orbited about the anode-getter apparatus within a tubular pump body to ionize gas molecules and strike the (getter source contained in the apparatus and having an inlet, the said apparatus comprising; an anode rod with a plurality of getter sources mounted on the rod thereon in a linear arrangement and a cap mounted over the getter source which is set on a portion of the rod to make that source the one which is closest to the inlet of the pump, the cap covering the face of said getter source nearest the inlet of the pump, said cap being made of a material having a lower vapor pressure than the getter source material.

2. The anode-getter apparatus of claim 1 in combination with a vacuum pump of tubular form and having an inlet at one end and wherein the said capped face of the inlet end tgetter source is located at a distance from the pump inlet greater than R and less than 2R where R is the radius of the tubular pump body.

3. The combination of claim 2 wherein said capped face of the inlet end getter source is located at least one-half inch from the inlet end of the rod.

4. The anode getter apparatus of claim 1 wherein the said cap has the form of a thin washer.

5. The anode-getter apparatus of claim 1 wherein the said cap has the form of a thick washer.

6. The anode-getter apparatus of claim 1 wherein the getter source is titanium and the cap is tantalum.

7. An orbiting electron vacuum pump comprising in combination:

(a) an annular cathode;

(b) an anode-getter apparatus containing getter sources and disposed within the annular cathode;

(c) electron emission means for introducing electrons within the annular cathode to ionize gas molecules therein and heat the getter sources by collision of electrons with said sources; and

(d) electron absorption means within the annular cathode constructed and arranged to absorb more electrons at the low pressure end of the pumps operating pressure range than it absorbs at the low pressure end of the pumps operating range and dissipating the heat generated by electron collision through radiation so that the heating of the getter sources is reduced at low pressure whereby getter material is conserved.

8. An orbiting electron vacuum pump, comprising in combination:

(a) an annular cathode;

(b) an anode-getter apparatus disposed within said annular cathode, said apparatus comprising an anode rod with getter slugs mounted along its length;

(0) electron emission means for introducing electrons within the annular cathode to ionize gas molecules therein and heat the getter sources by collision of electrons with said sources; and

(d) electron absorption means within the annular cathode comprising a slug of low vapor pressure refac' tory metal, relative to the getter slugs, mounted on the rod and constructed and arranged to absorb more electrons at low pressure end of the pumps operating range than at the high pressure end of the pumps operating range and to dissipate the heat generated by electron collision through radiation so that the heating of the getter sources is reduced at low pressure whereby getter material is conserved.

9. The pump of claim 8 wherein the said low-vapor pressure refractory metal slug is mounted closer to the inlet of the pump than any of the getter slugs.

10. The pump of claim 9 wherein the said low-vapor pressure refractory metal slug has a surface area facing the annular cathode of 1%20% of the surface area of the getter slugs facing the annular cathode and wherein the refractory metal slug has slightly smaller cross-section dimensions than the average cross-section dimensions of the getter slugs taken in a plane normal to the anode rod.

References Cited UNITED STATES PATENTS 3,244,969 4/1966 Herb et al. 324-33 HENRY F. RADUAZO, Primary Examiner. ROBERT M. WALKER, Examiner.

UNITED STATES PATENT OFFICE 4 CERTIFICATE OF CORRECTION Patent No. 3,357,634 December 12, 1967 Joseph C. Maliakal It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 3, after "tantalum" insert washer "A Column 5, line 35, "form" should read forms line 36, "sp'irallng" should read spiralling line 56, cancel "not". Column 6, line 28, "low" should read high ".Signed and sealed this 2nd day of December 1969.

( E Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

Claims (1)

1. AN IMPROVED ANODE-GETTER APPARATUS FOR USE IN GETTER ION PUMPS OF THE ORBITING ELECTRON TYPE WHEREIN ELECTRONS ARE OBITED ABOUT THE ANODE-GETTER APPARATUS WITHIN A TUBULAR PUMP BODY TO IONIZE GAS MOLECULES AND STRIKE THE GETTER SOURCE CONTAINED IN THE APPARATUS AND HAVING AN INLET, THE SAID APPARATUS COMPRISING; AN ANODE ROD WITH A PLURALITY OF GETTER SOURCES MOUNTED ON THE ROD THEREON IN A LINEAR ARRANGEMENT AND A CAP MOUNTED OVER THE GETTER SOURCE WHICH IS SET ON A PORTION OF THE ROD TO MAKER THAT SOURCE THE ONE WHICH IS CLOSEST TO THE INLET OF THE PUMP, THE CAP COVERING THE FACE OF SAID GETTER SOURCE NEAREST THE INLET OF THE PUMP, SAID CAP BEING MADE OF A MATERIAL HAVING A LOWER VAPOR PRESSURE THAN THE GETTER SOURCE MATERIAL.

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