US3437260A - Vacuum arc gettering pump - Google Patents

Description

April 8,1969 A. s. GILMOUR, JR

VACUUM ARC GETTERING PUMP Sheet I 0f2 Filed Aug. 16, 196? INVENTOR ALEXANDER S.GILMOUR JR.

ATTORNEY April 8, 1969 A. s. GILMOUR, JR. 3,437,260 VACUUM ARC GETTERING PUMP Filed Aug. 16, 1967 Sheet 2 of 2 HIGH VOLTAGE 23 s2' 50' J j HIGH VOLTAGE mgw ATTORNEY United States Patent US. Cl. 230-69 7 Claims ABSTRACT OF THE DISCLOSURE A gettering pump utilizing a solid material cathode as a source of gettering material, a coaxial anode, and circuitry for establishing a vacuum are between the cathode and anode to generate a plasma flow of gettering material particles that are deposited on the surface of a chamber to be evacuated.

Background of the invention The present invention relates to a pumping device and more particularly to a gettering pump that is capable of achieving and maintaining extremely low gas pressures.

There are many applications that require very high vacuums on the order of torr. or greater. For example, such low pressures or high vacuums are mandatory in simulating space environments for rocket or satellite testing. Presently there are three types of high vacuum pumps the ion pump, the

that have been relied upon; namely, diffusion pump and the gettering pump. In ion pumps, gas molecules are ionized and are drawn into a gettering electrode. Oil diffusion pumps coupled with adequate trapping function to discharge gas molecules by the action of bombardment by oil molecules. Gettering pumps continuously vaporize or sputter a gettering metal which deposits on a surface area to continuously getter random or ionized gas particles impinging thereagainst.

Not only is it necessary, in most instances, to generate a low pressure environment, but it is also necessary that such environment be maintained. Thus, evolved gases from the surface of a test body must be removed as fast as they are liberated. With gettering pumps this means, ideally, that the rate of gettering material production should be at least as great as the rate of gas production from the objects within the vacuum chamber.

Gettering pumps currently employed utilize, typically, a titanium wire or filament as a .source of gettering material. The filament is heated electrically causing the metal thereof to vaporize. In this type of pump the useful pumping life is extremely limited due to the small amount of gettering material available for proper vaporization. It is not unusual to replace the titanium filament once a day, with the attendant delays caused thereby, necessitating interruptions and delays in the use of the vacuum chamber.

In addition to having short lives, present gettering pumps utilizing electrical heating to vaporize the gettering metal have high power requirements. Moreover, the heat generated greatly increases the tendency for parts of the systerm to out-gas and thereby further increase the pumping load.

Summary of the invention The foregoing disadvantages of prior filament-type gettering pumps are overcome according to the present invention by the provision of a gettering material producer that is actuated by an electric are which functions to vaporize the gettering material into a continuous stream of plasma particles.

Basically the invention provides; means defining a chamber having a surface portion, a gettering material producer mounted within said chamber and having a forward end facing said surface portion, said gettering material producer comprising; a solid material cathode fabricated of a material having a high afiinity for gas, and a spaced c0- axial anode, and means for producing an electric are on said cathode over a suflicicntly small area thereof causing portions of said cathode material to ionize generating a plasma flow directed towards said chamber surface portion and forming a thin film of gettering material thereon.

Brief description of the drawings For a more fuller understanding of the present invention, reference may be had to the following detailed description of the same taken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a schematic sectional view of a vacuum chamber in which is installed in operative relation, the gettering material producing structure in accordance to the .present invention.

FIGURE 2 is an enlarged sectional View of a portion of FIGURE 1 illustrating a preferred embodiment of the present invention wherein provision is made for continuously supplying a source of gettering material and wherein the are for the production thereof is ignited by a removable igniter.

FIGURE 3 is a modified form of arc ignition usable with the device of FIGURE 2.

FIGURE 4 is a sectional view illustrating a modification of the device shown in FIGURE 2.

FIGURE 5 is a sectional view illustrating a modified form of ignition usable with the FIGURE 4 embodiment.

Description of the preferred embodiments Referring now to the drawings and more particularly to FIGURE 1, a vacuum housing generally indicated at 10, is shown as comprising side walls 12, 14 and a bottom wall 16 defining interiorly thereof a vacuum chamber 18. A closure plate 20 is securely and sealingly mounted on flanged ends of walls 12 and 14 as shown at 22 and 24 to enclose chamber 18.

A bracket 26 is fixedly mounted at one end by bolts or the like to an interior portion of side wall 12 and has an opposite end terminating in a cylindrical collar 28 through which is securely received the improved gettering material producer indicated generally at 30.

Gettering material producer 30 is thus mounted such that a forward flared end 32 thereof faces the bottom wall 16 of the chamber 18. Electrical lead wires 34 and 36 pass sealingly through the chamber in a conventional manner, not illustrated.

As will be discussed in greater detail hereinbelow, producer 30 functions to direct a plasma plume 38 of suitable gettering material upon bottom wall 16 forming a thin film 40 (shown greatly enlarged). In a manner commonly known the gettering material film 40 adsorbs the gaseous material within chamber 18, thereby lowering the chamber pressure.

As shown more clearly in FIGURE 2, the gettering material producer 30 is depicted as comprising a generally cylindrical metallic outer shell 42 that functions as an anode. Shell 42 has an outwardly flared conical forward face 32. Spaced axially inwardly from anode face 32 and coaxially contained within the shell 42 is a solid cylindrical metallic member 44 that functions as a cathode and as a source of gettering material for the pump. Cathode 44 has a generally planar forward face 46 that is adapted to be mounted in vacuum chamber 18 and directed towards bottom wall 16 thereof as shown in FIGURE 1. Cathode 44 may be fabricated of any suitable metallic material having an aflinity for gas such as titanium, for example. An annular sleeve 48 of suitable insulating material is located intermediate cathode 44 and anode 42 and extends the length thereof. Aluminum oxide is a suitable insulating material for sleeve 48.

A circuit shown schematically is adapted to create a potential diffeernce between the anode 42 and cathode 44. Thus, an electrical source 50 in series with a switch or the like 52 is in electrical communication with the anode via line 36 and the cathode via line 34.

In the operation of the FIGURE 2 embodiment, a conventional tungsten probe (not illustrated) is brought into contact in a rough vacuum environment with cathode surface 46 carrying a high electric current, of, say, 100 amperes. As the probe is pulled away from the surface 46 a very small spot thereon becomes the last part to remain in contact with the probe. The area of such spot may be on the order of 10 cm. or smaller. The high current passing through this very high resistance spot generates an intense source of heat which causes the material of the cathode spot to evaporate and ionize forming a high velocity plume, 38 of plasma particles, which as shown in FIGURE 1 is directed towards bottom wall 16 and forms a film 40 thereon. As the probe is removed, with switch 52 closed, an arc is sustained by the conductive path set up between cathode 44 and anode 42 due to the flow of electrons to the positive anode. The gettering material film 40 functions in the conventional way to remove gas from the vacuum chamber and to further decrease the pressure therein.

As the effectiveness of the film is reduced due to the adsorption of gas, new layers of film will be provided either by continuous or intermittent action of the arc.

Although not shown, the tungsten probe could obviously be remotely actuated by a solenoid or the like which would function to alternately move the probe in front of and away from the cathode.

The insulating sleeve 48 is required to prevent the are from running down the peripheral surface of the cathode and thereby destroying the operation of the pump. As the arc dances across the surface 46 of the cathode, new surfaces are exposed and may be forwardly moved by any suitable feeding mechanism.

It has been found that the conical face 32 of the anode facilitates arc ignition.

In FIGURE 3 a slightly different type of means to generate the vacuum arc is shown. Here, an annular groove is provided in insulating sleeve 48' in which is fixed a metal ring 49 of stainless steel or the like. A high voltage, about 10,000 volts, from a suitable source 23 is adapted to be impressed between the ring 49 and face 46' of cathode 44 via lines 25 and 27 and switch 29. This voltage generates a high electric field in the space between the ring and cathode. Since the face 46" of the cathode is microscopically composed of an irregular surface of peaks and valleys, an extremely intense field is generated at the tallest peak. This causes electrons to escape by field emission developing intense heat at said peak spot which evaporates to a plasma flow as previously described. Thus in this form, a stationary or fixed igniter is provided which operates on a high voltage as compared to the high current, movable contact igniter previously described.

Referring to FIGURE 4 a slightly modified producer is shown that requires no cathode feed system. Here, like numerals with primes refer to like parts of the FIGURE 2 embodiment. In place of the cylindrical cathode 44 of FIGURE 2, a thin disc or wafer cathode 44' is provided with a large surface area 48".

In FIGURE a ring igniter 49' is provided .in the insulator 48:: as in the FIGURE 3 embodiment. Otherwise, it is identical to FIGURE 4.

Gettering pumps with the improved gettering material producer of the present invention have been capable of very long periods of sustained operation without shut down. In fact, it has been determined that sustained operation of a year or more is possible, depending upon the gas load. This has been due to the virtually unlimited supply of gettering material as compared to the extremely 4 limited supply associated with the conventional filamenttype gettering pumps.

It has been found that high pumping rates are realized as a result of the increased rate of plasma material emanating from the gettering material cathode. Pumping rates on the order of 50,000 liters per second are now possible.

In addition, due to the absence of a continuous heating means to vaporize the gettering material as with prior devices, the present gettering pump can operate advantageously at a greater efiiciency because of lower power requirements, which may be two orders of magnitude less than other prior devices.

More importantly, less heating of the structure within the vacuum environment is realized. Since the tendency of a structure to out-gas or liberate gases that are combined therewith increases with increasing temperature, the lower temperatures of the present device means that the pump will not have to work against additional loads due to such out-gassing.

While the foregoing specifications have disclosed and described certain preferred embodiments of the present invention, other modifications will occur to those skilled in the art without departing from the spirit of the invention. For example, it is obvious that the structure could function as a deposition device. Accordingly, it is intended that the scope of the present invention is to be limited only by the scope of the appended claims.

I claim:

1. A gettering pump comprising;

(a) means defining a chamber having a surface portion,

(b) a gettering material producer mounted within said chamber and having a forward end facing said surface portion,

(c) said gettering material producer comprising; a

solid material cathode fabricated of a material having a high afiinity for gas, and a spaced coaxial anode, and

(d) means for producing an electric are on said cathode over a sufficiently small area thereof causing portions of said cathode material to ionize generating a plasma flow directed towards said chamber surface portion and forming a thin film of gettering material thereon.

2. The pump according to claim 1, further comprising;

(e) insulation means filling the coaxial space between said cathode and anode.

3. The pump according to claim 2, wherein;

(f) said means for producing an arc includes a ring of conducting material embedded in an annular groove of said insulation means.

4. The pump according to claim 2, wherein;

(f) said anode comprises a cylindrical shell,

(g) said cathode comprises a solid cylindrical member, and

(h) said insulation means comprises an annular sleeve.

5. The pump according to claim 4, wherein;

(i) said anode has an outwardly flared conical forward face.

6. The pump according to claim 2, wherein;

(f) said anode comprises a cylindrical shell,

(g) said cathode comprises a thin disc, and

(h) said insulation means comprises an annular sleeve.

7. The pump according to claim 6, wherein;

(i) said anode has an outwardly flared conical forward face.

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

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