US3694691A

US3694691A - Sublimation getter pump with cathode trigger means

Info

Publication number: US3694691A
Application number: US91978A
Authority: US
Inventors: James M Lafferty
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1970-11-23
Filing date: 1970-11-23
Publication date: 1972-09-26
Anticipated expiration: 1989-09-26

Abstract

Sublimation getter pump includes, in one embodiment, a cathode electrode made of an active gas gettering material, an anode electrode closely juxtaposed to the cathode electrode and defining therewith an electric discharge gap, and a trigger electrode assembly in close juxtaposition to the cathode electrode. Upon the application of a suitable pulse to the trigger electrode assembly, an electric discharge is established between the cathode and anode electrodes, causing the evolution of active gas gettering material from the cathode electrode. Active gas gettering material is deposited in a clean film upon a closely adjacent surface where it reacts with and removes from the atmosphere active gases, causing the reduction of pressure within the pumped volume.

Description

United States Patent Lafferty 1451 Sept. 26, 1972 [5 SUBLIMATION GETTER PUMP WITH CATHODE TRIGGER MEANS Primary Examiner-Palmer C. Demeo Atj qr gy lohn lil Ahem, Paul A. Frank, Richard R. [72] Inventor 2 3 Laflerty Schenectady Brainard, Jerome C; 8 61115115, Frank L. Neuhauser,

Oscar B. Waddell and Joseph B. Forman [73] Assignee: General Electric Company [22] Filed: Nov. 23, 1970 [57] ABSTRACT Sublimation getter pump includes, in one embodi- [211 App! 91978 ment, a cathode electrode made of an active gas gettering material, an anode electrode closely juxtaposed 521 vs. (:1. ..3l5/l08, 313/7, 313/178, to the cathode electrode and defining therewith 911 313/180, 313/198, 315/174, 417/49 electric discharge gap, and a trigger electrode as- [51] Int. Cl ..H0lj 7/18 sembly in close juxtaposition to the Cathode electrodfl 5 Field f Search 3 15 10 109 174; 313 17 Upon the application of a Suitable pulse to the trigger [56] References Cited UNITED STATES PATENTS 3,437,260 4/1969 Gilmour, Jr. ..417/49 3,489,336 l/l970 Schbnhuber ..313/7 X 3,583,829 6/1971 Schonhuber ..417/49 3,465,205 9/1969 Lafferty ..313/17s x electrode assembly, an electric discharge is established between the cathode and anode electrodes, causing the evolution of active gas gettering material from the cathode electrode. Active gas gettering material is deposited in a clean film upon a closely adjacent surface where it reacts with and removes from the atmosphere active gases, causing the reduction of pressure within the pumped volume.

12 Claims, 1 Drawing Figure III WAX wwwwm PATENTED SEP 26 1972 JAMES M. LAFFERT): by Mew H/S ATTORNEY PULSE 4/ CIRCUIT SUBLIMATION GETTER PUMP WITH CATHODE TRIGGER MEANS This invention relates to active gas sublimation getter pumps and, more particularly, to such pumps as operate for long operating lives without requiring repeated replacement of getter source elements.

Sublimation getter pumps have been used in vacuum technology for the removal of chemically active gases from closed systems. Prior art pumps, however, are severely limited in their operation. This is due to the general structure and mode of operation utilized by such pumps. In general, in such devices, the source of active gas getter material is a body or bodies of a gettering material such as for example, titanium, closely juxtaposed to an independent heater filament so as to be heated to sublimation temperature thereby. The gettering material is vaporized by the heat of the closely adjacent filament, forming upon an adjacent, cooled wall a clean fresh film which fresh film is an excellent getter for active gases in this form.

Due to the thermal heating of the active gas gettering sources of prior art pumps, such sources are short-lived and must be replaced. This is because the heat of the external heater filament, in order to drive off sufficient gettering material to perform the getter pumping function, drives off so much gettering material that the source is rapidly depleted. This is because of lack of control of the gross heating effects. The foregoing is particularly true if the source is provided in the form of thin wires of getter material closely adjacent the filament in order to reduce the thermal inertia thereof, since the thermal inertia of prior art devices imposes a severe limitation thereupon. Thus, for example, prior art pumps cannot be cycled or operated upon pulse voltages due to the slow thermal response of the filament, and consequently of the getter material, to heating or cooling, as compared with the voltage supplied to the heater filament. Therefore, designs directed to reducing the thermal inertia as much as possible cause the getter material to be formed in very small filaments, resulting in short operating life thereof.

A further disadvantage of filament-heated getter pumps is that the lifetime of the filament is greatly foreshortened in high pressure operation. This is because the filament tends to oxidize away in the presence of air at atmospheric pressure or pressures of air even as much as an order of magnitude lower than atmospheric. Due to this characteristic, getter pumps of the prior art are generally utilized in connection with mechanical fore pumps, which are utilized to lower the pressure of the system being pumped to the torr range, prior to the utilization of the getter pump.

Accordingly, it is an object of the present invention to provide new and improved sublimation getter pumps which overcome the aforementioned disadvantages.

Another object of the invention is to provide sublimation getter pumps wherein operating parts of the pump need not be repeatedly replaced during useful life.

Still another object of the invention is to provide sublimation getter pumps which vaporize a suitable gettering material without the necessity of getter heater filaments.

Yet another object of the invention is to provide sublimation getter pumps with a very short response time, permitting pulsed operation.

Briefly stated, in accord with one embodiment of the invention, 1 provide a sublimation getter pump including a cathode electrode of an active gas gettering material, an anode electrode disposed with respect to the cathode electrode so as to define an electric discharge gap therebetween, and a trigger electrode assembly closely adjacent the cathode electrode for initiating an electric discharge between the cathode and anode electrodes. When the trigger assembly is pulsed and an electric discharge is established between the anode and cathode electrode, cathode spots formed upon the surface of the active gas gettering cathode electrode cause the vaporization of a controllable amount thereof to supply to the pump the species which forms a clean sputtering surface upon a closely adjacent wall to remove thereat, active gases. Since the electric discharge may be initiated and extinguished rapidly, by switching voltages, and the response thereto is instantaneous, the inertia of the pump is negligible. Since the period of time during which the gettervaporizing discharge is in operation may be closely controlled to produce exactly the desired amount of gas gettering material required, the pump is very efiicient and cathode material is conserved. Since the cathode electrode may be made massive, the gettercathode lasts an extremely long time and repeated replacement of worn out sources of gettering material is unnecessary.

The novel features characteristic of the present invention are set forth in the appended claims. The .invention itself, together with further objects and advantages thereof, may best be understood with reference to the appended drawing in which the sole FIGURE is a vertical cross-sectional view of a sublimation getter pump constructed in accord with the invention.

In the drawing, a sublimation getter pump constructed in accord with one embodiment of the invention is represented generally at 10. Pump 10 includes a cathode assembly 11, an anode assembly 12, a trigger assembly 13, and a collector or shield member 14, all of which are mounted in, or on, a pump base member 15, which may readily be inserted in, and affixed to, a vacuum system by means of bolts inserted through mounting holes 16 in base assembly 15. Cathode as sembly 11 may conveniently comprise a metallic support and cooling member 17, conveniently a heavy copper tube which may contain cooling fluid ducts 18 therein, connected with fluid entrance tubulation 19 and exit tubulation 20. An active gas gettering electrode cylinder member 21 is mounted exterior of, and affixed to, the inwardly depending end of support member 17 and covers substantially all of the support member which is exposed within the volume of the pump 10.-Cathode electrode cylinder 21 may conveniently comprise an active gas gettering material such as beryllium, titanium, zirconium, cerium, magnesium, aluminum, hafnium, tantalum or uranium,

although it is preferably comprised of titanium, zirconium, beryllium or alloys thereof for maximum efficiency. Although cylinder 21 is massive as compared with the thin wire getter source member of the prior art and need not be replaced frequently, it may conveniently be removable so that after many hundreds of hours of operations, a new cylinder may he slipped over support 17 to provide a regenerated source of active gas-gettering material.

s in hermetic seal therewith.

Trigger electrode assembly 13 includes a metallic trigger anode 25 having a flanged edge 26 affixed to a ceramic spacer member 27 which is inserted in, and

pressed within, the upwardly extending end of cathode electrode cylinder 21. The trigger anode 25 is pressed over the exposed end of ceramic spacer 21 so that edge 26 thereof defines'with-the inwardly depending end of cathode electrode cylinder 21, an annular gap over the surfaceof spacer 27, which gap constitutes a trigger gap between the cathode electrode cylinder 21 and the trigger anode 25. A trigger anode lead 28 extends through a central aperture in electrode support member 17 and passes in hermetic seal through insulator 29 at the end thereof and emerges as trigger electrode lead 30. Insulator 29 is also hermetically sealed to cathode electrode support member end cap 31 to maintain an hermetic seal. End cap 31 is electrically, mechanically and hermetically sealed to the outwardly depending end of electrode support member 17.

-Anode electrode assembly 12 includes a helically coiled tubulation 32 supported upon apair of

tubular leads

33 and 34 which, are preferably of stainless steel, and which are hermetically sealed through base member 15. The helices of tubulation 30 comprise a generally permeable structure which surrounds cathode electrode member 11 in sufficiently close juxtaposition so as to define a discharge gap adapted to support an electric discharge therebetween when an appropriate voltage difference of several hundred volts, for example, is applied between the cathode and anode electrodes and conducting specie are supplied therebetween so as to permit the establishment of a discharge. In the illustrated embodiment, the anode electrode assembly 12 includes helically coiled tubulation 32 which is wound, for example, in ascending and descending helical coils and is conveniently fabricated of copper and is hollow to facilitate force cooling, preferably by a flow of water represented by inwardly and outwardly directed

arrows

35 and 36. In order to prevent short circuiting of the device by the coolant fluid, de-ionized water is preferably used. Alternatively, a non-conducting fluid such as alcohol may be utilized. Additionally, insulating

members

37 and 38 are interposed, exterior of the pump, inv the fluid conduit line in order that the electric potential applied to the anode electrode assembly is isolated from the fluid supply means.

Shield or collector member 14 is juxtaposed exterior of anode electrode assembly 12 and is adapted to provide a base for the deposition of a film of gettering material from cathode electrode cylinder 21 deposited thereupon after passage of the vaporized getter material through the particle-permeable anode electrode assembly 12. The deposition of a clean film of active gas gettering substance, as for example titanium, provides an ideal surface for a chemical reaction between active gases such as water vapor, carbon monoxide, carbon dioxide, nitrogen, oxygen, hydrogen and any gas, other than noble gases, which may be present within the system and which may be removed by the formation of stable, non-volatile reaction products with the titanium, zirzonium, beryllium, or other active gas gettering substance of which the cathode electrode cylinder 21 is made. The collector or shield 14 is not an essential element of the pump as is illustrated herein, since no electrical potential need be applied thereto and since the surface upon which the film of active gas gettering material is deposited may conveniently be aglass or other surface already provided within the vacuum system which is to be pumped by pump 10. Typical electrical connections for the pump 10 include a discharge voltage source, represented generally as generator 40 which may, if desired, contain means for limiting current when the arc is struck, connected between cathode electrode assembly 11, at the outboard end of electrode support member 17, and anode electrode assembly 12, at the outboard end of tubulation 33. A trigger voltage supply, which may conveniently be pulse source 41, is connected between cathode electrode assembly 11 and trigger electrode lead 30.

In operation, voltage source 40 is connected between anode and cathode electrodes and may conveniently comprise an alternating current source, providing for example, a sine wave form and having any predetermined frequency desired. Conveniently, a cycle alternating voltage of approximately 110 or 220 volts may be utilized for this purpose, although any suitable voltage may be utilized and the dimensions of the device adapted to accommodate the discharge characteristics thereof. A unidirectional pulsed voltage source may also be utilized as voltage generator 40 and may have any predetermined characteristic of pulse voltage v with any desired magnitude and time duration. A pulse trigger voltage source is connected between the cathode electrode and the trigger anode and is generally a voltage which is sufiicient to cause the establishment of a trigger are between flange 26 of trigger anode 25 and the inwardly depending end of cathode electrode cylinder 21 over the annular gap on the surface of ceramic spacer 27. Conveniently, with a voltage of 110 volts applied between anode and cathode electrode assemblies, a typical voltage for the trigger assembly may conveniently be 1,000 volts, although this voltage may be from several hundred to several thousand volts.

Operationally, the trigger voltage is synchronized to apply a trigger pulse between the trigger anode and the cathode electrode at a precise portion of the voltage cycle or the positive voltage pulse between the anode,

and cathode electrode so as to cause the amount of applied voltage, when the anode is positive with respect to the cathode, or alternatively, of a positive pulse of DC voltage, and the discharge continues for the remaining portion of the cycle or pulse until a current zero is reached. At that time, the discharge is extinguished and is not initiated again until the voltage between the cathode and the anode is such that the anode is positive with respect to the cathode and simultaneously, the trigger electrode is pulsed. If a continuous 60 cycle voltage is utilized, the pulse source may be synchronized, so that the trigger is actuated on each alternating half cycle, or on every other alternating half cycle, on every third, every fourth or any fraction of the desired number of alternating half cycles which is desired, or a pulsing of several cycles on, and several cycles off may be utilized. By conveniently adjusting the pulse repetition rate of the pulsed source, the pumping rate of the pump may conveniently be controlled to any desired value.

In operation, when the trigger arc is established across spacer 27, a cathode spot is established at the beginning of the inboard end of cathode electrode cylinder 21 and the discharge is immediately established between the cathode electrode 21 and the helical coil 32 of anode assembly 12. The discharge rapidly spreads out over the entire length of the cathode electrode cylinder and the helical anode tubulation 32. The current density is maintained sufficiently small by the spreading out thereof so that the titanium cylinder 21 erodes uniformly and non-destructively to provide a rapid pumping rate without substantial deterioration of the breakdown or other operating characteristics of the device. Since there is no incandescable filament which must achieve an operating temperature or cool down therefrom, the discharge of the devices of the invention may be established and discontinued as rapidly as voltages may be applied to the respective electrodes. Pulsed operation further permits the precise control of the pumping rate without any necessity of making design decisions based upon the necessity of maintaining a low thermal inertia in order to achieve rapid operation.

In one device constructed in accord with the present invention, the cathode electrode cylinder 21 was made of titanium and had a length of approximately 2 inches and an outside diameter of approximately one-half inch. The thickness of the cathode cylinder wall was approximately one-eighth inch. The anode electrode was one-fourth inch O.D. copper tubulation coiled in coils having an outside diameter of approximately 2 inches and included three turns upwardly and three turns downwardly over the 2 inch length of the cathode electrode. A voltage of 110 volts, 60 cycle alternating current was applied between cathode and anode electrodes and a 1,000 volt pulsed voltage of 100 microseconds duration was synchronized with the alternating current voltage applied between the cathode pumped at a rate of 8,400 liters per hour. A flow of water was maintained through the anode electrode at a rate of 10 liters per hour, but the cathode electrode was not force cooled.

Devices constructed in accord with the present invention have shown exceedingly long operating lives. Whereas the thermally heated filament type sublimation getter pumps of the prior art are generally only useful for approximately 6 to 8 hours of operation, devices in accord with the present invention have been operated routinely for as many as 200 or 300 thousand cycles of operation without any noticeable deterioration in pumping rate, electrical characteristics, or physical appearance of the devices.

A further advantage of devices in accord with the present invention, is the absence of any incandescable filament which is likely to oxidize destructively at atmospheric pressures or pressures at which tungsten and other incandescable filaments normally react with ambient atmosphere. Pumps in accord with the present invention may be started at atmospheric pressure and operated without the necessity of a mechanical fore pump to lower the pressure to a safe operating value.

In practice, pumps in accord with the present invention are readily calibrated. in order to calibrate the pump in accord with the present invention, it is merely necessary to assemble the pump into the desired system which is to be pumped and measure the pumping rate as a function of current through the discharge, noting the value at which saturation begins to occur. The optimum pumping current is then a value just slightly below the current value at which saturation begins to appear.

By the foregoing disclosure, 1 have described improved sublimation getter pumps which are suitable for operation at high and low pressure, which have exceedingly long operating lives and which do not require repeated replacements of operating parts. Pumps in accord with the present invention are highly advantageous in that they have no thermal time lag and are highly reliable to pump an exact rate of active gas without unduly wasting active gas gettering material.

While the invention has been disclosed herein with respect to certain preferred embodiments and examples thereof, it is readily apparent that many modifications and changes will immediately occur to those skilled in the art. Accordingly, by the appended claims, I intend to cover all such modifications and changes as fall within the true spirit and scope of the foregoing disclosure.

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

1. An active gas getter pump comprising:

a. an elongate annular cathode electrode of an active gas gettering metal;

b. a substantially open anode electrode juxtaposed laterally about said cathode electrode;

c. said anode and cathode electrodes defining a gase ous discharge gap therebetween;

d. trigger means disposed adjacent said cathode electrode and adapted to initiate a trigger arc therebetween when actuated; and

e. means for applying a voltage between said cathode and anode electrodes sufficient to sustain an electric discharge therebetween when said trigger means is actuated and said trigger arc is established. 1 D

2. The pump of claim 1 wherein said anode electrode is of relatively open structure and is disposed coaxially about said cathode electrode.

3. The pump of claim 1 wherein said cathode electrode is a massive member adapted to erode away progressively during the active life of the pump.

4. The pump of claim 1 wherein said anode electrode is force-cooled in order to reduce the effects of discharge erosion thereof.

5. The pump of claim 1 wherein said cathode electrode is force-cooled.

6. The pump of claim 3 wherein said cathodeelectrode is centrally disposed, said anode is an open structure laterally surrounding said cathode electrode; and a collector member is disposed beyond said anode electrode to receive electrode material removed therefrom by said electric discharge.

7. The pump of claim 6 wherein said anode and cathode electrodesare water cooled.

8. The pump of claim 1 wherein said cathode electrode is fabricated from an active gas .gettering material selected from the group consisting of titanium, zirconium, beryllium, cerium, magnesium, aluminum, hafnium, tantalum and uranium.

9. The pump of claim 1 wherein said cathode electrode is fabricated from an active gas gettering material selected from the group consisting of titanium, zirconium and beryllium.

10. The pump of claim 9 wherein the cathode electrode is titanium.

1 l. The pump of claim 3 including means for periodically pulsing said trigger electrode to initiate a trigger are.

12. The pump of claim 1 1 wherein said means for applying a voltage between said cathode and anode electrodes is an alternating current source connected between said cathode and said anode electrodes, said means for periodically pulsing being connected between said alternating current source and said trigger electrode.

Claims

1. An active gas getter pump comprising: a. an elongate annular cathode electrode of an active gas gettering metal; b. a substantially open anode electrode juxtaposed laterally about said cathode electrode; c. said anode and cathode electrodes defining a gaseous discharge gap therebetween; d. trigger means disposed adjacent said cathode electrode and adapted to initiate a trigger arc therebetween when actuated; and e. means for applying a voltage between said cathode and anode electrodes sufficient to sustain an electric discharge therebetween when said trigger means is actuated and said trigger arc is established.

5. The pump of claim 1 wherein said cathode electrode is force-cooled.

6. The pump of claim 3 wherein said cathode electrode is centrally disposed, said anode is an open structure laterally surrounding said cathode electrode; and a collector member is disposed beyond said anode electrode to receive electrode material removed therefrom by said electric discharge.

7. The pump of claim 6 wherein said anode and cathode electrodes are water cooled.

8. The pump of claim 1 wherein said cathode electrode is fabricated from an active gas gettering material selected from the group consisting of titanium, zirconium, beryllium, cerium, magnesium, aluminum, hafnium, tantalum and uranium.

10. The pump of claim 9 wherein the cathode electrode is titanium.

11. The pump of claim 3 including means for periodically pulsing said trigger electrode to initiate a trigger arc.

12. The pump of claim 11 wherein said means for applying a voltage between said cathode and anode electrodes is an alternating current source connected between said cathode and said anode electrodes, said means for periodically pulsing being connected between said alternating current source and said trigger electrode.