US3330472A

US3330472A - Vacuum device

Info

Publication number: US3330472A
Application number: US492057A
Authority: US
Inventors: Joseph C Maliakal
Original assignee: National Research Corp
Current assignee: National Research Corp
Priority date: 1965-10-01
Filing date: 1965-10-01
Publication date: 1967-07-11
Anticipated expiration: 1984-07-11

Description

United States Patent I I 3,330,472 VACUUM DEVICE Joseph C. Maliakal, Newton, Mass., assignor to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed Oct. 1, 1965, Ser. No. 492,057 Claims. (Cl. 230-69) The present invention relates to electrical devices of the orbitron type and particularly to orbitron pumps.

Orbitrons are Well known from the work of the University of Wisconsin. These devices provide an orbiting cloud of electrons which ionize gas in an enclosed annular pumping region through electron-molecule collisions.

I have developed an improved mode 'of construction of such devices which allows increased mean free paths of electrons for higher ionization efiiciency, operation at higher currents if desired, and simplifies the construction of the filament and filament supports of the electron sources for orbitron devices. These features afford the important economic and technical advantages of higher inert gas pumping speeds in orbitron pumps and reduced cost of construction and operation of all orbitron devices utilizing my invention.

It is therefore the object of the invention to provide an improved orbitron device aifording the above features and advantages.

Other objects, features or advantages of the invention will, in part, be obvious and will, in part, appear hereinafter in the following description of my invention, which is made with reference to the accompanying drawings wherein:

FIG. 1 is a sectional view of an orbitron pump.

FIGS. 1A and 1B are alternative top Views of the FIG. 1 pump corresponding to two alternate modes of construction;

FIG. 2 is a detailed drawing of the anode showing its relation to the filaments and the shield electrode of the orbitron device.

FIG. 3 is a typical speed-pressure curve showing performance of the improved device in pumping argon.

Referring now to FIG. 1, the orbitron pump is connected to a vacuum chamber 12, a portion of which is shown in cutaway form, through a spool piece 14. Ports in the spool piece provide connections to roughing pumps,

inlet valves and other conventional accessories of a vacuum system.

The pump 10 comprises an outer annular electrode traced by cooling coils 22 and capped by a cover flange 24. The cover 24 contains vacuum feedthroughs with electrical connector pins 31-35, mounted therein. Within the pump body 20 are a glass header 40, which contains seven metal pins 41-47, and an anode rod 50. A titanium cylinder 56 is mounted on the anode rod 50. A filament 52 is mounted from the

pins

44, 45. A reflector shield 58 is mounted on the three remaining pins (not shown) of header 40. The

pins

41, 42, 44, pass through slots 59 in the shield 58.

In operation, the outer annular electrode 20 of the pump is grounded and a potential on the order of 10,000 volts positive is impressed on the center rod electrode 50. Water is pumped through coils 22'. The

filaments

52, 54 are heated by an alternating current source (not shown) and biased slightly positive with respect to ground. Electrons are thermionically emitted from the filaments. These electrons orbit around the rod until they collide with a gas molecule in the pump body or strike the rod 50 or a getter cylinder 56 disposed along the rod 50. Electron-molecule collisions produce positive ions which are attracted to the outer annular electrode 20. Electron bombardment of getter cylinders 56 heats the getter material (titanium) to its vaporized temperature and titanium vapors are emitted and condensed on the cooled outer annular electrode 20. The fresh deposits of titanium on the outer annular electrode readily combine with ions or molecules of active gases and ions of the inert gases by directchemical combination and physical burial as the titanium vapors continuously stream onto the outer annular electrode. Inert gases require electron collisions for efficient pumping.

It is desirable to maximize the average trajectory length of the electrons in the pump body in order to increase their inert gas pump efficiency. In the present invention this is accomplished by increasing the distance I from the filaments to the inner electrode 50, by the utilization of smaller dimensions for the cross-sections of the fila ments and their supporting lead wires, and decreasing the potential bump caused by the presence of the filament and its supports. All these steps are made possible by the use of thoria-coated iridium filaments. An additional benefit is that the design of support pins is simplified, as described below.

The'l spacing is described herein as an optimum fraction of D which is the minimum cross section dimension of the pump body in a plane passing through the central electrode 50. Generally, the orbitron 10 has the form of a cylinder and D is the diameter of outer annular electrode 20 as shown in FIG. 1A. But the pump can also have other shapes such as an ellipse or a rectangle as shown in the pump 10' of FIG. 1B, where the characteristic dimension D is the smallest dimension across the outer electrode 20 passing through the rod anode 50'. It is also in point to note that the electrode 50 or 50' (FIG. 1A or 1B, respectively), may be a grid or cylinder contained in a larger pump body. In that case D still refers to the dimension across the annular electrode 50 5 or 50'. Dimensions 1 in excess of 7 D are made possible With the use of thoria-coated iridium filaments. The maximum l is held to less than %D to avoid excessive collision of the electrons with the electrode 50 or a necessity of high bias on the filament to avoid such collisions. Generally, an I spacing of 4 inch is preferred for fourinch pumps and one inch for six-inch pumps. Throughout the range of positions of I from about to D, high emission current can be obtained from thin filaments (up to milliamperes and typically 30 to 40 milliamperes) for high emission and good average trajectory lengths obtained and filament temperatures are held very low (about 1400 C. compared to prior art filament temperatures of about 2700 (3.). Also the

filaments

52 and 54 may be formed with welded lead connections at the ends of the helix. This allows close spacing of their wire leads (typically inch compared to about 4 inch in prior art devices) and spacing between the filament (e.g. 52) and the shield lead (e.g. 42) can be held very close between A and inch), thus limiting the disturbance caused by the filament structure in the orbitrons electrostatic field. The desirable range of filament to shield pin spacing may be expressed as between one and two helix diameters.

Referring to FIG. 2, there is shown a preferred form of anode, for utilizing the orbitron device as a high speed six-inch pump. The rod 50 is .08" diameter tungsten. The titanium cylinders 560 and 56F are 7 diameter by 1" long and separated by a ,4 spacing. The cylinders 56N and 56M are diameter by 1" long and also apart. The letter dimensions are as follows (in inches):

a .250 b .375 c 3,250 d 4.0 e 1.0 Radius l-1.0

Springs 57 are used to hold the titanium cylinders in place. The filament helices were /3 in length. The

filaments

52, 54 were made of .005" diameter thoria-coated iridium wire made as described in Redhead; Can. J. Physics, vol. 40: p. 1814 (1962) and Hanley, Journal of Applied Physics, vol. 19: p. 583 (1948) and coiled in a helix diameter of .030" with a pitch of 16 turns per inch.

FIG. 3 shows a typical performance curve for a (nominal) six-inch pump constructed as described above. The abscissa of the curve is pressure in torr and the ordinate is argon pumping speed in liters per second. The speed of 21-22 liters per second at low pressure is very high for this size pump in comparison to prior art devices. The same pump was tested for air speed and pumped air at speeds in excess of 1,000 liters per second at low pressure (below l torr). The argon speed tests were conducted with an anode potential of 10 kilovolts and filament emission current of 50 milliamperes and filament bias of 120 volts positive. The air speed tests were conducted with the anode at 10 kilovolts, filament emission current -50 ma. and filament bias -l30 volts positive. Both

filaments

52 and 54 were operated at temperatures of about l300 C. to 1400 C.

These significant improvements in performance are due to the use of the thoria-coated iridium filaments. The l spacing of one inch would not be feasible with the conventional tungsten filaments. The large I spacing gives a high electron average trajectory length for ionization of gas molecules and this mechanism accounts for substantially all the pumping of argon.

While I have described my improved orbitron device in terms of its usage as a pump, usage of the device is also possible as a gauge, amplifier, or in other forms, utilizing the features of the present invention. It is therefore intended that the above description shall be read as illustrative and not in a limiting sense.

What is claimed is:

1. An orbitron device comprising, in combination,

(a) an outer annular electrode,

(b) a straight inner electrode located centrally within said outer electrode so that a space is formed between the inner and outer electrodes, said inner and outer electrodes occuping a length in excess of the minimum cross section dimension of said outer electrode,

(c) means for producing a positive high voltage potential on said inner electrode,

((1) at least one electron emitting filament made of thoria-coated iridium and located at an end of the axial space between electrodes and at a radial distance from said inner electrode of between about and the minimum cross section dimension of said outer electrode, the filament being essentially parallel to the inner electrode, and

(e) means for shielding the filament from the inner electrode.

2. The device of claim 1 wherein the filament has the form of a helix with its axis parallel to the inner electrode, the helix being supported on a pair of lead pins with one of said pins constructed and arranged as said shielding means.

3. The device of claim 2 with said pins being parallel over a portion of their lengths with a spacing, the coil diameter of the filament being less than one pin diameter.

4. The device of claim 2 with the helix filament being essentially parallel to its shield pin and having a spacing therefrom of between one and two helix diameters.

5. An orbitron vacuum pump comprising, in combination,

(a) an outer annular electrode,

(b) a straight inner electrode located centrally within said outer electrode so that a space is formed between the inner and outer electrodes, said inner and outer electrodes occuping a length in excess of the minimum cross section dimension of said outer electrode, the inner electrode having getter material mounted thereon,

(d) at least one electron emitting filament made of thoria-coated iridium and coiled into a straight helix which is located at an end of the axial space between electrodes and at a radial distance from said inner electrode of between about and the minimum cross section dimension of said outer electrode, the filament helix being essentially parallel to the inner electrode, and

(e) a pair of lead pins supporting the helix filament at its ends, one of said pins being disposed between the filament and inner electrode and parallel thereto to serve as a shield, the diameter of said shield pin being greater than the helix diameter, and the shield pin and helix having a spacing therebetween of from one to two helix diameters, the lead pins being parallel along the portion thereof axially beyond the filament and having a spacing of about three pin diameters.

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

Claims

1. AN ORBITRON DEVICE COMPRISING, IN COMBINATION, (A) AN OUTER ANNULAR ELECTRODE, (B) A STRAIGHT INNER ELECTRODE LOCATED CENTRALLY WITHIN SAID OUTER ELECTRODE SO THAT A SPACE IS FORMED BETWEEN THE INNER AND OUTER ELECTRODES, SAID INNER AND OUTER ELECTRODES OCCUPING A LENGTH IN EXCESS OF THE MINIMUM CROSS SECTION DIMENSION OF SAID OUTER ELECTRODE, (C) MEANS FOR PRODUCING A POSITIVE HIGH VOLTAGE POTENTIAL ON SAID INNER ELECTRODE, (D) AT LEAST ONE ELECTRON EMITTING FILAMENT MADE OF THORIA-COATED IRIDIUM AND LOCATED AT AN END OF THE AXIAL SPACE BETWEEN ELECTRODES AND AT A RADIAL DISTANCE FROM SAID INNER ELECTRODE OF BETWEEN ABOUT 3/16 AND 3/8 THE MINIMUM CROSS SECTION DIMENSION OF SAID OUTER ELECTRODE, THE FILAMENT BEING ESSENTIALLY PARALLEL TO THE INNER ELECTRODE, AND