US2796555A - High-vacuum pump - Google Patents

Description

June 1957 R. J. CONNOR HIGH-VACUUM PUMP Filed June 29, 1954 FIG. I

Peltented June 18, 1957 2,796,555 HIGH-VACUUM PUMP Richard 3. Connor, Medsvay, Mass.,- assignor to High Voltage Engineering Corporation, Cambridge, Mass, at corporationof Massachusetts Application June 29, 1954, Serial No. 440,137

14 Claims. (Cl. 315-108) This invention relates to high-vacuum umps, and in particular to a novel high-vacuum pump of the continuous-gettering type.

A major use of high vacuum pumps is in connection with the evacuationof those regions of charged-particle accelerators wherein charged particles are accelerated to high energy. Such regions must be highly evacuated in order to minimize collisions of the charged particles being accelerated with gas molecules or ions, since such collisions tend to prevent the individual charged particles from attaining the high energy desired.

At the present time, acceleration tubes are commonly evacuated and a high degree of vacuum maintained by means of diffusion pumps in conjunction with cold traps. A new type of high-vacuum pump to replace such diffusion pumps and cold traps has been developed at the University of Wisconsin. in the Wisconsin pump, the system to be evacuated is first partially evacuated by a fore pump, and the system is then sealed on. Titanium in wire form is evaporated continuously inside a chamber which forms a part of the se'aled-ofi system. The gettering action of the titanium, which is thus continuously deposited on the inner wall of the chamber, results in a pumping action, and a high vacuum may be obtained in this manner. A brief description of the Wisconsin pump appears in volume 89' (second series) of The Physical Review at page 897 (issue of February 15, 1953).

Major advantages of the continuous-gettering-type pump are its simplicity and the fact thatit makes possible the construction of an acceleration tube as a sealed-01f system.

Generally pumps of this type must include means for producing ionization within the system to be evacuated, in order to remove substances, such as argon and helium, which do not otherwise readily respond to the get-tering action.

My invention comprehends an improved high-vacuum pump of the continuous-gettering type, wherein is provided novel means for evaporating the getter material. In particular, my invention includes a continuous-gettering pump in which, although the gettering action is continuous during operation of the evacuated unit, the getter material is only intermittently evaporated. My invention also includes the incorporation of a vacuum gauge in an improved high-vacuum pump of the continuousgettering type, so as to provide a simple and compact unit.

'In the drawing:

Fig. 1 is a diagram illustrating the essential components of my improved high-vacuum pump;

'Fig. 2 is a View illustrating in detail novel means for evaporating the getter material; and

Fig. 3 is a somewhat diagrammatic view in vertical cross-section of a preferred embodiment of my invention, and illustrates the incorporation of a vacuum gauge in my improved high-vacuum pump; and

Fig. 4 is a somewhat diagrammatic view in vertical cross-section of a closed-01f multiple-electrode accelera tion tube incorporating my invention.

Referring to the drawings, and first to Fig. 1 thereof, the essential components of my improved high-vacuum pump" are enclosed within a chamber 1 which is connected to the system 2' to be evacuated by a suitable length of tubing 3. A second length of tubing 4 connects the volume, enclosed by the chamber 1 and the system 2 to be evacuated, to a forepu'mp' 5. After the system 2 has been partially evacuated 'by the fore-pump 5, it is closed ofi from the atmosphere by a' suitable valve 6.

Further evacuation is then accomplished by the gettering action of a suitable material, preferably titanium, assisted by suitable means for ionizing at least some of the residual gases. Any suitable ionization means may be employed, such as a conventional ionization gauge, and the ionization means is therefore indicated merely diagrammatically at 7.

In accordance with my invention, a tungsten filament 8' is supported Within the chamber 1 by conductive members 9' which are connected to a suitable voltage source 10"by leads 11. Thetungsten filament 8 is covered by titanium, which may be deposited thereon-by an electrolytic plating process or in any other convenient manner. A simple manner of obtaining the required titanium covering is to overwind thetungsten filament 8 with smalldiameter titanium wire, as indicated at 12 in Fig. 1. The tungsten filament '8, overwound withtitanium wire 12, is illustrated in detail in Fig. 2.

The'high-vacuum pumping action is commenced merely by setting the ionization means 7 in operation, and by turning. on the voltage source 10, so that sufiicient electric current flows through thetungsten filament 8 to heat the same and cause evaporation of titanium from thetitanium wire 12. The titanium vapor is deposited on the walls of the chamber 1 and, by a gettering action, pumps the residual gases from the system to be evacuated.

Tests conducted by me have'indicated that a continuousgettering pump constructed as hereinbefore described vgives excellent performance, particularly in the evacuation of acceleration tubes of the type shown in Fig. 4 and including a multiplicity ofelectrodes 21 separated byinsulators 22 and cemented thereto by an organicplastic cement 23. In one such test, an acceleration tube having 3 1 electrodes separated by insulators %-lI1Cl1 thick and sealed thereto by vinylseal cement, and including .ametal tube extension 18 inches long, could be evacuated at will to a pressure of about 1 l0 mm. Hg solely by means of the high-vacuum pump herein described and claimed. After 1000 hours the apparatus had to be dismantled for other reasons, and the pump was still operating satisfactorily at that time. During the 1000 hours, the acceleration tube was periodically operated at l megavolt with an electron beam current of 250 microamperes for a total of 60 hours.

The acceleration tube just described evolves gases at a rate of about 10*" mm.-liters per second when the tube is not in operation, and at a rate of about 5 l0- mm.- liters per second when the tube is in operation. Hence, during the hours of operation of the acceleration tube, the pump absorbed a total of more than 2 atmosphere-cc. of gases.

A suitable forepump can evacuate such an acceleration tube to about 10- mm. Hg in about 15 minutes. After this preliminary evacuation, the system may -be closed ofi and the forepump disconnected. Referring to Fig. 1, the voltage source 10 may be turned on for a period of about 10' minutes to 1 hour, and the resultant layer of titanium deposited on the inner walls of the chamber 1 is sufiicient to provide the proper pumping action for about 8 hours operation of the acceleration tube. The ionization means 7, however, must be in operation continuously while the acceleration tube is lacing operated,

3 and should be turned on at least 3 minutes before the acceleration tube is to be utilized.

In the aforementioned test, a .045-inch-diameter tun sten filament 3 inches .in, length was overwoun'dwith .OZO-inch-diameter titanium wire (as illustrated in Fig. 2), and the voltage source (Fig. l) was a transformer with a variac, operating at 4 volts and 65 amperes. The ionization means 7 (Fig. l) was a conventional Penningtype ionization gauge. a

It will thus be noted that, while the gettering action of the titanium acts continuously during operation of the acceleration tube, it is not necessary to evaporate the titanium continuously. Consequently, very little power is required to produce the desired pumping action, and the necessity for cooling parts of the acceleration tube structure which would result from prolonged heating of the titanium is avoided. Moreover, the undesirable increase in the rate of gas evolution from organic plastics used to cement the components of the acceleration tube, which results from the decomposition of complex gas molecules in the presence of heated surfaces within the tube, is also minimized by evaporating the titanium only intermittently. It is possible to use a carbon filament in place of the tungsten filament 8 (Fig. 1), but a carbon filament will require about twice as much power from the voltage source 10 as thetungsten filament 8. 7

Titanium is by far the best material to provide the necessary gettering action, although it would be possible to use a suitable filament 8 (Fig. l) overwound with zirconium ormagnesium wire 12. a

As hereinbe-fore stated, any suitable device may be employed as the ionization means 7 (Fig. 1). However, a simple arrangement is illustrated in Fig. 3, wherein a hot cathode, comprising atungsten filament 13 energized by asuitable voltage source 14, emits electrons which are attached to a tantalum plate 15, which is maintained at a positive potential with respect to the cathode by a platevoltage source 16. The electron flow from thecathode 13 to the plate provides the necessary ionization. Thetungsten filament 8 overwound withtitanium wire 12 may then be positioned as shown in Fig. 3. The entire highvacuum pump may thus be enclosed within the chamber 1, which comprise, for example, a 25-mm. diameter Pyrex glass bulb.

Moreover, the arrangement of Fig. 3 may also serve as an ionization gauge, in addition to its function as a highvacuum pump. To that end, the plate 15 may be perforated, as indicated by the multiplicity of apertures 17, so that many of the electrons attracted from thecathode 13 to the plate 15 continue to travel beyond the plate 15 and towards thetungsten filament 8 overwound withtitanium 12. If a small negative voltage is applied to thetungstentitanium filament 8, as by abattery 18, then any positive ions produced by the electrons after traveling through the apertures 17 will be collected by the tungsten-titanium filament 8. By means of twomicroammeters 19, 29, one 19 of which measures the current between thecathode 13 and the plate 15, and the other 20 of which measures the ion current collected by the tungsten-titanium filament 8, the pressure within the chamber 1 may be measured.

The voltages supplied by the plate voltage source 16 and thebattery 18 are not critical. Merely by way of example, the plate 15 may be maintained at +200 volts and the tungsten-titanium filament 8 at 22 volts with respect to thecathode 13. 7

Having thus described the preferred construction of a high-vacuum pump in accordance with my invention, toward with several modifications thereof, including the incorporation of a vacuum gauge in such a high-vacuum pump, it is to be' understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

I claim: 7

I 1. A high-vacuum pump comprising in combination 4 with a chamber to be evacuated: means for ionizing at least some of the residual gases within said chamber, a filament supported within said chamber, a coating of getter material about said filament, and means to create sufiicient electric current through said filament to heat said getter material and cause evaporation thereof.

2. A high-vacuum pump of the continuous-gettering type, comprising in combination: a chamber in communication with the space to be evacuated, said chamber and said space forming a clolsed-ofi system, means for ionizing at least some of the residual gases within said system, a filament supported within said chamber, a coating of v :getter material about said filament, and means to create suflicient electric current through said filament to heat said getter material and cause evaporation thereof.

3. A higl1-vacuum pump comprising in combination with a chamber to be evacuated: means for ionizing at least some of the residual .gases within said chamber, a

' cient electric current through said filament to heat said titanium and cause evaporation thereof.

, 5. A high-vacuum pump in accordance with claim 4,

wherein said filament comprises a length of tungsten wire. 6. A high-vacuum pump in accordance with claim 4,

wherein said filament comprises a length of carbon in filamentary form. I

7. A combination high-vacuum pump and vacuum gauge comprising in combination with a chamber to be evacuated: an electron-emitting cathode supported within a said chamber; a filament supported within said chamber and spaced from said cathode, said filament being overwound with titanium wire; means to create suflicient electric current through said filament to heat said titanium and cause evaporation thereof; means for maintaining said filament at a negative potential with respect to the potential of said cathode; a perforated plate of conducting material supported within said chamber between said cathode and said filament; means for maintaining said plate at a positive potential with respect to the potential of said cathode; means for measuring the current between said cathode and said plate; and means for measuring the ion current collected by said filament.

8. A combination high-vacuum pump and vacuum gauge in accordance with claim 7, wherein said perforated plate is composed of tantalum, and wherein said filament is composed of tungsten.

9. In combination with an acceleration tube for the acceleration of electrons to high energy, the interior of said acceleration tube being closed olf from the surrounding atmosphere: means other than the high-energy electron beam accelerated by said acceleration tube for ionizing, during at least some of the time that the acceleration tube is closed off from the surrounding atmosphere, at least some of the residual gases within said acceleration tube, a filament supported within said acceleration tube, a coating of titanium about said filament, and means intermittently to create sufiicient electric current through said filament to heat said titanium and causes evaporation thereof. a

10. In an electron acceleration tube the interior of which is closed olf from the surrounding atmosphere: means other than the high-energy electron beam accelerated by said acceleration tube for ionizing, during at least some of the time that the acceleration tube is closed off from the surrounding atmosphere, at least some of the material about said filament; and means to create, during only a small fraction of the time during which said acceleration tube is closed ofi from the surrounding atmosphere, sufficient electric current through said filament to heat said getter material and cause evaporation thereof.

11. Apparatus in accordance with claim 10, wherein said getter material comprises titanium.

12. In an electron acceleration tube the wall whereof is composed of a multiplicity of apertured-disk electrodes and insulating rings alternating therewith and sealed thereto by an organic plastic material: means other than the high-energy electron beam accelerated by said acceleration tube for ionizing, during at least some of the time that the acceleration tube is closed olf from the surrounding atmosphere, at least some of the residual gases within said acceleration tube; a filament supported within said acceleration tube; a coating of titanium about said filament; and means to create, during only a small iraction of the time during which said acceleration tube is closed off from the surrounding atmosphere, sufiicient electric current through said filament to heat said titanium and cause evaporation thereof.

13. In combination with a sealed-off electronic vacuum tube at least a portion of the interior surface of which contains organic plastic material: means for ionizing at least some of the residual gases within said vacuum tube; a filament supported within said vacuum tube; a coating of getter material about said filament; and means intermittently to create sufiicient electric current through said filament to heat said getter material and cause evaporation thereof.

14. Apparatus in accordance with claim- 13, wherein said getter material comprises titanium.

References Cited in the file of this patent UNITED STATES PATENTS 2,550,498 Rice Apr. 24, 1951 2,573,005 Glyptis Oct. 30, 1951 2,653,620 Morgen Sept. 29, 1953 OTHER REFERENCES Evapor-Ion Pump, by H. G. Herd, R. H. Davis, A. S. Divatis and D. Saxon, Physical Review, 2d Series, vol. 89. No. 4, p. 897, Feb. 13, 1953.

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