US3140820A - Method for maintaining very high vacuum in a system - Google Patents

Description

July 14, 1964 c usme 3,140,820

METHOD FOR MAINTAINING VERY HIGH VACUUM IN A SYSTEM Filed May 51, 1962 2 Sheets-Sheet 1 {l i 4 g fig m /7Z l J) f-wro VACUUM PUMP T W i q. 1. 7

INVENTOR. Robe/"f E. C/ausing BY ATTORNEY July 14, 1964 R. E. CLAUSING METHOD FOR MAINTAINING VERY HIGH VACUUM IN A SYSTEM Filed May 31, 1962 2 Sheets-Sheet 2 (OHS/$83117) OHBdS WELLSAS mIO 546 m0 mwqoiv 0 mvizmon Roberf E. C/ausing S NOllQVHd smmous ATTORNEY.

United States Patent 3,140,820 METHOD FOR MAINTAINING VERY HIGH VACUUM IN A SYSTEM Robert E. Clausing, Oak Ridge, Tenn, assignor to the United States of America as represented by the United States Atomic Energy Commission Filed May 31, 1962, Ser. No. 199,205 4 Ciaims. (Cl. 230-69) This invention relates to vacuum systems and more particularly to an apparatus and method for maintaining very high vacuum in a system where a gaseous material is continuously introduced into the system.

The research on controlled thermonuclear fusion requires the disposal of large quantities of hydrogen or hydrogen isotopes (as well as other gases) so as to achieve and maintain very low pressures. Vacuum pumping requirements range between and 10 liters/sec. at pressures from 10* to 10 mm. Hg. The attainment of very large pumping speeds with liquid-nitrogen-trapped or zeolite-trapped diffusion pumps is feasible but would require very large and expensive equipment. The space required by such equipment places severe restrictions on the design of experimental apparatus and often would require compromising the most desirable experimental arrangement to permit the necessary vacuum manifolding and equipment.

As a result of these considerations, several other pumping methods were investigated. The present development relates to the improvements in the method and equipment, and results obtained, on a large-scale getter pumping arrangement.

Applicant with a knowledge of the problems of the prior art has for an object of his invention a method and an apparatus for providing very high pumping speeds in connection with the maintenance of a very high vacuum.

Applicant has as another object of his invention the provision of a system for maintaining very high vacuum in a device by employing an efficient large-scale getter pumping facility in combination with a modest size diffusion pump.

Applicant has as a further object of his invention the provision of a high vacuum pumping facility employing titanium as a getter for high speed pumping by evaporation of the titanium and subsequent deposition on a surface at a very low temperature.

Applicant has a further object of his invention the provision of a high vacuum pumping facility employing titanium as a getter for high speed pumping by evaporation of the titanium and subsequent deposition on a surface in the presence of an inert gas at a temperature maintained through circulation of water at room temperature.

Applicant has as a still further object of his invention a method of increasing the sorption rates of getter deposits more than tenfold by depositing the getter material on a liquid nitrogen cooled surface in the presence of an inert gas.

Other objects and advantages of my invention Will appear from the following specification and accompanying drawings, and the novel features thereof will be particularly pointed out in the annexed claims.

In the drawings, FIGS. 1, 2 show vertical sections of a large vacuum system with the essential auxiliary components associated therewith.

FIG. 3 is a set of graphs showing the sticking factor for hydrogen (which is directly proportional to pumping speed) resulting from the getter surface prepared by:

(a) deposition on a water-cooled surface, (b) deposition on a water-cooled surface in the presence of helium,

(c) deposition on a liquid nitrogen-cooled surface, and

(d) deposition on a liquid nitrogen-cooled surface in the presence of helium.

A getter pump is one where an active metallic surface is provided on which sorption of gases occurs. Such surfaces may be prepared in several ways, but in this instance getter vapors are deposited on suitable substrates at low pressures by evaporation from a heated source. Since the getter surface must be prepared at low pressures, a suitable means must be supplied for bringing the system to this pressure and maintaining it there during getter preparation. The getter does not have any appreciable capacity for sorption of inert or chemically inactive gases. therefore, an auxiliary method of removing these gases to very low pressures is also required. These and other considerations lead to the very useful, novel combination of a large getter pumping speed for the bulk of the gas load with a modest size diffusion pump system to provide the initial pump-down and the pumping of chemically inert gases.

Referring to the drawings in detail, FIG. 1 shows the arrangement used to study getter pumping. In one embodiment, the vacuum tank 1 has the dimensions 36 inches in diameter and 36 inches high. It is made of aluminum and has a removable, cooled copper liner 2,2',2" which fits loosely on the inside. The cooling is accomplished by circulating coolant, such as water or liquid nitrogen, through tubes 12. The liner 2,2,2" is in three sections and its original purpose was to permit easy removal of the getter deposits. It was not, however, necessary to clean the liner during the first 150 days of operation. A medium size (such as a 6 inch MCF 700) diffusion pump 3 is separated from the tank 1 by a conventional Freoncooled trap 4 (at about 40 C.) and an isolation valve 5. A mechanical backup or fore-pump (not shown) is connected to the duct 18 of the diffusion pump 3. A vacuum lock 6, which may comprise a series of cylindrical conduits 50, 51 and valve 16 are provided to permit changing the Ti-Ta or Ti-Cb getter evaporator assemblies with out losing the vacuum inside the tank. A suitable vacuum gage 15, (e.g., Veeco RG- ion gage) communicates with the interior of the tank 1 through a manifold 17. The lock 6 is provided with a vacuum roughing valve 11 for pump-down after insertion of an evaporator assembly. The vacuum seals (not shown) throughout the system are neoprene gaskets and O-rings with silicone vacuum grease lubrication. Other seals with less outgassing would be preferable.

The getter is evaporated from a source 7 near the center of the liner and deposited on the cooled liner. During getter evaporation an inert gas may be introduced through a suitable valve 13 to bathe the liner 2. A shutter mechanism on the end of vacuum gage manifold 17 protects the vacuum gage 15 during the deposition of the titanium upon the surface of the liner. The shutter may be formed, for example, by coaxial tubes which may be rotated to align corresponding apertures near their lower extremities. Once the deposit is established, the inert gas flow is stopped. To determine sorption characteristics of the getter deposit, appropriate gas (hydrogen) is admitted through leak valve 8 at a known rate. The pressure increase is noted, using the gage 15, as the gas is introduced through line 9 and diffusion 10 into tank 1. The sticking factor, a, is calculated from this data by means of suitable methods based on the geometry of the system.

The evaporation source 7 is electrically heated by direct current passing therethrough from a suitable power source (not shown) connected to water-cooled leads 14, 14. About 450-500 amperes. are passed through the filament at a low voltage. Approximately 10001500 w. are required during evaporation. A suitable evaporation source 7 consists of a 10-inch tantalum rod, 0.170 inch in diameter, the center 7 inches which is wrapped with one layer of close-Wound 0.030 inch diameter columbium wire and then two layers of 0.035 inch diameter titanium wire. The wound assembly was bent into the shape shown in FIG. 1 for convenience in passing it through the vacuum lock. The purpose of the colnmbium wire is to alloy with the titanium and raise the melting point of the source, thereby permitting operation at higher temperature, thus achieving faster, more easily controlled evaporation. This source will evaporate about 3 g. of titanium per hour and may be used for at least four hours at this rate. A larger version of this source, with an active length of 35 inches, required 7500 W. power input at 500 amperes and evaporated at a rate of 16 g./hr.

The operations using this equipment consisted of evapcrating titanium and admitting a gas to determine its rate of sorption. For simplicity in report ng, all of the data reported in terms of the sticking factor, a (the fraction of molecules absorbed compared to those that strike the surface). The sticking factor, a, was: determined as a function of the gas leak rate for a constant titanium evaporation rate. Specific speeds in liters per second per square centimeter at 25 C. can be obtained by multiplying a by the constants F listed in Table I. Total surface pumping speeds for this system are obtained by multiplying a by the constants, K also listed in Table I.

Data for the sorption of hydrogen and other gases on titanium deposits are shown in Table II. In these experiments it was noted that substantial variations in the sticking factor were obtained when conditions of operation (e.g., liner temperature, gas compositions, etc.) were varied. These results are given in Table II with additional comments below, referring to FIG. 2:

Curve a.-Sorption onto films deposited on watercooled surface-The titanium films formed in high vacuum by deposition on the water-cooled substrate are metallic in appearance and have a well-developed crystal structure as evidenced by sharp X-ray diifraction patterns.

Curve b.Sorpti0n onto films deposited on watercooled surfaces in the presence of inert gas-Evaporation of titanium films in a pressure of approximately two microns of helium or argon produces deposits which sorb gases much more rapidly than films deposited under a imilar circumstances in high vacuum. This improved sorption rate is associated with a very pronounced change in the appearance and structure of the film. The films obtained in these evaporations have a velvety black appearance and are very poorly crystallized.

Curve c.--S0rpti0n onto films deposited on substrates at liquid nitrogen temperature (195 C.).The highvacuum deposition of titanium onto substrates at liquidnitrogen temperatures produces films which have a metallic appearance but have poorly formed crystal structures. These films exhibited high gas sorption rates.

Curve d.S0rpti0n onto films deposited on substrates at liquid nitrogen temperature in the presence of helium.-The sorption rate of hydrogen onto films deposited in a 2.5 helium atmosphere, with the substrate at liquid nitrogen temperature, was the highest of all of the films tested. Titanium films produced under these conditions had the same velvety black appearance and poorly developed crystal structure as the films deposited at 10 C.

From the foregoing, it is apparent that the combination of improved getter pumping with diffusion pumping is well suited for obtaining high speeds for hydrogen, deuterium oxygen, nitrogen, carbon dioxide, and carbon monoxide. Sticking probabilities, as indicated in Table II, have been measured for several gases on titanium films produced under a variety of conditions. Maintenance is not a problem, and the over-all reliability, practicality, and limitations of the systems have been established. Several methods of increasing the sorption rates of the getter deposits have been discovered, as described herein, including evaporation in the presence of an inert gas, evaporation onto liquid-nitrogen cooled substrates and the combination of both methods. Sticking factors for hydrogen in excess of 0.8 have been consistently obtained on films deposited in the presence of helium at C. as compared with sticking factors of approximately 0.05 on films deposited in high vacuum onto a water-cooled substrate.

FIG. 2 shows in more detail, the mechanism for inserting and withdrawing the source 7 of FIG. 1. The lead Wires 14, 14 enter through apertures in top cover 53 and top plug 52, and pass through the length of tube 51. The tube 51 passes through bushing 50, which is filled with packing to provide a vacuum seal around the tube wall. Tube 51 also passes through vacuum lock 6, which is a hollow cylinder provided with a closure valve 16. To change sources, tube 51 is pulled upward, bringing source 7 up to the position shown in the dotted lines, above valve 16. The valve seat is moved in the direction of the arrow and seals-oh. the large chamber volume from the upper part of the vacuum lock. Then the unit may be disassembled, a new source installed, the lock 6 pumped down through valve 11, valve 16 reopened, and source 7 reinserted into the large chamber, all without losing the vacuum in the large chamber.

Such arrangement is provided only for convenience in source changing, and is not essential to the fundamental principles of the invention herein described, the scope of which is limited only as set forth in the appended claims.

Table I.C0nstants for Conversion of the Sticking Factor (a) to Specific Speeds and t0 Surface Pumping Speeds Table 2.Initial Sticking Factors for Various Gases 0n Titanium Films Deposited Under Several Conditions Gas Species High Vacuum, High Vacuum,

10 C. 2 I-Ielium, 195 C. 2 Helium 10 0. 195 0. Batch Batch Continuous Batch Evapo- Continuous Batch Evapo- Evapo- Evapo ration Evapo- Evaporation ration ration ration ration Hydrogen Dcuteriurm.

Nitrogen Carbon Monox- Methane "l:

Having thus described my invention, I claim:

1. A system for maintaining high vacuum in a vessel during the introduction of gaseous material comprising a substantially closed vessel having an inner surface cooled by liquid nitrogen, pumping means providing a reduced pressure in the vessel, means for evaporating getter material in the vessel to contact said cooled surface to pro duce condensation thereon, and means to introduce an inert gas into the vessel for bathing the cooled surface while the getter material is being deposited.

2. A system for maintaining high vacuum in a vessel during the introduction of gaseous material comprising a substantially closed vessel for receiving a gaseous material, a dilfusion pump for producing a vacuum in the vessel, means for cooling an inner surface in the vessel to a low temperature, means in the vessel for evaporating a getter material to contract said surface to condense thereon, and a leak valve for introducing inert gas near the cooled surface to bathe it during the condensation of the getter material thereon.

3. An apparatus for maintaining high vacuum in a system comprising a vessel, a liner having a surface disposed therein, cooling tubes carried by the liner for passage of a coolant to lower the temperature of the liner, a filament of getter material located in an intermediate portion of the vessel, means for heating the filament to evaporate the getter to permit condensation of the vapors on contact with the surface, and an inlet adjacent the surface for introducing inert gas to bathe the surface while the getter is deposited thereon and a further inlet for introducing gaseous material to the vessel for sorption by the getter in such a Way as to permit accurate measurements of the sorption processes.

4. An apparatus :for maintaining high vacuum in a system during the introduction of gaseous material comprising a chamber, a getter filament disposed centrally of said chamber, a liner disposed along the inside Walls of said chamber, means for cooling said liner at liquid nitrogen temperature, means for electrically heating said filament to evaporate getter material therefrom for deposition upon said liner, and means for introducing an inert gas into said chamber for bathing the cooled liner while the getter material is being deposited, said getter material on said liner serving as a means for sorption of said gaseous material at very high rates thereby maintaining the vacuum Within said chamber in spite of the introduction of gaseous material.

No references cited.

Claims (1)

1. A SYSTEM FOR MAINTAINING HIGH VACUUM IN A VESSEL DURING THE INTRODUCTION OF GASEOUS MATERIAL COMPRISING A SUBSTANTIALLY CLOSED VESSEL HAVING AN INNER SURFACE COOLED BY LIQUID NITROGEN, PUMPING MEANS PROVIDING A REDUCED PRESSURE IN THE VESSEL, MEANS FOR EVAPORATING GETTER MATERIAL IN THE VESSEL TO CONTACT SAID COOLED SURFACE TO PRODUCE CONDENSATION THEREON, AND MEANS TO INTRODUCE AN INERT GAS INTO THE VESSEL FOR BATHING THE COOLED SURFACE WHILE THE GETTER MATERIAL IS BEING DEPOSITED.

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