US2858972A - Ionic vacuum pump - Google Patents

Description

. 1958 A. M. ukEwlTscl-y 2,358,972

IONIC VACUUM PUMP Filed March 27, 1956 In ve ntorz- A na to/e M Gu rsewitsch,

/"//'s Attorney.

United tates Patent IONIC VACUUM PUMP Anatole M. Gurewitsch, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application March 27, 1956, Serial No. 574,226

2 Claims. (CL 230-69) This invention relates to ionic vacuumpumps and, more particularly, to a method whereby the elficiency. of such devices is increased for the pumping of inert gases at very low pressures by means of a working gas, a gas which may be added to the system and removed at a later time by gettering action without any resulting net increase in the amount of gas in the system.

In the copending application, Serial No. 350,964, filed April 24, 1953, now Patent 2,755,014, dated July 17, 1956, of Willem F. Westendorp and Anatole M. Gur'ewitsch, and assigned to the same assignee as this application, there is disclosed an ionic pump of the type to which this invention is applicable. p

Such a pump may comprise a ring-shaped anode and opposed cold cathode plates enclosed in a vacuum tight envelope and means to provide a magnetic field axially of the electrodes. In operation, after a rough vacuum has been established by a separate fore pump, the ring electrode is energized at a positive potential relative to the cathodes to initiate a cold-cathode electric gas discharge. Free electrons accelerated toward the anode are constrained to move along spiral paths by the axial magnetic field. Gas molecules from the system under evacuation are ionized by electron bombardment and the resulting positively charged ions are accelerated into and absorbed by the cold cathodes. Since the pumping action does not necessarily depend upon any chemical gettering, 'it is especially adapted to the removal of inert gas residues.

A limitation of the operation of such devices is that the pumping speed depends upon the discharge current which is a function of the gas pressure in the system. Consequently, for given values of accelerating potential and magnetic field strength, there is a limiting pressure below which the discharge current becomes irregular and tapers oif, resulting in a decreased and ineffectual pumping speed.

Therefore, it is an object of this invention to provide a method whereby the efiiciency of an ionic pump is increased for the pumping of inert gases at extremely low pressures.

According to this invention, a working gas which may be gettered chemically is introduced into a system being evacuated by an ionic pump when the limiting pressure for the cold cathode gas discharge is approached. The amount of this working gas is controlled to maintain optimum pumping of the mixture of the working gas and the system gas until the partial pressure of the system gas has been reduced to the desired limit. The working gas is then gettered chemically to produce the final vacuum.

The features of the invention desired to be protected herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in comiection with the accompanying drawings, in which:

Fig. 1 is an elevational view partially broken away inert gases in a system under evacuation.

sure decreases the discharge current also decreases. is found in practice that the pumping speed diminishes asymptotically as the pressure is reduced to the limiting 2,858,972 Patented Nov. 4, 1958 of an ionic vacuum pump and apparatus for performing the method of this invention; and

, invention.

The ionic pump may comprise a pill-box-shaped evacuable envelope 3, a ring electrode 4, and disc electrodes 5 and 6 of a gas absorbing material such as carbon.

Soft iron pole pieces 7 and 8 and permanent magnet 9 supply a magnetic field axially of electrode 4. Communication between the pump 1, auxiliary device 2, and the system to be evacuated (not shown) is by means of tubulations 10 and 11. The high voltage source 12 is provided to maintain the ring electrode 4 at a positive potential relative to cold cathode discs 5 and 6 which are in contact with the envelope at ground potential. Connection to the ring electrode 4 is by means of a metallic cap 13 and electrode support 14, the cap 13 being insulated from the envelope 1 by a glass member 15.

The pumping speed for the ionic pump is substantially proportional to the dischargecurrent, which is a function, ia(V,B,[7), of the anode voltage V, the magnetic field strength B and the pressure p. In general, as the prespressure for a given pump and gaseous system. For example, with V=5,000 volts and B=800 gauss, the limiting pressure, p;,, is.about 0.006 micron in a closed helium system. Under the same conditions, with the voltage increased to V=10,000 volts, 11 is found to be 0.0025 micron. It is seen that, to a certain extent, an increased discharge current and hence a lower limiting pressure is obtained by increasing the anode voltage. However, this control is limited by attendant difficulties in the supply of such high voltages and because an increase of potential beyond a certain limit without an accompanying increase in magnetic field causes electrons to be collected at the anode before they are able to produce a sufficient number of ions. This results in a net loss in efficiency.

Applicant has discovered that a working gas may be employed to reduce the otherwise limiting pressure for Apparatus 2, for the supply and removal of the working gas may comprise, for example, a foil 16 of titanium or other material containingan adsorbed gas. The working gas such as hydrogen is chosen from those gases which are not inert, especially hydrogen, oxygen and nitrogen, which are known to be easily gettered-chemically. The properties of gases suitable as working gases and of getters therefor may be found described in the text by Saul Dushman entitled, Scientific Foundations of Vacuum Technique, published in 1949 by John Wiley & Sons, Inc.

As the pressure in the system is reduced by the pump 1 to a point where the discharge current becomes insufiicient for satisfactory pumping speeds, a current is passed through the leads 17 and 18 to heat the foil 16 and evolve a portion of the absorbed gas, so that the sum of the partial pressures of this working gas and the gas of the system is suflicient for the maintenance of a satisfactory discharge current. Pumping may then be continued at an efficient speed. When the desired partial pressure of the system gas has been reached, the working gas is removed from the system by means of a getter 19, for example a titanium filament, in the case of hydrogen, which is cycled through a suitable temperature range by a current supplied through the leads 20v and 21 according to conventional steps. The free hydrogen or other shown) of the working gas which'again may advantageously be hydrogen, oxygen, or nitrogen. Supply of the working gas may be regulated to maintain the discharge current at some optimum value while the pumping is continued to produce the desired evacuation. The valve 22 is then closed and a getter 24, illustrated as a nickel tube 25 having a thin wall portion 26 and containing barium 27 is heated as by passing current through the leads 28 and 29. The barium permeates the thin wall portion 26 and is evaporated upon the surrounding surfaces to getter the working gas. 1

While the invention has been described by reference to particular embodiments thereof, it will be understood that numerous changes can be made by those skilled in the art without actually departing from the invention, and it is the aim of the appended claims to 'cover all such equivalent variations as come 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. The method of evacuating a system containing inert gases by means of an ionic vacuum pump requiring sufficient gas pressure for the maintenance of an electric discharge for efficient pumping speed, which method comprises the steps of reducing the pressure of the system falls below an-optimum value, introducing into the system a working gas which can be gettered chemically to increase the total pressure in the system to a level where an electric discharge within the pump is readily maintained to restore and maintain the pumping speed at an optimum value, pumping the mixture of the system gases and the working gas until the partial pressure of the system gases has been reduced to a predetermined value, and removing the working gas from the system by means of a getter.

2. The method of evacuating a system containing inert gases by means of an ionic vacuum pump requiring sufiicient gas pressure for the maintenance of an electric discharge for etficient pumping speed, which method comprises the steps of reducing the pressure of the system gases by means of the pump until the pumping speed falls below an optimum value, introducing into the system a flow of a working gas to restore the pumping speed to an optimum value,'maintaining the flow of the working gas into the system at a rate to maintain the pressure in the system at a level where an electric discharge is readily maintained to maintain the pumping speed at an optimum value, pumping the mixture of the system gases and the working gas until the partial pressure of the system gases has been reduced to a predetermined value, interrupting the flow of the working gas into the system, and removing the working gas remaining in the system by means of a getter.

References Cited in the file of this patent UNITED STATES PATENTS 1,651,386 Gustin Dec. 6, 1927

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