US2838676A - Ion source - Google Patents

Description

ION SOURCE I June 10, 1958 F. RAIBLE ETAL 2,838,676 v 1 Filed July 9. 1953 5 Sheets-Sheet a FIG. 7

INVENTOR FRANK RAIBLE A ORNEY United States Patent ION SGURCE Frank Raible, Sands Point, Joseph E. Nemeth, Syosset, and Robert C. Nemeth, Huntington, N. Y., assignors to Vacuum-Electronics Engineering Co., New Hyde Park, N. Y., a partnership Application July 9, 1953, Serial No. 367,040

Claims. Cl. 250-419 This invention relates to ion sources and more particularly to ion sources which may be employed to ionize the molecules of the constituent gases which are drawn into instruments for purposes of analysis, as exemplified by ion sources employed in the mass spectrometer. Still more particularly, the invention relates to methods of operation of analytical instruments using electron beams as a means of ionization and employing oil diffusion pumps as evacuating means in devices, such as a mass spectrometer for leak detection in which a probe gas is employed.

Still more particularly, this invention relates to ion sources used in devices incorporating oil diffusion vacuum pumps generally referred to by K. C. D. Hickman in the articles entitled Vacuum pumps and pump oils, Journal of the Franklin Institute for February 1936, vol. 221, No. 2, page 215, and March 1936, vol. 221, No. 3, page 383, as well as in United States patents as follows: 1,857,506; 1,857,508; 2,080,421; 2,147,488; 2,147,479, as examples disclosing various diffusion pumps and pump oils employed for evacuation in connection with which the ion sources of this application may be employed, it being understood that the oil diffusion pumps and pump oils so described are referred to merely as examples.

Still more particularly, our invention relates to ion sources used in devices exemplified in the mass spectrometer in an arrangement described in the publication Journal of Applied Physics, vol. 18, pages 3033, January 1947, an article entitled Mass spectrometer for leak detection. V r 1 It is known'to us that some ion sources have the inherent defect of being subject to'harmful contamination when used in spaces evacuated' by oil diffusion pumps, due to the formation of insulating deposits which accumulate'on certain electrodes. These deposits progressively cause a decrease in the-amount of the useful ions formed, due to distortions in the desired electrical field distribution within theion'source itself. V

Known to us also is theexpedient attempting to correct this distortion in systems employing oil evacuating pumps byperiodically varying the-potential applied to the source Known. to us also are the efforts to influence the electron beam of the ion source by the influence of a collimating magnetic field in the ionization chamber. Such expedient is not only unsuccessful but, at its best, is con- .finedto a source providing a limited arrangement of the cooperating components of electrodes in relation to the deposition .ofthe deposits of insulating material.

In'each of the foregoing modes of operation, after a perio'd of continued operation of such ion sources, the amount of contamination reaches a degree whereit is impossibleor impractical evenv partially to offset itseifect by the expedients described, aiming to correct the distori 2,838,676 Patented June 10, 1958 tion. At such stage of operation, the quantity of ions formed is but a small fraction of that which is required and which can only be obtained when the ion source is in its initial uncontaminated condition. Thereupon it is necessary to break the vacuum in the instrument in which the ion source is connected, to remove it from its enclosure and to disassemble its component parts. The contaminating deposits must then be removed from the ion source, the parts reassembled and the source reinstalled and again re-evacuated. This type of corrective procedure is extremely laborious, time-consuming, and results in taking the apparatus out of service during the period for effecting the repairs. In addition, the contaminating efiect progressively alters the sensitivity of the related apparatus and where the mechanism is employed in connection with amass spectrometer leak detector, or the like instrumen "in devices which are evacuated by oil diffusion pumps with all of the incident advantages thereof.

Still more particularly it is an object of this invention to provide a mode of operation of devices employing ion sources using oil diffusion pumps as the evacuating me dium which assures that the quantity of ions formed by the ion source is substantially constant, to avoid diminishing sensitivity of the instrument employing the ion source and assuring constant operation over extended periods of time without interruptions previously caused by the necessity to clean and remove the contaminating influence of carbonaceous or like insulating deposits.

Still further it is an object of this invention 'to provide an ion source which provides substantially a constant number of ions which may be employed in measuring instruments without special attention or adjustment by reason of periodic changes in sensitivity which have characterized prior devices and thereby to assure continuous operation with the minimum of care and attention.

Still further it is an object of our invention to provide an ion sourceand method of using the same in the presence of an oil diffusion pump characterized by passing the electron beam adjacent to a repeller for the ions under conditions wherein the ion repeller or the like is heated to prevent insulating or deleterious deposits being formed.

To attain these objects and such further objects as may appear herein or be hereinafter pointed out, We make reference to the accompanying drawings, forming a part hereof, in which Figure 1 is a wiring diagram and fragmentary section of an ion source illustrating our invention;

Figure 2 is a fragmentary, longitudinal section of the source end of a tube employing our invention, taken along the line 2-2 of Figure 1;

Figure 3 is a fragmentary section taken on the line 33 of Figure 2;

Figure 4 is a wiring diagram and fragmentary section of another embodiment of our invention;

Figure 5 is a fragmentary, longitudinal section of the source endof a tube taken on the line S5 of Figure 4;

Figure 6 is a section taken on the line 6-6 of Figure 5;

Figure 7 is a diagrammatic representation of a mass spectrometer incorporating the ion source.

Referring to the drawing, in Figure 1 there is shown a Wiring diagram and fragmentary section of an ion source which are the essential featu res of the source and of a tube in which 11 is a tungsten filament arranged to be heated to form the cathode and which is supported on posts 12 and 13, which also serve as electrical condoctors to pass the heating current through the filament 11 so as to cause it to reach a temperature sufficiently high to emit electrons. Filament heating current is supplied to the filament 11 by the center-tapped filament transformer 14 through conductors 15 and 16, leading to the posts 12 and 13. The posts 12 and 13 are sealed into the vacuum system by the seal 17 Encasing the filament 11 is a shield box 18, having an open end 19 and a closed end 20. The shield box serves to enclose the filament and posts which are projected through the opening 19. The shield box 13 serves as the anode, having a potential which is positive with respect to the filament 11 so that the electrons emitted from the filament are directed to the closed end 20 of the shield box, which is positioned in close proximity to the filament 11.

The closed end 20 is provided with a small slit or aperture 21 which is directly opposite the filament 11. Some of the electrons attracted toward the closed end 20 pass through the slit 21 and enter the ionizing charnber 22 where ionization of the gaseous molecules is effected by collision of the electrons with the molecules.

The ionizing chamber 22 is bounded on its lower side by a shield plate 23. The shield plate 23 and the shield box 18, by means of the conductors 24 and 25, respectively, are operated at approximately the same potential. Shield plate 23 is provided with a beam slit 26 through which the ions formed are expelled from the ion source.

In opposite position to the shield plate 23, around the path of the electrons entering the chamber 22, there is provided the grid 27. This grid consists of a fine heater wire or ribbon, such as Nichrome, which is distributed along the path of the electrons in zig zag fashion in a plane parallel to the shield plate 23. The ionization chamber 22 is further defined by the end angle shield 28 and the side angle shields 29 and 30. The end angle shield 28 and the side angle shields 29 and 30 are welded to the shield plate 23 to be spaced from the grid 27 and to remain insulated from it.

The method of supporting the grid 27 and insulating the various components from it may be by well-known techniques, it being sufficient to state that the grid may be supported upon a sheet of mica, although other expedients will readily suggest themselves to the skilled technicians in this field. I Also, it is to be noted that the methods of sealing the electrical leads into the vacuum systems are well known and are merely diagrammatically shown for an understanding of the invention.

Spaced from the shield plate 23 and in parallelism therewith is a focus plate 31 and .a source plate 32, having a source slit 33 for the ion beam B. The assembly is held to a source flange 34 in spaced relation by a plurality of insulating spacers 35 through which the bolts 36 pass. These bolts are covered with insulation 36a along their entire length. The shield box 18 is supported from these bolts in spaced position from the focus plate 23 by means of a pair'of extended arms 18:: formed with ears 18b, through which the bolts pass. Only a fragment of the source flange 34 is shown, as other details for connecting the assembly, including connections to power supply and internal leads, are such well-known techniques as not to require illustration. I The grid 27 is connected by leads 37 and 38 to a direct current source of heating current 39, to heat the grid 27 to temperatures ranging up to 1800" F. The end of the grid 27 nearest the filament 11 is connected by the lead 40 to the voltage divider 41, to which the lead 24 from the plate 23 is likewise connected. The

grid 27 is operated so as to be more positive in potential than the shield plate 23, by an amount diagrammatically identified as E The component E is adjusted so that the average potential of the grid 27 is sufiiciently more positive than that of the shield plate 23 to repel positive ions formed between the two toward the plate 23, and then project them out of the ion source through the slit 26. Because of this repelling action, grid 27 is hereinafter referred to as ion repeller grid.

In the construction as shown, the ion source is characterized by electrons projecting into the chamber 22, so that they pass between the plate 23 and the ion repeller grid 27 which is operated preferably at elevated temperatures, thereby preventing contaminating insulating materials from forming on the repeller surfaces of the wire, when associated with an oil diffusion pump to draw the ion and constituent gases into an atmosphere evacuated to a pressure of from 2 l0- to 10* millimetres of mercury. The result achieved is that when this ion source is operated continuously in conjunction with oil diffusion vacuum pumps, the number of ions in the ion beam remains constant and there is no necessity for periodic dismantling of the assembly for cleaning. Likewise, frequent adjustment after the initial potentials have been set for appropriate operation is avoided.

It is to be understood that in the wiring diagram given, the direct current sources 39, 43 and 44 have been indicated as batteries having the polarity as identified. In practice, rectifier power supply circuits having suitable filters are used. Similarly, the electron emission of the filament 11 is usually regulated by a suitable regulatory circuit which is only diagrammatically illustrated, since such circuits are well known to the skilled technician and need not be particularized to a greater degree.

Without intending to be bound by the explanation of what occurs, We have found that the absence of a red hot to incandescent condition particularly as on the plate 23 does not upset the desired field distribution in the ion source because the electrons are directed not toward plate 23 but to the grid 27. The electrons passing through the space between the plate 23 and the grid 27 will have a tendency to strike the heated surface because it has a potential which is more positive and has an attraction for the electrons. The possible accumulation of a deposit on plate 23 appears to be immaterial.

While we have shown and described an ion repeller which is heated along the path of the electron beam to a temperature in its specific form above the actual carbonization point of the oil used in the diffusion pump and in a range above the boiling point of the oil when used as described, we may both heat the walls of the ionization chamber and contour them to provide an accelerator of the electrons. This we accomplish by surrounding the ionizing chamber with a coiled heater which, with regard to its size and the number of turns as well as the spacing and potential, achieves a focusing effect upon the electron beam.

For this purpose we now refer to the embodiment illustrated in Figures 4, 5 and 6 in which the source end of the tube 10 is provided with the same arrangement of filament 11 to be heated to form the cathode and is supported similarly on posts 12 and 13 to pass the heating current through the filament 11. As in the prior embodiment the filament 11 is extended into the shield box 18, having an open end 19 and closed end 20, the shield box serving as the anode. The shield box 18 has a pair of extended supporting arms 18a formed with ears 18b. The closed end is provided with an emission slit 21, as before, with its long dimension in parallelism with the filament 11 and in close adjacency to and opposite the filament 11.

In this embodiment, however, surrounding the slit 21 and the path taken by the electrons passing through the slit is an ionizing chamber 22a. Sfilch chamber is defined by a helical coil 27a having longitudinally spaced convolutions of rectangular section and which is heated to incandescence or red heat by means of an electric current supply by direct current source 39a through the conductors 37a and 38a. The electrons passing through the slit 21 now enter the space 22a surrounded by coil grid 27a, where the accelerated electrons produce ions by collision with the gaseous molecules, to one side of the end shield 28a.

The end of the coil grid 27a nearest the filament 11 is maintained at a slight positive potential Ea with respect to the anode by means of a voltage divider 41a. Thus, the field of the coil grid 27a penetrates through the slit 21 more effectively to draw electrons out into the ionization space.

Shield plate 23a has a slit 26a closely adjacent to the coil grid 27a and slit 26a has its longest dimension in parallelism to the longitudinal axis of the grid coil 2711. Plate 23a is operated at a slight negative potential with respect to the coiled grid 27a by means of voltage divider 41b. The negative field of shield plate 23a penetrates into the coil grid 27a, thereby attracting the positive ions which are formed and causing them to be expelled from the source through the slit 26a.

Since the coil grid 27a is operated at incandescence or red heat, the surface of the wire is prevented from being contaminated, with the result that the ion source maintains constant the number of ions formed, fulfilling one of the objectives described in connection with the first embodiment. However, this embodiment has the additional advantage that by an appropriate design of the coil 27a as regards the size of the turns as well as the spacing and potential difference between turns, a focusing effect can be imparted to the electron beam within the coil 27a. This focusing effect, which may be explained as due to the electrostatic field distribution within the coil, influences the beam to prevent divergence from a path corresponding to theaxis of the coil 27a. The effect produced is to have more extensive collision between the electrons and the molecules of the gaseous elements along the axis of the coil. The ions resulting from the collisions, since they are directly opposite the slit 26a, have the greatest probability of passing through the slit and being expelled from the source rather than striking the shield plate 23a. By providing an ionizing chamber which is heated along the entire path of the bombarding electrons and which simultaneously effects a focusing action of the electron beam, the number of useable ions produced by the source is greatly increased.

In Figure 7 there is shown a diagrammatic representation of a mass spectrometer S used for leak detection, the same incorporating the ion source above described and including filament 11 with leads 15, 16; shield plate 23 formed with beam slit 26; the grid 27; the focus plate 31; and the source plate 32 formed with source slit 33 for passing the ion beam B. The spectrometer further includes analyzer magnet M; a collector plate or target C with electrical leads therefrom for connection to the usual amplifier and measuring devices, not shown; an inlet H for the incoming helium gas; and a diffusion pump P for evacuating the system. These instruments employ ion sources for the primary purpose of ionizing helium which gas is used as the tracer or probe gas in the detection of leaks. The helium ions formed in the ion source are magnetically separated from other ions which are formed by magnet M and they are caused to impinge upon conducting target or collector C. Here they give up their charge, causing an electrical current to flow, which is called the collector current.

The mechanism of mass spectrometer leak detection may be summarized as follows:

The magnitude of the collector current is a measure of the number of helium ions formed which, in turn, is a measure of the quantity of the helium entering the mass spectrometer which, in turn, is a measure of the quantity of helium passing through the leak being detected, and which in turn depends upon the size of the leak.

While we have shown and described the heating of the walls of an ionization chamber by the provision of an ion repeller in the form of a heater filament which may be heated to elevated temperatures and which may be located to one side of the ionization chamber or to effect focusing of the ion beam, it will be understood that our invention contemplates providing means to heat the effective components externally applied, such as by radiation, conduction or by bombardment by electrons while applying potential difference in relation to other component parts to repel the ions,

While we have described as the preferable operating condition and as a separate expedient, the application of heating for the grid 27 to achieve a condition of red heat or incandescence to insure against the deposition of the diffusion pump oil and also any other vapors, separate heating means may be supplied to the grid 27, and coil 27a, with or without additional heat along the path of the electron beam passing through the ionization chamber, with due regard to the nature of the diffusion pump oil, and the constituents of gas mixture being ionized in the use of the ion source. Thus we may use as a conditioning operation and employ as separate means to apply heat at a temperature sufiicient to prevent the condensation of the pump oil vapor on the surface of the wire and at least one of the walls defining the path for the electron beam. The temperature contemplated may be within a range between the boiling point of the diffusion pump oil used, of about 250 F. and that of incandescence of about 1800 F., the exact temperature being determined by such factors as the constituents of the gas being ionized. Accordingly, in the claims where we use the expression of applying decarbonization temperatures or heating above the carbonization point, or the like terminology, we mean to include thereby the broader phases of our invention encompassing temperatures which remove or prevent the formation of carbonization deposits, by volatilization or evaporation of the diffusion pump oil above its boiling point or conditions for separately applying heat in the operations described which prevent condensation of the pump oil vapor and which, but for the heat applied to remove or prevent condensation, would carbonize or form insulating or contaminating deposits. The terms defined are accordingly used in the sense of preventing a condition of formation as well as of positive removal of the contaminating deposits.

Having thus described our invention and illustrated its use, what we claim as new and desire to secure by Letters Patent, is:

1. In an ion source for purposes described, a cathode providing an electron emitting source, an ionizing chamber providing an electron beam, and heating means and electrical potential means therefor to the side of said chamber applying decarbonization temperatures and ion repeller influence along the path of the electrons for the reduction of insulating deposits and providing a uniform source of useable ions.

2. An ion source for the purposes described having an electron emitter and heat source therefor and an ionizing chamber for projecting an electron beam along a path through said chamber and heating means independent of said emitter and its heat source having decarbonization temperature effective along the, path of the bombarding electrons passing through said chamber, whereby the number of useable ions produced by the source is increased.

3. An ion source in accordance with claim 1 wherein said heating means and electrical potential means therefor simultaneously impress an electrical potential for directionally influencing the ions.

4. An ion source in accordance with claim 1 wherein said heating means simultaneously has means to impress an electrical potential for directionally repelling 76 ions.

A alsslie" length of the beam an ion repeller including means to heat the chamber and ion 'repeller at a decarbonization temperature directionally to influence the ions.

7 7. In an ion source for the, purposes described, a cathode providingan electron emitting source, an anode surrounding said cathode and defining to one side thereof an ionization chamber, an ion repeller along the path of the electron beam projected in said ionizing chamber, said ion repeller comprisinga heater grid and including means to provide heat thereto attemperatures ranging up to about 1800 F.

8. In an ion source for the purposes described, a cat11- ode providing an electron emitting source, an anode surrounding said source and having an outlet through which an electron beam may project, an ion repeller and shield plate along the path of the electron beam, the ion repeller comprising a heater grid including means for setting up a repeller potential and hcating'the grid to elevated temperatures.

9. In an ion source comprising an anode, an ionization chamber and a cathode spaced in relation to each other with means to provide an ion beam, the combination therewith comprising an ion'repeller, together with means to heat the repeller to elevated temperatures and connected with means for impressing a repeller potential.

10. In an ion source of the character described comprising an encasing anode, a cathode within said anode, an emission outlet through-said anode for directing electrons into an ionization chamber for ionization of gaseous molecules, the combination wherein said ionization chamber includes ionbeam focusing means and ion repelling means provided with heater means to heat the ion repeller to elevated temperatures.

11. An ion source in accordance with claim 6 wherein said ion repeller includes fine heater filament positioned in close zig zag fashion, including means to'heat the same to; elevated temperatures.

' 12.: An ion source in accordance with claim 1 wherein said heater'means'includes a heater filament coiled about said ionizing chamber, including means to heat the same to temperatures ranging upwardly to about 1800 F, the coil comprising loops spaced from each other and means to provide electrostatic fields between adjacent loops of said coil. 7

13. In a massjspectrometer for leak detection having a spectrometer tube including an oil diffusion pump as an evacuator and having an ion source, which includes an electron generator, an ionization chamber through which an electron beam is passed and including an ion repeller, the combination therewith or" means to heat the chamber to 'decarbonization temperatures along the path of the beam through said chamber.

14. In a mass spectrometer for leak detection having a spectrometer tube including an oil diffusion pump as an evacuator and having an ion source, which includes an electron generator, an ionization chamber through which an electron beam is passed, the combination therewith of means 'toi'heat the chamber to decarbonization temperatures along the path of the beam through said chamber, and means to impress a potential difference on said heater means.

15. In a mass spectrometer for leak detection having a spectrometer tube including an oil ditr'usion pump as an evacuator and having an ion source, which includes an electron generator, an ionization chamber through which an electron beam is passed, the combination therewith of means to heat the chamber to decarbonization temperatures along the path of the beam through said chamber, and means to impress a potential difference on said heater means, said heater means comprising a coil having spaced convolutions and the potential diiierence being impressed between elements of said heater means.

References Cited in the file of this patent UNIT ED STATES PATENTS 2,624,845 Thompson Jan. 6, 1953

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