US3385965A - Ion source having a hollow cylindrical permanent magnet maintained at a positive potential relative to the electron emitter - Google Patents

Description

y 28, 1963 I w. D. DAVIS 3,385,965

IoN SOURCE HAVING A HOLLOW CYLINDRICAL PERMANENT MAGNET MAINTAINED AT A POSITIVE POTENTIAL RELATIVE To THE ELECTRON EMITTER Filed Aug. 10, 1965 His Afro/nay.

High Voltage 0. 6. Power PP y United States Patent [0N SOURCE HAVING A HOLLOW CYLINDRKCAL PERMANENT MAGNET MAINTAINED AT A POSITliVE PDTENTHAL RELATK" E TO THE ELEC- TRON EMITTER William D. Davis, Schenectady, N.Y., assignor to General Electric Company, a corporation of New Yorir Filed Aug. 1t), 1965, Ser. No. 478,668 5 Claims. (Cl. ISO-41.9)

ABSTRACT (9F THE DISCLGSURE A mass spectrometer ion source for highly efficient production of ions at very low pressures having substantially no energy spread utilizes a magnetic field to enhance ionization. Electrons emitted from a hairpin filament situated within the interior of a hollow cylindrical permanent magnet are constrained by electric fields to remain within the interior of the magnet and thereby increase the probability of collisions with gas molecules.

This invention relates to mass spectrometer ion sources and more particularly to an improved electron bombardment type ion source which is especially efiicient at very low gas pressures. The invention herein described was made in the course of or under a contract or subcontract thereunder with the Air Force Department.

Gas analysis by mass spectrography requires production of sufficient numbers of ions to achieve reasonably high output signals. This requirement becomes more difficult to meet as gas pressures are decreased. In electron bombardment type ion sources operating at very low pressures and producing electrons thermionically, it is necessary to keep electron emission and filament heater power low in order to avoid outgassing; however, enough ions must still be produced in order to enable measurement of the partial pressures. At these very low pressures, therefore, the ratio of ion current to electron emission must be made as high as possible.

Heretofore, mass spectrometers utilizing conventional ion sources have been unsuited for applications at extremely low pressures, since the attendant outgassing necessitated much pumping of the ion source. Moreover, mass spectrometer ion sources utilizing magnetic fields for enhancement ionization exhibit diminished resolution of the spectrometer to below satisfactory values because of a high energy spread among the ions. Moreover, because use of mass spectrometers at very low pressures requires that the spectrometer first be baked at temperatures of about 400 C. in order to remove extraneous particles, materials used within the spectrometer, such as organic insulation for wire, as well as the material of the spectrometer itself, tend to emit contaminant particles during the baking process. Thus in mass spectrometers utilizing conventional ion sources, there eXists a level of gas pressure below which it is impractical to attempt to make an accurate gas analysis.

Accordingly, one object of this invention is to provide a mass spectrometer ion source for permitting accurate gas analyses to be made at pressures lower than have heretofore been possible.

3,385,955 Patented May 28, 1968 Another object is to provide an efficient source of ions having substantially no energy spread.

Another object is to provide a mass spectrometer ion source using a permanent magnet as the anode thereof.

Another object is to provide a mass spectrometer ion source having a minimal possibility of contamination from the materials of which the ion source is constructed.

Briefly stated, in accordance with one aspect of the invention, a mass spectrometer ion source is provided having a first end plate containing an aperture therein and a hollow cylindrical permanent magnet having one end concentrically aligned with the aperture. At the other end of the permanent magnet, a second end plate is situated in orthogonal relation to the longitudinal axis of th magnet. Electron emitting means are situated within the hollow interior of the magnet and are maintained at a potential which is positive with respect to the end plates and negative with respect to the magnet.

The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

The single figure is a cross section view of the portion of a mass spectrometer tube containing the novel ion source, with some of the detail illustrated in cutaway and some illustrated schematically.

In the drawing, the ion source assembly is shown mounted within an evacuable envelope 10. One mass spectrometer tube adaptable for use with the novel ion source described herein, is similar to that shown in the application of W. D. Davis et al., Ser. No. 327,617, filed Dec. 3, 1963, now Patent No. 3,230,362 issued Jan. 18, 1966, and hence only the portion of the mass spectrometer tube containing the ion source is shown herein. The ion source basically comprises a thermionic electron emitting filament 11 having its apex portion 12 protruding into the hollow interior 13 of a cylindrically-shaped permanent magnet 14 through an aperture 15 in a metallic end plate 16 situated at one end of the cylindrical permanent magnet. The permanent magnet is preferably constructed of Alnico-8, although Alnico-S is also a satisfactory magnet material. A second metallic plate 17 having an aperture 18 therein is situated at the opposite end of the cylindrical permanent magnet. The longitudinal axis of cylindrical permanent magnet 14 is concentrically aligned with apertures 15 and 18 of plates 1t; and 17 respectively, and is situated in orthogonal relation to the plates. Apertures 15 and 18 are preferably circular in configuration.

Drawout and focus electrodes 19 and 20 are situated above end plate 17. Each of these electrodes comprises a pair of opposed, substantially semicircular discs located in a common plane. The planes of electrodes 19 and 20 are parallel to the plane of end plate i7. Each half of lower focus electrodes 19 has an upward-turned flange 21 along the entire chordal or diametral portion of the electrodes; similarly, each half of upper focus electrode 20 has a downward-turned flange 22 along the entire chordal or diametral portion of the electrode. Focus electrodes 19 and 20 are situated so that the transverse spacing between flanges of the respective portions of each electrode is bisected by the longitudinal axis of cylindrical permanent magnet 14.

Situated above the focus electrodes and in parallel relationship thereto are a first metallic disc 23 and a second metallic disc 24. Each disc has a coliirnating slit therein, with slit 25 in disc 23 being several times wider than slit 26 in disc 24 in order to narrow the beam of ions passing therethrough. The lengthwise portions of these slits are oriented parallel to the chordal or diametral portions of electrodes 19 and 29, and their widths are bisected by the longitudinal axis of cylindrical permanent magnet 14. Although the functions of the focus and collimating electrodes are well-known in the art, means for applying the requisite voltages thereto are described herein.

The ion source is supported by means of four support rods 27, only two of which are shown. These support rods are fabricated of dielectric material, such as quartz. A collar 28 forming an integral portion of each support rod abuts collimating disc 24, which is Welded or brazed to a metallic plate 29. An extension portion 30 of each support rod 27 passes through mounting holes in both collimating disc 24 and metallic plate 29. Metallic plate 29 comprises a flange which is welded or brazed to a sector tube 31, which passes through a metallic cap 32 to form the characteristic deflection portion of the mass spectrometer. Cap 32 is fitted to envelope 10 in order to form a hermetic seal therewith, and sector tube 31 is welded or brazed to cap 32 for the same purpose. A tubulation is provided for the evacuable assembly, as for example, tubulation 33 of evacuable envelope 10, for connection to suitable pumping apparatus in order to obtain the required low operating pressure therefor.

A plurality of spacers in the form of collars fabricated of a dielectric material, such as quartz, are fitted around each of support rods 27 for electrically isolating the various electrode structures. Thus, spacers 34 separate collimating electrode 23 from drawout and focus electrode 20, spacers 35 separate drawout end focus electrodes 19 and 20 from each other, spacers 36 separate drawout and focus electrode 19 from end plate 17, spacers 37 separate end plate 17 from a first anode electrode 43, spacers 38 separate first anode electrode 43 from a second anode electrode 44, spacers 39 separate second anode electrode 44 from an end plate support ring 45, and spacers separate end plate support ring 45 from a non-conducting ceramic ring 46. The ceramic ring is retained in position by coil springs 47 which are tightly wound around each of support rods 27, respectively.

Four holes 48, only two of which are shown, are bored through ceramic ring 46, and a shaft 59 is respectively inserted therein. Each shaft 50 is joined to a mounting pin 51, respectively. Alteratively, shaft 56 may be constructed as an integral extension of mounting pin 51. At each junction of shaft 5t with mounting pin 51 there is a flanged collar 53, affixed by welding or other means, to mounting pin 51. An elongated coil spring 54 is wound in compression, around each shaft 50, respectively, thus exerting an upward force against ceramic ring 46, tending to compress spacers 34-40. Because of the force exerted by each of springs 54, the ion source assembly is retained as an integral unit. In addition, springs 54 also function as shock absorbers in the event any of mounting pins 51 are moved vertically due to temperature changes or other reasons.

Support for cylindrical permanent magnet 14 is provided by a pair of tabs 55 which are affixed to opposite sides of the outer surface of the permanent magnet by spot welding or other means. First and second anode electrodes 43 and 44 respectively are preferably welded to the upper and lower portions of tabs 55. Anode potential may then be supplied to the permanent magnet through a conductor wire 56 which is attached to a mounting pin 57 by welding or other-wire. A flexible loop of wire, or pigtail 58 may be inserted in the conducting path of wire 56, in order to permit slight movement of the ion source, when necessary, without causing damage to the source. in similar fashion, a potential is supplied to end plates 16 and 17 through a conductor 60 having a pigtail 61 inserted therein, through a mounting pin 62. Because positive potential exists on permanent magnet 14 with respect to potential applied to end plates 16 and 17, end plate support ring 45 must have a sufliciently large aperture therein in order to avoid contact with the cylindrical permanent magnet. Therefore, less positive voltage for end p. e i6 is supplied through a conductor 63 which is welded between end plate support ring 45 and end plate 16. A pair of nonconducting ceramic discs 64 are affixed to end plate support ring 45 such as by a bolt 74, and each disc 64 is used to provide support through a bolt '75 or other means, for a respective conductor 65. A conductor 6-6 is welded between one end of each of conductors 65 and separate sides of filament 11, respectively. Welded between each of a pair of filament support pins 67 and each of the other ends of conductors 65, respectively, is a conductor 68, respectively. Each of conductors 63 contains a pigtail 70 therein.

Each portion of drawout and focus electrode 19 is connected through a conductor 76 containing a pigtail 77 therein to a mounting pin 78, and are thereby maintained at a common potential. Each portion of drawout and focus electrode 20 is separately connected through a conductor 80 containing a pigtail 82 and a conductor 81 containing a pigtail 83 respectively, to mounting pins 84 and 85 respectively. Collimating electrodes 23 and 24 are connected through a conductor 86 containing a pigtail 87 and a mounting pin 88 to ground.

D.C. filament voltage is supplied to support pins 67 from a D.C. power supply or storage battery 71, although A.C. filament voltage may alternatively be supplied. A high voltage D.C. power supply 72 provides a large positive potential through mounting pin 57 to cylindrical magnet 14, and a lesser positive voltage to end plates 16 and 17 through a bucking battery 73 connected to mounting pin 62. By connecting a battery 90 between filament D.C. power supply 71 and high voltage D.C. power supply 72, of voltage less than that of battery 73 and poled so as to buck the high voltage supply, filament 11 is maintained at a positive potential intermediate the mode and end plate potentials.

Positive voltage is supplied to both halves of focus electrode 19 from high voltage power supply 72 through a variable resistor 91 connected to mounting pin 78. Each half of focus electrode 20 is maintained at a separate positive voltage supplied from high voltage D.C. power supply 72 through respective parallel-connected variable resistors 92 and 93 connected to mounting pins 84 and 85 respectively. Hence, voltage magnitudes and polarities may be established independently on electrodes 19 and 20. It should be noted that the mounting pins penetrate envelope 10 in a manner which maintains the hermetic seal of the envelope.

The composition of the ion source permits removal of extraneous particles by baking the system at high temperatures, without influencing the amount and composition of the gas to be analyzed. These extraneous particles have proven to be a source of inaccuracy, par ticularly in mass spectrometer tubes requiring an electromagnet within the ion source assembly, due to spurious emission of particles from the insulation required for the windings of the electromagnet. Elimination of the electromagnet, therefore, substantially obviates this problem.

Because the apex portion of the filament extends into the hollow interior of magnet 14, which produces a relatively weak magnetic field, lower anode voltages than heretofore deemed practical may be used. Since electrons emitted from the filament thus have very little lateral distance to travel before striking the anode, the anodic field must not be of sufficient magnitude to immediately draw electrons to the inner surface of the anode. This condition is especially important at extremely low pressures, when it is desirable that electrons travel over great distances within the interior of the cylindrical permanent magnet in order to enhance the possibility of colliding with a gas molecule and producing an ion. Due to presence of end plates 16 and 17, the electrons emitted within the interior of permanent magnet 14 are constrained to remain therein, increasing the probability of obtaining a longer mean electron path within the interior of the magnet.

Utilization of a permanent magnet as the anode results in a more confined magnetic field than would be obtainable from an electromagnet surrounding the anode and producing the same flux density. This is advantageous in that the magnetic field thus has less disturbing effect upon the portion of the mass spectrometer tube outside the ion source. Moreover, because the magnetic field is so confined, proper ope-ration of the ion source requires less total magnetic flux from the permanent magnet anode than from an electromagnet surrounding the anode. This lesser value of required flux is an amount which is readily obtained from a permanent magnet.

In typical operation, the anode is maintained at approximately +1000 volts by high voltage power supply 72, while the end plates are maintained at approximately +750 volts due to a bucking potential of 250 volts supplied by battery 73 in series with power supply 72. The filament may be operated at 1 or 2 volts DC. from power supply 71, while being maintained at approximately +950 volts above ground potential by virtue of a 50 volt potential introduced by battery 90 in series opposition with power supply 72. Electrons emitted by the hot filament are repelled by the end plates which are thus 200 volts negative with respect to the filament, and attracted to the anode which is 50 volts positive with respect to the filament. The axial magnetic field produced by permanent magnet 14, which may be in the order of about 500 gauss, constrains the movement of electrons to a reciprocating motion within the cylinder, thereby making the path of the electrons much greater than the dimensions of the cylinder. Although the exact behavior of the electron cloud within the cylinder is uncertain, the net result is a large increase in the ionization efficiency of the electrons. The ions produced within the interior of cylindrical permanent magnet 14 are withdrawn through aperture 18 in end plate 17, and are focussed and accelerated in conventional fashion, such as described in the store-mentioned Davis et al. Patent No. 3,230,362.

It is noted that conventional electron bombardment type ion sources for mass spectrometers have typical sensitivities for generating ions of about torr* senitivity being measured by the ratio of ion current to the product of electron emission and pressure. On the other hand, by using the novel ion source described herein, a source sensitivity of 1.2 10 torrhas been attained at a pressure of 3x10 torr and electron emission current of 10" amperes. Similar results have been obtained at a pressure of 3X10 torr. This drastic increase in sensitivity permits generation of less electrons, thus allowing use of low heater power in order to avoid outgassing without reducing the number of ions generated to a value less than that required to accurately read the partial pressures. Moreover, use of low heater power greatly extends filament life. Further, under the aforementioned conditions, resolution of the mass spectrometer has been normal, indicating that the ions are supplied to the spectrometer with no energy spread. In addition to the aforementioned advantages, the novel ion source disclosed herein greatly increases the ratio of ordinary gas ions to those ions such as F Cl+, Na etc., which appear to be formed by electrons striking the surfaces of the ion source. At very low pressure, such surface ions could interfere with measurements of the gas ions,

in the event the number of ordinary gas ions were low.

While only certain preferred features of the mass spec trometer ion source of the present invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art. It is, there fore, to be understood that the appended claims are intended to cover all such modifications and changes which fall with the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a mass spectrometer tube, an ion source assembly comprising: a pair of end plates, each said end plate containing an aperture therein; a hollow cylindrical permanent magnet; support means mounted within said tube for maintaining said permanent magnet longitudinally disposed between said end plates and concentrically aligned with said apertures; an electron emitter afiixed to said support means so as to protrude through one of said apertures into the hollow interior of said magnet; and circuit means coupled to said pair of end plates, said electron emitter, and said permanent magnet to maintain said pair of end plates at a negative potential with respect to said electron emitter and said magnet at a positive potential with respect to said electron emitter.

2. In a mass spectrometer tube, an ion source assembly comprising: a pair of end plates, each said end plate containing an aperture therein; a hollow cylindrical permanent magnet; support means mounted within said tube for maintaining said permanent magnet longitudinally disposed between said end plates and concentrically aligned with said apertures; a conducting filament adapted to be heated to incandescence by passage of current therethrough and afiixed to said support means so as to protrude through one of said apertures into the hollow interior of said permanent magnet, said filament constituting a thermionic source of electrons; and circuit means coupled to said pair of end plates, said filament, and said permanent magnet to maintain said pair of end plates at a negative potential with respect to said filament and said magnet at a positive potential with respect to said filament.

3. In a mass spectrometer tube, an ion source assembly comprising: a pair of end plates, each said end plate containing a circular aperture therein; a hollow cylindrical permanent magnet; support means mounted within said tube for maintaining said permanent magnet longitudinally disposed between said end plates and concentrically aligned with said circular apertures; a hairpin-shaped electron emitting filament affixed to said support means so that the apex portion thereof extends through one of said circular apertures into the hollow interior of said magnet; circuit means coupled to said pair of end plates, said filament, and said permanent magnet to maintain said pair of end plates at a negative potential with respect to said filament and said magnet at a positive potential with respect to said filament; and means coupled to said filament for heating said filament.

4. In a mass spectrometer tube, an ion source assembly comprising: a first end plate containing an aperture therein; a hollow cylindrical permanent magnet support means mounted within said tube for maintaining one end of said magnet concentrically aligned with said aperture; a second end plate, said support means maintaining said second end plate in orthogonal relation to the longitudinal axis of said magnet at the other end of said magnet; electron emitting means affixed to said support means so as to be situated within the hollow interior of said magnet; and circuit means coupled to said end plates, said electron emitting means, and said permanent magnet to maintain each of said end plates at a negative potential with respect to said electron emitting means and said magnet at a positive potential with respect to said electron emitting means.

5. In a mass spectrometer tube, an ion source assembly comprising: a pair of end plates, each said end plate con- 7 taining an aperture therein; a hollow cylindrical permanent magnet; support means mounted within said tube for maintaining said magnet longitudinally disposed between said end plates, said end plates being afiixed to said support means so as to be in substantially orthogonal relationship with the longitudinal axis of said magnet; electron emitting means afiixed to said support means so as to extend into the hollow interior of said magnet through the aperture of one of said end plates; and circuit means coupled to said pair of end plates, said electron emitting means, and said permanent magnet to maintain said pair of end plates at a negative potential with respect to said electron emitting means and said magnet at a positive potential with respect to said electron emitting means.

References Cited UNITED STATES PATENTS 2,831,996 4/1958 Martina 31363 2,911,531 11/1959 Rikard et a1. 25041.9 3,109,115 10/1963 Lafferty 324-33 X FOREIGN PATENTS 665,094 1/1952 Great Britain.

WILLIAM F. LINDQUIST, Primary Examiner.

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