US3296481A

US3296481A - Ion source having both deflection and repeller electrodes for directing an electron stream

Info

Publication number: US3296481A
Application number: US367044A
Authority: US
Inventors: John L Peters
Original assignee: Individual
Current assignee: Individual
Priority date: 1964-05-13
Filing date: 1964-05-13
Publication date: 1967-01-03
Anticipated expiration: 1984-01-03

Description

J. L. PETERS 3,296,481 ION SOURCE HAVING BOTH DEFLECTION AND REPELLER ELECTRODES Jan. 3, 1967 FORDIRECTING AN ELECTRON STREAM Filed May 15, 1964 Rm m .w w Ne w w A N L 7w I n h I 2 a 3 3 3 2)7.\ 313 3 I THW w m 2 6 3 V O 0 0 V 7 53 1211?. o w w 4 4 2 4. r 2 m m m m m m n m k m3 1 n n I m m Q G W "w. m 3 3 III I. 21L 4 4 w t O m D m 2 Wm /l\ c D. a w G AGENT United States Patent 3,296,4551 ION SGURCE HAVENG BQTH DEFLETION AND REPELLER ELECTRODES F012 DKRECTING AN ELEQTRON STREAM John L. Eeters, 114 Dikeman St Hempstead, NY. 11550 Filed May 13, 1%4, Ser. No. 367,644 11 (Ilaims. (Cl. 313230) The present invention relates to ion sources and, more particularly, to means for producing a positive ion beam.

In apparatus such as analytical mass spectrometers and mass spectrometer leak detectors, for example, a source of positive ions and a means for focusing and accelerating the ions are employed to produce a positive ion beam. One type of widely used ion source comprises a box-like ion chamber that contains molecules of gas, means for producing and directing a beam of electrons into the chamber, an ion outlet at the bottom of the chamber, and a grid positioned at the top of the chamber for repelling ions toward the ion outlet. The production of usable ions and, consequently, the current in an ion beam that is formed from the ions leaving the ion outlet depends upon the number of electrons and focusing or controlling of the electrons within the ion chamber.

It is a primary object of the present invention to provide improved means for producing a positive ion beam with increased current.

Another object is to provide an improved ion source for increasing the usable positive ions that are produced in the ion chamber of the ion source.

Still another object is to provide improved means for focusing or controlling an electron beam in an ion chamber of an ion source for increasing the usable positive ions which are produced in the chamber.

A further object is to provide improved means for preventing contamination in an ion source so that a maximum amount of usable ions can be produced.

The foregoing objects as well as other objects and advantages of the inventtion are achieved by an ion source, as is described above, to which is added a deflection electrode that is positioned adjacent the ion repelling grid on the far side of the grid from the bottom of the ion cham ber. The focusing electrode and the grid are operated at different voltages to optimize the trajectories of the elec trons in the ionization chamber thereby increasing the number of positive ions that are produced in the chamber. An internal heater is positioned in back of the focusing electrode. Means are provided for turning the heater off when the filament of the means for producing and directing a beam of electrons into the chamber is switched on and vice versa, thereby preventing contamination.

In the drawings,

PEG. 1 is a sectional view, partly schematic of ion beam producing means that is provided in accordance with the present invention;

FIG. 2 is an end view, taken from the line 22 in FIG. 1, of the focusing electrode that surrounds the filament of the electron stream producing means;

FIG. 3 is a sectional view, which is taken at right angles with the view shown in FIG. 1, of the ion beam producing means; and

FIG. 4 is an enlarged plan view, taken from the line 44 in FIG. 3, of the repeller electrode.

Referring to FIGS. 1 and 3, there is shown a positive ion beam producing means comprising an ion chamber 11 that contains molecules of gas, means including a filamerit 12 and focusing electrode 13, shown in FIG. 1, for producing and directing a beam of electrons into the chamber 11 for producing positive ions, at repeller electrode 16 positioned at the top of the chamber for repelling positive ions toward the bottom of the chamber, and the combination of focusing

electrodes

18a and 18b and an accelerating electrode 19 positioned below the ionization chamber for focusing and accelerating an ion beam that is formed from the ions which pass out of the ion chamber. In accordance with important features of the inention, a deflection electrode 22 is positioned in back of the repeller electrode 16 on the far side of the electrode 1e from the bottom of the ionization chamber and a heater 23 is positioned in back of the deflection electrode 22.

The heater 23 is a coil or zig zag heater winding which is insulated from the electrode 22. A heat shield 26, made from polished metal, is positioned in back of the heater 23 for reflecting heat back into the ion source. The heater 23 is energized by a voltage supply 25 which may be either a DC. supply as is indicated in FIG. 1 or an AC. supply. The electrode 22, which is a plate made from non-magnetic metal like all of the parts of the ion source, distributes the heat produced by heater 23.

Suitable insulator means, not shown, are employed to support the various elements of the ion beam producing means in insulated relationship with each other within a vacuum envelope 20 that is made from either nonconducting material or from metal. A vacuum pump, which is identified in FIG. 3, is connected to the envelope 2% for drawing gas from the gas inlet, which is identified in FIG. 3, through the ion chamber 11.

The ionization chamber 11 is a rectangularly shaped box-like electrode. It is open at the top and is closed at the bottom except for a rectangularly shaped slit 24 which forms an ion outlet from the chamber. One of the narrow side walls 27 of the chamber 11 contains a rectangularly shaped slit 28 for passing a beam of electrons into the ionization chamber.

The beam of electrons which passes into the ionization chamber 11 is produced by the filament 12 and the focusing electrode 13. The filament 12 is a rectangularly shaped strip of tungsten, for example, which is approximately twice as wide as filaments heretofore employed in ion sources. The increased surface area of the wider filament directs more electrons into the ionization chamer 11 to produce more usable positive ions to produce a larger current in the positive ion beam, compared with filaments which have been used in the art heretofore. The filament 12 is energized by a voltage supply 29 shown in FIG. 1. A DC. voltage supply is shown, but instead an A.C. supply could be used.

The focusing electrode 13 is shaped to form an electron stream that has a rectangular cross section A suitable design for this focusing electrode is shown and de scribed in an article entitled Mass Spectrometer Leak Detector With Improved Sensitivit by John L. Peters, on page 1094 of the Review of Scientific Instruments, vol. 30. N0. 12, December 1959, for example. The focusing electrode 13 is supported mechanically by

struts

34 and 35 which extend from a fiat plate 36 that is supported within the tube envelope 20 by suitable insulator means, not shown.

As is shown both in FIG. 1 and in FIG. 3, the various electrodes of the ion source are connected respectively to a plurality of adjustable taps (represented by the arrow heads) along a resistor 30. The resistor 30 is connected across the output terminals of a regulated DC. voltage supply, not shown. One end of the resistor is connected to ground, for example, the other end of the resistor being at a positive voltage of 700 volts (for example) with respect to ground.

The repeller electrode or grid 16 comprises a metal plate or frame 31 and fine wires 32 which are stretched across a rectangularly shaped aperture 33. The wire portions are accurately positioned and firmly anchored to the frame 31 by welding, for example. The repeller electrode 16 is biased by connecting it to the adjustable tap 37 so that electrode 16 is approximately ten volts more positive than the voltage of the ion chamber 11. The chamber 11 is connected to the resistor 30 by an adjustable tap 38. As an example, the repeller electrode 16 contains approximately ten wires 32 from .005 inch to .010 inch in diameter, made from Nichrome V, which are spot welded at each end to the frame 31 and are spaced evenly apart by approximately .060 inch. In constructing the eiectrode 16 the wires 32 are straightened, stress relieved and then held straight under slight tension when welded.

Beyond the repeller electrode 16 on the far side of the repeller from the bottom of the ion chamber 11, the second focusing electrode 22 is supported in insulated relationship with respect to the repeller electrode. The deflection electrode 22 is a flat metal plate that is insulated both from electrode 16 and from the first focusing electrode 13 so that operating voltages of these electrodes can be adjusted separately for obtaining optimum trajectories of the electrons in the ion chamber 11 for producing a maximum number of positive ions. The deflection electrode 22, the heat shield 26 and the filament 12 all are connected to adjustable tap 40 which is approximately -50 to 100 volts more positive than the potential of the focus electrode 13 at tap 41. The distance from the second deflection electrode 22 to the repeller electrode 16 is chosen so that the electrode 22 can be operated at filament potential. This eliminates the need for another control and another adjustment. This distance is determined experimentally and is 0.100 inch, for example, in an operative embodiment of this invention.

The heater element 23 behind the focusing electrode 22 indirectly heats the electrode 22 and the whole ion source. About watts of power, for example, is supplied to the element 23 by the power supply 25 through the switch 38. About 20 watts of power also supplied to the filament 12 by the power supply 29 through the switch 39.

In accordance with one of the features of this invention, the switches 29 and 38 are ganged, as is illustrated by the dashed line 44 in FIG. 1, so that the heater 23 is turned off when the filament 12 is switched on, and vice versa. This provides approximately constant ion source temperature and sufficient heat at all times to prevent contamination of the ion source regardless of whether or not the filament 12 is on or is 01f. External heater elements, not shown, also are mounted on the tube envelope at positions where contamination otherwise would occur. The temperature of the housing in the ion source region is maintained at from 80 degrees to 90 degrees centigrade, for example. By heating the ion source, as is described above, contamination is prevented and the spectrometer tube, in which the ion source is used, for example, will retain its sensitivity and optimum performance for a considerable length of time.

By using the two heaters (filament 12 and coil 23) alternately, a maximum usable amount of heat is put into the ion source at all times. In prior art ion sources the heat for decontaminating the ion source is produced both by a hot repeller and by the filament of the electron beam producing means. Therefore, the watts of power supplied to the repeller added to the watts of power supplied to the filament must be the maximum allowable. In this prior art ion source when the filament is off (which is most of the time, nights, etc.), maximum heat to avoid contamination is not being used.

In operation, when the filament 12 is energized by closing the switch 39 there is produced a beam of electrons 42 having a rectangular cross section. The beam is shaped and accelerated by the focusing electrode 13 and by the accelerating electrode formed by the boundary of slit 28. As the electron beam 42 crosses the ion chamber 11, it collides with molecules of gas which come into 4. the chamber from the gas inlet, FIG. 3, to produce positive ions.

Within the chamber 11 there is a tendancy for the electrons to penetrate the repeller electrode 32 to be partly collected and partly deflected back into the ion chamber 11. By providing a focusing electrode 22 behind the repeller electrode 16 and by adjusting the tap 40 to which electrode 22 is connected, the trajectories of the electron stream within the ion chamber 11 are controlled for achieving an optimum rate of ion production and an ion beam having maximum current.

The positive ions produced in the ion chamber 11 are repelled toward the bottom of the chamber by the repeller electrode 16. The positive ions are formed into a beam having a rectangular cross section which passes through the rectangularly shaped slit 24 at the bottom of the ion chamber through the focusing

electrodes

18a and 18b, and through the accelerating electrode 19. The ion beam is wedge-shaped and comes to a line focus in the vicinity of the accelerating electrode 19. Below the accelerating electrode 19 the ion beam passes through a rectangularly shaped slit 46 in an iron bafile 43 which is provided for intercepting stray ions. The baffle 43 is made from metal and is connected to ground as is illustrated in FIG. 3.

The improved ion source, as is described above, produces an order of magnitude increase in positive ion current with a corresponding increase in sensitivity of a mass spectrometer using this improved source. Optimum performance is maintained by avoiding contamination by alternately heating the source by the internal heater 23 and by the filament 12 as is described above.

Since changes could be made both in the illustrated embodiments of the invention and in the above description, and different words of description might be used without departing from the scope and spirit of the invention, it is understood that the invention is limited solely by the accompanyng claims.

What is claimed is:

1. An ion source comprising an ion chamber, means including a filament and first focusing electrode for producing and directing a stream of electrons into said chamber, an ion outlet on one side of said chamber, a grid positioned on the opposite side of said chamber for repelling ions toward said one side, and a deflection electrode positioned on the far side of said grid from said one side of said chamber and substantially parallel to said grid for controlling the trajectories of electrons in said chamber to increase the amount of positive ions which are produced in said chamber, said deflection electrode imposing operative field forces in said chamber through the interposed grid.

2. The ion source as is set forth in claim 1, in which said deflection electrode is physically separated from said grid.

3. The ion source as is set forth in claim 2, further including means for supplying dilferent voltages to said deflection electrode and said grid.

4. The ion source as is set forth in claim 1, further including a heater positioned behind said deflection electrode, and means for energizing said heater and said filament alternately for maintaining an approximately uniform temperature in the ion source for preventing contamination.

5. An ion source comprising an ionizing chamber, means including a filament and an electron focus elec trode positioned laterally of said chamber for producing and directing an electron stream into said chamber to produce positive ions, an opening in the bottom of said chamber for passing ions out of said chamber, a repeller electrode positioned at the top of said chamber, said repeller electrode comprising a plurality of spaced conductive elements between which electrons can escape from said chamber, and a deflection electrode positioned above said repeller electrode, spaced physically therefrom in substantially parallel relation, and operative therethrough for controlling the trajectories of the electron stream in said chamber.

6. An ion source as is set forth in claim 5, further including a heater positioned behind said deflection electrode, and means for energizing said heater and said filament alternately for maintaining an approximately uniform temperature in the ion source for preventing contamination.

7. An ion source comprising an ion chamber having a bottom wall and upwardly extending side walls, an aperture in one of said side walls, means for producing and directing a stream of electrons through said aperture into said chamber, an aperture in said bottom wall, a grid positioned at the top of said ion chamber for repelling ions through the aperture in the bottom wall of said chamber, a deflection electrode positioned adjacent said grid on the far side of said grid from the bottom wall of said chamber, said deflection electrode being physically spaced from said grid and in parallel relation thereto, and operative therethrough for controlling the trajectories of the electron stream in said chamber.

8. The ion source as is set forth in claim 7, further including adjustable means for supplying voltages to said repeller electrode and to said deflection electrode to control the trajectories of the electron stream in said cham ber for optimizing the production of positive ions in said chamber.

9. The ion source as is set forth in claim 7, further including a heater positioned behind said deflection electrode for indirectly heating said second focus electrode and said ion source, and means including a switch for energizing said heater.

10. The ion source as is set forth in claim 7, wherein said grid comprises a metal frame having a plurality of spaced wire portions stretched across said frame in at least one plane and firmly anchored to said frame.

11. The ion source as set forth in claim 7, said means for producing electrons comprising a filament having a width substantially equal to the height of the electron beam aperture in the side wall of the ion chamber.

References Cited by the Examiner UNITED STATES PATENTS 2,694,152 11/1954 Berry 3l3230 X 2,732,500 1/1956 McLaren 25041.9 2,967,239 1/1961 Zarnany 25041.9

JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, S. SCHLOSSER,

Assistant Examiners.