US 2754422 A
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y 10, 1955 E. J. LOFGREN ET Al.
SOURCE OF HIGHLY STRIPPED IONS Filed July so, 1952 POWER SUPPLY POWER SUPPLY POWER SUPPLY INVENTORS. EDWARD J. LOFGREN WARREN W. EUKEL ATTORNEY.
United States Patent 2 SOURCE OF HIGHLY STRIPPED IONS Edward J. Lofgren, Albany, and Warren W. Enkel, El Cerrito, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Application July 30, 1952, Serial No. 301,722
11 Claims. (Cl. 250-419) The present invention relates to an improved ion source and particularly to the production of fully ionized or highly stripped ions.
There are many applications of ionized particles and innumerable different ion sources have been developed to produce ions for such uses; however, conventionally singly charged ions are employed. It is generally much easier to remove a single electron from a molecule or atom than to remove a plurality of electrons and for the majority of applications singly charged ions have proven adequate. Thus, in measurement or identification procedures with apparatus, such as a mass spectrometer, it is only necessary for the molecules that are to be operated on to have a known electrical charge and an increase in the charge is not particularly advantageous. In addition to such applications, ions or ion beams may be used as bombarding projectiles which are accelerated to some high energy and directed upon a material to produce a change therein. For this use of ions, as well as many others, it is advantageous to generate highly charged ions, and the ultimate in this respect is the production of stripped nuclei wherein all electrons have been removed from their orbits and only the positive nucleus remains.
Accordingly, it is an object of the present invention to provide an improved ion source for removing a plurality of electrons from a molecule or atom to produce a highly charged ion.
Another object of the present invention is to produce ions having more than a single positive charge.
A further object of the invention is to provide an improved ion source generating stripped nucleus having no orbital electrons.
A still further object of the present invention is to provide an improved ion source generating a beam substantially composed of ions having a plus charge in excess of one.
Yet another object of the present invention is to provide an improved ion source generating C .ions and establishing a beam thereof.
In addition to the above exemplary objects of the invention, there are numerous other possible objects which together with the advantages of the invention will become apparent from the following description of the preferred form of the invention. The description to follow is referenced to the attached drawing wherein Figure 1 illustrates a frontal view of an ion source constructed in accordance with the principles of the invention, and Fig. 2 is a sectional view taken at 22 of Fig. 1.
Considering now the invention in some detail and referring to the drawing, there is provided a housing 11 which may have integral back and bottom walls, and optional end walls, defining a rectangular box having 2,754,422 Patented July 10, 1956 same is a face plate 14 which is removably secured in closing relation to the remainder of housing 11 by screws or the like 16. Face plate 14 is provided with an aperture 17 therein which is preferably elongated in the vertical direction, as shown, and the position of which is related to aperture 13 in collimating plate 12 as explained below. Housing 11 thus comprises an enclosed rectangular box defining a chamber 18 therein which communicates with the exterior of housing 11 through collimating plate aperture 13 and face plate slit or aperture 17.
Within chamber 18 of housing 11 there is disposed a solid block 19 of a material to be ionized, in this respect carbon will be taken as an example in the following description. Carbon block 19 may be cut to slip fit the lower portion of chamber 18 and is preferably formed with a slanted face 21 which is some few degrees, such as 10 or 15, oif of vertical. Carbon block 19 is formed to fit into chamber 18 of housing 11 in such a manner that the slanted face 21 thereof is disposed to refiect a vertical projection of collimating aperture 13 through v the center of face plate slit 17, as shown in Fig. 2. The
an open front and top. Attached to the top of housing present invention is adapted to operate in conjunction with apparatus having an evacuated envelope in which the invention is disposed and a vacuum is maintained within chamber 18 by communication between same and the evacuated volume about housing 11. Rapid evacuation of chamber 18 is facilitated by dispensing either partially or wholly with the end Walls of housing 11 so that maximum communication is provided between chamber 18 and the evacuated volume about housing 11.
A further portion of the present invention is a cathode 24 which includes a pair of rigid electrical conductors 26 carrying an electron emissive member 27, or themselves being in part electron emissive. Conductors 26 are disposed atop housing 11 with the electron emissive surface 27 thereof being aligned with collimating aperture 13 above collimating plate 12. Cathode 24 may be mounted in the above position by means of a clamp 28 engaging conductors 26 and mounted upon the exterior of housing 11 by means of screws 29 or the like. Also associated with housing 11 and cooperating therewith to form the ion ejection means of the invention is an accelerator plate 31 which is disposed substantially parallel to face plate 14 of housing 11 and displaced therefrom. Mounted upon accelerating plate 31 is an accelerating electrode 32 which preferably has somewhat the shape of a hollow truncated cone with the base thereof attached to accelerating plate 31 and tapering therefrom toward housing face plate 14. Accelerating plate 31 has a relatively large elongated aperture 33 therein in alignment with aperture or slit 17 in face plate 14 and hollow accelerating electrode 32 is disposed about aperture 33 so that there is defined an ion path from carbon block face 21 through aperture 17 in face plate 14 and thence through accelerating electrode 32 and aperture 33 in accelerating plate 31. Accelerating plate 31 and accelerating electrode 32 carried thereon may be mounted in position by any suitable means and upon any convenient member and there is illustrated a mounting including insulators 34 connected to housing 11 and accelerating plate 31.
Attached to the above-described elements of the source are various power supplies to enable the source elements to carry out their designed functions and completing certain electrical circuits, as noted below. Across the conductors 26 of cathode 24 is connected a filament power supply 36 providing heating current for cathode 24 whereby the temperature thereof is raised to the point of electron emission. In this respect cathode 24 may comprise a single U-shaped conductor 26 with electron emissive 7 surface 27 being formed by reducing the area of conductors 26 where they pass over collimating aperture 13in alignment therewith whereby onlythis portion of cathode 24 is raised to a sufiicient temperature for electron emission and no discharge occurs to collimating-plate 12. An electron accelerator power supply '37 is connected between cathode 24 and housing 11, which has all of the elements thereof in electrical contact. The positive terminal -of power supply'37 is connected to housing 11-so 'that'electrons will be attracted from cathode 24. One further powersupply 38 is provided to produce an ion accelerating field and is connected between housing 11 and accelerating plate 31 with the negative terminal being joined to accelerator plate 31. Power supplies 37 and 38 are of the direct current type in order that the electrostatic fields established by the potentials thereofwill not reverse polarity; however, both of these'power supplies may be pulsed and the pulsed operation thereof 'may 'be controlled from a common pulser or control circuit (not shown).
Considering now the operation of the illustrated embodiment of the invention, operation -of filament supply 36 causes a heating current to pass through conductors 26 of cathode 24 to raise electron emissive surface 27 thereof to the temperature of electron emission. The potential impressed between cathode 24 and housing 11 causes electrons emitted from cathode 24 to be attracted therefrom toward collimating plate 12. As the electron emissive surface 27 of cathode 24 is aligned with aperture 13 in plate 12 the electron discharge attracted from cathode 24 is accelerated through aperture 13 into chamber 13 and is collimated thereby to a desired cross section. The collimated electron discharge passes through a relatively field-free region Within housing 11 and strikes a portion of the slanted face 21 of carbon block 19 with a resultant heating of the portion of block 19 so bombarded. There are produced, substantially at the'area of bombardment, carbon ions which are thus subjected to the attractive force of ion accelerating electrode 32 caused by the electrostatic field produced by power supply 38. These ions are thus attracted toward ion accelerating electrode 32 through exit slit 17 in the face plate 14 of housing 11 and thence through ion accelerating electrode 32 and through aperture 33 in accelerating plate 31 to emerge as a beam of ions. This beam of ions may then be operated upon in any desired manner and for Whatever purpose may be desired, such as target bombardment.
in order to produce multiply charged ions it is necessary to prevent the formation of an are within chamber 18, as noted below, and consequently the material of which block 19 is formed must be substantially non-volatile, such as a refractory material. Further insurance against the establishment of an arc is provided by the use of an open ended housing which is thus readily evacuated by the vacuum system of the apparatus in which the source is disposed.
In connection with-the mechanism of ionization, it is to be noted that the removal of each electron from a molecule or atom requires a certain energy whichmay be defined in terms of electron potential. Thus for carbon it requires about 24 electron volts to remove a first orbital electron, 45 electron volts for the second, 64 electron volts for the third and increasing potentials for successive electrons up to the sixth. These energies are commonly termed ionization potentials and the ionization potential required to remove all of the orbital electrons from carbon is the sum of the ionization potentials of each electron or somewhat less than 500 electronvolts.
While this ionization potential will theoretically strip the carbon nucleus by removing all six of the orbital electrons it is necessary to provide some greater potential to obtain a high probability of large scale production of stripped ions. It has been found that the provision of energy that is some three to five times the theoretical minimum ionization potential will cause large scale tflpped ionproduction.
The-presentinvention operates to provide the-requisite energy-of some -1S0Oto 2500 electron volts by accelerating an electron beam with an electrostatic field of about three kilovolts, for example, established by electron accelerating power supply 37. By impressing at least 1.5 to 2.5 kilovolts between cathode 24 and housing 11 the electron discharge from cathode 24 is accelerated to this potential range and collimated ,into .a beam to impinge upon a desired area of the slanted face 21 of carbon block 19. The mechanism .of ionization includes the production of at leasta substantial portionof the ions by direct impingement of high energy electrons upon the surface of block 19. Ions are thus made directly from the solid block and :the energy .of the bombarding electron beam insures complete stripping of orbital electrons to produce highly charged ions.
in the design of any particular source according to the present invention, it must be realized that ion exit slit 17 is to be positioned adjacent the area of block 19 that is bombarded by the electron :beam. Also, exit slit 17 defines the extent of the ion accelerating field within chamber 18 and 'thusslit v,17'should not be so large as to allow this extracting field .to distort or disrupt the bombarding electron beam. .A further consideration is the angle at which the ion beamimpinges the surface 21 of block 19. With large scale .ion production a considerable quantity of block material is removed and if the electron beam strikes surface .21 at a large angle, say in the vicinity of ninetydegrees, an .undue amount of pitting of the surface .21 will -.result;,however, block 19 may be moved past the beamto prevent this. Also, by decreasing the angle .Of incidence of :the electron beam .a greater area of surface 21 is :subiected .to bombardment for ion production and ion-extractiongis simplified.
In addition .to the above, it is particularly important thatzan arc discharge .does not occur within chamber 18. Asset out above an electron beam of specified energy, dependent 'upon the total ionization potential of the material to be ionized, is employed to bombard the surface of the material; however, should this electron beam be transformed into an arc discharge the surface 21 would not be bombarded with electrons of sufiicient energy to strip the atoms ;for the are potential would differ but slightly'from' that of block :19. The establishment of an arc is prevented-by employing a substantially non-volatile material for block 19, and the continuous evacuation of chamber 18, while there may be further provided additional collimatingplates between the one shown and surface 21 of block -19for the purpose of confining gas molecules and ions to the vicinity of the surface of block 19.
With the present device, it is possible to produce copious quantities of highly charged ions and in particular Wholly stripped atoms having .no orbital electrons by bombardingtheatoms. with electrons having an energy in excess by a factor of from three to five of the total ionization potential :of the'material (sum of the ionization potentials of each electron).
While the invention has been described with respect to a single embodiment it will be apparent to those skilled in the art that numerous modifications and variations of the illustrated form of the invention are possible within the scope of the invention and thus attention is directed to the following claims for a precise definition of the invention.
What is claimed is:
'1. ,An improvedion source comprising a material to be ionized, an electron emissjive cathode, electron accelerating means .coactiing with aid cathode to accelerate electrons therefrom toward said material .to be ionized, and meansmaintafininga ,siibstantial'length ofelectron traverse ion free, said electron accelerating means imparting to said electrons an energy in excess of the total ionization Potential ofatoms ofsaid material whereby atoms .0f.said mater al aretstripped .of orbital electrons.
in pronedgion source for generating ions .having noflibitall electrons comprising a material to be ionized in the form of a solid, an electron emissive cathode spaced from said material, means accelerating electrons from said cathode toward said material whereby electrons bombard said material, said means accelerating the electrons to a potential greater than the total ionization potential of atoms of said material whereby stripped ions are produced herefrom, and ion accelerating means attracting ions from said material away from said ionizing electrons whereby the electron path is substantially free of ions.
3. An improved ion source for producing stripped ions having no orbital electrons from a solid block of material comprising means producing an electron discharge, means directing said discharge upon a solid block of a material to be ionized, and means accelerating the electrons of said discharge to potentials in a range of three to five times the total ionization potential of atoms of said material whereby stripped ions are produced therefrom, said means including electron accelerating means and means defining a substantially field-free region in the path of said electron discharge adjacent said solid block of material whereby ions do not traverse the path of said electron discharge.
4. An improved ion source for producing multiply charged ions comprising means establishing a high energy electron beam, a non-volatile material to be ionized, evacuated housing means enclosing and containing said material, means directing said electron beam into said housing upon said material to produce by electron bombardment multiply charged ions thereof, and ion ejection means maintained at a negative potential with respect to said material and in communication therewith to attract ions therefrom.
5. An improved ion source for producing multiply charged ions from a material to be ionized comprising a housing containing the material to be ionized and having first and second openings therein, means producing a high energy electron beam directed through the first of said housing apertures to bombard said material therein, means defining an evacuated path for said electron beam to the vicinity of said material whereby said beam impinges said material at a high energy to produce multiply charged ions, and ion ejection means electrically influencing the ions so produced to eject same through said second housing aperture.
6. An improved ion source for producing multiply charged ions from a material to be ionized comprising a housing containing said material and having first and second apertures therein, an electron emissive cathode disposed in alignment with said first aperture exterior to said housing, high voltage power supply means connected between said cathode and said housing to produce an electron discharge from said cathode passing through said first housing aperture wherein same is collimated to produce a high energy electron beam directed upon said material disposed within said housing, said power supply means impressing between said cathode and said housing a voltage greater than the total ionization potential of said material to be ionized, whereby multiply charged ions thereof are formed by the bombardment of said material by said electron beam, and means electrically attracting the ions so formed through said second housing aperture whereby said ions are ejected from said housing.
7. An improved ion source for producing multiply charged ions from a material to be ionized comprising housing means containing said material to be ionized and having a collimating aperture therein, an electron emissive cathode disposed exterior to said housing in alignment with said collimating aperture, power supply means connected intermediate said cathode and housing for producing an electron beam directed through said collimating aperture into said housing and into contact with said material to be ionized whereby multiply charged ions thereof are formed by electron bombardment, and means electrically attracting said ions from said housing.
8. An improved ion source as claimed in claim 7 further defined by means preventing the establishment of an arc discharge between said cathode and said material to be ionized and including means continuously evacuating said housing to remove vapor of said material produced by electron bombardment thereof.
9. An improved ion source for producing ions constituting atomic nuclei without orbital electrons and comprising a material to be ionized, housing means containing said material and having first and second apertures therein, an electron emissive cathode disposed exterior to said housing in alignment with said first aperture therein, a power supply connected between said cathode and said housing for producing an electron discharge collimated by said first aperture into an electron beam directed upon said material to be ionized, said power supply impressing between said cathode and said housing a voltage that is three to five times the summation of the ionization potentials of each of the orbital electrons of an atom of said material to be ionized whereby said bombarding electron beam strips the atoms of said material of all orbital electrons, and ion ejection means disposed adjacent said second housing aperture for removing ions from said housing.
10. An improved ion source as claimed in claim 9 further defined by said material to be ionized having the form of a solid with one face thereof disposed in the path of said ion beam and inclined with respect thereto and said second housing aperture being aligned with the bombarded area of the face of said solid material.
11. An improved ion source as defined in claim 10 further characterized by the face of said solid material being inclined with respect to said bombarding electron beam to provide an angle of incidence of 10 to 15 degrees.
References Cited in the file of this patent UNITED STATES PATENTS 2,498,841 King Feb. 28, 1950 2,576,601 Hays Nov. 27, 1951 2,600,151 Backus June 10, 1952 OTHER REFERENCES In Source for Mass Spectrography, by R. F. K. Herzog and F. P. Viehbock, published in Physical Reviews, vol. 76 (1949), pages 855, 856.