US3663852A

US3663852A - Double cell high intensity ion source

Info

Publication number: US3663852A
Application number: US24941A
Authority: US
Inventors: John S Luce
Original assignee: US Air Force
Current assignee: US Air Force
Priority date: 1970-04-01
Filing date: 1970-04-01
Publication date: 1972-05-16
Anticipated expiration: 1989-05-16

Abstract

The instant invention involves a system for generating high current density charged particle beams. The system includes a plurality of high intensity ion sources in combination with two charge exchange cells aligned linearly with one another. The ions are converted from one type of charge to an opposite charge in two stages. In the first stage, several beams of positive ions, for example, derived from a plurality of positive ion sources are beamed into a charge exchange cell and converted into neutrals. The several beams of neutrals from the first cell are then directed into a second charge exchange cell where the neutrals are converted into negative ions.

Description

United States Patent Luce [4 1 May 16, 1972 54] DOUBLE CELL HIGH INTENSITY ION 3,424,904 1/1969 Donnally ..3l3/63 x SOURCE Primary Examiner-Roy Lake [72] Inventor: John Luce, Cahf- Assistant Examiner-Darwin R. l-lostetter [73] Assignee: The United S m f America as Attorney-Harry A. Herbert, Jr. and William J. O'Brien re resented b the Secreta of the Air Fo ice y W [57] ABSTRACT [22] Filed: Alm l, 1970 The instant invention involves a system for generating high current density charged particle beams. The system includes a PP 24,941 plurality of high intensity ion sources in combination with two charge exchange cells aligned linearly with one another. The [52] U s 313/63 250/84 313/1 ions are converted from one type of charge to an opposite [5 I] "fi 27/00 39/00 charge in two stages. In the first stage, several beams of posi- [58] Fieid 6 tive ions, for example, derived from a plurality of positive ion sources are beamed into a charge exchange cell and converted into neutrals. The several beams of neutrals from the first cell [56] References cued are then directed into a second charge exchange cell where UNITED STATES PATENTS the neutrals are converted into negative ions. 3,136,908 6/ l964 Weinman ..3l3/63 9 Claims, 1 Drawing Figure Patented May 16, 1972 3,663,852

. AAA A INVENTOR. vfd/IN .9- .(Z/CA' M wwm DOUBLE CELL HIGH INTENSITY IONSOURCE BACKGROUND OF THE INVENTION The present invention relates to an improved system for the generation of charged particle beams. More particularly, the present invention concerns itself with an apparatus and method for converting a first beam of charged particles of one polarity to a charged beam of the same or opposite polarity by a two stage conversion technique. In this technique, the charged ions are first converted to neutrals, followed by a second conversion or ionization of the neutrals to form a charged particle beam of the same or opposite polarity as that possessed by the first beam.

In the past the generation of negative ions, generally depended on a double charge exchange in a single charge exchange cell of either the gas or the mercury jet type. These ion sources developed a total current of I to 200 microamperes. However the limited current-delivery capabilities of these prior ion sources prevented their utilization for modern technical applications. The technical applications of today require negative ion sources capable of delivering a total negative ion current of to 100 milliamperes, to come from a focal spot 1 millimeter square. This means that even with an optimistic l0 percent efficiency of conversion from positive ions to negative ions, in a conventional negative ion source, an initial positive ion beam of from 100 milliamperes to one ampere must be concentrated in an area 1 millimeter square. The mutual repulsion of the positive ions makes this impossible.

In attempting to overcome the problems associated with these prior art devices, it has been found that a two stage technique for converting positive ions to negative ions circumvents these difficulties. In the first stage positive ions derived from several ion sources are converted into neutrals in a charge exchange cell. The beam of neutrals then enters a second charge exchange cell where the neutrals are converted into negative ions. As many neutrals as desired can be focused into the second cell because the neutrals, being uncharged will not interact or repel one another.

SUMMARY OF THE INVENTION In accordance with the present invention, negative ions are generated from a system including a high intensity positive ion source and two charge exchange convertors which convert positive ions to neutrals and then to negative ions.

The essential elements of the system of this invention is a high intensity ion source, a first charge. exchange cell to convert a beam of charged particle to neutrals and a second charge exchange cell to convert or ionize the neutrals to a charged particle beam of the same or opposite polarity as that of the beam emanating from the ion source.

Preferably, the beam emanates from a plurality of ion sources and is directed toward and passes through a first charge exchange cell, where it is converted into a beam of neutrals. The several beams of neutrals are then directed into a second charge exchange cell, where they are converted into a charged particle beam.

The approach conceived by the present invention has several advantages.

For example, the multiple neutral beams can be directed to intersect within a small volume without space charge blow-up, thereby achieving higher current densities than are possible with state of the art devices. In addition, the positive ion sources can be placed far enough from the focal point to provide them with room and considerable freedom exists in the arrangement of the several ion sources and the charge exchange cells as well as the beam-defining baffles which help to minimize the angular divergence of the finally emerging high current density ion beams.

Accordingly, the primary object of this invention is to provide an efficient high current density ion source.

Another object of this invention is to provide an improved system for converting a plurality of positive ion beams to a high current density negative ion beam.

Still another object of this invention is to provide an improved system for generating negative ions through the utilization of a two stage technique in which positive ions are first converted to neutrals followed by the conversion of the neutrals to negative ions.

A further object of this invention is to provide a technique of utilizing a plurality of intersecting neutral beams to achieve high density charged particle beams that overcome space charge effects thereon.

Still other objects and advantages of the present invention will become more readily apparent after consideration of the following detailed description thereof when taken in conjunction with the accompanying drawing.

A BRIEF DESCRIPTION OF THE DRAWING The drawing: The FIGURE represents a schematic illustration of preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT It is believed that a brief discussion of the formation of a negative ion will be of help in understanding the present invention. The formation of a negative ion involves the addition of an electron to a neutral atom according to the equation A e E A, where A and e are the masses (in energy units) of the neutral atom and the electron, respectively, E is the electron affinity, and A is the mass of the ion, again expressed as energy.

For the ion to be stable, E must be negative, that is, energy must be expended to remove the electron. It is also possible to create negative ions directly from positive ions by double electron capture:

Several methods have been utilized for the generation of negative ions. In view of the very large currents desired for present applications, methods such as thermionic emission, surface ionization, and secondary negative ionic emission are not feasible. The large electron loss cross sections of negative ions in molecular and electron collisions preclude the achievement of an intense ion beam from gas discharge sources.

One method, however, appears and uses charge exchange in gases and vapors, metallic jets, and metallic foils. In the charge exchange process, a moving positive ion, for example, in passing close to a neutral atom, will remove an electron from it; the ion thereby becomes a neutral while the former neutral becomes charged. This event can be expressed symbolically as A B l5 A. The probability of this event occurring is measured, as with other atomic events, by the numerical value of a cross section. Such cross sections have been measured for several types of charge exchange collisions between many pairs of elements.

When a beam of particles, say H moves through a gaseous target a great many charge exchange collisions will take place. Some of the H will become H", which in turn will charge exchange back to l-I or possibly II. If the target has enough atoms, an equilibrium composition of the beam containing the highest possible fraction of negative ions will result. The numbers of H*, H", H3, and H in the equilibrium beam will be related to each other in a complicated fashion. A target which contains enough atoms to cause this equilibrium composition of the beam is called a thick" target. Once the equilibrium composition of the beam is achieved, further collisions scatter out of the beam, thereby reducing the intensity and increasing the angular divergence. In the production of intense ion beams it is important, therefore, to provide a target just thick enough to achieve equilibrium composition without unnecessary scattering or attenuation.

With the foregoing background information in mind, attention is now directed to the present invention as represented in the FIGURE of the drawing.

The Figure is a schematic illustration of a preferred embodiment of the invention and discloses three individual high intensity positive ion sources, l0, l2 and 14 positioned in linear alignment with a charge exchange cell 16. Electrostatic lens 18 19 and 21, which are conventionally included in ion sources, are positioned to direct the ion beams into cell 16 which in turn is positioned in linear alignment with a second charge exchange cell 20. Pairs of

electrostatic deflection plates

22, 23 and 24 are positioned between the two charge exchange cells for removing unwanted charged particles from the neutral beam. An analyzing chamber 26, which includes an electromagnet and beam calorimeter, is positioned to intercept and measure the charged components of the emerging beam. Beam defining baffles can be positioned to help minimize the angular divergence of the ion beam. The entire system, including the ion sources, charge exchange cells, electrostatic lens, beam defining baffles if utilized and electrostatic deflection plates is enclosed in a vacuum assembly not shown.

The ion sources are positioned at one end of the vacuum assembly and a conventional small hydrogen bottle charged to 150 psi and a conventional palladium leak assembly, neither of which are shown, are mounted adjacent thereto. The pressure in the ion source, measured by means of a conventional thermocouple gauge, can be modified during operation by adjusting the palladium leak heater current. Preferably, the ion source is of the von Ardenne duoplasmatron type which normally operates at a positive potential of the order of 40 kv. However, any conventional ion source would be suitable.

The electrostatic lenses l8, l9 and 21, which forms part of the ion source, are einzel lense which consists of an extractor at or near ground potential, a focusing electrode at a high positive potential several kilovolts less than that of the source, and a second focusing electrode at extractor potential. The lens is an accel-decel-accel arrangement which has a focusing action on the beam emerging from the source.

In a conventional charge exchange cell, certain difficulties are encountered. For example, conventional cells comprise a target gas within a chamber having two apertures arranged to permit free passage of charged particle beam. Extensive baf fling and differential pumping must be used to handle the large amounts of gas leaking from the relatively large apertures. The very large ion currents required in modern technical applications, therefore, oftentimes precludes the use of conventional charge exchange cells with their high gas leakage rates. The gas cannot be pumped away fast enough to maintain the required vacuum.

One way of circumventing this problem is the use of metal vapors instead of gas in the cell. Many metals, such as aluminum, magnesium, titanium, chromium, or lithium, may be used. These metals have sufficiently low vapor pressures at room temperature such that they will condense on most surfaces instead of loading the vacuum pumping system. At the same time, their vapor pressures at achievable high temperatures are sufficient to attain vapor densities of to 10 atoms/cm. in reasonably small cells. If a metal such as titanium is used, the pumping load could actually be decreased by the gettering action of the metal condensed on the walls of the system. It is this type of cell which has been found preferable for use with the present invention. One of the simplest and most reliable cells uses an electrically heated furnace. The technology of these cells is well developed and, as a consequence, a discussion of the details of the cell is not presented herein.

The metal vapor charge exchange cells and a pair of electrostatic deflection plates (located between the cells to remove charged particles from the beam entering the second vapor cells are positioned in linear alignment with the ion sources. Furnace power for the vapor cells can be brought in by means of insulated water-cooled copper tubing, not shown.

The high electric fields associated with the ion sources can be terminated in a conventional manner by use of a copper screen which serves to shield the vapor cells from high voltage. A second copper screen can be positioned to surround the metal vapor cells and is biased to a positive potential of 300 volts to collect stray electrons which might otherwise be accelerated by the high voltage of the einzel lens or the ion sources. The analyzing chamber 26, including an electromagnet, is used to separate and measure the various charged components of the beam.

in the operation of the preferred embodiment of this invention, a plurality of beams of positive ions are generated by individual

positive ion sources

10, 12 and 14 which are directed by means of a conventional electrostatic lens 18, located within the sources to a first charge exchange cell 16 where more of the positive ions are converted to neutrals. The several beam of neutrals are then directed to a second charge exchange cell located at the focal point of the converging beam of neutrals. A portion of the neutrals are converted to negative ions by the second charge exchange cell resulting in the generation of a negative ion beam of an intensity considerably higher than that achieved by previously known ion generating systems.

Having thus described a preferred form of the present invention, it is to be understood by those skilled in the art that the description was for purposes of illustration only and that all such modifications and alterations as come within the scope of the appended claims is intended to be included herein. For example, although the present invention discloses components which change positive ions to neutrals and thence to negative ions, it would also be within its scope to change negative ions to neutrals and thence to either positive or negative ions as desired.

What is claimed is:

l. A system for generating a high current density charged particle beam comprising:

a plurality of ion sources for generating a plurality of charged particle beams;

means for neutralizing most of the ions of each of said charged particle beams generated by each of said plurality of ion sources; and

means for ionizing a portion of the neutralized ions of the beams coming from said means for neutralizing, said plurality of ion sources and said means for neutralizing being arranged such that said neutralized particle beams converge into and intersect at said means for ionizing.

2. An apparatus in accordance with claim 1 wherein said ion sources generate positively charged particle beams.

3. An apparatus in accordance with claim 1 wherein said ion sources generate negatively charged particle beams.

4. An apparatus in accordance with claim 1 wherein said means for ionizing is adapted to convert said neutralized beams to negatively charged particle beams.

5. An apparatus in accordance with claim 1 wherein said means for ionizing is adapted to convert said neutralized beams to positively charged particle beams.

6. An apparatus in accordance with claim 2 wherein said means for ionizing is adapted to convert said neutralized beams to negatively charged particle beams.

7. An apparatus in accordance with claim 2 wherein said means for ionizing is adapted to convert said neutralized beams to positively charged particle beams.

8. An apparatus in accordance with claim 3 wherein said means for ionizing is adapted to convert said neutralized beams to positively charged particle beams.

9. An apparatus in accordance with claim 3 wherein said means for ionizing is adapted to convert said neutralized beams to negatively charged particle beams.

Claims

1. A system for generating a high current density charged particle beam comprising: a plurality of ion sources for generating a plurality of charged particle beams; means for neutralizing most of the ions of each of said charged particle beams generated by each of said plurality of ion sources; and means for ionizing a portion of the neutralized ions of the beams coming from said means for neutralizing, said plurality of ion sources and said means for neutralizing being arranged such that said neutralized particle beams converge into and intersect at said means for ionizing.