US2926251A - Ion acceleration system - Google Patents

Description

Feb. 23, 1960 J. 5. LUCE arm. 2,926,251

ION ACCELERATION SYSTEM Filed July 18, 1956 2 Sheets-Sheet 1 I I n I IIIIIII I I I v P- um 4 24 f:

IN VEN TORS BY John S. Luce and John A. Marfin ATTORNEY Feb. 23,, 1960 J. 5. LUCE ETAL ION ACCELERATION SYSTEM 2 Sheets-Sheet 2 Filed July 18, 1956 IN VEN TORS v BY John S. Luce and 7 John A. Mari-In Maya/4M ATTORNEY United States Patent ION ACCELERATION SYSTEM John S. Luce and John A. Martin, Oak Ridge, Tenn.,

assignors to the United States of America as represented by the United States Atomic Energy Commission Application July 18, 1956, Serial No. 598,725 7 Claims. (Cl. 250-413) The present invention relates to production of relatively large beams of positive ions, and more especially to a method of and apparatus for forming intense beams of positive ions including an ion source and means to accelerate ions from said source through grid electrodes to a desired region of focus. One form of the apparatus may be incorporated with conventional'structure to form an improved embodiment of the isotopeseparator called the calutron.

The calutron is a device for accomplishing isotope enrichment or mass separation of positive ions by accelerating them through an intense magnetic field between a source and a collector. In the calutron, a solid sample material is vaporized by heating, the vapor is ionized by electron bombardment inside an ion source which is maintained at a high positive potential, ions are pulled out of the source by a highlynegative accelerating electrode disposedadjacent the mouth of the source, the ions passing through the accelerating electrode are decelerated by causing them to pass through a second grounded electrode, and the resulting beamof ions is passed through the intense magnetic field to the collector. A complete description of a suitableembodiment of the calutron is contained in US. Patent 2,709,222, issuedto 2,926,251 Patented Feb. 23, 19 0 point is termed the saddle point. It was believed that positive electric fields penetrated the negative accelerating slit, and accordingly a flat grid was placed in the accelerating slit to straighten the positive fields. As a result, the focal region was moved to the grid openings and the beam diverged badly, so that unsatisfactory focusing of the beams on the collectors resulted. Other studies suggested that the beam might be focused if the arc plasma were bowed convex, rather than concave, as if a virtual focus point existed behind the are, but such focussing was never achieved. Widely divergent beams resultedfrom operation with a single focussing electrode, because the electric field was so weak in thecenter that .the arc bowed toward the accelerating electrode. With such divergent beams, increasing arc current or voltage only increased the numbers of ions draining to the electrode.

With a knowledge of the shortcomings of the known methods and apparatus for obtaining well-focused, intense ionbeams, applicants have, as a primary objectof their invention, provision of a method of and means for obtaining a relative intense, well-focused ion beam. Another object of the invention is to stabilize the arcplasma in an ion source while simultaneously withdrawing an ion current of relatively high intensity therefrom. ,A further objectof the invention is to provide a generator of positive ions including an ion source provided With.a grid-like ion exit aperture forming a plurality of small, discrete beams, and an accelerating grid disposed closely adjacent the source to further focus and define the discrete beams. Another objectis. to provide a well-focused intense beam of ions with a single accelerating electrode and a single voltage source. .Yet another object of the invention is to provide means for obtaining large ion beam by means of an ion source and an accelerating grid structure which has a small convex curvature to obtain greatly improved focusing of the ion beam at the collector. The major object of the invention is to eliminate the disadvantages inherent in ion beam generators having a narrow focal region in front of the source by providing novel means for withdrawing the beam from the ion source and for focusing it in such a manner that all the beam in the acceleration region, resulting from the coulomb repulsion accompanying large beams of charged particles. he observed calutronperformanceis in :aceord with theory, sinceapplication of .Ghilds 3/ 2-power law indicatesthat no mo re than .about 46 ma/in. of emission could be-expectedto pass through a focal: point. In, prior calutronoperation, a positive-ion sheath forms between the arc plasma and the accelerating electrodes, the boundary between the arc plasma and the sheath being known as the center of the meniscus. The meniscus is pushed back toward ,the arc plasma, away from the source grid by the accelerating field which extends into l-the ,iorieiiit slit region and presents a concave surfacetoward the arc plasma, so thatall ions leaving the me sens normal to thesurface thereof must passthorugh narrow ionexinslit in thesource and a small focal region the vicinity of the accelerating electrode. The focuss ing effect of the dimple in the arc plasma as caused 'by the'meniscus being pushedback as discussed above results in excellent separation factors for different isotopes collected at the "180 position.

In prior calutronstudies, a'region of ionization due-to oscillatory electrons was notice in the accelerating elec trode slit which caused the ion bearnto pass'thro'ugh a focal point in the accelerating electrode slit and ions no longer must pass through any small focal regions.

These ,objectsare attained'by providing a series of small apertures in the front'face of the ion source, rather than the one continuous ion exit slit, and a single corresponding grid spaced very .closely adjacent the ,ion sourceto replaceboth. the prior accelerating and decelerating electrodes. With the sourceata positive potential, the accelerating grid may be grounded. With such arrangement,.intense normally emergent ion beams maybe 1 p'roducedifrom the source and focussed at a desired location without first passing through a common focal point, so that no beam blow up occurs due to coulomb repulsion. One or moreofthe grids may be convex relative to the front face of-the ion source, to improve the focus of the beam ,at .the desired location. By providing a grid across the ion-exit slit, the are is strengthened in the center and shielded from external fields, so that relatively large currents may be obtained with relatively low accelerating potentials. Preferably, the accelerating grid is curved to improve the focus, while the source grid is .flat, tending to push'back the center of the arc and thereby preserve the focus,

;Means by which the above objects may be attained are illustrated in the appended drawings, wherein: Figure 1 illustrates schematically one form of my improved 'fo lsystem;

Figure 3 illustrates a partial plan view of one form of an accelerating grid;

Figure 4 illustrates a partial plan view of one form of a corresponding source grid;

Figure 5 illustrates a preferred embodiment of the grid focal system as it is utilized for isotope separation.

Figure 6 illustrates a partial plan view of the accelerating grid of Fig. 5;

Figure 7 illustrates a partial plan view of the source grid of Figure 5, and

Figure 8 shows schematically a flat-grid source used to inject ions into a system. Referring now to Fig. 2, a first grid focal system is shown incorporated in a conventional calutron ion source box, including a rectangular graphite box 20 defining an arc chamber 21 therewithin. One wall 22 of the box is provided with a defining slot 24 through which electrons are accelerated from a filament mounted adjacent the wall 22, across the arc chamber, through an aperture 25, to an anode mounted externally of the wall 23. A suitable source box is shown in my prior Patent No. 2,700,107, issued January 18, 1955. A potential of 50-300 volts between box 20 and the filament may be provided to accelerate electrons into the chamber. A vapor-entry slot in the bottom of the box receives vapor as described in the patents cited above. Electrode 26 bridges the vapor exit slit in the top wall of the box, and may be provided with a plurality of aligned small aper- 'tures 29 as shown in the plan view, Figure 4. The source grid 26 may be a separate grid structure placed in a corresponding aperture in the top wall of the box 20, as shown in Fig. 2, or it may be a separate plate as shown in Fig. 4, provided with a grid like portion and also a portion without apertures, such that it may be fastened to the holds the box 20, through a suitable insulator. The spacing between grids is made very small, but may vary from about ,4 to going from mass 1 to mass 235. One satisfactory source comprised a grid 27, A thick at its outer edges, and tapering to 4," thick in the center, grid-like portion, with a grid 26 only & in thickness. In one suitable arrangement, the apertures in the source grid were x while the slots were ,4 apart along the line of their shortest dimension and apart along the line of their longest dimension. Corresponding apertures in the accelerating grid were A x 7 and the apertures were spaced apart along the line of their shortest dimension and apart along the line of their longest dimension. From the above exemplary dimensions it may be noted that the apertures in the grid 26 are slightly shorter than those of the grid 27, and that the walls between the apertures in grid 27 are correspondingly thinner to allow for alignment of apertures in the two grids.

Referring now to Figure 1, an improved embodiment of the grid focusing system is illustrated schematically. Rectangular graphite box 1 defines an arc chamber 2 in which is established an arc plasma 3 comprising positive ions and electons. A flat source grid 4 forms the front wall of the arc chamber and is provided with a plurality of apertures to divide the ion beam into a plurality of discrete, small beams. The grid is maintained at the same potential as the arc chamber box 1. Spaced closely adjacent the grid 4 is an accelerating grid 5, the central grid portion of which is slightly convex in curvature. A negative accelerating potential is applied between the grid 5. and the box 1 to form an ion beam and to accelerate it into the analyzing field along the paths illustrated into a focus along the focal line 6, where a collector may be positioned. It will be noted that the focal line is no longer 180 from the arc chamber or from the accelerating region as it is in the normal calutron operation. Rather the curvature of the accelerating grid causes ions of like mass-to-charge ratio to be focused at a point less than 180 from the accelerating region, as shown. This focusing arrangement may be better understood by considering that a virtual focus is formed at a point behind the arc chamber box 1 along the line 7, displaced 180 from the focal line 6. It may be seen that by virtue of this novel apparatus, the actual ion beam never goes through a real focal point until it reaches the collector, so that there is not now a space-charge imposed limitation on the amount or current density of the beam going through a saddle point.

Figure 5 illustrates schematically how large ion currents are withdrawn from the arc plasma, accelerated into an analyzing field, separated into discrete beams of ions having different rnass-to-charge ratios, and collected in suitable collector pockets. The substance to be separated may be vaporized in any conventional calutron furnace, such as that shown in the Lawrence patent, supra, and allowed to enter an especially constructed, one-piece graphite source box 31 through vapor entry slit 32 in graphite plate 33 which defines the rear of the arc chamber 34. Rather than being provided with the conventional ion exit slit in the forward wall, source box 31 is provided with a slightly convex grid portion having a plurality of small, parallel slots therein. Figure 6 illustrates the front view of the source box 31 looking into the grid portion. The slots 35 are drilled into the slightly convex portion 36, which may be machined in the same block of graphite forming box 31. The electron collimating slits, allowing entrance of the electron beam from the filament and exit of the electron beam to the anode are placed in line with the inner edge of the grid portion as indicated by the slot-shaped arc plasma 37, so that the arc will be formed adjacent the grid portion, and just behind the slit 35.

The accelerating grid 38 may also be a unitary graphite structure in which is machined a grid portion 39 which is slightly concave in the direction facing the convex grid 36. A front view of grid 38, looking into the grid from the left in Fig. 5, is shown in Fig. 7. A plurality of parallel, aligned slots 40 are provided in the curved portion 39 of the grid, the front portion of the member 38 adjacent the slits being cut away to form a recess 41 to allow ample clearance for the ion beam. The grid 38 may be held in place adjacent source 31 through an insulating spacer member fastened to the same frame as is the source, or it may be held in place by a separate support or any other suitable means.

It has been found that the arc plasma is stabilized by the curved grid 36 and that large quantities of ions may be withdrawn from the plasma, since there is no focal point in the accelerating region to limit the numbers of ions which may pass therethrough. Ions may be collected in suitable collector boxes 42 provided with pockets 43, 44 for receiving ions of different masses. It is to be noted that the collector 42 is disposed forward of the source 31 rather than directly above it, as is usually the case because of the virtual focus method of operation achieved by the described curved grid system. With the system illustrated, a broad, intense beam originates at the ion source and narrows to a focus only when it arrives at the collector. Relatively low accelerating potentials are required for the accelerating electrode.

In a preferred embodiment of the apparatus of Figure 5 adapted for separation of isotopes having respective mass to charge ratios of 7 and 6, collection has been achieved with a separation of 1% inches at the collector, the collector being 3 1 inches forward of the source box.

netic field of 3.100 gauss, .collection is readily achieved,

--although it willtbe ap arent'ithaLother*suitable fieldtintensities and zaccelerating voltages may heiprovided.

.By way ofrfurther illustration, :the slits :35 :may be PA inch in height while :the :slits :40;may be 7% :inch in -:height. Those :slits Y may be .inch :across and separated by spacer members A inch wide. the bars between the slits are preferably beveled *att45", the bars being' A inch in thickness andzthe bevel extending approximately /2 :of this .thickness.. -.In'one embodiment-o'f an ion source used, thegrid members were 16% inches in'length, with 13 inches between center lines of the first and last of the parallel grids. Thegridsiin the cut-away .portion of member 38 may be inch in thickness, or twice the thickness of thegrid spacer membersinthe source 31. While considerably largertotal ioncurrents can be consistently received, it has been found thatthe beam focus is sufliciently sharp to assure acceptable isotopic enrichment at total currents of around .700'niilliamperes with uranium. Beam currents of about ing 'vertical, rather than horizontal grids, beam disper- -The outward edg -Sci sion=vertically-is about i 2%, rather than 27%, allow- I ing 180 isotope separation;

In the preferred embodiment shown the -source grid outer surface is machined to a 4 inch radius,the outer surface of-the accelerating grid is machined to a4l s inch-radius, and the inner surface of-the accelerating grid is machined to a 4 inch radius. The cutawayportion of the accelerating'grid is 1% inches at its outermost point. t

It hasbeen found that replacing the high potential arecelerating-electrodeof the calutron by the 'present grid,

closely adjacent the arc and maintained at the'same potential'as thearc. chamber, greatly-stabilizes the 'arc,"so that better operation of the entire unit results; It has been'further found thatwith this novel acceleration systemthe source may be operated at 20 kilovolts rather than at 38 kilovolts as required in'standard'ca-lutron operation, the accelerating .grid may be operated at ground potential, and the'high negative potential supply formerly used for the accelerating slit is no longer required. Elimination of the requirement of thehigh negative voltage thus eliminates a'costly power supply and also makes operation more efficient, sincethe ion drainto the highly negative electrode is eliminated. Reduction the potential requirements of the accelerating voltage source*from "38' to 2()kilovolts achieves a further major cost savings in both original cost and operating expense.

Further, as pointed'out above, the ratio of power supply drain to the useful ion beam, which forms an index to the cost of operation, is appreciably smaller, that is 1.3- 2.5, as compared with 4.5 for a conventional calutron source.

It has further been found that sources of the character described herein are highly useful as sources of large currents of ions for injection into a circulating charged particle system. Since the ions of the beam emerging from the flat grids of the source come to a focus at the 90-degree point, such point is a convenient point of entry for the ions into a circulating system.

As shown in Fig. 8, ions are formed in a source chamber defined by the walls of box 51 by means of an electron beam passing from an external filament to the box wall or to a cathode. The are discharge 52 so formed is shown in cross-section. A flat source grid 53 is formed by vertical slots in the front face of box 51, corresponding to the grid shown in Fig. 7, except that grid 53 is parallel with the rear wall 54 of box 51. A corresponding flat accelerating grid 55 is provided closely adjacent grid 53 in the same closely-spaced relationshipas between the curved grids of Fig. 5. Ions from the source are accelerated by a potential applied between the grids, and are brought to a focus at the 90 point by the applied magnetic field, which is shown by the dot H as coming out of the plane ofthe paper. The broad 1 focused. The long dimension of the grids may bejeither vertical orhorizontal, that is, normal or parallel to the magnetic field. It has been found that better focused beams may the .obtained, for purposes such as isotope separation, by using vertical grids. The individual ion beams-.emetgingfrom the slits appear to repel each other so=as.to:reduce.divergence,parallel to the magnetic field, providing .a-narrower, more uniform beam than can he obtained by horizontal grids, where there is not that unifying effect bunchingthe beams together.

It :may .be 1seen that' the novel means for generating ions herein described-may be applied in various ways to differentproblemssuch as injection of ions into accelerators,wisotopetseparations, and the like. Otherapplications whereintense beams of singly and multiply charged ions will becapparent torthose skilled in the art. 'It is, therefore,-intended that the scope of the invention not be .1limited to the physical embodiments described, but only by the attached claims.

Having {described :the invention what is claimed as novel is:

:1. .In isotopeseparation equipment comprising an ion .sourcer-having .an ion exit passage, an ion collector, means forr-providingaa strongmagnetic field between said source and collec.tor,.-;and1a source of potential for ac- .celeratingionsyfrom,said ion source to said collector through said field, the improvement comprising a first multitapertured plane .;.member forming a grid disposed acrosssaid exit aperture and contacting the Walls of said source, :a second 'multi-apertured convex member forming a-focussinggrid disposed in spaced relation-to said firstgridzbetween saidsource and collector, said grid aperturesrhaving.their-longest dimension normal to the direction .of said magnetic field, and means connecting said-source ;of accelerating potential between said first and saidsecond grids.

-2. In..-isotope separating means provided with an are chamber, a source of -.ions, means, for establishing an arc withinas'aidrchamber, .one:wall of said. chamber being provided with an aperture for the escape of ions, an ion collector disposed in spaced relationships from said source, and means for establishing a strong magnetic field between said collector and said chamber, the improvement comprising a first plane grid member disposed across said aperture and contacting the walls of said chamber, a convex grid member disposed adjacent said plane grid member between said chamber and said collector, said grids having openings the longest dimensions of which are normal to the direction of said magnetic field, and a source of potential coupled to said chamber and convex grid to accelerate ions to said collector, the radius of curvature of said convex grid member, the potential established by said source, and the strength of said magnetic field being so related that ions of a selected mass from said chamber pass through said grids and come to a focus upon said collector.

3. In ion generating means of the character described including an ion source provided with an ion exit aperture, an ion collector, and means for establishing a magnetic field between said source and said collector, the improvement comprising a first convex grid member disposed across said ion exit aperture, a second convex grid member disposed adjacent said first grid member and aligned between said first grid and said collector, said grid membershaving openings the longest dimensions of which are normal to the direction of said magnetic field, both of said grid members having radii of curvatures operative to focus ions of selected masses from said 7 source upon said collector, and means coupled between said grids for accelerating positive ions from said source through said grids to said collector. I

4. In an ion producer comprising means defining an arc chamber having an ion exitaperture, means for establishing an arc therethrough, means for feeding vapor into said arc, an ion receiving means disposed in spaced relationship to said chamber, means for establishing a strong magnetic field encompassing said chamber and receiving means, the improvement comprising; means for stabilizing said arc within said chamber, said means comprising a first m-ulti-apertured grid member disposed across said ion exit aperture and contacting said arc; means for accelerating ions from said are through said field to said ions receiving means comprising a second multi-apertured convex grid member disposed closely adjacent said first grid and aligned with the apertures therein; and a source of potential having a positive terminal connected to said are chamber and a negative terminal connected to said second grid to accelerate positive ions from said chamber through said grid to said collector, at least said second grid having a radius of curvature such that ions of a selected mass from said source passing therethrough will be focused upon said collector, said grid apertures having their longest dimension normal to said magnetic field and to the direction of ion travel therethrough.

5. In isotope separating means provided with a source of gas to be ionized, said gas including at least two isotopes to be separated, an evacuated tank, means for establishing a magnetic field in said tank, and electron beam means for ionizing said gas within a chamber, the improvement comprising a back plate having a central aperture for receiving gas from said source; an elongated graphite block provided with a generally U-shaped' cavity defining an ionization chamber and having a back surface contacting said back plate to receive gas therethrough, the ends of said block being provided with curved slits conforming to the bottom of said cavity to allow passage of said electron beam therethrough, the bottom of said cavity and front surface of said block being correspondingly arcuate and convex and provided with a plurality of spaced slits; and an accelerating electrode provided with a corresponding arcuate, convex apertured portion disposed closely adjacent said front surface of said block, a plurality of collectors, means connected to said front surface and accelerating electrode for accelerating ions from said electron beam through said 'field to said collector, said collectors being disposed forward of said ionization chamber at substantially from the virtual focal point defined by said arcuate surfaces and said front surface slits and electrode apertures having their longest dimension normal to the direction of said magnetic field.

6. In a source of ions for particle accelerators and the like, provided with a source box defining an ionization chamber, means for admitting vapor to be ionized to said chamber, and means for establishing an arc discharge across said chamber, the improvement comprising: means for establishing a magnetic field parallel to said arc, means to receive said ions disposed in said field in spaced relation to said box, a first planar ion exit grid electrode provided with a plurality of parallel apertures, a second planar grid electrode provided with corresponding apertures aligned with said first grid apertures disposed adjacent said first grid, and means for establishing an electrical field between said electrodes to accelerate ions from said arc, said apertures being aligned normal to the direction of said field and to a line drawn between said electrodes. 7 In an ion producer operating in a magnetic field and comprising an arc chamber provided with an ion exit aperture in one wall and means for establishing an arc discharge adjacent said wall and parallel to said wall and said magnetic field to produce ions, the improvement comprising a first multi-apertured grid disposed within said exit aperture in said wall to shield said arcfrorn external electrical fields, a second multi-apertured grid disposed closely adjacent said first grid and outside said chamber, said apertures of both said grids being elongated with their longest dimension normal to said magnetic field and being aligned to permit ion egress, and a source of accelerating potential connected between said arc chamber and said second grid to accelerate positive ions out of said chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,624,845 Thompson Ian. 6, 1953 2,732,500 McLaren et al Jan. 24, 1956 2,743,370 McLaren et al Apr. 24, 1956 2,774,882 Wells Dec. 18, 1956 2,785,311 Lawrence Mar. 12, 1957

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