US2677060A - Ion source - Google Patents

Description

Z50-426:. XR 216771060 5R April 27, l954 w. M. wooDwARo ETA. 2,677,050

ION SOURCE Filed Jan. 13, 1953 4 Sheets-Sheet 1 ,/y fyfff, 1

April 27, 1954 w M. wooDwARD :TAL 2,677,060

ION SOURCE Filed Jan. 13, 1953 4 Sheets-Sheet 2 [ha M INVENTol-.

G- mf Fig-5 /W APY 27, 1954 w. M. wooDwARD E1- Ax. 2,677,060

10N souacs Filed Jan. 13, 1953 4 Sheets-Sheet 3 Fig. 6 Fig. 7

INVENTUM. william M Woodward BLYinpoln @.Smi'k MQW April 27, 1954 w. M. wooDwARD E-r AL 2,677,060

10N souRcE Filed Jan. 13, 1953 4 Sheets-Sheet 4 NVENTORS. ufllarn, M. coda/aal ola @.Smil'h Patented Apr. 27, 1954 ION SOURCE William M. Woodward, Ithaca, and Lincoln G. Smith, Center Moriches, N. Y., assignors to the United States of America as represented by the United States Atomic Energy Commission Application January 13, 1953, SerialNo. 330.952

3 Claims.

This invention is a continuation-impart of the invention disclosed in application Serial No. 557,821 filed October 9, 1944 and relates to isotope separation techniques and more particularly to an apparatus and method for separating isotopes of metals such as, for example, copper, cobalt, iron and uranium. The separation each from the other of copper isotopes `63 and 65; or cobalt isotopes 59 and 60; or the uranium isotopes 234, 235, 23'? and 238 is important to provide quantities or a selected isotope for research, medicinal, or industrial purposes such as radiography.

The modus operandi of the apparatus and method of the invention herein disclosed is the ionization of the metal of mixed isotopes, propelling the ions through a specially arranged and controlled magnetic or electrostatic deflectingfield and collecting the ions in selected zones of deflection. For such purposes, in order to obtain a substantial yield of the desired isotope, or isotopes, ion beams of high intensity or very high ion content are required, i. e. ion currents of the order of ten milliamperes or preferably much more as distinguished from the small ion currents, of one hundred microamperes or less, obtained in ordinary mass spectrometers.

It is sometimes the case that in a mixture of several isotopes in a metal, either as occurring in nature or produced through irradiation, the desired isotope may be a minute fraction of the total with the result that a great deal of metal must be vaporized and ionized to separate even moderate useful amounts of the desired isotope. Further, it is necessary for quantity production that the ion source be capable of operating continuously and uniformly for long periods of time. Apparatus of the character described is usually of the evacuated sort, operating at relatively low pressures, so that some structural problems of the ion source, as in cooling, in replacing worn parts, and in feeding the metal to be ionized, are necessarily complicated by vacuum conditions.

Among various conceivable types of ion sources, an arc is of particular advantage in that it not only provides a supply of electrons to eiect ionization by collision but also affords a relatively high temperature to facilitate the necessary vaporization of metals for production of ions from solid or molten bodies of the metal. However, extensive tests have now revealed that a number of difficulties are encountered in attempting to employ an arc for such purpose, particularly Where itis necessary to provide forthe feeding of fresh metal to the arc as the ions are evolved and (Cl. Z50-41.9)

Withdrawn for use. In the first place, it has been found that an arc source for generating ions from metals such as copper or uranium must run at a very high temperature, the melting points of uranium and copper being found to lie near 1100 degrees C., and the vapor pressure of uranium being not more than about 19x10-2 mm. of mercury even at temperatures of the order of 1900 degrees C. Cobalt requires a temperature several hundred degrees C. higher. Some of these metals react with many materials, and uranium in particular forms extremely stable carbides and oxides, so that attempts to use graphite or many of the refractory oxides as supports for the molten metal in the arc have given trouble leading to a contamination of the ion beam and a waste 0f the metal (as well as damage to the support). Furthermore, these metals form alloys with a great many other metals including aluminum, iron, titanium, vanadium, molybdenum, tantalum| tungsten and a number of others, so that the alloying inuence of the molten metal is a serious factor. For example, in the case of uranium, the alloying influence is evidenced by a tendency to rapid destruction oi any support or supporting structure made of metals commonly used in vacuum apparatus. In view of these diilculties, and other electrode-deteriorating influences, such as are due to the severe electronic or ionic bombardment commonly characteristic of an arc, the possibility of providing a stable, long-lived arc source of metal ions, generated directly from the metal, has not been promising and preliminary tests indicated that arrangements such as may have been employed heretofore in other types of arc discharge devices would not be satisfactory for producing ions of a metal such as uranium, copper, iron or cobalt, for example.

Accordingly, an important object of the present invention is to overcome the described diiculties and to provide a stable, abundant and efficient source of metal isotope ions, operable for long periods of time without contamination oi the evolved ions or rapid deterioration of the metalsupporting or other structure.

A further object is to provide an improved are source of metal ions, wherein a continuous ionization may be obtained as successive quantities of the metal are supplied during operation, and wherein loss of the metal by reaction or in other Ways may be reduced to a minimum.

Another and particularly important object is to provide an improved procedure for the generation of ions of natural uranium or of other uranium which comprises a mixture of isotopes such as for example, uranium 237 and 238, obtained by a chemical separation process from uranium 238 irradiated in a high neutron flux, which it is desired to separate by appropriate classification of the ions in accordance with their atomic masses, the improved procedure being such as to provide a copious supply of the ions and to be capable of operation continuously and with substantially uniform results.

Still further objects include the provision of apparatus and procedure for generating metallic ions, and the provision of efficient structure of ion sources adapted for stable operation and for the transformation of successive quantities of metal, fed to the source, into ions of a desired character.

Although this invention provides an apparatus and method for the separation of isotopes of various metals, for the sake of brevity, the apparatus and method'as applied to the separation of uranium isotopes is selected for explanatory purposes. An important feature of the invention resides in the discovery that the stability of an arc struck to a body of uranium metal may be greatly enhanced and the difficulties of reaction or alloying between the molten uranium and its support may be satisfactorily overcome, by supporting the metal on a suitable metal of relatively refractory nature, particularly tungsten, While maintaining the concentration of the uranium metal suiliciently low relative to the amount of tungsten constituting the support, so that any alloy of the uranium with tungsten has a high enough melting point to withstand the arc conditions. Although tungsten advantageously has a much higher melting point than uranium, it appears that large quantities of molten uranium will take up tungsten, forming an alloy melting within the arc temperature, but the resulting diiculty may be obviated by controlling the relative amount of uranium present. Thus it has specifically been found, for example, that ii the uranium is maintained in a relatively thin molten layer on a relatively massive tungsten support, an arc may be satisfactorily run from a heated cathode to the support and its uranium coating, without melting or otherwise rapidly damaging the support and without appreciably contaminating the resulting positive ions (which may be withdrawn as a beam of high intensity, e. g., by a suitable accelerating field) with ions of other than the desired uranium atoms.

These and other features of the invention, including a number of useful structural arrangements which are believed to be novel, are more particularly disclosed in the following description of certain presently preferred embodiments of the invention, set forth by way of example and illustrated in the annexed drawings, wherein:

Figure l is a largely diagrammatic view, in section, of an ion source embodying the principles of this invention;

Figures 2 and 3 are respectively plan and vertical sectional views of an ion source of this general type included in Figure l, Figure 3 taken on line 3-3 of Figure 2;

Figures 4, 6, and 8 are elevational views of other arc structures embodying the invention;

Figure 5 is a sectional view in plan taken on line 5--5 in Figure 4;

Figures '7 and 9 are vertical sectional views taken on lines 1-1 in Figure 6 and lines 9-9 in Figure 8 respectively;-

Figure 10 is a plan view of another form of ion source of the invention; and

Figure 11 is a vertical sectional view taken on line Il-H of Figure 10.

It will be appreciated that ion sources of the character herein described may be employed for a wide variety of purposes, including the isotopic fractionation of copper or other metals, including uranium or the like by appropriate classifying operation upon the resulting ion beam with various types of apparatus, such as magnetic separating structures of the general character of mass spectrometers. One especially useful type of device having certain novel ion-classifying and other features is particularly described and claimed in the U. S. Patent 2,606,291 of Robert R. Wilson, issued August 5, 1952.

Referring in particular to Figure l the ions are generated in the arc between the filamentary cathode 20 and an anode 2i, and are accelerated in a beam along the evacuated tube generally designated 22, by appropriate electrode structure which is represented, for simpliiication o illustration, by the cylinder 23 and which is maintained at ground potential and thus strongly negative relative to the arc-type ion source 2li-2l, the latter being insulated from the tube 22 and kept at a high positive potential, say of the order of 500 to 250'0 volts, or in some cases as high as l0 to 2O kv. This potential is conveniently applied to one of the arc electrodes, e. g. the cathode, by means of a suitable voltage source 24.

Returning to Figure l, the arc structure embodying the cathode 20 and anode 2| is energized by a suitable source of direct current 25, for instance providing a voltage of to 250 volts, the arc circuit including an appropriate millianimeter or ammeter 26, and if desired, an adjustable ballast resistor 21. The lamentary cathode 2li is preferably energized from a source of direct current 28 which may be connected to the cathode through a rheostat 29. It will be appreciated that in this, as Well as in other illustrated forms of the invention, the battery sources shown may g be, and indeed usually will be, replaced by other appropriate voltage or power sources, such as carefully regulated D. C. supplies of the usual rectified A. C. type. It will also be appreciated that the apparatus shown in Figure 1 is appropriately evacuated. The ion beam and its direction (but not its size or configuration) is indicated by the arrow H. (I

Referring particularly to Figures 2 and 3, the are source comprises an anode 30 consisting of a tightly coiled helix of tungsten wire, or more generically, tungsten rod, disposed a short distance above the lamentary cathode 3i which may actually consist of a single tungsten wire mounted in appropriate supports 32, 33 as shown in Figure 2. A small amount of metal containing a mixture of isotopes such as uranium of which it is desired to produce a supply of ions, is placed on the upper surface of the anode structure 30. At the outset of the operation, the filament is energized, and upon reaching the temperature of electron emission, provides electron bombardment of the anode, with the aid of the potential applied to the latter from the source 25 as shown in Figure l. An are is quickly established, and the feed metal then melts over the upper surface of the anode and hows down through the cracks between the convolutions of tungsten rod and forms a. thin layer over the bottom of the anode structure, the molten metal e. g., uranium, being thus shown. considerably exaggerated in thickness, at 34 and` 35, in Figure 3. It has also been found that in some cases improved results, in confining the discharge to the intended electrodes, are had by running the arc in a magnetic field, for instance, in a vertical eld when the electrodes are shaped and disposed as shown in Figure 3. Such eld is indicated by the magnetic pole pieces 36, 31 disposed above and below the arc in Figure 3. It should be noted that because the magnetic field is preferably localized to the arc region and also because the electron velocities in the arc are so much higher than the heavy ion velocities, the field is useful to conne the electrons of the arc without appreciably impeding the withdrawal of ions and the advance of the latter down the tube 22 in Figure 1.

As explained hereinabove, it has been found that the arc runs at a high temperature, but, at least in the case of uranium, by limiting the amount of uranium metal presented to the are conditions in relation to the amount of tungsten constituting the support 3U, the melting point of the alloy which presumably results between the uranium and the tungsten is maintained at a sumciently high value so that the tungsten support 3B is not appreciably melted or vaporized or otherwise damaged. Indeed, it may be that insofar as any alloy is formed it is merely in the nature of a lm intermediate the uranium end the tungsten support, and there is apparently no substantial solution or alloying of the two metals together such as might tend to destroy the structure or otherwise impair the arc or the character of the evolved ions. The uranium is vaporized and ionized by the arc, and a copious supply of ions may be continuously withdrawn. Additional metal can be fed to the upper surface ci the support 3B from time to time, the amount being conveniently controlled, as explained hereinabove, so that the support maintains its shape and size and is not corroded or otherwise darn aged.

By way of specic example, one remarkably successful arrangement of the type shown in Figure 3 consisted of an anode 3D of tightly cciicd 40 mil tungsten wire and a cathode 3| comprising a single 40 mil tungsten wire, spaced about six millimeters apart. Operated in a magnetic field of approximately three hundred gauss, the

arc drew a current of about ve amperes with a drop ci approximately twenty volts. About fifty milligrams of uranium were loaded on the anode at one time, and after about ten minutes or so, the arc voltage would commence to risc. Upon feeding a further like quantity of uranium to the upper surface of the anode, the arc voltage would again drop to twenty volts and the operai tion would continue satisfactorily. With such a structure and the described mode of operation, a continuous supply of uranium ions, consisting predominantly of singly charged ions of the several isotopes present in the uranium sample employed, was obtained over a period of several hours, without any marked signs of deterioration of the anode. In one instance, where natural uranium was supplied to the arc, mass spectroanalysis with a Nier-type (120 degrees) mass spectrometer showed that the only ions present were those of uranium, both singly and doubly charged, the doubly charged being only about percent of the singly charged and decreasing somewhat in abundance with lower arc voltage.

Since the procedure and apparatus have been found by extensive tests to operate very satisfactorily in the described manner, the invention is thus not limited or bound by any particular theory or present theoretical understanding. It definitely appears, however, that the use of tungsten, and the control of the amount of isotopic metal mixture relative to the tungsten support, prevent deleterious effect on the tungsten support, even though metals such as uranium ordinarily tend to alloy quite readily with tungsten. At the same time the procedure avoids contamination and loss of feed metal, e. g., uranium, such as are due to the formation of carbides or oxides in the use of graphite or various refractory supports, reactions which may even consume practically all of the supplied metal. Furthermore, by employing the preferred arrangement of a hot cathode, the arc is easily started and is very readily localized and maintained over long periods of time.

It will be understood that the mechanism ci ionization in arc sources of the type here dcscribed is believed to involve heating of the metal to a point of substantial vaporization, by the electron bombardment and particularly by tho high temperature of the arc discharge (since bombardment by thermionic electrons alone appears to be insufficient, without arc conditions, for generation of the desired intensity of ion beam), with subsequent ionization of the metal lic gas, presumably by collision in the arc itself. The vertical magnetic field shown in Figure 3 has been .found helpful particularly with ano-des of small area, to collimate the arc and to oonfine it to the surface of the metal sought to be ionized. Apparently the magnetic field prevents diversion of electrons away from the intended are by oonning them to tight spirals along the magnetic field lines between the electrodesA Figures 4 and 5 illustrate a somewhat modified structure which has been found to provide a larger arc current and a correspondingly im creased supply of ions. Here the anode comprises a platform 38 consisting of several forty mil tungsten rods placed side by side and welded to a pair of tungsten rods 39 to form a grid-like structure which is supported by the rods 39 secured in cooled tubes which are suitably mounted in an end plate of the device in which the ion source is to be used. Two cathodes are included, provided respectively by the tungsten laments 40 and 4I spaced a short distance, e. g., about onehalf inch, respectively above and below the platform 38, the filaments being carried by the cooled tubes 42 of copper or other suitable material, mounted to the end plate. To feed metal to the arc as needed, an appropriate supply device may be included in this, as well as in all others of the arc structures herein described. By way of ample, one such device may comprise a cooled feed tube or duct 43 extending to a point just behind and slightly above the platform 38. A wire or ribbon of feed metal 44 is advanced continuously or in successive steps as desired, through the tube 43 to the upper surface of the platform. It will be appreciated that the various supporting and feeding structures, as mounted in the plate may include suitable cooling insulating and sealing or packing means where needed in accordance with the usual practice for vacuum-type apparatus. Although mechanical wireadvancing means, e. g. including a reel and motor-driven rollers drawing the wire from the reel and pushing it through the tube i3 (such parts being enclosed in a communicating evacuated housing if desired), may be employed in some cases, satisfactory results can be had simply by pushing the wire through the tube by manual operation.

In Figure 4, it will be apparent that the feed metal, under the heat of the arc structure as described in connection with Figures 2 and 3, spreads out in relatively thin layers 45 and 46 respectively, on the upper and lower surfaces of the anode platform 38. The arc runs from both filaments 40, 4| to the anode platform and thus occupies a considerable region lengthwise of the filaments and platform. Considerable further advantage is obtained in some cases by providing a vertical magnetic field, such magnetic eld being provided, for example, by the pole pieces 41. 48 (of a suitable electromagnet) disposed as shown. With wide electrodes of the sort illustrated in Figure 4, having a transverse extent equal to several times the distance between them, it was found that a vertical magnetic field tends to conne the arc to a single locality.

By way of specic example, one arc source emobdying the construction shown in Figures a and 5 was found to operate very satisfactorily with an arc current of about ten amperes and an arc drop of about one hundred and twenty volts, without a magnetic field. With the source so operated, a beam of ions providing an ion current of from twenty to sixty milliamperes was :w

obtained in a separating tube, specifically, a tubo having a diameter of six inches and having a correspondingly substantial isotope-fractioning capacity; and when a vertical magnetic iield was added, as explained above, the ion output became even greater.

Figures 6 and 7 show a simplified and compact arrangement of an arc source, wherein the anode 49 is a single tungsten rod, say of a diameter of one-fourth of an inch, extending between two tungsten filaments 50, 5I disposed about onefourth of an inch from the rod. Feed metal, e. g., uranium. supplied in the form of a wire 52 from a feed tube 53, was found to flow out around the entire circumference of the rod 49, and satisfactory arc operation and production of ions was obtained. A somewhat similar arrangement is shown in Figures 8 and 9, including a like single rod 54 utilized as the anode and fed with, e. g., uranium, metal supplied in the form of wire 55 to the top of the rod. The cathode here consists of a ring or partial ring of tungsten wire 5G encircling the rod 54 a short radial distance from it, as shown. This arrangement provides an extremely compact arc source such as might be desirable where a battery or series of such sources is to be employed to provide an ion beam of considerable cross section. In each of the arrangements of Figures 6, '7, 8, and 9 the anode rod provides a relatively extensive area over which a suitably thin layer of the feed metal may be melted, to aiord the desired relationship which prevents undesirably alloying and deterioration of the anode in the arc region, especially in the case of uranium, in the manner explained above.

It will be understood that Figures 6, 7, 8 and 9 are somewhat diagrammatic, and that if desired, the feed of wires 52, 55 may be such (here and also in other forms of the invention) that the wire is brought directly into the arc region of the anode surface, as in Figures 4 and 5, without touching the anode at a locality back of the arc, thus avoiding possible erosion at such locality by successive increments of flow oi some alloyed anode metal toward the arc. That is, while a Lil) slight alloying of the anode at the arc region may not actually impair the anode there (since the isotope source metal in effect boils out of the alloy), even slight alloying at other places may result in a transport of metal. to deposit in the arc region of the anode, so that there is an erosion at such other places. To avoid the eflect further, the structure of Figures 8 and 9 may be disposed vertically, with the tip of rod 54 uppermost, and the metal wire fed (otherwise than as shown) down on the tip. It will be further understood that the various lead-in electrodes, supports, feed-tubes, and shield plates may be cooled by squirt tubes and cooling coils (some of which are diagrammatically shown) to effect a more rapid transfer to the exterior of the evacuated chamber or to the end plate of the device of the heat supplied to these members from the high temperature arc region.V

Figures l0 and il show a further form, including additional structural features of advantage in some cases, particularly where compaetness is desired and where at the same time a magnetic iicld is of advantage for the arc. Here the onefourth inch anode rod 5l is carried within a pair or" concentric tubes 58, 59 (communicating at their otherwise closed ends adjacent the arc), through which water o1' other uid may be circulated for cooling purposes, and the entire tube and rod structure is mounted within a massive cylindrical member i of iron. A pair of lament wires 6i are mounted a short distance, say about one-fourth inch, in front of the end of the rod 51, in the general relationship shown. The filament wires are carried by copper clamping members 62, 63 which are in turn mounted on supporting members 65 extending through glass or other insulating tubes 65 traversing the iron member 69. To protect the filament leads 64 and the ends of the glass tubing 65 from deteriorating. effects ci the nearby arc, shield sleeves 6B may extend rearwardly from the supporting menibers 62, 53 shown in Figure l0. A copper shield plate Eil surrounds the rod 5'! and the tip or head of the member 6U, and for further liquid cooling of this part of the apparatus a loop of copper tubing 68 or the like may be soldered to t. e outer face `of the disk 61, as shown.

Supply of isotope source metal Wire to the anode rod 51, is achieved through a feed tube 59 conveniently mounted in a slot in the upper portion of the iron member Eil, the adjacent structure of the shield plate 61 and of a mounting tip 'i8 for the member G being such as to provide a passage 'H through which the wire 12 may be advanced toward the exposed end of the tungsten anode rod 5l. It will be appreciated that the member 60 may be permanently magnetized or may be appropriately surrounded with a solenoid (not shown) or otherwise embodied in a magnetic circuit so that a magnetic field extends in the direction indicated by the arrow H of Figure 11. For concentration of the field in the vicinity and along the path of the arc between the rod and the exposed filaments 6i, the member Bil may be appropriately tapered as shown in the drawings. It will be noted that the supporting structures for the several arc electrodes, viz., the rod 51 and the filament rods El are shaped to constitute shield sleeves spaced from and surrounding the electrodes for an appreciable distance from the arc region to the locality of actual support, thus minimizing conduction of heat from the arc and preventing deterioration of the adjacent struetural and supporting elements, aswell as facilitating replacement of the electrodes by preventing their being arc-welded to the supports.

The are in Figures 10 and 11 extends, as will now be appreciated, from the cathodes 6i to the end and adjacent cylindrical surface of the rod 5l, the isotope source metal spreading out over the end and adjacent surface of the rod in a thin layer, with the advantages explained above. At the same time, the magnetic field confines the arc to the electrodes as desired and prevents attack on the adjacent supporting structure. It will be noted that cooling means are provided for the several elements, and it will be understood that tubular or like cooling structure (not shown) may also be arranged for the cathode support and connecting members 64 in a manner which may be similar to that for the rod 5T and which is therefore not shown in detail. A particular advantage 'of structures such as shown in Figures l0 and l1 and likewise in Figures 6, 7, 8, and 9 is that if and as some deterioration or corrosion of the anode rod taires place at its extremity, which may occur after prolonged operation, the rod can be moved lengthwise las by mounting in a suitable gland, not shown) so as to bring a fresh portion of rofl into desired position for the arc anode, i, e., by moving the rod to the left as it appears in Figures l0 and l1. Furthermore, as previously indicated, compact devices of this sort may be arranged side by side to provide a gang type of source where a beam of considerable cross section area is desired. It will be appreciated that in all of the illustrated embodiments, for 'utilization or' the evolved ions as a beam or other stream, suitable accelerating means may be provided such as a grid or screen, or an accelerating tube of the sort indicated at 23 in Figure 1, maintained at an appropriate negative potential relaiive to the are electrodes. The ions may be con.- veniently so withdrawn from the sides of arc, e, g. toward or away from the observer in Figure ld; and i1 desired, the illustrated structure may be mounted vertically, i. e with the rod 51 in a vertical position.

En some useful results may be obtained with hot-cathode arcs of the general type herein described employing somewhat modified means for preventing attack of the arc on adja` cent structure and for preventing reaction or alioying between the feed metal and its support, for example. by using non-metallic refractory materials.

It will now be apparent that the present invention provides eilicient and economical means and procedure for generating metallic ions, of metals containing a mixture of two or more isotopes 'The operation is readily controlled in such manner as to prevent waste of the material by reaction and also to prevent injury or wearing out of the arc electrodes, while at the same time contamination of the resulting ow of ions is avoided; an important generic feature involving not only the use of a refractory supporting material such as tungsten or a refractory oxide, but especially the control of the extent to which the metal (e. uranium) and reactive materials ithe term being here used generically to include alioying eiiects) are juxtaposed in the presence of are conditions, such control being preferably by limitation of the amount of the metal on a tungsten support, but in some cases by limitation of the amount of arc discharge directly present, or by selection of a spcifie, unexpectedly inert refractory, via., zirconium oxide.

Although the invention has been described to ill include by way of example, the conversion of natural uranium metal into a stream or flow of ions which would thus have the isotopic composition characteristic of the natural metal, the same features and steps are equally applicable i to like advantage) to metallic uranium having other isotopic composition, for instance, uranium `which is partially enriched and from which further isotope fractionation or concentration is desired, and the invention is similarly applicable to uranium consisting of a single isotope, in any case where a supply of such ions is desired. Thus the term uranium, unless otherwise characterized, is used broadly here and in the claims, to mean the element of atomic number 92, whatever its atomic mass or composition as to isotopes. The invention is likewise applicable to any other metal, such as copper, cobalt, nickel, iron, thorium or plutonium, particularly those which exhibit the property of alioying with other metals when molten.

In view of the foregoing description, it will now be understood that the optimum rate at which isotope source metal should be fed to arc structures, for example of the sort shown, can be determined very easily for any given case, and indeed will necessarily depend on the dimensions and physical characteristics of the given electrode structure, as well as to some extent upon the quantity of ion flow to be withdrawn from the source. For many purposes, it appears that structures of the type, illustrated in Figures 2 to l1 inclusive, are somewhat self-regulating (in that any large excess of metal iiows off) and are more accurately regulated, if desired, simply by preventing the molten metal from forming incipient drops on the under side of the anode; thus in general, good results are had where the molten layer of uranium, for example, is no thicker than can be maintained by the surface tension of the metal and against gravity, on surfaces (such as shown) from which the molten metal is free to flow away, i. e., is not otherwise restricted from flowing away. But in any event, by very simple tests with a particular apparatus it may be easily determined just how fast the metal can be fed to the arc without appreciable deterioration of the anode. and the invention can be practiced in accordance with such determinations, the arrangement being such, of course, as to provide for effective spread of the molten metal over the tungsten support in the path of the arc discharge. Of course, reference herein to the avoidance of appreciable melting, alloying or deterioration of an anode or support is intended (unless other characterization appears) to mean such avoidance over reasonable periods of time, bearing in mind, for example, that an anode or anode portion lasting several hours when operated in accordance with the principles of the invention represents a great improvement over prior arc structures which have been found to be destroyed in twenty or thirty minutes of operation.

It is conceived that in substantial measure the described procedure and structures may be applied to other uses including the generation of beams or other volumes of ions of other metals, as by supporting such metal on another which is wetted by the molten metal to be ionized but which is kept from melting, by controlling the quantity present of the molten metal. the invention being of particular advantage for the production of ions from solid materials which are both refractory and highly reactive.

said metal on the opposite faces and also the in- 10 terstices thereof, thermionically emissive cath` ode means spaced from and facing said opposite faces of said platform, an evacuated envelope enclosing said platform and cathode means,

with metal to be vaporized and ionized by the arc discharge, and thermionically emissive cathode means facing said coated surface, means for maintaining an arc discharge between said surface and said cathode means to vaporize and ionize said metal coating, and means including an ion-accelerator for withdrawing the ions of said metal from said discharge means to a point of use.

3. In an ion source, in combination, arc discharge means comprising an anode including a tungsten rod having a renewable metal containing a cobalt isotopic coating on an end surface thereof, thermionically emissive cathode means means including a SOUICG 0f Current OI mairl- 15 facing said coated end surface. and mounting taining an arc discharge between said cathode means and both faces oi said platform as anodes, and meas providing a magnetic field perpendicular to said platform and parallel to the arc means extending from said rod and positioning said surface in proximity to said cathode means and remote from said mounting means, means for maintaining an arc discharge between said dSChI'ge t0 distribute Said discharge Over the 20 anode and Said cathode means t() vaporize and faces of the platform.

2. In apparatus of the character described, in combination, arc discharge means comprising a tungsten anode member having a surface coated ionize said coating, and means for withdrawing cobalt ions from said arc discharge.

No references cited.

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