US2621296A - Ion source - Google Patents

Description

4 Sheets-Sheet l R. w. THOMPSON 10N SOURCE INVENTOR. iQoreff ZO Dea 9, 1952 Filed Sept. 2, 1944 www Dec. 9, 1952 R. w. THOMPSON 2,621,296

10N SOURCE Filed Sept. 2. 1944 4 Sheets-Sheet 2 Fig. Z

INVENToR.

Dec. 9, 1952 R. w. THOMPSON 2,521,296

10N soURr:

Filed Sept. 2. 1944 4 Sheets-Sheet 5 Fig. 3

IN V EN TOR.

Dec. 9, 1952 R. w. THOMPSON 2,621,295

ION SOURCE Filed Sept. 2, 1944 4 Sheets-Sheet 4 IN VEN TOR.

/aW//W Patented Dec. 9, 1952 UNITED STATS ION SOURCE Robert W. Thompson, Minneapolis, Minn., assignor to the United States of America as represented by the United States Atomic Energy Commission Application September 2, 1944, Serial No. 552,553

(Cl. Z50-41.9)

1d Claims.

This invention relates to methods and apparatus for producing ions in vacuum, particularly to a new and improved method and apparatus for producing the ions of the heavier metals and their compounds, such as uranium and uranium halides, for use primarily in the ionic separation of the isotopes of these metals.

In producing ions of these heavier metals for isotope separation by ionic methods in vacuum, it has been preferable to utilize a volatile metal compound, to vaporize the compound, and to produce ions by passing an arc through the vapor.

The investigation of previous ion sources of this type showed that these ion sources were unsuitable for the production oi ions of the heavy metals due to four independent major causes:

1. The ion output as shown by mass spectrographic analysis lacked reproducibility;

2. The ion output of the early sources was of unsatisfactory ion composition;

3. It was not possible to maintain a high vacuum while the early ion sources were in operation; and

Li. rIhe early sources operated at low eiciency which meant that the amount of neutral salt vapor leaving the ion source Was unnecessarily large.

The object ci the investigation which led to the present invention Was to study these four difficulties with the end in view of producing a satisfactory method of producing ions in vacuum and, particularly, to perfect a method of reproducible operation of an ion source of this type and then to investigate the remaining diiculties.

It appeared that the lack of reproducibility in the early Work might have been due to lack of control of pressure in the arc chamber and/or to impure vapor of the material to be ionized.

As to the lack of control of pressure in the arc chamber, the operation of the early ion sources depending on the assumption that the vapor pressure cf the volatile salt in the arc chamber was a unique function of the temperature of the zone in which the solid salt is converted to vapor. In general, this is true only in the idealized case of a one component system in Aa :closed vessel at temperature equilibrium.

As a result of the present investigation, it appears that in the ion sources heretofore employed deviation from these idealized conditions occurred so as to resuit in lack of a one to one correspondence between pressure in the arc chamber and the temperature in the zone of evaporation. rEhe vessel could not be closed and the system could not be vapor tight because openings obviously had to be provided in the arc chamber for ions to get out. in addition, one or more of the sources heretofore used had nonuniform temperature conditions and/or had cold points in the vapor system at which condensation of vapor could occur.

With reference to the impure vapor, all of the sources heretofore used were inadequate with respect to the purity of the vapor in that the solid salt to be volatilized was not pure, and/or its vapor came into contact with materials which Were not inert with respect thereto.

The second major diiculty With sources heretofore operated was that the ion output Was of an unsatisfactory composition. Although in these sources reproducibility in ion output was not attained, it was noted that under most conditions under which the ion output was observed, that the composition thereof was unsatisfactory. A mass-spectroscopic examination of the ions produced by such an ion source showed the ionization products of even the simpler compounds of the heavier metals are, in general, numerous and complex. For example, When utilizing uranium tetrachloride as the source material, the ions produced include uranium ions, complex uranium chloride ions and chlorine ions. For the application for which the present investigation was carried out, that is metal isotope separation, it is necessary to have a pure source of metal ion, but even in an application in which this is not necessary, it may be desirable or necessary to have an output of controllable composition.

The third major difficulty encountered With the sources heretofore employed was that high vacuum could not be maintained while these sources were in operation. The cause for this seems to be specific to the particular volatile salts employed. As a result of the present investigation, it has been shown that many salts, for example, uranium tetrachloride, must be very pure and in strictly anhydrous form in order that they may be converted to vapor While maintaining a high vacuum as to all other material, for example, decomposition products.

The last major diniculty encountered With previous sources was that unnecessarily large quantities of neutral salt escaped from the ion chamber and this neutral salt in a number of Waysy It is another object of this invention to pro'- l vide a method of ion generation wherein the ratio of different ions generated may be made more constant and may be controlled to a greater degree than heretofore.

It is a further object to provide an ion source of high stability capable of constant ion development.

The present invention accomplishes these objects and results as disclosed hereinafter.

The essential features of the process comprise volatilization of the desired element in the form of a compound, for example, uranium tetrabromide, uranium tetraiodide, or uranium tetrachloride; conduction of the resultant vapor to an arc chamber, ionization of the vapor to metallic ions in the arc discharge; and removal of positive ions from the arc chamber by effusion and by the action of the transverse electric fields which exist in the arc plasma,

The novel features of this invention include use of dry pure source material, lo-w pressure in the arc zone, control of the pressure (or density) of the vapor in the arc zone at the desired level, apparatus which throughout is inert to the vapor, a properly designated arc chamber, and the operation of the arc under the proper conditions of low voltage and high current density.

Actually, it is the density of the vapor in the arc chamber which should be controlled rather than the pressure of the vapor. An accurate control of density of vapor would require an accurate control over both pressure and temperature of the vapor in the arc chamber. However, the variations in temperature cause little difference in density since the thermal expansion of the Vapor at constant pressure is slight over the temperature range concerned.

The rst feature of using dry pure source material is of great importance. It has been found that thisv is intimately connectedv with the production and maintenance of a high vacuum. Many salts are hydroscopic and/or unstable in the presence of air or moisture; particularly at elevated temperatures. Hygroscopic materials, such as the heavy metal salts considered for use in this invention, often do not reverse their Waterabsorption readily but slowly yield their moisture over long periods of time, thus preventing theA establishment andV maintenance of a high vacuum until the material is dehydrated. Further'- more, the instability of many compounds in the presence of air or moisture is a serious factor in the operation of an ion source.

On several occasions when operating with uranium tetrabromide, after the ion source had been out-gassed, it was necessary to break vacuum. It was found that even if the source was' out of vacuum only a few minutes so that the tetrabromide was in contact with normal atmosphere, the pumping and out-gassing time was increased several-fold. Furthermore, at the higher temperatures of out-gassing the compounds are often unstable in 'the presence of oxygen and/or moisture forming non-volatile oxyhalides, oxides and hydroxides, and corrosive vapors. These non-volatile substances formed on the surface of the halide. reduce the effective surface thereof. In addition, the presence of other impurities in the material to. be ionized will cause. the pressure for a particular temperature of operation to be different from the vapor pressure of the pure source material at that pressure. This obviously will affect conditions inthe arc plasma.

With pure source material, it is possible toA opf erate the apparatus in a chamber maintained at high vacuum. The source material to be vaporized is preferably introduced into the vapor generator zone in an air-tight evacuated frangible capsule prior to the application f the vacuum pumps to the system. The system is out-gassed for a sufficient period at an elevated temperature of the order of operation of the apparatus or even higher. The apparatus is brought to the desired operation temperature and high vacuum and then the capsule is 'broken so that the pressure is the vapor pressure of the source material at the predetermined temperature. In this way, the source material is kept free from contact with oxygen and/or Water, hence has a clean free surface and does not yield corrosive decomposition products inthe vapor generator.

The low pressures employed in this process, e. g., of the order of two to ten microns of mercury, have been found especially suitable for the production of a plasma having positive ions predominantly single consisting of charged metallic ions, such as U+ ions from uranium tetrabromide.

Turning to the controlv of the pressure at the desired level, a first method ofv attaining this without condensation is (a) to maintain a vaporizing surface in a first zone at a predetermined temperature at which the compound has a vapor pressure corresponding to the desired pressure; (b) to maintain all surfaces in subsequent zones, through which the vapor passes, at the same or preferably, a higher temperature than the first or vaporizing zone; and (c) to maintain a suicient vaporizing surface inthe first zone lthat the pressure throughout the first and subsequent zones remains substantially the saturated vapor pressure at the temperature of the first zone, in spite of the effusion of vapor and plasma from the final orarc. zone.

Quite obviously the rate of vapor drain cannot be greater than the product of 1A; ACp (in which A is the area of the surface for evaporation, C the mean speed of the vapor molecules at temperature T and p the density of the saturated vapor at temperature T), since this product is equal to the total mass rate of formation of the vapor fromr the vaporizing surface, i. e., mass rate being used to indicate mass per unit of time, and does not include the rate at Which vapor recondenses at the vaporizing surface. The net rate of formation of vapor, Which is equal to the difference of these. two terms is always equal to the vapor drain.

If the situation is notidealizedas noted above, it is. clear that nevertheless the pressure in the arc chamber will be maintained constant, if the mass rate of formation of vapor in the vapor generator is constant. This constitutes a second method of keeping the pressure in the arc cham- 'ber constant.

A third method for controlling the pressure of the vapor in the arc chamber is to locate therein a suitable, pressure gauge (such as tungsten filamentv Pirani gauge)` and either to regulate the pressure of the arc chamber. by suitable feed valves from a source of vapor or to regulate the temperatureof the vapor-generating zone directly from this gauge. A decrease in pressure in the arc chamber due to a reduction in the rate of evaporation ofthe compound in the evaporating zone would result in an increase in resistance of the Pirani filament, and this change of resistance can be used automatically to produce an increase in pressure.

Clearly, a fourth method of controlling the pressure is an intermediate between the first and second methods above. If the mass-rate of formation of vapor is only, say, several times the vapor drain; and if themass-rate of formation of vaporl does notvary by a largev factor, then the 5. pressure in the vapor system Will remain substantially constant.

It has also been found that the usually outstanding heat-conducting construction metals, particularly copper, which is preferred in the construction of the vapor system, show considerable attack from the vapors of uranium halides or other metal salts, uranium tetrachloride and tetrabromide vapors being particularly corrosive. Furthermore, it was found by mass-spectographic analysis that Cui and complex copper chloride ions in considerable abundance were formed with the prior art apparatus. Accordingly, it is a feature of this invention to employ lining for the copper surfaces which will not be attacked by the vapor, e. g., graphite, tungsten, quartz and/or molybdenum. Graphite is preferred on the basis of cost, heat conductivity and workability.

As to the operation of the arc and design of the arc chamber, an important feature of this invention is to arrange to remove the positive ions from a central region of the arc only, so as to have a uniform plasma composition from the arc source, that is, to avoid the removal of ions from the immediate vicinity of the cathode and/or anode. In addition, an arc of a low voltage of about twelve to forty volts and a high current density of the order of two-tenths to ten amperes per square centimeter is employed with lower pressures than were previously used with any arc of equivalent intensity. It should be emphasized at this time that these conditions are inter-related and that it is the feature of low uniform pressure throughout the arc chamber with high current density that effects the uniform positive ion current having exceptionally high percentage of metallic ions.

The apparatus for carrying out the process of this invention and which achieves important ob- `iects thereof comprises essentially a vapor generator wherein the source material to be ionized, such as a solid uranium tetrahalide, is contained and is converted from a solid or liquid to a vapor phase by evaporation; means, such as a chamber or reservoir, which receives the vapor from the vapor generator and serves as a distribution manifold for transferring the vapor from the vapor generator into an arc chamber so as to distribute the vapor uniformly therein; and an arc chamber wherein the vapor is substantially uniformly ionized to a uniform plasma composition consisting of a high percentage of metallic ions; and one or more openings in said arc chamber for removing the plasma from the region of the center of the arc.

Additional objects, features and advantages of this invention will be apparent from the following detailed description and accompanying drawings (forming part of the specification) wherein:

Figure l is a longitudinal sectional view of one embodiment of an apparatus for carrying out this invention; said section being taken on the line I-I of Figure 2;

Figure 2 is a plan view partly section of the apparatus shown in Figure l;

Figure 3 is a sectional view along the line 3-3 of Figures l and 2;

Figure 4 is a partial sectional view of one means for breaking the vaporizable material capsule while the apparatus is under vacuum.

Referring now to these drawings, the entire apparatus is contained within an envelope 5 which may be evacuated to a relatively high degree with respect to air, such as -5 mm. Hg through an evacuation manifold t. rhe envelope 5 is closed at one end by a water-cooled plate 'l on which the entire ion source is mounted rigidly through a heat-conducting support member` 8 attached to vapor generator 9 and distribution manifold and heater block I0.

The component parts of this structure comprising the vapor generator 9 and the distribution manifold and heater block I0 are constructed of massive blocks of metal of high heat conductivity with the vapor contact surfaces coated with graphite or other inert material, preferably of high heat conductivity. Referring to Figures 1 and 2, the vapor generator 9 is preferably constructed of a massive block of copper having a cylindrical hole II therethrough. This hole is lined with a material I2, such as graphite, which is inert to the vapors produced. The hole I I with graphite cylindrical liner` I2 is closed at its opposite ends with inert covers I3, e. g. graphite, providing a chamber enclosing the material, such as uranium tetrabromide or other volatile metal compound Ill to be vaporized. The vapor generator 9 is provided with a vapor duct I5 of a material which is inert to the vapor, such as graphite, said duct I5 communicating from the hole II through the wall of the heater and distribution manifold block I0 and into cylindrical hole IB extending through the distributing manifold and heater block It). The manifold and heater block Ill is also constructed of a massive block of material highly conductive to heat, such as copper, and the hole I6 therein likewise is provided with a cylindrical coating or sleeve I7. The hole I6 is closed by graphite covers I8. The vapor duct I5 is made with thin Walls in order to minimize heat transfer from the vapor manifold IU to the vapor generator 9.

On the side of the manifold block I9, opposite from the vapor duct I5, is provided one or more vapor channels I9 from the hole I6 into a hot ionizing arc chamber 20 which is made of some inert material, e. g., graphite. These channels I9 may be formed of small diameter sleeves of graphite. As illustrated, six of these vapor channels I9, each one-quarter inch internal diameter, are employed to provide a total crosssectional area of about 0.295 square inch for the discharge of the vapor from the reservoir I0. In the structure shown, the duct I5 is one-half inch in internal diameter to give a total crosssectional area for Vapor flow of approximately 0.197 square inch. The ionizing chamber is at the highest temperature of the ion source by reason of the arc therein. In this embodiment it supplies heat to the heater and manifold block I0 by conduction.

Auxiliary means are provided to heat the manifold and heater block I0. For example, one or more auxiliary heaters 2l (e. g., one-quarter inch diameter graphite rods carrying large A. C. currents) are positioned in wells in the heater and manifold block I0 but which do not communicate with the hole I6. The vapor generator 9 is heated solely by conduction from the heat conducting supporting member 8 which in turn is heated solely by conduction from the heater and manifold block I0 connected at one end thereof. The manifold block is heated by the higher temperature arc and by the auxiliary heaters 2l. The heat conducting supporting rod 8 has a uniform linear temperature gradient between the predetermined temperature of the heater block and the water-cooled plate 1. The vapor generator receiving its heat by conduction intermediate these extremities of the support 8, will have a predetermined temperature intermediate the extremity temperatures of the supporting rod'8, depending on the generators position along said supporting rod. Preferably the point 'of support of the vapor generator along the member 8 is so adjusted that the temperature of the vapor generator is 50 C. or more below that of the manifold.

Although very satisfactory operation has been obtained with the embodiment, wherein the amount of powerdissipated in the arc discharge inuences the temperature-in the vapor-generating zone, this requiring adjustable auxiliary heating and cooling means, it is believed to be advantageous to have the temperature in the vapor generator completely independent of said power dissipation in the arc discharge.

It has also been found that the transfer of energy between the vapor generator and the source material is largely radiative. Hence, the material to be vaporized for supplying vapor to the arc is screened from the arc region so that there is no radiation from the arc or other higher temperature source back to the material in the vapor generator. vapor generator is thus at temperature equilibrium with the vapor generator.

The front 22 of the arc chamber 20 is shown in Figure 3 as being provided with a plurality of openings for the'eifusion of the ions. It should be clear that the size and number of holes is limited only insofar as they effect the control of pressure. It is thus'clear that the method of controlling pressure in the embodiment is the fourth method discussed hereinbefore. ionization chamber 2U is constructed 'of graphite plates forming a box-like enclosure containing means for developing an electric discharge in the atmosphere ofthe vapor flowing into the enclosure through the channels I9. More-particularly, the electric discharge means may comprise electrodes between which an electric arc may be struck in the vapor atmosphere. Various electrodes and electrode configurations may be used although it is preferred to provide a carbon anode 23 at one side of the enclosure 2) and an oppositely disposed cathode comprising a 40 mil coiled tungsten filament 24, each insulated from the chamber walls to form a discharge path between the anode and cathode.

More particularly the lament terminals 25 are brought out of the arc chamber through the bushings 26 formed of alundum into which is pressed a thin wall graphite lining to protect the alundum from contact with the hot filament. The bushings are assembled tightly around the terminals and pressed into the arc chamber 29 in such a way that there are no vapor leaks or cold spots at which condensation of the vapor may occur.

In a satisfactory form of' construction for the apparatus vshown in the drawings, the arc chamber 29 is attached to the manifold block I9 and has ducts in the rear wall thereof which are in alignment with the channels i9 to the manifold chamber IB. Inasmuch as considerable power is dissipated in the arc chamber, it is desirable to provide a relatively large area of contact between the back plate 21 of the arc chamber and the face of the manifold block I0. In addition, the high end losses from the cathode make desirable the use of water-cooling of the cathode leads which may be double walled tubular conduits 28 so designed that water may be circulated the full length of the conduits from outside The source material in the 'i The of'the end wall 1. These water cooled leads 28 are brought up near the ends 25 of the filament 24 to remove the heat therefrom so that the temperature of the filament in contact with the graphite will be below the sublimation temperature of the graphite. In a similar manner, the power lead for the heaters 2| is likewise preferably enclosed in or comprises one wall of a double walled conduit 29 for water cooling of this lead. Because of the heat developed by the heaters 2i and by the discharge in the ionization chamber, the entire envelope 5 or at least that part thereof adjacent the apparatus described is preferably water-cooled such as by a helically wound tubing 3U in heat conductive relation with the envelope. Furthermore, the heat conducted to the end wall 1 by the massive copper rod 8 which carries the vapor generator and manifold is preferably absorbed by the water flowing through copper tubing 3| in heat conductive relation with the said end wall.

Figure 4 illustrates a specific device employing an advantageous method for introducing the source material in which the vapor generator block 9 consists of a massive block of metal of high heat conductivity having a cylindrical hole therethrough, a liner or coating I2 cf an inert material such as graphite, a top cover or closing plate 32 of graphite, or other inert substance, the underside of which has an annular formed portion in which arc` imbedded or aiixed a plurality of pointed molybdenum rods 33. The hole il is closed at the bottom by a plate or disk 34 of an inert material, such as graphite and a highly heat conductive metallic plate 35 with a threaded bushing 36 secured thereto through which passes a threaded molybdenum rod 31, said plate 35 supporting the inert plate 34 and being removably attached to the underside of the vapor generator block by screws. The plate 34 has a hole of a diameter sufficient to permit a translatory movement of the rod 31 and allowing a minimum escape loss in pressure. The inner end of thev rod 31' has attached thereto a disk or plate 38 of molybdenum which disk is rotatable relative to said rod. On this disk 3S rests the thin fracturable glass vial or envelope 39 containing the source material. Translatory motion is given to the rod 31 by engagement with and rotation of the rod 49, said rod 40 passing through the envelope 5 by means of a sliding compression seal 4i of the type illustrated or of a similar type known to the art.

Although the drawings do not show insulators and seals for each of the various leads extending through the end wall 1, it is obviously apparent that these leads are sealed to maintain the vacuum within the envelope 5 and are insulated from the wall by suitable insulated bushings through which they extend.

.Particularly good results have been obtained with an ion source of the following specific description: The vapor generator 9 was a rectangular copper block measuring two and onehalf inches in a direction axially of the chamber, one and one-half inches in width transverse to the vapor outlet duct l5, and one and one-quarter inches in the direction parallel to the vapor duct leading into the heater and manifold block l0; the diameter of the chamber Il being approximately one and one-eighth inches to provide a large surface area for evaporation. The vapor generator block 9 was separated approximately one-quarter inch from the manifold block i0 which had dimensions of three inches, two and five-eighths inches and one and three- '9 eighths inches taken 'respectively along the same directions as in the specification of the dimensions of the vapor generator block 9, the manifold chamber I6 being about one inch in diameter. The support member 8 was made of one inch diameter copper, silver-soldered at opposite' ends to the manifold and heater block I and the end wall l. With this size of apparatus, the manifold and heater block I0 were designed to be heated up to a maximum temperature of about 650 C. by radiation from the two heaters 2I comprising vtwo one-quarter inch graphite rods through which passed up to several hundred amperes A. C. The total power input was of the order of 2.2 kilowatts, and under these conditions the temperature of the heaters was about 2200" C. For control of the temperature to obtain various vapor pressures within the system, the heaters EI may be energized through a variable autotransformer or other controlling device, the temperature being noted by imbedding a conventional thermocouple in the vapor generator 9, although this is not shown in the drawings.

To place the apparatus in operation, the end wall I and the various component parts supported thereon are removed from the envelope 5, and a quantity of the source material is placed within the chamber II of the vapor generator 9, such as by removing the cover I3.

A preferred method for placing the source material in the vapor generator 9, while avoiding conditions which adversely effect said material, is to use the anhydrous source material to be vaporized, as shown in Figure 4, and discussed hereinafter.

After the source material has been inserted in the vapor generator 9, the apparatus is then assembled as shown in Figures 1 and 2, evacuation of envelope through manifold 6 is begun. Circulation of water in the various cooling tubes 3i! and 3l, is started, and the ion source is heated to at least 400 C. by means of the auxiliary heaters 2I, when the vacuum has reached, say, A'l0-4 mm. of Hg. At this time, the cathode filament 24 is also heated by passing an A. C. current therethrough to assist in the out-gassing. Sufcient water is circulated in the various cooling tubes 30, 3I, to keep the envelope. etc., from overheating.

When the vacuum has reached 1 or 2 105 mm. of Hg, the source is brought to the desired operating temperature, which, for a uranium tetrabromide source, is of the order of 400 to 600 C. for the manifold I0, and correspondingly of the order of 350 to 450 C. for the vapor generator 9. Since the temperature gradient is linear along the support member 8, the temperature of the vapor generator 9 is determined by positioning on this support 8 at such a position that its temperature is approximately 50 C. less than that of the heater block I0 at the one extremity of said supporting rod 8. This temperature differential is, however, not critical.

The iilamentary cathode 24, which had been heated as noted before by passing of electrical current therethrough, is also adjusted to a proper temperature to emit suiiicient electrons. Then the capsule 39 is broken by engaging rod 40 in the rod 3l, and rotating the rods so that plate 33 is forced against the capsule 39 by the translatory movement caused by the threaded bushing 3S. The capsule 39 is fractured by the pressure against the molybdenum prongs or pointed rods 33, releasing a iiow of vapor through the apparatus including the arc chamber. If a closed l0 vial or envelope containing the vaporizable material is used as noted, the' container is usually ruptured following evacuation and out-gassing. By thus loading a salt of the material, the ions of which are desired, into the source in sealed glass capsules 39, which have been filled, for example, by distillation under vacuum, it is ensured that the charge of 'source material I4 is anhydrous and free of oxygen and water at the start of operations. Consequently, the source material I4 does not present vacuum difficulties, and no change in pressure is observed (for example, as shown by an ionization manometer communicating with the envelope 5) when the capsule 39 is ruptured.

Next, the arc is struck by bringing the anode 23 to a potential of severalV hundred volts, positive with respect to the cathode 24. The arc power supply must have a negative voltage characteristic, this being achieved in one case by connecting the arc in series with a constant voltage supply and a suitable ballast resistor. In this circuit, there is inserted a large inductance which increases the stability thereof. This inductance plays an important part in getting the arc started, for in transition from glow discharge to low voltage arc, the discharge is unstable. However, once a current is owing in the circuit any sudden decrease in arc current isaccompanied by an induced voltage across the inductance which restrikes the arc.

The starting vapor pressure using vaporized uranium tetrabromide as a source material I4 may be of the order of fifteen microns, and after starting, the pressure may be reduced to values of the order of two to ten microns. If the arc does not start immediately under the above conditions, the graphite arc chamber 20 may be shorted to the anode 23 by a suitable switch. This effects a shorter cathode-anode path (i. e., making the chamber 20 the anode) thereby initiating the arc which may then be transferred to the anode by opening the switch.

The positive ions in the plasma of the arc are drawn from the arc chamber 20 by in insulated electrode, such as the electrode 42, maintained at a negative potential with respect to the cathode 24, and may be utilized as desired. Thus, the charged ions may be collected in accordance with their charge and mass by the use of suitable apparatus such as a mass-spectrograph or other device for large scale separation of isotopes, in the utilization chamber 43. Consequently, one of the principal uses in which this apparatus may be employed is as a component part of a separator for the various isotopes of the elements comprising the source material.

Since as indicated above, the present apparatus is of particular advantage in developing ions of matter which is normally solid but capable of being vaporized such as the various halides of uranium, it will be appreciated that the invention does not relate particularly to methods and means for utilizing the developed ions and consequently there has not been shown in the drawings any particular structure for acting upon the ions such as separating one type of ion from another. In any apparatus utilizing this novel ion source there would usually be provided an accelerator such as an insulated electrode 42 maintained at a negative potential with respect to the cathode to withdraw the developed ions `from the arc and to direct positive ions into another portion of the envelope 5, such as a utilization chamber 43. This utilization chamber 43 may be of any configuration and volume. The ion source of this invention is designed specifically for use in apparatus for separating in quantity the isotopes of heavy elements, particularly the isotopes U235 and Um. It may be used for the production of other heavy metal ions, such as those of plutonium, neptunium, thorium or the like by use of the appropriate volatile compound. Although the halides have been specifically mentioned, other volatile compounds can be used in this ion source.

While the invention has been described with reference to a particular structure, it will be appreciated that the invention provides a method and apparatus for developing ions, and that many modifications may be madeboth in the apparatus and method of this invention without departing from the spirit and scope thereof.

The structure .disclosed hereinbefore uniquely satisfies the objects of this invention in that the uniformity and control of ion development is more reproducible than when utilizing ion sources previously described in the art. The uniformity of pressure which may be obtained provides a controllable ratio of ion .production as well as f uniform arc starting and operating characteristics. However, adaptation of this apparatus to large scale operation might entail substitution of other convenient methods and automatic devices available in the art for controlling the pressure, the temperature and/or the arc.

An operative embodimentof this invention of a two dimensional ion source has been described above, so that others may be capable of obtaining the good results and objects thereof, with the understanding, however, that the described embodiment is for the purpose of illustration and not limitation; reference lfor this latter purpose being had to the appended set of claims.

I claim:

1. An apparatus for developing ions from a vapor comprising a vapor generator adapted to enclose the source material to be ionized, means for heating said vapor generator to Vaporize the source material, an apertured arc chamber, a

cathode andan anode insulatingly supported in said arc chamber, means for impressing a voltage across the anode and cathode to establish an electric arc means for transferring vapor from said vapor generator and for feeding a uniform vapor at a predetermined pressure into the zone of the arc in said arc chamber, means for maintaining said transfer and feeding means and said arc chamber at temperatures above that of the vapor generator, and means for applying f a vacuum to the system through the apertures of the arc chamber.

2. An apparatusfor developing ions from a vapor comprising a vapor generator adapted to enclose the source material to be ionized, means for heating said vapor generator to vaporize the source material, an aro Chamber, at least one electrode insulatingly supported in the arc chamber, and means for impressing a potential between said electrode and at least a portion of the arc chamber to establish an electric arc, said arc chamber having at least one aperture for removing ions produced by the arc, means for transferring vapor from said vapor generator and for feeding a uniform vapor at a predetermined pressure into the zone of the arc in said arc chamber, and means for applying a vacuum to the system through the apertures of the arc chamber, said heating means and arc having sufficient heating effect to maintain said transfer l2 and feeding means and said arc chamber at temperatures above that at Vwhich condensation of the vapor will occur.

3. An apparatus for developing ions from a vapor comprising a vapor generator adapted to enclose the source material to be ionized, an apertured arc chamber, a pair of arc electrodes supported in the arcchamber, means for impressing a difference of potential on said'electrodes to establish an arc zone, means for transferring vapor from -said vapor generator and for feeding a uniform vapor at al predetermined pressure into the zone of the arc in said arc chamber, means for heating said transfer and feeding means and said arc chamber, means for maintaining the vapor generator at a temperature at which a substantial proportion of vapor will be generated but below the temperatureof the arc chamber andthe transfer and feeding means, and means for applying a vacuum to the system through the aperture of the arc chamber.

4. An apparatus for developing ions from a vapor source comprising a vapor generator adapted to enclose a source material to be vaporized and ionized, a vapor distribution manifold adjacent said vapor generator, means to heat said manifold, heat conductive means attached to said manifold and supporting said vapor generator toheat said vapor generator to a temperature lower than that of said manifold by heat conduction therefrom, an arc chamber having insulatingly supported therein arc electrodes, means for impressing a potential on said electrodes to establish an electric arc, said arc chamber having apertures to withdraw ions generated in the arc adjacent said manifold, and means to provide a flow of vapor from said vapor generator to said arc chamber through said distribution manifold.

5. An apparatus for developing ions from a vapor source comprising a vapor generator adapted to enclose a material to be vaporized and ionized, an enclosure adapted to receive vapor from said vapor generator, means to heat said enclosure, heat conductive means supporting said vapor generator from said enclosure whereby said vapor generator is heated to a lower temperature than said enclosure, an arc chamber, electrodes supported in said arc chamber, means for impressing differences of potential on said electrodes to establish an electric arc, a communicating vapor duct between said enclosure and said arc chamber, means to withdraw ions developed in said chamber for utilization thereof, and means for applying a vacuum to the system through the means for withdrawing ions from the are chamber.

6. An apparatus for developing ions from a vapor comprising a vapor generator adapted to enclose the material to be vaporized, a distribution manifold for said vapor, an apertured arc chamber adjacent said manifold, means to provide a flow of vapor from said vapor generator to said distribution manifold and therefrom to said arc chamber, means for heating said vapor generator, manifold and arc chamber, said vapor generator being of large mass and of a material highly conductive of heat whereby upon applying heat to said vapor generator any temperature differential within said vapor generator will be minimized, and means for applying a vacuum to the system through the aperture of the arc chamber.

7. An apparatus for developing ions from a vapor comprising a vapor generator adapted to enclose the material to be vaporized, a distribution manifold for said vapor, an apertured arc chamber adjacent said manifold, means to provide a ilow of vapor from said vapor generator to said distribution manifold and therefrom to said arc chamber, means for heating said manifold, and means responsive to manifold temperature for providing a flow of heat from said manifold to said vapor generator and arc chamber, said vapor generator and manifold being of large mass and of material highly conductive f heat whereby upon heating said manifold to a predetermined temperature the temperature gradients within said manifold, and vapor generator will be minimized, and means for applying a vacuum to the system through the arc chamber.

8. Apparatus for developing ions comprising an envelope having a heat conductive wall portion, a vapor generator to receive the material to be vaporized and ionized, a massive enclosure to receive vapor from said vapor generator, means to heat said enclosure, heat conductive means attached to and extending between said massive enclosure and said wall portion, means to support said vapor generator on said heat conductive means intermediate said enclosure and said wall portion whereby said vapor generator is maintained by said heat conductive means at a lower temperature than said massive enclosure, an

apertured arc chamber adjacent and communicating with said massive enclosure and having therein electrodes impressed with differences in potential whereby an electric arc is established to generate ions of the vapor, and means to withdraw ions through the apertures in said ionization arc chamber for utilization.

9. Apparatus for developing ions from a solid vaporizable material comprising an envelope having a heat conductive wall portion, a hollow vapor generator within said envelope, a vapor distribution manifold of heat .conductive material, means to heat said manifold to a temperature above the desired vaporizing temperature of the material to be ionized, elongated heat conductive means extending between said wall portion and said manifold to conduct heat from said manifold to said wall portion whereby a temperature gradient is produced along said heat conductive means, said vapor generator being supported by said elongated means intermediate its length whereby the generator is heated by said means and an apertured arc chamber adjacent and communicating with said manifold and having therein a pair of electrodes adapted to support an electric arc therebetween to develop ions for utilization.

10. An apparatus for developing ions from a vaporizable source material comprising a vapor generator for vaporizing said source material, a vapor distribution manifold communicating with said vapor generator, a massive heat conductive member for supporting said vapor generator and manifold in spaced relation so that the vapor generator will be thermally conductively insulated from said manifold except through said member, heater means for said manifold, an apertured arc chamber supported by and communicating with said manifold, and arc forming means in said chamber shielded from said manifold and vapor generator, whereby vapor pressure will be independent of variations in arc current.

l1. In an apparatus for developing ions from a vapor source, the combination of a vapor generator for containing source material to be vaporized, a distribution reservoir for receiving the vapor of said source material from said vapor generator, means for heating said reservoir, and means for effecting a heat transfer from said reservoir to said vapor generator to maintain said vapor generator at a temperature lower by a substantially predetermined amount than the temperature of said reservoir.

l2. An apparatus for developing ions from a vapor source comprising a vapor generator for containing source material to be vaporized, a heater for vapor issuing from the generator, an ionizing chamber adjacent said heater, means for utilizing a transfer of heat energy from said heater to said vapor generator to provide the heat for vaporizing said material, and means providing for a flow of vapor from said vapor generator to said heater and therefrom to said ionization chamber.

13. In an apparatus for generating ions in vacua from vaporizable source compounds, means forming an arc chamber, means for providing an arc of law voltage of the order of twelve to forty volts and a high current density of the order of two-tenths to ten amperes per square centimeter, and means for maintaining the pressure in the zone of the arc at about two to fifteen microns, said arc chamber being elongated and having means providing for uniform flow of vapor to be ionized into the arc chamber along its length.

14. In an apparatus for generating ions in vacua from vaporizable source compounds, means for providing an ionizing arc, means for generating vapor of the source material, means for feeding the vapor from the generating means to the zone of the arc, means for withdrawing ionized vapor from the arc zone and means for controlling the temperature in the vapor generating means so that the pressure in the arc zone is maintained at the desired level.

ROBERT W. THOMPSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,746,670 Meyer Feb. 11, 1930 1,841,034 Ives Jan. 12, 1932 1,930,132' Reger Oct. 10, 1933 2,027,405 Smede Jan. 14, 1936 2,219,033 Kuhn Oct. 22, 1940 2,221,467 Bleakney Nov. 12, 1940 2,285,622 Slepian June 9, 1942 2,355,658 Lawlor Aug. 15, 1944 2,374,205 Hoskins April 24, 1945 FOREIGN PATENTS Number Country Date 350,069 Great Britain June 1l, 1931 OTHER REFERENCES Marden: Article in Journal of the Optical Society of America, May 1940, page 184.

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