US2682611A - Ion source - Google Patents

Description

June 29, 1954' w. M. WOODWARD ET AL 2,682,611

ION SOURCE 4 Sheets-Sheet 1 Filed Jan. 29, 1955 5% E5 z z 1 INVENTORS W l am M Woodward Lincoln G. Smil'h BY WZM Qh W on EWWW/qv n1" 3 L S? 25:54 3 2 WW /h-V Z fifififi Q I w wl/l dl/ *m r 0 3 I 4 a on G QM an Wm 9/) 22:22.22: 5' g Z mm 0 mm m mm mm m I m m N a an on mm vw m wk w. M. WOODWARD ET AL 2,682,611

June 29, I954 ION SOURCE 4 Sheets-$heet 2 Filed Jan. 29, 1955 IN VEN TORS William M. Woodward Ely-(1200112- G. Smii'b June 29, 1954 w. M. WOODWARD ET AL 611 ION SOURCE Filed Jan. 29, 19.55

'4 Shee ts-Sheet s INVENTORS William M. Woodward F15 7 c ln G. Smz'fb J 1954 w. M. WCODWARD ET AL 2,682,611

ION SOURCE Filed Jan. 29, 1953 4 Sheets-Sheet 4 Fig, 6

I I I II I|- II- INVENTORS HI I William M. Woodward a Blz'ncoln G. Smiilz Patented June 29, 1954 UNITED STATEd i ATENT OFFICE ION SOURCE William M. Woodward, Ithaca, and Lincoln G. Smith, East Moriches, N. Y., assignors to the United States of America as represented by the United States Atomic Energy Commission Application January 29, 1953, Serial No. 334,013

11 Claims. (01. 25041.9)

This invention is a continuation-in-part of application S. N. 547,918, filed August 3, 1944, now abandoned, and relates to an apparatus for developing ions and more particularly to a new and improved apparatus for producing in quantity a substantially continuous supply of ions of metals including the heavy metals and the compounds of such metals such as copper, cobalt or uranium or the halides of such metals, for use in the ionic separation of the isotopes of such source metals.

Prior to our invention, one procedure has been to produce these heavy ions by vaporizing suitable volatile salts within a partially evacuated chamber and directing the vapors into a region wherein the vapor molecules become ionizedby the bombardment of accelerated electrons. When the salts of the metals such as the halides are used,

the ions which are formed by electron collision are generally complex, consisting not only of the singly and doubly charged ions of the metal, but also of compound ions, such as the lower halides. It is desirable that the current from the ion source be nearly free from these complex ions if the ions produced are to be used in large scale ionic separation of the isotopes of the metal.

Since it appeared logical that the ions derived from electron collision within a vapor of the metal would be less complex than the ions derived from electron collision within a. vapor of the metal salts, the development of an ion source employing the metal rather than the salt was undertaken. It was found as a result of investigations conducted to determine the vapor pressure and melting point of the source metals such as zinc and uranium, that the ion sources employing the metal would have to run at a temperature much higher than required in the ion sources employing the more readily volatilized salts. The necessity of these high temperatures impeded the development of the metal source because of the difiiculties encountered in finding a material capable of holding the metal to be vaporized. It was found that non-conducting refractories reacted with the source metal and that metallic receptacles alloyed with the source metal and burned out. Furthermore, it was noted that an oxide layer formed on the liquid surface in almost all cases where the metal had been liquefied in a refractory substance.

To obviate the above difficulties, it was suggested that an arc discharge be employed, running directly to the metal to both volatilize and ionize the metal. It Was thought that the high temperature required to volatilize the metal could be thus localized by directing the source of heat to the place where it was needed, and further that the arc would reduce the oxide layer to give a good clean surface for evaporation. These advantages did result, but localizing the temperature did. not avoid the difliculties previously experienced in finding a material for supporting any one time did not exceed a small and predetermined quantity. It was known from prior research that the metal of the source would alloy with the tungsten and that the alloy thus formed would have a melting point lower than the tungsten. However, continued research led to the discovery that tungsten could be used by keeping the concentration of the uranium low, so that the resulting alloy would have a melting point high enough to withstand the are conditions.

It :will be apparent that a source employing such a small supply of metal would be of little advantage, in the quantity production of heavy ions for isotope separation, since in operating it would be necessary to frequently break vacuum to replenish the feeble supply. Consequently, we set out to develop a means for continuously and automatically replenishing the supply of heavy metal without breaking vacuum.

In the course of our research, it became evident that the successful and continued operation of such a metal source is absolutely dependent upon keeping a correct balance between the source metal and tungsten. An important feature of our invention is the discovery that the loading operation, if automatic, must be controlled in response to some condition indicative at least of depletion in the supply of metal on the anode, and in a manner consistent with maintenance of this proper balance between the material to be vaporized and the material of the anode. It is known that the voltage of an arc is a function of the boiling temperature of the anode material. Accordingly, a further important feature of our invention was the discovery that the change or approaching exhaustion in the amount of source metal relative to the tungsten would be accompanied by a change, in efiect, in the boiling temperature of the anode, and that this change in boiling temperature would in turn produce a change in the voltage at the are which could be utilized to control apparatus for feeding on to the anode a predetermined quantity of the metal to be ionized. Thus, an accurate control of the correct balance between the source metal and tungsten could be maintained.

Accordingly, in one important aspect, the invention provides an automatic loading device operable to intermittently feed onto the anode a predetermined quantity of the material to be ionized only when the supply of this material, already on the anode, has been exhausted to the extent that the added supply would not exceed the desired or correct balance. The loading apparatus of our invention is further characterized by the advantage that substantially immediately upon replenishing the supply of the source metal to the anode, a change in the arc voltage is obtained which may be utilized to stop the operation of the loading mechanism, preventing any further deposit onto the anode of the ionizable material which if continued would cause the proper balance to be exceeded and which would result ultimately in the melting or burning away of the anode material.

Accordingly, a principal object of the invention resides in the provision of a metallic vapor ion source which is capable of operation substantially continuously and uniformly for long periods of time.

A further and important object of this invention is to provide an ion source employing an arc discharge for vaporizing and ionizing a material supported on the anode, which, when alloyed with the anode material, will have a melting temperature lower than that of the anode material by an amount dependent upon the concentration of said ionizable material, wherein con" trolled quantities of the material are automatically and intermittently fed onto the anode so that its concentration relative to the anode material forms an alloy having a melting point high enough to withstand the arc temperatures.

It is a still further object of this invention to provide an improved means for feeding onto the anode of an arc discharge in an ion source a measured or predetermined amount of vaporizable material to be ionized by the arc discharge.

It is another object of this invention to provide apparatus for automatically and intermittently feeding a predetermined quantity of ionizable material onto an anode support of an ion generator.

It is still another object of this invention to provide apparatus operating in response to the voltage rise of an arc discharge which accompanies the depletion of a supply of ionizable material that is supported on the anode and that is subjected to vaporization by the heat of the arc, whereby the supply of this material is replenished by a predetermined amount, and further, to utilize the decrease in voltage accompanying the deposit on the anode of a predetermined quantity of the material, to stop the loading operations, whereby an accurate and automatic control of the concentration of the material to be ionized relative to the material of the anode structure may be maintained.

Further objects and many of the attendant advantages of this invention'will be appreciated more fully as the same become better understood by reference to the following detailed description or" a preferred embodiment, when taken with the accompanying sheets of drawings wherein:

Figure 1 is an elevational view in longitudinal section of a preferred embodiment of this invention, the section being taken on lines i-l of Figure 2;

Figure 2 is a sectional view taken on lines 22 of Figure 1;

Figure 3 is a sectional view taken on lines 3-3 of Figure 1;

Figure 4 is a sectional view of the arc chamber taken on lines 4-4 of Figure 1;

Figure 5 is a sectional detailed view of the flanged bushing 14 shown in Figure 1;

Figure 6 is a sectional detailed view of the squirt tube 54 shown in Figure 1;

Figure '7 is a sectional detailed view of the sliding seal 38 shown in Figure 2, and

Figure 8 is a circuit diagram and schematic view in perspective, of the apparatus shown in Figure 1, illustrating its mode of operation.

In accordance with this invention, we provide a sealed chamber evacuated to a subatmospheric pressure for containing an emissive cathode of the filamentary type, preferably tungsten, and a grate-like anode also preferably of tungsten, positioned above the filament to support the metal of the ion source. The metal of the source is melted by the heat of an electron controlled are established within the chamber, running from the cathode to the anode, and the molten metal flows down between the rods forming the gratelike anode so as to be vaporized and ionized by the are which is struck to the underside of the anode. To keep the source operating, small pieces of the metal are dropped onto the anode from a loading tube located within the chamber directly above the anode. This tube, formed preferably of some refractory material or else water cooled concentric metal tubes, is filled, substantially with a number of small pieces of the metal and serves as a guide directing the metal pieces onto the anode. A rod or plunger extending into the tube and through a sliding seal in the chamber is moved in the direction of the anode to transmit movement to pieces of metal, thus pushing these metal pieces out, one at a time, to fall upon the anode and become liquefied by the heat of the arc. (The liquid metal of course runs down between the rods of the anode as described above.) To effect translation of this rod in the same direction as that of the feed of the metal pieces, we connect the end of said rod to a screw mechanism by having the end of the rod project out of the vacuum through a sliding seal, and impart to said screw mechanism a translatory movement in the desired direction by a threaded split collar driven by suitable electrical device, such as the motor shown in the drawing.

One of the characteristics of an arc source such as that described above is that when the material on the anode is almost completely evaporated, the arc voltage begins to rise. We utilize this increase in arc voltage with decrease in the amount of material in the source to control the operating circuit of the electric motor. Specifically, we apply the potential of the arc to a negatively biased grid of a thyratron tube. When the arc voltage attains a certain predetermined value, determined by the amount of initial bias applied to the grid, the thyratron is rendered conducting. The plate circuit of the thyratron is connected into an alternating current supply source in series with a coil of a motor-control circuit relay. Thus the motor is energized in response to are voltage to feed onto the anode a piece of metal of the source. The renewed supply of metal causes the voltage of the arc to drop almost immediately to a lower value so that the grid will again become biased to cut off and then promptly will again regain control, i. e., when the alternating plate potential next approaches or reaches the zero point of its cycle.

As stated above, the apparatus of this invention is of particular advantage in developing ions from the pure or substantially pure metal, normally solid but capable of being vaporized at high temperatures. It will be appreciated consequently that the invention is not particularly concerned with the method and means for utilizing the developed ions, Thus the drawings do not show any particular structure or apparatus for acting upon the ions produced. It may be stated, however, that the apparatus disclosed is especially useful as an ion source in the ionic methods of separating the isotopes of the metal and that the apparatus disclosed in the drawing was designed especially for use with an ion-classifying apparatus of the type described and claimed in U. S. Patent 2,606,291, issued August 5, 1952.

Referring now to the drawing and particularly to Figure 1, the ion source illustrated is contained within a chamber formed preferably of ceramic or metallic tubing ID sealed to a supporting plate I I by means of the gasket I2, to be compressed by an annular packing gland, not shown, so that the interior may be evacuated to subatmospheric pressure. The supporting plate I I has a drilled opening IS in which the flanged bushing I4 (shown in enlarged detail in Figure 5) is supported. To securely retain the bushing I4 in place and to effectively seal the opening I3, the flange I5 of the bushing is shown provided with a plurality of screws I9 which are tapped into the face of the plate II serving to draw the flange down tightly, compressing the ring gasket I6 between the under surface of the flange and the face of the plate I I. The flanged bushing I4 is also shown provided with a centrally drilled and tapped opening IT for receiving the packing gland I8 through which a feed rod is adapted to extend. The packing material contained within the space between the gland and the rear wall 2 I of the tapped opening I! is forced to fit snugly about the rod 20 as the gland is threaded into the drilled opening II. While we do not wish to be limited to the use of any particular type of packing to seal this rod 20, we have found the packing of the type illustrated in detail in Figure 5 to be remarkably satisfactory.

The principle of the seal illustrated is that the rod 20 slides through an elastic sealing medium 22 such as thin disks of sheet rubber or synthetic rubber in which has been punched a hole of a diameter considerably smaller than the diameter of the rod. The rubber close to the rod is thus distorted and bent out from its normal plane on translation of the rod. The gas pressure (from the top of the device as seen in Figure 5) as well as the elasticity of the rubber forces the inner edge of each rubber-like disk or washer intimately against the sliding rod so that no air leaks past the seal on translation of the rod. The outer edge of each of the rubber washers is pressed tightly down by the gland I8, against the metal washer 23 and the end wall 2! of the drilled opening II. It is important that the rubber near the rod be free so that the rod can slide freely through the seal without jamming. The effectiveness of the seal is further increased through the translation of the rod, whereby the inner, or freely moving, edge of the rubber disk is bent into and plugs the hole 24 drilled into the end wall of the bushing I4, the hole 2% being of slightly larger diameter than the rod 20.

Thus, the rod 26 may be made movable longitudinally through the end plate II of the evacuated chamber to feed in train-like fashion the pieces 25 of the particular heavy metal of the ion source. tained in alignment within a guide means such as the tube 25 formed of quartz or other suitable refractory material capable of providing a smooth sliding surface for the metal and capable also of withstanding the high temperatures of the source.

The pieces of source metal within the loading tube The pieces of metal 25 are shown re-.

source. To prevent this softening of the metalv pieces which would at least render difficult the loading operation, we provided a cover for the refractory tube in the form of a metallic sheath of high thermal conductivity. The sheath is in heat transfer relation with the end plate II, through the bushing I4. As illustrated, a copper sheath 2'! is utilized and is tightly fitted about the quartz tube 26; the sheath and tube are retained within the bushing l4 bymeans of the collet 28, which is threaded onto this bushing.

As is viewed in the drawings, particularly in Figures 1 and 4, the anode is made up of a plurality of transverse rods 29, supported side by side on the two parallel and spaced-apart rods 3'1 which extend through but do not touch the copper plate 3| which forms the rear wall of an enclosure 32 for the arc region. Since the apparatus disclosed in the drawings was developed specifically for generating the ions of copper, cobalt or uranium, we have disclosed, in accordance with present preference, an anode support made entirely of tungsten, with the understanding, however, that the use of an anode cifically, an end of each of these tungsten rods is respectively inserted into an opening drilled into an end of a threaded rod t l. The rods 35 are then hard soldered respectively into the tubes 33. A pair of collets 35 are threaded onto the respective ends of these rods 39 to retain the tungsten rods 30 within the drilled openings into which they have been inserted. It is thus apparent that a good electrical connection is proided from the anode structure to the supporting plate II. By reason of this supporting arrangement, the anode will be maintained at a ground potential relative to the other electrodes, as will be apparent from an inspection of the wiring diagram illustrated in Figure 8.

Since a source of potential 68 is to be applied to the tubular supporting leads 3? leading from the filamentary cathode 345 to heat the filament to an electron emission temperature, it is apparent that these leads must be insulated from each other where they project through the charm ber wall. The lead-in conductors 31 are preferably formed of copper tubing, and as will be noted in the cross-sectional showing of lead-in 3?, are provided with a second tube which is supported concentrically therein for permitting the flow of cooling fluid through these tubes to remove the heat from the ends of the filamentary cathode where they contact the lead-in conductors. The flow of the cooling liquid is maintained through the medium of a squirt tube 54 shown in enlarged detail in Figure 6. leads from the chamber and to connect the anode terminals to a common ground through the end plate I I. Any number of insulating seals known to the prior art may be used for the leads 3?. By way of specific example, one markedly successful seal is illustrated in Figure 1 and in enlarged detail. in Figure 7 and consists essentially of the metallic insert 38 in the form of a cylindrical bushing which is silver soldered or otherwise securely retained in the drilled opening of the We prefer to insulate both plate H. The end of the bushing extending to the high pressure side of the plate H is of an enlarged diameter and is threaded to receive the packing gland $9. interposed between the lead 3'5 and the insert 38 are the two bushings MI and H of which bushing fill is formed preferably of a refractory insulating material. The insulated bushings til and ll each have a uniform diameter over a greater portion of their length but are also provided with enlarged sections 42 and d3 respectively, to provide flange surfaces for receiving the sealing rings 4% of rubber or other suitable material which is compressed between the opposing surfaces formed by these enlarged sections, the outer end surfaces or the enlarged sections, the end surface of the gland and an appropriate shoulder in the metallic insert 38 when the gland 39 is threaded into the metallic insert.

To provide additional support for the end plate 3i of the enclosure 32, the tubes 3? which extend into the chamber for supporting and conducting the heating current to the filamentary cathode are insulatably secured to the plate 34 by means or the bushings t and 26 formed of suitable ceramic insulating material such as quartz or lavite, the latter consisting of a mineral talc, capable of withstanding temperatures of the order of 1600 C. These bushings are shown clamped firmly in place by the flanged end of the cylinder 4?, which is interposed between the bushings and the tubes ill, and the threaded collar 48. The end of each tube 3'! is provided respectively with a plug insert 49 to which a clamp for the tungsten filament 35 is secured. The enclosure 32 of the arc region is formed preferably of a cylindrical Nichrome cap 59 secured to the end plate 3! by means of screws, for example. The front surface of the cap 56 has a rectangular opening 5| over which is spotwelded a tungsten screen 52 to provide an ion transparent covering for the opening 5i. The enclosure 32 is supported by a pair of tubes 53 which project out of the vacuum through suitable insulating seals in the supporting plate ll, so that a high negative potential El may be applied to the screened opening of the cylindrical cap for withdrawing and accelerating the ions formed within the arc region. The supporting tubes 53, to provide a means of removing the heat from the plate 3!, and similar in construction to the leads 3! and as is apparent from the drawings, are provided with a squirt tube 54. Specifically, the seals for the leads 53 are similar to the insulated seals for the cathode terminals 3?, shown in the drawing in longitudinal section.

The squirt tube 51!, illustrated in enlarged detail in Figure 6, consists of a cylindrical metallic body, the enlarged portion of which contains the packing gland 55, the metal washer 5d, the packing material 51 and the exhaust connection Eat; the smaller portion 59 which connects to the cooling fluid supply and supports the inner tube concentrically within the tubes 3'5 and 53'. The squirt tube 54 is supported by and sealed to the lead-in tubes 3! and 53 by the packing gland 55 and the packing material 57. The cooling fluid, under moderate pressure, is forced through the inner tube (which terminates a short distance from the cathode and plate ends of the lead-in tubes as is apparent from the drawing) and returns around the inner tube heat conductively, to the squirt tube exhaust for recooling and recirculating or waste.-

That the arc may be initiated, a source of potential is applied to the terminals 3? to heat the filament 36 to electron emitting temperatures, as shown in Figure 8. The cathode is then connected to a source of negative potential relative to the grounded anode by being connected to the negative terminal of the direct current source 62. Inasmuch as the high side of the direct current source 52 is connected through the impedance 88 to ground, the cathode and anode may be considered as effectively connected across this source of potential through the resistive impedance 8B. The terminals 53 which extend from the plate 3i supporting the enclosure 32 of the arc region are connected to a negative potential higher than the negative potential of the cathode by being connected to the low side of the direct current potential source 6i A magnetic field having its lines of force in the direction of the arrow H, i. e. in the general direction of the arc, is preferably provided, since the arc voltage is found to be decreased through the use of such a magnetic field. The source magnet utilized to produce this field is not shown in the drawing in the interests of clarity, it be ing understood however that any magnetic field source may be utilized.

In operating sources similar to the one disclosed in the drawing, a discharge was noted to appear, at times, around the leads extending to the source and in the region behind the arc enclosure 32, where at least part of the arc assembly was surrounded by an enclosing metallic wall structure (not shown in the present drawings) at a high negative potential relative to the arc. This discharge was believed to be attributable to the ionization and excitation of gas in the crossed fields of the accelerating potential and the source magnet, i. e., apparently by the action of electrons oscillating in the crossed fields. We have provided in this source, as well as in other sources, similar to the type illustrated in a copending application (Serial No. 335,190, filed February 5, 1953) a shield for these leads, which has been found to be effective in eliminating this discharge phenomenon. As illustrated, the shield for the leads comprises two cylindrical metal tubes 53 and 64 and an intermediate metallic supporting ring 65 by means of which the tubes are joined together electrically and mechanically. The tube 63 is shown supported in the plate ll so that the tube 64 of smaller diameter will encircle the leads at the rear oi the arc region. At times, in operating sources of the type disclosed, a discharge, believed attributable to the reasons as given above, was noted over the bushings of the insulating seals 33. A metallic shield 89 for these bushings has been found to be effective in eliminating this discharge. A cooling tube 66 is shown supported in heat conductive relation with the front of the plate H. A suitable cooling fluid is circulated through the tube 66 to remove the heat conducted from the arc region to the end plate by the anode supporting leads and shielding tubes 83 and 6d.

To impart a translatory movement to the push rod 20, which movement effects the loading or the metal pieces onto the anode in response to the increase in arc voltage, we have connected the end of the rod 20, which projects from the vacuum through the sliding seal, to a threaded shaft 61. This shaft is supported for translatory movement only in the bearing block '68 by being provided with a longitudinal keywav into which projects the set screw 69. This set screw by engaging the keyway, prevents rotation of the shaft 6! butpermits translation in either forward or rearward direction. That .this .shaft may be supported in alignment with the rodzil three tubes l are provided for supporting the bearing block -58. These tubes have .plugged'ends by means of which they are secured respectively and in equi-angularly spaced relation to thesup porting plate H, by being threaded respectively onto the projecting studs, tapped into the face of this end plate. A plug TI having a stud pro jecting therefrom isinserted also into the free end of each of'the tubes 1-!) for supporting the bearing plate 12 (to which thebearing block .68 is secured) by means of the wing nut 13, for example. We haveshown in the drawings an electricmotor I4 supported by the bearing plate 1.2 having on its shaft a pinion gear '55 which meshes with the ring gear 16. .This ring gear has a cylindrical extension 17 which is iournaled for rotation in the bearing plate 12 and extends into the bearing block 68. A circular groove is cut on .the surface of this cylindrical extension into which the pin 78 rides to prevent movement longitudinally of thering gear 16. A splitnut 1-9 is :clamped about the shaft 6'! and is secured to the ring gear 16 for rotation therewith. Upon rotation of this split nut by the ringgear .76 the threaded shaft 61 will be moved forwardly to feed the metal pieces 25 onto the anode.

In Figure 8, we have shown schematically a wiring diagram for efiecting automatic energization of the motor to automatically drive .the threaded shaft 6'! in the direction of feed of the metallic pieces 25. As illustrated thearc electrodes 36 and 29 are energized by a source of direct current potential 62 through a series resistor 88. The. anode of the arc electrodes is connected to the-grid of a thyratron tube 8%) through a'series resistor 8! and an adjustable grid biasing potential 82 consisting specifically'of a resistor 83 shunting the. battery 84. The platezcircuit of the thyratron 80 is connected to analternating current source through a series resistor 85 and the relay coil 86 which serves to operate the contacts 81' in the energizing circuit of the motor 14. The negative biasing potential 82 is adjusted to cut off, at the preferred and normal arc voltage of the source. When themetal of .thesource is exhausted or nearly so, the :voltage across the arc rises until it reachesa critical valueat which the grid is sufficiently less negative to. permit the thyratron tube Bil-to conduct. Thecircuit containing the relay coil '8fieis thus rendered conductive and the plate current flowing therein is suificient to close the contacts 8'! ini'the energizationcircuit of the motor l4, starting :the motor and driving the threaded shaft 6'! forwardly to push the rod 20 through the loading tube or guide tube 25 to feed onto the anode 29 a piece of the metal 25. Immediately with the depositing of this metal onto the anode, the voltage of the arc drops to its normal preferred value and the proper .bias is again applied to the tube to render the thyratron tube 30 non-conductive the instant the alternating current plate voltage approaches or passes through the zero. 'Thegrid thus regains control of the thyratron so that it may :again close the contacts in the motor control circuit on the exhaustion'of the renewed supply of heavy metal. In the schematic wiring diagram referred to above, a number of batteries are shown for simplicity of circuit, with the understanding that other types of voltage and. current supply may be used, and in fact suit-able potentials and curinch I below the anode.

10' rents may be derived from the same source in a manner wellknown to the art.

-We have obtained particularly good results with a source-metal vapor source wherein the arc electrodes consistedof an anode made up of four rodsof 80 mil tungsten, supported side by side on two 30 mil tungsten rods, a 50 mil tungsten cathode supported five-sixteen hs of an Pieces of source metal one-sixteenth inch-square and about one-eighth inch long were loaded on to the anode, one piece at a time,.from a-z quartz ,tube four and one-half inches long having a one-quarter inch overall diameter and aninternal diameter of threethirty seconds -of an inch. An 80 mil tungsten rod was usedto push the pieces of source metal onto the anode. The supporting leads for the cathode, anode and are region enclosur were formed of one-quarter inch copper tubing, and

three one-half inch brasstubes each six and-onequarter inches long were utilized to supports. one-eighth inch brass bearingplate. The opening in the frontof the Nichrome cap forming the enclosure for the arc region was one-half by one inch and was covered with a fifteen mesh tungsten screen.

Satisfactory operating conditions for use with an isotope'separating device are, attained with an arc voltage drop of forty to fifty volts, an arc current of six amperes and a magnetic field of between ZOO-to 300 gaussin-which case an ion drain "current of-=about ten 'milliamperes is ob tained. This drain :will, of course, vary with the transparency of the tungsten screen.

Thearc voltage depends on the strength of the magnetic field Hand rises as the field is decreased. Thus a good'control of the source magnet is essential. The .arornay be operated Without amagnetic field byincreasing the arc currentto about twenty to .thirtyamperes or even higher. .'A ten ampere arc :current would raise the-arc voltageto about one hundred and ten volts with no field. ,Suchamount of powenwoul'd ordina-rily'cause the anode and source shield to melt, with the specific structural arrangement and dimensions described above.

It was found that one piece of source metal ofthe size given above, would. allow the arc to run for from five to ten minutes. Thus, runs of from two to three hours are possible with a loading of about twentypieces.

It should be'apparent, that we are not limitedfito the use of the above number of pieces, or for that matter'it isnot essential that pieces be utilized sincethe sourcemetal may, for example, be granulated and suitably fed onto t; e anode. Thus the detailed description of a preferred embodiment is set forth above only so that others may-readily understand how'to achieve the good results and objects of this invention, with the understandinghowever-that changes in the construction and combination of parts may be made 7 without'departing from the spirit and scope of this invention as defined in'the subjoined set of claims.

We claim:

1. An ion source including in combination a chamber sealed at subatmospheric pressure enclosing an anode and a cathode, said anode b ing adaptedto receive a vaporizable material the heavy ions of which are desired, means for establishing an arc discharge between said anode and cathode for both vaporizing and ionizing said materiahsaid anode being formed of a metal having a melting point higher than the melting 11 point of said material but alloying with said ma terial to lower the anode melting point by an amount dependent upon the concentration of said material, means operable to feed said vaporizable material to said anode, and means responsive to the concentration of said material alloying with the metal of said anode, for controlling said feedin means.

2. An ion source including in combination a chamber sealed at subatmospheric pressure, enclosing an anode, cathode, and supported on said anode, a vaporizable material the ions of which are desired, means for establishing an are discharge between said anode and said cathode for vaporizing and ionizing said material, means for withdrawing the ions from the region of said arc, means responsive to a predetermined increase in the voltage of said are accompanying the depletion of the said material on said anode, for increasing the supply thereof on said anode, and means responsive to a predetermined decrease in the voltage of said are following the increase in supply of said material on said anode for arresting increase in the supply of said material on said anode.

3. An ion source including in combination a chamber sealed at subatmospheric pressure enclosing an anode, cathode, tubular guide means and a supply of vaporizable material the ions of which are desired, said supply of vaporizable material being contained within a guide means supported to deposit said material on to said anode, a rod movable longitudinally of said guide means and extending through said chamber in sealed relation therewith for feeding said material onto said anode, means for establishing an arc discharge between said anode and said cathode for vaporizing and ionizing the material on said anode, means for withdrawing the ions from the region of said arc, means responsive to a predetermined increase in the voltage of said are accompanying depletion of the supply of said material on the anode, for imparting longitudinal movement to said rod to increase the supply of said material on said anode, and responsive to a decrease in voltage accompanying the feed of said material on to said anode for stopping the longitudinal movement of said rod.

4. A heavy ion source including in combination, a chamber sealed at subatmospheric pressure enclosing an anode, a cathode, and a tubular guide means containing a plurality of solid pieces of vaporizable material the ions of which are desired, said anode being adapted to receive the solid pieces of material from said guide means,and means exterior of said chamber having a motion transmitting element projecting through the chamber into the guide means in vacuumtight relationship with said chamber for intermittently feeding said pieces onto said anode.

5. An ion source including in combination, a chamber sealed at subatmospheric pressure enclosing an anode and a cathode, a guide means containing a supply of vaporizable material the ions of which are desired, said guide means being supported to direct said material onto said anode, electrically operated translating means exterior of said chamber having a motion transmitting element in said guide means projecting through the walls of said container in vacuumtight relationship therewith for feeding said material onto said anode, an electronic discharge device having a cathode, anode and control electrode, a control circuit for said translating means connecting the anode and cathode of said discharge device and means for applying between the control electrode and cathode of said discharge device a voltage variable with the voltage of said arc, whereby the operation of said translating device will be governed by predetermined changes in said are voltage.

6. An ion source including in combination an evacuated chamber enclosing an anode, a cathode, a guide tube and a supply of vaporizable material, the ions of which are desired, means operable for feeding said material through said tube onto said anode, means for establishing an arc discharge between said anode and said cathode, and means responsive to the voltage of said are for controlling the operation of said feeding means.

7. The apparatus defined in claim 6, wherein said vaporizable material consists of solid pieces of source metal and said anode consists of tungsten.

8. The apparatus defined in claim 6 wherein said feeding means includes a plunger extending into said tube in vacuumtight relationship with said chamber wall portion and a screw threaded drive means exterior of said chamber for efiecting longitudinal movement of said plunger through said tube to effect feeding of said material.

9. The apparatus defined in claim 6 wherein said feeding means includes a member within said tube movable to feed said material onto said anode, electrical translating means exterior of said chamber for transmitting motion to said member, and said are voltage responsive control means includes a grid controlled electrical discharge device having a grid biasing potential variable with are voltage for controlling the energization of said electrical translating device.

10. An ion source including in combination an evacuated chamber enclosing an anode, a cathode and a supply of vaporizable solid material the ions of which are desired, a guide means supported within said chamber for directing the feed of said material onto said anode, a member movable to feed said material in the direction of said guide means, means for establishing an arc discharge between said anode and said cathode for vaporizing and ionizing thematerial on said anode, and means exterior of said chamber for imparting motion to said member to maintain a supply of said material on said anode.

11. A heavy ion source including in combination, a chamber sealed at subatmospheric pressure enclosing an anode, a cathode, a tubular guide means of quartz tubing having thermally conductive envelope thereon in heat transfer relation with said chamber walls, said tubular guide means supporting internally thereof a plurality of solid pieces of vaporizable material, the ions of which are desired, said anode being adapted to receive the solid pieces of material from said guide means, and means exterior of said chamber having a motion transmitting element projecting through the chamber into the guide means in vacuumtight relationship with said chamber for intermittently feeding said pieces onto said anode.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Backus June 10, 1952 Number

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