US2717963A

US2717963A - Arc discharge device

Info

Publication number: US2717963A
Application number: US582090A
Authority: US
Inventors: Wilson M Brubaker
Original assignee: Individual
Current assignee: Individual
Priority date: 1945-03-10
Filing date: 1945-03-10
Publication date: 1955-09-13
Anticipated expiration: 1972-09-13

Description

Sept. 13, 1955 w. M. BRUBAKER 2,717,963

ARC DISCHARGE DEVICE 7 Filed March 10, 1945 2 Sheets-Sheet l HEATER JUPPL Y g I F/LAME/VT SUPP]. y

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MAGNET/C FIELD /00 r 1 ILAME/VT Fig. 3

P TROL ELECTRODE can/m0 Pa 01 moms ELECTRODE SUPPL y /7 COLL/MAT/NG ma CATHODE 1 ELECTRODE SUPPLY 24 V N5 T /o o mausr/c 5 ARC HELD COLLECTOR SUPPLY mzwzuva ELECTRONS /Z 7 A/VODE INVENTOR. IZ L 601V 11!. BRUBAKER BY- M 5m ATTORNEY.

United States Patent ARC DIStIHARGE DEVICE Wilson M. Brubaker, Crescent Hills, Pa., assignor to the United States of America as represented by the United States Atomic Energy Commission Application March 10, 1945, Serial No. 582,090

Claims. (Cl. 25049.5)

The present invention relates to electric discharge devices and more particularly to calutron ion sources.

It is an object of the invention to provide an electric discharge device including an improved electron source arrangement operable on alternating current for producing an arc discharge that may be easily and simply regulated.

Another object of the invention is to provide a calutron ion source having an improved electrode arrangement productive of a uniform and copious supply of ions.

A further object of the invention is to provide a calutron ion source of improved construction and arrangement that is eificient in operation and subject to minimum wear and erosion.

In carrying out the objects of the present invention, a calutron ion source is provided with an electron source including a filamentary cathode, a plate-like arc cathode, and a control electrode arranged in alignment with the magnetic field of the calutron, the arc cathode being positioned immediately above a collimating electrode of the ion source are chamber, which chamber contains a vapor through which an arc discharge is established. The filamentary cathode is raised to thermionic emissive temperature and an alternating voltage is applied between the filamentary cathode and the arc cathode whereby electrons are accelerated from the filamentary cathode during alternate half cycles of the applied voltage and bombard the arc cathode raising the arc cathode to a thermionic emissive temperature. Further, a negative potential is applied to the control electrode with respect to are cathode to minimize bombardment of the filament by electrons emitted from the arc cathode during the half cycles when the arc cathode is at a negative potential with respect to the filamentary cathode. Thus the present invention provides an electron source arrangement of the indirectly heated cathode type wherein the arc cathode is heated by electron bombardment, a major part of the electrical energy is derived from an alternating current source, the electron emission of the arc cathode may be. easily regulated and varied and the electrodes constituting the improved electron source are subjected to minimum wear. The indirectly heated type of arc cathode provided with grid control is not new and is shown in the copending application of Louis F. Wouters, Serial No. 528,818, filed March 31, 1944. HOW- ever, the use of alternating current entails many advantages in that alternating current is readily available, easy to vary and transform, and requires none of the usual rectifying and filtering necessarily attendant to obtaining a high voltage direct current. Moreover, the provision of a grid electrode which may be biased with respect to the arc cathode makes the use of alternating current readily feasible in that electron bombardment of the fila mentary cathode can be reduced thus increasng the life thereof, and the electron bombardment of the arc cathode can be easily controlled. Further, by properly biasing the grid electrode the flow of current to the electron source during that portion of the cycle when the filament is positive with respect to the arc cathode may be minimixed, thereby increasing the efiiciency of the electron source.

The invention, both as to its organization and method of operation, together with other objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which Figure 1 is a diagrammatic plan view of a calutron into which there may be incorporated an ion source embodying the present invention; Fig. 2 is a diagrammatic sectional view of the calutron taken along the line 2-2 in Fig. 1; Fig. 3 is a schematic perspective view of the electrode arrangement in a calutron ion source embodying the present invention; Fig. 4 is a longitudinal sectional view of a calutron ion source embodying the present invention; Fig. 5 is a front elevation view of the calutron ion source shown in Fig. 4; and Fig. 6 is a sectional view of the calutron ion source taken along the line 6-6 in Fig. 4.

At the outset, it is noted that a calutron is a machine of the character of that disclosed in the copending application of Ernest 0. Lawrence, Serial No. 557,784, filed October 9, 1944, now U. S. Patent No. 2,709,222, and is employed to separate the constituent isotopes of an element and, more particularly, to increase the proportion of a selected isotope in an element containing several isotopes in order to produce the element enriched with the selected isotope. For example, the machine is especially useful in producing uranium enriched with U Such a calutron essentially comprises means for vaporizing a quantity of material containing an element that is to be enriched with a selected one of it several isotopes; means for subjecting the vapor to ionization, whereby at least -a portion of the vapor is ionized causing ions of the several isotopes of the element to be produced; electrical means for segregating the ions from the un-ionized vapor and for accelerating the segregated ions to relatively high velocities; electromagnetic means for deflecting the ions along curved paths, the radii of curvature of the paths of the ions being proportional to the square roots of the masses of the ions, whereby the ions are concentrated in accordance with their masses; and means for de-ionizing and collecting the ions of the selected isotope thus concentrated, thereby to produce a deposit of the element enriched with the selected isotope.

Referring now more particularly to Figs. 1 and 2 of the drawings, there is illustrated a representative example of a calutron 10 of the character noted, that comprises magnetic field structure including upper and lower pole pieces 11 and 12, and a tank 13 disposed between the pole faces of the pole pieces 11 and 12. The pole pieces 11 and 12 carry windings, not shown, to produce a substantially uniform and relatively strong magnetic field therebetween. Also, vacuum pumping apparatus, not shown, is associated with the tank 13.

Disposed within the tank 13 is an ion source unit, generally designated 20, adapted to produce an abundance of ions. An ion accelerating structure 39 is disposed in spaced-apart relation with respect to the source unit 20 and is adapted to withdraw the ions as a beam and project the same into the tank 13 where the ions are acted upon by the magnetic field. A tubular liner 34 disposed within the tank 13 substantially encloses the path of the ion beam and serves as an electrostatic shield.

At the exit end of the liner 34 within the tank 13 is positioned a collector block 29 for receiving and discharging the desired ions of the beam. The liner 34 is electrically connected to the collector block 29 and to the accelerating structure 39, so that substantially all of the ion beam path is maintained at a uniform potential.

Associated with the various electrical elements of the calutron are suitable electrical connections supplying the necessary operating potentials thereto. In operation, with the various electrical elements energized, the vacuum pump is utilized to obtain a predetermined degree of vacuum in the tank 13. A charge of material containing isotopes to be treated in the ion source is vaporized and forced through an are which ionizes the vapor. The potential of the accelerator structure 39 is such that the ions are Withdrawn from the source 20 and projected as a beam into the liners 34 where they are subjected to the force of the magnetic field. The effect of the foregoing is to curve the ion beam about a radius which may be readily calculated and thence into the collector block 29. Thus the desired portion of the ion beam is intercepted and retained and may be reclaimed for future use. Of course, it will be understood that the various dimensions of the parts of the calutron 10, the various electrical potentials applied between the various electrical parts thereof, as well as the strength of the magnetic field between the pole pieces 11 and 12, are suitably correlated with respect to each other, depending upon the material to be treated therein. In this connection reference is again made to the copending application of Ernest 0. Lawrence for a complete specification of such a calutron.

In the operation of the calutron 10, it is highly desirable that a relatively intense stable beam of positive ions be projected by the ion accelerating structure 39, through the liner 34, toward the collector block 29; which operating condition requires that the source unit 20 be productive of a steady and copious supply of positive ions. To accomplish this end in the source unit 20, the are discharge through the cavity in the arc-block 22 must be both relatively intense and uniform. Moreover, it is desirable that such an arc discharge should be steady and free from both intensity and position variations in order that the ion source unit 20 be productive of a highly continuous, copious and uniform supply of positive ions. Furthermore, the ion source unit 20 should be so constructed and arranged that the parts thereof are subjected to minimum wear and erosion, whereby the unit has a long life and an efiicient operating characteristic.

While the source unit in the calutron disclosed in the previously mentioned copending application of Ernest 0. Lawrence is satisfactory in operation, it does not possess the characteristics noted to the extent desired; nor has it the long life required for most etficient commercial operation.

Referring now more particularly to Fig. 3 of the drawings, the electrode arrangement in the calutron ion source unit 100 embodying the features of the present invention is illustrated schematically in perspective. More particularly, this ion source unit 100 comprises an electron source 101, a plate-like collimating electrode 124 formed of tungsten, molybdenum, or the like, and a plate-like anode 127 formed of tungsten or the like. The electron source 101 and the

electrodes

124 and 127 are arranged with the discharge paths therebetween in alignment with the magnetic field of the calutron, the direction of the field between the north pole and the south pole of the field structure being indicated by the arrow 53.

Considering now the electron source 101, there are provided a substantially U-shaped filamentary or first cathode 121 formed of tungsten or the like and arranged in a vertical plane parallel to the magnetic field, a platelike are cathode 117 formed of tungsten or the like and positioned above and adjacent to the collimating electrode 124, and a plate-like control electrode 114 formed of tungsten or the like having a longitudinal slot 104 formed therein, the control electrode 114 being disposed between the filamentary cathode 121 and the arc cathode 117. Further, it should be noted that the electrodes constituting the ion source unit 100 are all arranged in alignment with the magnetic field 53 of the calutron and that the

electrodes

114, 117, 124 and 127 are disposed in planes normal to the magnetic field 53. The longitudinal slot 104 formed in the control electrode 114 and a longitudinal slot 125 formed in the collimating electrode 124 are parallel one to the other. Moreover, it should be noted that the filamentary cathode 121 is disposed in a vertical plane and the longitudinal slot 104 formed in the control electrode 114 is of such configuration and is so positioned relative to the filamentary cathode 121 that the projection of the filamentary cathode along the magnetic field entirely passes through the longitudinal slot 104.

Considering now the structure and dimensional details of the electrodes constituting the electron source 101 of the ion source unit 100, satisfactory results have been obtained employing a filamentary cathode of circular cross-section having a diameter of 0.050", the length of. the central portion being slightly less than 0.625. The control electrode 114 is disposed approximately 0.125" below the filamentary cathode 121 and approximately 0.030" above the arc cathode 117. Further, the control electrode 114- is of 0.060" thickness, the longitudinal slot 104 formed therein being 0.625" x 0.125" and being aligned with the central portion of the filamentary cathode 121, the width of the control electrode 114 being 0.675. The are cathode 117 is of 0.125 thickness and 0.875" wide and the front edge thereof extends about 0.094 beyond the front edge of the longitudinal slot 104 formed in the control electrode 114.

The filamentary cathode 121 is connected by way of an adjustable resistor 102a to any suitable source of filament supply either A. C. or D. C. but here indicated for the purpose of illustration as a battery 1030; while the arc cathode 117 and the control electrode 114 are respectively connected by way of a potentiometer to the positive and negative terminals of a suitable source of control electrode supply. Further, the filamentary cathode 121 and the arc cathode 117 are respectively connected to the terminals of a suitable source of alternating current cathode supply. The above connections constitute the electrical arrangement of the electron source 101 of the are mechanism; whereas the arc cathode 117 and the anode 127 are respectively connected to the negative and positive terminals of a suitable source of arc supply, the collimating electrode also being electrically connected to the anode 127, thus completing the electric network of the ion source unit 100.

Considering now the general principle of operation of the ion source unit 100 and more particularly the electron source 101, the current flowing through the filamentary cathode 121 is adjusted by means of the adjustable resistor 102a until the filamentary cathode 121 becomes electron emissive. The control electrode supply circuit is completed, thus applying a bias to the control electrode 11d. Thereafter, the cathode supply circuit is thence completed so that electrons emitted by the filamentary cathode 121 are accelerated to and bombard the arc cathode 117 during alternate half cycles of the alternating current cathode supply, thereby raising the temperature of the arc cathode 117 to incandescence and to electron emission. It Will be understood that by varying the potential of the cathode supply the energy with which the electrons leaving the filamentary cathode 121 bombard the arc cathode 117 may be regulated. Further, by varying the potential of the control electrode supply and consequently the impressed bias on the control electrode 114 with respect to the filamentary cathode 121 the number of electrons which are emitted by the filementary cathode 121 and bombard the are cathode 117 may be regulated. Inasmuch as the filamentary cathode 121 is so positioned that its projection along the magnetic field passes through the longitudinal slot 104 of the control electrode 114, and since the electrons traveling from the filamentary cathode 121 to the are cathode 117 are confined to paths aligned with the magnetic field, the control electrode current will be small even though a large positive bias is applied thereto. This feature is of great advantage since the current density of the electron stream bombarding the arc cathode 117 may be simply adjusted with a minimum dissipation of current. Consequently, the electron source arrangement 101 affords a variable source of electrons for an arc discharge of the ion source unit 100 which requires little additional apparatus of low current carrying capacity. Moreover, the electron emission of the arc cathode 117 may be also adjusted by varying the cathode supply voltage; but since the current output of the cathode supply is substantially high and the voltage swing necessary for a given change in the electron emission of the arc cathode 117 is greater than the voltage swing required on the control electrode 114, this method is not as facile as the method employing the control electrode. Furthermore, inasmuch as the potential of the control electrode 114 relative to the arc cathode 117 is negative, the electrons emitted from the upper face of the arc cathode 117 will not bombard the control electrode 114.

It should also be noted that by properly positioning and biasing the control electrode 114 relative to the arc cathode 117 electrons emitted by the upper surface of the arc cathode 117 are prevented from bombarding the filamentary cathode 121 when the filamentary cathode 121 is positive relative to the arc cathode 117, which occurs during alternate half cycles of the arc cathode supply. For example, in the electron source 101 employing the dimensions above set forth, substantially no electron bombardment of the filamentary cathode 121 was present, an alternating potential of about 2100 volts being applied between the filamentary cathode 121 and the arc cathode 117, and a 580-volt negative bias being applied to the control electrode 114 realtive to the arc cathode 117. Employing the above arrangement, sufficient eletcron emission was obtained from the lower surface of the arc cathode to produce a 95-volt are having a current of two amperes.

The electrons emitted from the lower face of the arc cathode 117 are accelerated toward the collimating electrode 124, some of the electrons passing through the longitudinal slit 125 formed in the collimating electrode 124, which collimating slit defines the cross-section of the ionizing electron stream which is projected into the field-free gaseous region betweenthe collimating electrode 124 and the anode 127. The electrons thus projected into the gaseous region between the collimating electrode 124 and the anode 127 result in a discharge therethrough. This discharge through this region occurs between the arc cathode 117 and the anode 127 and is of the arc type, being characterized by a high current, a low voltage drop and a luminous plasma. Accordingly, there is defined a zone of intense ionization having a cross-section of the same shape as the longitudinal slit 125, this zone constituting part of the arc discharge between the arc cathode .117 and the anode 127. It should be noted that the aforesaid zone of intense ionization has a ribbon-like configuration and is believed to be a plasma region with little variation in potential.

Referring now more particularly to Figs. 4 to 6, inelusive, of the drawings, there are illustrated the structural details of the source unit 100 which is arranged in the magnetic field between the pole pieces of the calutron in the manner previously explained, the source unit 100 comprising a charge receptacle 99 and an arc-block 102. The charge receptacle 99 comprises wallstructure, including a removable cover 99a, defining an upstanding cylindrical cavity 103 therein, that is adapted to receive a removable cylindrical charge bottle 104a containing a charge 105 that is to be vaporized. The arcblock 102 comprises wall structure defining an upstanding vapor distributing chamber 106 and an upstanding arc chamber 107 therein, the cavity 103 communicating with the vapor distributing chamber 106 through a tubular member 108 supported by the wall structure ofthe charge receptacle 99 and the wall structure of the arc-block 102.

The wall structure of the charge receptacle 99 carries an exteriorly arranged electric heater 109 of any suitable form, whereby the charge receptacle 99 and consequently the charge bottle 104a may be appropriately heated in order to vaporize the charge contained in the charge bottle 104a. Similarly, the wall structure of the arc-block 102 carries an exteriorly arranged electric heater 110 of any suitable form, whereby the arcblock 102, and more particularly the vapor distributing chamber 106 therein, may be heated in order to prevent condensation of the contained vapor, as explained more fully hereinafter.

More particularly, the wall structure of the arc-block 102 comprises a substantially inverted U-shaped frame member 111, supporting an upstanding baffle plate 112, the frame member 111 and the bafile plate 112 being formed of carbon or graphite. The frame member 111 is secured to the Wall structure of the arc-block 102 by an arrangement comprising two upstanding strips 113, and comprises a top wall 128, two upstanding substantially parallel spaced-apart side walls 115 and a front wall 116, the front wall 116 having a centrally disposed longitudinal slot 117a formed therein and communicating with the arc chamber 107. The'side edges of the baflle plate 112 are spaced a short distance from the side walls 115 of the frame member 111 in order to provide communication between the vapor distributing chamber 106 and the arc chamber 107, the bafile plate 112 defining the boundary between the chambers men-v tioned.

The wall structure of the charge receptacle 99 carries a standard 118 which supports cathode structure 119 in cooperating relationship with respect to the arcblock 102. More particularly, the cathode structure 119 comprises two terminals 120 supporting the opposite ends of the substantially U-shaped filamentary cathode 121, the opposite ends of the filamentary cathode 121 being removably clamped in place by the respective terminals 120, and the two terminals being connected to the source of filament supply, as previously noted. The control electrode 114 is disposed immediately below the filamentary cathode 121 and is connected by way of the lead 115a to a terminal of the control electrode supply, the other terminal of the control electrode supply being connected to the filamentary cathode 121. Further, the arc cathode 117 is positioned below the control electrode 114 and the cathode supply is applied between the filamentary cathode 121 an dthe arc cathode 117 as has been heretofore described. sulating members 129 are inserted between the terminals of the filamentary cathode 121 and the control electrode 114, and between the terminals of the control electrode 114 and the arc cathode 117.

The central portion of the top wall 128 of the frame member 111 has a transversely extending slot 122 formed therethrough communicating with the arc chamber 107. The upper end of the transverse slot 122 is provided with a counter recess 123 extending thereabout which receives the collimating electrode 124, the collimating control electrode 114 and the transverse slot 125 formed in the collimating electrode 124 being in vertical alignment. Further, a laterally extending slot 126 is formed in the front wall 115 of the frame member 111 adjacent the lower end thereof, and supports the anode 127 extending into the are chamber 107 in vertical alignment with the transverse slot 125 formed in the colli- It should be noted that inmating electrode 124. The negative and positive terminals of the arc supply are respectively connected to the arc cathode 117 and to the arc-block 102, the anode 127 and the collimating electrode 124 being connected together by the frame member 111, and consequently, by way of the arc-block 163 to the positive terminal of the arc supply mentioned, as previously noted.

Considering now the detailed operation of the source unit 100, when the electric circuit for the heater 109 is completed, the charge receptacle 99 and consequently the charge bottle 104a are heated, whereby the charge 105 is vaporized, filling the cavity 103 in the charge receptacle 99. The vapor passes through the tubular member 108 into the vapor distributing chamber 106, whereby this chamber is filled with the vapor. The vapor is thoroughly diffused in the vapor distributing chamber 106 and passes around the side edges of the baflle plate 112 into the arc chamber 107, whereby this chamber is filled with the vapor.

ionized, due to the arrangement of the vapor distributing chamber 106 and the battle plate 112.

When the circuit for the filamentary cathode 121 is completed, the filamentary cathode 121 is heated and rendered electron emissive; and when the control electrode supply circuit is closed and the arc cathode circuit is completed between the filamentary cathode 121 and the arc cathode 117, electrons are projected from the filamentary cathode 121 through the slot 104 formed in the control electrode 114 and bombard the arc cathode 117, thus rendering the arc cathode 117 electron emissive. Further, by varying the potential applied to the control electrode 114 the number of electrons bombarding the arc cathode 117 and consequently the electron emission thereof may be adjusted. Thereafter the arc supply circuit is closed and some of the electrons emitted by the arc cathode 117 are projected through the transverse slot 125 formed in the collimating electrode 124 into the arc chamber 107 and proceed toward the anode 1277 Accordingly, the collimating electrode 124 causes a stream of electrons having a ribbon-like configuration to be projected through the arc chamber 107, whereby the vapor in the arc chamber 107 is ionized. Moreover, the width of the stream of electrons is greater than the width of the upstanding slot 117a formed in the front wall 116, whereby any vapor flowing through the arc chamber 107 and the slot 117a must traverse the electron stream and thus be subjected to its ionizing influence. The positive ions produced in the arc chamber 107 are drawn through the upstanding slot 117a formed in the front wall 116 of the frame member 111 by the associated ion accelerating structure, whereby a beam of positive ions having a substantially ribbon-like configuration is projected into the adjacent end of the associated liner and directed toward the cooperating collector block.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An electric discharge device comprising a first cathode being substantially U-shaped, an anode disposed transversely of said first cathode, a second cathode being plate-shaped disposed intermediate said first cathode and said anode and parallel to said anode, means for rendering said first cathode electron emissive, means for applying an alternating potential between said first cathode and said second cathode, whereby electrons emitted by said first cathode bombard said second cathode to raise said second cathode to an electron emissive temperature, a control electrode having a single elongated aperture formed therein disposed parallel to said second cathode between said More particularly, the arc chamber 107 is thoroughly and uniformly filled with the vapor to be first cathode and said second cathode, said aperture being disposed parallel to said cathode, means for applying a potential to said control electrode relative to said second cathode, said second cathode and said anode having a gaseous region disposed therebetween, means for establishing a magnetic field in the direction of the electron flow, and means for producing an arc discharge between said second cathode and said anode through said gaseous region.

2. An electric discharge device comprising a substantially U-shaped filamentary cathode, an anode disposed transversely to said filamentary cathode, a plate-shaped second cathode disposed parallel to said anode intermediate said filamentary cathode and said anode, means for rendering said filamentary cathode electron emissive, means for applying an alternating potential between said filamentary cathode and said second cathode, whereby electrons emitted by said filamentary cathode bombard said second cathode to raise said second cathode to an electron emissive temperature, a plate-shaped control electrode having an elongated slot formed therein positioned parallel to said second cathode between and aligned with said filamentary cathode and said second cathode, means for applying a potential to said control electrode relative to said second cathode, said second cathode and said anode having a gaseous region disposed therebetween, means for establishing a magnetic field in the direction of flow, and means for producing an arc discharge between said second cathode and said anode through said gaseous region.

3. An ion source unit comprising means for establishing a magnetic field, a substantially U-shaped first cathode disposed in alignment with said magnetic field, a plateshaped anode disposed transversely of said first cathode in spaced-apart relation, a plate-shaped second cathode disposed parallel to said anode between said anode and said first cathode, a hollow arc-block defining a gaseous region therein positioned between said second cathode and said anode and aligned with respect to said magnetic field, means for rendering said first cathode electron emissive, means, for applying an alternating potential between said cathodes, whereby electrons emitted by said first cathode bombard said second cathode when said second cathode is more positive to raise said second cathode to an electron emissive temperature, a control electrode having a single elogated slot therein disposed between said cathodes with said slot parallel to said first cathode for defining the cross section of such electron emission, means for applying a potential to said control electrode in relation to said second cathode, means for establishing an arc between said second cathode and said anode through said gaseous region, means for supplying a gas to be ionized into said region, said arc-block having a slit in a wall thereof in alignment with said magnetic field, and means for drawing a beam of ions from said gaseous region through said slit transversely of said magnetic field.

4. An electric discharge device comprising a substantially U-shaped filamentary cathode, a plate-shaped anode disposed transversely of said filamentary cathode, a plateshaped cathode disposed intermediate said filamentary cathode and said anode and parallel to said anode, means for rendering said filamentary cathode electron emissive, means for applying an alternating potential between said cathodes, whereby electrons emitted by said filamentary cathode bombard said plate-shaped cathode to raise said plate-shaped cathode to an electron emissive temperature, a plate-shaped control electrode having a single elongated aperture therein disposed parallel to said plateshaped cathode between said cathodes, said aperture being disposed parallel to said filamentary cathode, potential means connected between said control electrode and said plate-shaped cathode for rendering said control electrode negative with respect to said plate-shaped cathode, structure forming a gaseous region disposed between said anode and said plate-shaped cathode, potential means connected between said plate-shaped cathode and said anode for producing an arc discharge therebetween, means for establishing a magnetic field in the direction of said are, and means for withdrawing ions from said gaseous region.

5. An ion source unit comprising means for establishing a magnetic field, a substantially U-shaped first cathode, a second cathode disposed transversely of said magnetic field with the discharge path between said cathodes aligned with said magnetic field, an anode mounted parallel to said second cathode with the path therebetween aligned with said magnetic field, a hollow arc-block defining a gaseous region therein positioned between said second cathode and said anode, means for rendering said first cathode electron emissive, means for applying an alternating potential between said cathodes, a plateshaped control electrode having an elongated slot with dimensions substantially equal to said first cathode disposed between said cathodes parallel to said second cathode, said control electrode slot being positioned parallel to said first cathode, means for applying a negative potential to said control electrode relative to said second cathode, means for applying a positive voltage to said anode with respect to said second cathode for establishing an arc therebetween, a plate-shaped collimating electrode having an elongated aperture therein disposed parallel to said anode between said second cathode and said arcblock, said aperture also being disposed parallel to said first cathode, said plate-shaped collimating electrode being electrically connected to said anode, means for supplying gas to be ionized to said gaseous region, and means for withdrawing a beam of ions from said region through a slit in said arc-block transversely of said magnetic field.

References Cited in the file of this patent UNITED STATES PATENTS 1,419,547 Ehret June 13, 1922 1,864,591 Foster June 28, 1932 2,250,511 Varian et a1. July 29, 1941 2,355,658 Lawlor Aug. 15, 1944 2,373,151 Taylor Apr. 10, 1945 2,374,205 Haskins Apr. 24, 1945 FOREIGN PATENTS 149,193 Britain of 1922