US2956195A - Hollow carbon arc discharge - Google Patents

Description

Oct. 11, 1960 J. 5. LUCE I 2,9

HOLLOW CARBON ARC DISCHARGE Filed Aug. 1 4, 1959 TO VACUUM ION SOURCE TO VACUUM TO VACUUM GAS SOURCE INVENTOR. John S. Luce BY i ATTORNEY Un ted States. Patfi f HOLLOW CARBON ARC DISCHARGE John S. Luce, Oak Ridge, Tenn., assignor to the United States of America as represented by the United States Atomic Energy Commission Filed Aug. 14, 1959, Ser. No. 833,897

Claims. (Cl. 313-153) against the instreaming of low-energy neutral particles into a plasma formed within the hollow discharge. Reduction of incoming neutrals reduces the number of charge exchange reactions and thereby increases the probability of creating and maintaining adequate reaction temperatures to produce neutrons from a thermonuclear plasma.

This invention is an improvement over the carbon arc discharge set forth in my co-pending application, Serial No. 738,242, filed May 27, 1958, now Patent No. 2,920,234, issued January 5, 1960. The arc discharge set forth in that application is very efficient as a dissociating and/ or ionizing mechanism, but in order to achieve and maintain adequate thermonuclear reaction temperatures, it would be necessary to trap a very large quantity of high energy ions to ionize the neutrals in the system. This phenomenon is commonly called burnout.

One major problem associated with establishing and operation of a neutron producing or thermonuclear plasma by trapping of atomic ions inside a magnetic mirror field is the effect of cooling of energetic ions by collision with low energy neutral particles. The magnitude of this problem of charge exchange is such that a thermonuclear plasma cannot be obtained unless burnout of the residual neutral gas occurs, as set forth in the application of Albert Simon, Serial No. 732,770,

filed April 28, 1958. h

It is recognized that when energetic particles strike the walls of any containment vessel, there will be low energy neutral particles produced that will degrade the temperature of the plasma. In addition, similar cooling neutrals will arise from outgassing and other sources within the vessel and thus will cause untenable charge exchange and ionization losses. Many means for reducing charge exchange in the plasma have been suggested. These include evaporating a getter over the entire inner surface of the machine; burying ions in plates with means provided for renewing the surface thereof; and providing semi-porous liners to permit the escape of these particles but retard the reverse flow into the machine. None of these means, however, appear as effective as the means set forth herein to reduce charge exchange.

Another problem associated withignition of a thermonuclear plasma by the Luce method of dissociation of molecular ions to form atomic ions by passing the former I through an energetic arc discharge is the subsequent capture of electrons by the hot ions, forming neutrals which are not confined and are lost to the system; 'The carbon ions formed in carbon discharges where the anode is within the mirror region are predominantly singly ionized (C+) ions, with fewer multiply ionized ions. The anode 2,956,195 Patented oer, 1 1 .1960

cannot, in solid arcs, be placed any substantial distance behind the mirror. without the arc becoming serpentine and kinking due to the interaction of the mirror field and the circular field around the arc.

An unexpected advantage of the hollow. carbonarc herein described is that it will remain, stable even when the anode is placed .a long distance behind the mirror region. With such spacing, I have foundi(l) that substantially 'fewericarbon ions enter the plasma region to contaminate it, and (2) that the ratio of multiply ionized to singly ionized carbon ions is greatly increased. 'The highly ionized carbon atoms compete forelectrons with the plasma gas atoms, so actually help to reduce charge exchange losses.. Thus the hollow arc can operate with the arc anode substantially distant behind amirror coil to achieve .even greater protection for the plasma it surrounds. I J 7 Some advantagesof a hollow arc discharge have been set 'forth in other applications. One example of such discharges is set forth in my co-pending' application, Serial No. 748,771, filed July 15, 1958, now Patent No. 2,927,232, issuedMarch -1, 1960. The discharges set forth in that application are sustained by gas fed to the discharges. .The devices for producing such discharges,

as .a .barrierfor low. energy neutral particles attempting to enter the hollow interior ofthe discharge, and to pro vide means for assisting in removing excess gas particles fromtheapparatus.

- It is .another object of this invention to provide a hollow, gas-fed carbon arcdischarge in an evacuated container and in a-strong confining magnetic field for dis 'sociating and ionizing an injected molecularion beam, to

form a trapped plasma of-ener getic ions and electrons withinthe regionboundedby the discharge, and to reduce ion losses from the plasma due to the charge ex change process, whereby the density of the plasma may be increased sufilciently to reach thermonuclear temperatures, producing neutrons. j h V V -It is still another object of this invention to provide' a hollow carbon arc discharge which is effective as an ion pump for low-energy ions formed Within the discharge.

These and other objects and advantages of this invention will become apparent from'a consideration of the following detailed specification and the accompanying drawing, wherein the single figure is a cross-sectional view of a mirror-type device for accomplishing the above objects. Q1

The above objects have been accomplished in the present invention by initiating and sustaining'an energetic hollow carbon arc discharge between a thin hollow carbon cathode and a hollow carbon anode in an evacuated enclosure and within a strong confining magnetic field provided by two spaced magnetic mirror coils. The electrodes are preferably positioned with the axes thereof alongthe axis of magnetic symmetry.' 'The electrode faces are positioned in regions ofcompressed magnetic field so as to form an are which is enlarged indiameter midway between the electrodes due to the .bowing out of the magnetic field lines in this region; .One method 0f forming a thermonuclear-Plasma of. trapped atomic hollow 'arc discharge is to inject a beam of molecular ions at near grazing angle to the inside region of the arc wall, where a portion of the molecular ions are dissociated and/or ionized by the discharge and thus form a trapped plasma of atomic ions and electrons within the volume of the arc discharge. The are wall then serves as a shield or barrier to the cold neutral particles formed outside of the hollow arc.

Another method for forming a thermonuclear plasma of trapped atomic ions and electrons within the volume established by the hollow arc discharge would be to provide a gas fed arc discharge in axial alignment with and in the center of the hollow arc discharge and to provide a potential gradient between the discharges in a manner as set forth in my copending application, Serial No. 790,- 031, filed January 29, 1959. In this arrangement, ions would be accelerated from the more positive arc toward the negative arc and electrons from the negative arc to the more positive arc and they would be magnetically trapped between the arcs to form a plasma of energetic ions and electrons.

The use of carbon electrodes in the present invention has at least two major advantages over discharges using other materials. One of these advantages is that due to the low atomic number of carbon, the arc discharge formed between such electrodes is well defined and produces less scattering of ions from the discharge. By keeping this scattering at a minimum, the ion losses from the plasma are reduced. Another advantage is economy. Carbon pieces and particles are torn from the electrodes during arc operation, and these pieces and particles, together with the carbon ions, absorb gas to such an extent that it is possible to feed a large amount of gas to the discharge and still not require as large a pumping system as would otherwise be required.

Refer now to the single figure in the drawing Which illustrates one embodiment in which the principles of this invention may be carried out. A hollow carbon cathode is supported within and afiixed to a tubular member 16. Member 16 is enclosed partly by an insulating. sleeve member 17 which has a flange portion. This flange portion is clamped against one end wall of housing 1 by a clamping ring 35 With retaining bolts therethrough which areafiixed to the end wall of housing 1. A hollowcarbon anode 11 is supported within and affixed to a tubular member 19. Member 19 is enclosed partly by an insulating sleeve member 20 which is provided with a flange portion. This flange portion is clamped against the other end wall of housing 1 by a clamping ring 36 with retaining bolts therethrough which are afiixed to said end wall of housing 1.

A hollow cup-shape bafide 12 is disposed within and spaced from cathode 10., and extends beyond the leading edge of cathode 10. Baffle 12 is supported by an insulating rod 14, and rod 14 is in turn supported by a member which is fitted within tubular member 16. A hollow sleeve-like baflle 13 is closely fitted within and supported by hollow anode 11, and it extends beyond the leading edge of anode 11. Baffles 12v and 13 are extended beyond the leading edges of cathode 10 and anode 11, respectively, to prevent carbon particles and carbon pieces, which are produced at the electrode edges, from reaching the inner region of the hollow are 31 formed between the cathode and anode. Bafiie 12 must be insulated so as to prevent it from serving as the cathode for the are discharge. Insulating rod 14 is provided with a central conduit 40, to which is connected a gas feed tube,38, which in turn is connected to a source of feed gas 37. Hollow baflle 12 has a plurality of openings 39 adjacent to the leading edge of cathode 10 so that gas may be fed to the arc discharge formed between cathode 10 and anode 11.

The gas is used not only in assisting in the formation of the discharge, but in helping to sustain the discharge and reduce scattering from the discharge.

The container 1 is divided into three separate chamthe conductance between the chamber 33 and the higher I pressure end chambers 32 and 34. The annular passageway between the cathode 10 and baflle 12 is provided to permit escape of the ions produced in the arc and thus permit the arc to more effectively act as anion pump. These ions may then escape through a plurality of openings 18 in tubular member 16. An additional portion of the ions formed in the arc leave the inner region 33 through an annulus between arc discharge 31 and the outer bafiles 21.

An annular magnetic mirror coil 23 is disposed adjacent to and surrounding cathode 10 and baffies 21 in such a position that the face of cathode 10 is in a region of compressed magnetic field. An annular magnetic mirror coil 24 is disposed adjacent to and surrounding anode 11 and baffles 22 in such a position that the face of anode 11 is in a region of compressed magnetic field. The cathode 10 and anode 11 are positioned withthe axes thereof along the axis of magnetic symmetry. Thus the arc discharge 31 formed between cathode 10 and anode 11 is a symmetrical hollow are which follows the 'magnetic field lines, and the arc is enlarged in diameter midway between the electrodes due to the bowing out of the magnetic field lines in this region.

Chamber 33 is connected to a vacuum pump, not shown, through a connection 5 and opening 8 therein. Chamber 32 is connected to a vacuum pump, not shown, through a connection 3 and opening 6 therein. Chamber 34 is also connected to a vacuum pump, not shown, through a connection 4 and opening 7 therein.

Cathode 10 is electrically connected through tubular member 16 and lead 27 to one side of a source of a variable D.C. voltage 25. The other side of source 25 is connected by a lead 26, and tubular member 19'to anode 11. Source 25 and gas from source 37 are used for initiating and helping to sustain the arc discharge between cathode 10 and anode 11. This source 25 and/or any other conventional means may be employed for striking the arc discharge as described in my 'co-pending application Serial No. 728,754, filed April 15, 1958. Such other means include heating the cathode and anode electrodes to outgas said electrodes, providing a movable auxiliary electrode adjacent to the cathode and applying a RF. voltage across said auxiliary electrodeand cathode until an arc is struck and then removing the auxiliary electrode, or by making the position of the anode adjustable with respect to the cathode until an arc is struck and then varying the relative position of the anode and cathode to a desired operating position.

An energetic, high current arc discharge in a confining magnetic field is an efficient mechanism for dissociating and/or ionizing a molecular ion beam to form a magnetically trapped plasma of energetic ions and electrons as set forth in my aforementioned co-pendingapplication Serial No. 738,242, now Patent No. 2,920,234, issued January 5, 1960. That application, however, does not disclose the use of a hollow arc discharge for confining the thus formed plasma within the hollow region of a hollow arc discharge. The hollow discharge in the present invention in conjunction with the magnetic field is used as a shield or' barrier for the low energy neutral particles, formed outside the discharge, from entering the plasma region, and thus to more readily effect burnout of the neutral particles in the plasma region within the discharge. Any such low energy neutral particle,whatever its origin, is ionized in the discharge upon contact therewith and as such is pumped along the discharge and carried out of the reaction zone of the device.

One typical wayof igniting a high energy plasma in the device set forth above, is to inject a beam 30 of molecular ions from a source 28 and through an accelerator tube 29 into the path of the discharge 31 at near grazing angle to the inside wall of the discharge, where a portion of the molecular ions are dissociated and/or ionized to form a magnetically trapped ionized plasma of atomic ions and electrons in a manner as set forth in my aforementioned co-pending application Serial No. 728,754. The accelerator tube 29, referred to above, is energized by a suitable high voltage generator. A suitable high current source of molecular ions from source 28, may be provided by apparatus such as set forth on page 18 of Nucleonics, vol. 9 (3), 1951; Review of Scientific Instruments, vol. 24, p. 394, 1953, for example; or that described by Von Ardenne, Tabellen der Elektronephysik, Ionenphysik, and Ubermikroskopie, VEB Deutscher Verlag der Wiscenschaften, Berlin 1956 (Duo- Plasmatron), or by means such as disclosed in my copending application Serial No. 833,895, filed August 14, 1959, now Patent No. 2,933,630, issued April 19, 1960. Accelerator tube 29 is fitted in a member 9 in outside wall 1, and extends into the interior of chamber 33 and fitted within liner 2, as shown. The molecular ions from source 28 may be D or D for example, and have an accelerated energy of about 600 kev., for example. The magnetically trapped atomic ions will then have an energy of about 300 kev. if D is used, or 200 kev. if D is used.

In one embodiment of the device, described above, the cathode 10 was about 4.5 inches in diameter and had a wall thickness of about 0.062 inch. The annulus between the cathode 10 and the inner baflle 12 was about 0.25 inch, and the annulus between the outer battles 21 and the discharge 31 and between the outer baflles 22 and the discharge 31 was about 0.25 to 0.30 inch. The spacing between the anode 11 and cathode 10 was about 42 inches. Arcs obtained with this geometry have been about 5.7 inches in diameter at the mid-plane between the magnetic mirrors provided by coils 23 and 24. The current in such arcs ranges between 3500 and 4000 amperes, with a voltage drop between the electrodes of about 160 volts. 5X10"- mm. Hg, and the vacuum in chambers 32 and 34 was about 5 10 mm. Hg.

The coils 23 and 24 each provide a magnetic mirror field, the value of which is as high as can be obtained, say 3000 to 10,000 gauss.

In operation of the apparatus having the parameters set forth above, a hollow carbon arc discharge 31 is initiated between cathode 10 and anode 11 by feeding gas through openings 39 in baffle 12 and byapplying any of the arc initiating assisting means set forth above. After the arc is struck, the potential source 25 is adjusted to a desired operating level, which will provide a voltage drop between the electrodes of about 160 volts. The pressure in chamber 33 is adjusted to a vacuum pressure of about 5 l0 mm. Hg, and the pressures in chambers 32 and 34 are adjusted to a pressure of about 5 10-- mm. Hg. The coils 23 and 24 are energized to provide an average field strength of about 3500 gauss. The discharge 31 is sustained partially by gas particles in the discharge and partly by carbon ions produced as a result of electron bombardment of the anode and/or other ion-forming means. Vaporized carbon particles from the cathode absorb gas and thus produce a high pumping capacity for neutral particles ionized in the discharge. The are current was about 3500 amperes for the above operating parameters. Very stable arcs are obtained when the power input is about 35 kw., or greater, per circumferential inch of the are at the cathode.

Due to the severe erosion of the cathode, it may be desirable to substitute a composite cathode, e.g., a tungsten-graphite electrode for the carbon cathode. Ions for the arc would be supplied, as before, primarily from the The vacuum in chamber 33 was about,

obtainable pressure.

It has been determined that by regulating the rate of gas feed to the discharge through the openings 39 in baffle 12, it is possible to provide a generous quantityof gas particles to the discharge without having to provide a large pumping system for removing excess gas particles since they are, to a large extent, removed by being absorbed by carbon pieces and particles released from the electrodes during arc operation. It is estimated that the number of gas particles may be varied from 50 to 75 percent of the total number of gas particles and carbon particles present during arc operation, while the number of carbon particles would correspondingly vary from 50 to 25 percent of said total. These percentages may readily be determined by spectrographic analysis, and the ratio of gas to carbon particles may be controlled by regulating the feed rate of the gas fed to the discharge. Thus, it can be seen that gas to establish and sustain the discharge may be fed to the discharge in a generous quantity and the benefits of the presence of such gas in the system, as discussed above, may be realized without the use of a large pumping system for removing excess gas particles, because of the gas absorbing properties of the carbon particles released from the electrodes.

In one way of igniting a highly energetic plasma in the above device, a beam 30 of energetic molecular ions, D or 133 for example, is injected from a source 28 and through accelerator tube 29, with an energy of about 600 kev., into the inside wall region of the hollow arc discharge 31, where a portion of the molecular ions are dissociated and/or ionized into atomic ions, electrons, and neutrals. The atomic ions thus formed are magnetically trapped by the magnetic field, provided by coils 23 and 24, in a manner as set forth in my aforementioned co-pending application Serial No. 728,754. The highly energetic plasma thus formed may be increased in density to a degree sufficient to thereby produce a thermothus greatly reducing the charge-exchange process within the plasma region. Losses are further reduced by reducing scattering from the discharge as discussed above, and by maintaining the pressure'in chamber 33 at the lowest Any low-energy neutral particles that strike the discharge 31 are immediately ionized by the discharge and pumped out of the system along the discharge as discussed above. As set forth above, a carbon arc discharge is a very eflicient ion pump.

Where the anode is moved outside the mirror, the arc spacing may be 73", for example, with the same other arc parameters above recited.

Thus, it can be seen that I have provided an improved arc discharge which is useful as a dissociating and/or ionizing mechanism for forming-a thermonuclear plasma. The substantial reduction of scattering and. the charge exchange process by use of the hollow carbon, gas-fed, arc discharge, as discussed above, will permit a more rapid formation of a thermonuclear plasma, since burm out of the low energy neutral particles in the system may be accomplished in a shorter time.

The use of the carbon arc to ignite an energetic plasma of atomic ions and electrons has certain applications in the particle accelerator art also. For example, the breakup of D ions and trapping of protons froma point from the point of dissociation may be utilized as a method of injecting protons into a proton synchrotron.

When used for dissociation of molecular ions, the high current are can also be 'used for producing large well defined neutral beams. Since the cross section for dissociation decreases slowly as the energy increases, this process can be used at any desired energy. These neutral particles can be injected into accelerators and then converted into ions.

It has been determined that for the above operating conditions, there is at least a 25 percent breakup of the molecular ions into atomic ions. This percentage increases linearly as the arc current is increased.

This invention has been described by way of illustration rather than limitation, and it should be apparent that the invention is equally applicable in fields other than those described.

What is claimed is:

1. A device for producing an energetic, hollow, carbon arc discharge, comprising a container, said container being provided with a central chamber and two end chambers; an elongated, hollow carbon cathode provided with an annular face and mounted in one of said end chambers and extending to within said central chamber; a cup-shaped baffie mounted within, spaced from, and extending beyond the face of said cathode, said bafile being provided with a plurality of radial apertures positioned just beyond the face of said cathode; an elongated, hollow carbon anode provided with an annular face and mounted in the other end chamber and extending to Within said central chamber, the face of said anode being in confronting relation tothe face of said cathode and being widely spaced therefrom, said cathode and said anode having a common axis; a sleeve-like bafile closely fitted within said anode and extending beyond the face 'of said anode; a first plurality of annular bafiles disposed just beyond the face of said cathode and being affixed to said central chamber; a second plurality of annular baffles disposed just beyond the face of said anode and being affixed to said central chamber; a first annular magnetic mirror coil disposed about saidcathode and said first plurality of annular baffles to provide a first, constricted magnetic field having field lines with an axis in alignment with said common axis; a second annular magnetic mirror coil disposed about said anode and said second plurality of annular baffles to provide a second constricted magnetic field having field lines with an axis in alignment with said common axis; a source of feed gas; means connected between said source and the interior of said cup-shaped bafile for feeding gas thereto and then through said apertures at a controlled rate; means connected to each of said chambers for evacuating said chambers to selected pressures; and means connected between said anode and said cathode for initiating and sustaining a hollow, gas-fed, arc discharge therebetween which follows the magnetic field lines, said discharge being sustained partly by carbon particles released from said anode and cathode and partly by gas particles fed to said discharge through said apertures when said discharge is operating, the ratio of gas particles to carbon particles being a function of the rate of gas flow to said discharge, said extensions of said bafiies disposed within said anode and said cathode preventing said carbon particles from entering the interior region of said discharge, each of said first plurality of annular baffles defining an annular space between each of said first baffles and said discharge, the discharge after being formed serving to pump the ions formed in the discharge out of the central chamber into said one end chamber through said annular spaces.

2. The device set forth in claim 1, wherein the magnetic field of each of said mirror coils is maintained at an average flux density of about 3500' gauss, the pressure in said central chamber is established at a value of about 1O* mm. Hg, the pressure in each of said end chamhers is established at a value of about 5 10 mm. Hg, said means for initiating and sustaining said discharge including a source of voltage at an operating value of about 160 volts to produce an arc discharge current of about 3500 amperes.

3. The device set forth in claim 1, and further including a source of molecular ions; and means connected to said ion source for accelerating and injecting a beam of said ions with a selected energy and density into the inside wall region of said discharge where a portion of said molecular ions are dissociated and ionized to form a highly energetic plasma of magnetically trapped electrons and atomic ions within the region bounded by said hollow discharge, whereby the charge exchange between cold neutral particles and the ions of said plasma is greatly reduced by the natural barrier of said discharge to cold neutral particles formed outside said discharge, in that said neutral particles are ionized by said discharge and pumped out of said central chamber by said discharge through said annular spaces between said first lurality of annular baflles and said discharge.

4. A device for producing an energetic, hollow, carbon arc discharge and for producing and shielding a plasma, comprising a container; a pair of widely spaced, hollow, carbon electrodes mounted on a common axis and within said container, each of said electrodes being provided with an annular face, said annular faces being in confronting relation within said container; a first annular electromagnetic mirror coil disposed around one of said electrodes; a second annular electromagnetic mirror coil disposed around the other of said electrodes, said mirror coils providing magnetic field lines which are oriented in a direction parallel to said common axis and which are constricted about said electrodes; evacuating means connected to said container for establishing a selected pressure within said container; a source of gas; means connected to said source of gas for feeding gas at a controlled rate to the annular face of one of said electrodes; means connected across said electrodes for initiating and sustaining a gas-fed, hollow, carbon are discharge therebetween, said discharge being sustained partly by carbon particles released from said electrodes and partly by gas particles fed to said discharge during operation of said discharge, the ratio of gas'particles to carbon particles being a function of the rate of gas flow to said discharge, said discharge following said 'magnetic field lines; means disposed within and extending beyond the annular faces of said electrodes for preventing said carbon particles from entering the interior region of said hollow discharge; a source of molecular ions; and means connected to said ion source for accelerating and injecting a beam of said ions into the inside wall region of said discharge, where a portion of said ions are dissociated and ionized to form a highly energetic plasma of magnetically trapped electrons and atomic ions within said interior region, thereby isolating said plasma from cold neutral particles formed outside the boundary of said discharge, in that when said neutral particles contact said discharge they are ionized thereby and pumped by said discharge away from said plasma, such that said plasma may acquire sufiicient density to produce thermonuclear neutrons.

5. The device set forth in claim 4, wherein said coils provide a magnetic field having an average flux density of about 3500 gauss, the pressure in said container is established at about 5X10 mm. Hg, said means for initiating and sustaining said discharge including a source of voltage at an operating value of about volts to produce an arc discharge current of about 3500 amperes; said molecular beam comprising D ions, and the energy of said molecular beam is maintained at about 600 kev.

References Cited in the file of this patent UNITED STATES PATENTS 2,675,470 Wideroe Apr. 13, 1954 2,719,240 Walker Sept. 27, 1955 2,855,537 Mendel Oct. 7, 1958 2,919,370 Giannini Dec. 29, 1959

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