US2920234A - Device and method for producing a high intensity arc discharge - Google Patents

Description

Jan. 5, 1960 J LUCE 2,920,234

DEVICE AND METHOD FOR PRODUCING A HIGH INTENSITY ARC DISCHARGE Filed May 27, 1958 2 Sheets-Sheet 1 I \T L 009 R. F. VOLTAGE SOURCE VACUUM CHAMBER Fig.

Q I INVENTOR.

m BY John S. Luce ATTORNEY J. S. LUCE DEVICE AND METHOD FOR PRODUCING A HIGH INTENSITY ARC DISCHARGE Jan. 5, 1960 2 Sheets-Sheet 2 Filed May 27, 1958 INVENTOR. John S. Luce ATTORNEY rates DEVICE AND mrnon FOR PRODUCING A mes INTENSITY ARC DISCHARGE Application May 27, 195s, Serial No. 738,242

4 Claims. (Cl. 315-111 Ihis invention relates to a device for providing an energetic direct current discharge between widelyspaced electrodes in a magnetic field under high. vacuum, and more especially to means for establishing and "maintaining a novel electrical discharge which has certain characteristics distinguishing it from a normal high-current arc. Establishment of such discharges is of fundamental importance in studies of electrical discharge phenomena, and has found further usefulness specifically in dissociation and ionization of high energy (20 to 600 kev.) molecular ion beams. In most vacuum arcs, a tungsten or tantalum filament is heated to emission temperature by passing a large current through it. Electrons are accelerated from the 'filament towards the anode by a potential gradient which "'is applied between the cathode and a defining electrode intermediate the anode and cathode. This electrode de 'fines the shape of the are as desired. Approximately half the electrons pass through the defining slot into the 'arc chamber, and an arc is formed. The are may be stabilized by a magnetic field oriented parallel to'the am. At low arc currents approximately half the electrons pass through the defining slot and half drain to it directly. If the arc current is raised, a critical point is reached when a direct short exists between the filament and defining electrode, and only very few electrons pass through. Other arc geometries have been developed that .donot use defining electrodes, but shorts always appear some place to limit the usable current. The limit for stable operation in the conventional arc is about 100 amperes. Moreover arcs could not heretofore be established and operated in intense magnetic fields of about 3000 gausses, for example, at extremely low pressures such as 2. mm. Hg.

With a knowledge of the difficulties encountered by prior workers in attempting to achieve useful arc currents above 100 amperes, applicant has as a primary object of this invention to provide means for establishing a high intensity direct-current arc, and more particularly a carbon arc'discharge which is to be maintained in a magnetic field under high vacuum between widely-spaced electrodes. I

It is a further object of this invention to provide an improved apparatus andv method of establishing a carbon arc discharge without using a defining electrode and thus overcoming the shorting problem between the filament and defining electrode that existed in prior art devices. Other objects and advantages of the invention will be apparent from a consideration of the following detailed [specifications and. the accompanying drawings, wherein:

Fig. 1 is a sectional drawing of one embodiment of components used to provide a discharge, and

v Fig. 2 is a schematic diagram of a use for the arc discharge 'for ionization or dissociation in a thermonuclear device;

' The invention described herein has overcome the shorting problem aforementioned so that stable direct-current latented Jan. 5, 191

vacuum arcs have been operated in the heretofore imcant 'diflrerence between this device and prior arc souri is that the cathode is'heated, not by an external curre but bybombardment of positive ions from the arc its:

In the embodiment of Fig. 1, a hole is bored throu the center of the cathode, and gas is fed directly to 1 arc. This method of feeding the discharge allows t pressure to be relatively high in the discharge its without appreciably increasing the pressure inthe volun thus avoiding shorts due to high pressure. These a; have been-operated with nitrogen, argon, hydrogen, 2 and several vaporized solids such -as carbon, tungsn etc. Many different materials can be used for cathor and anodes, for example, copper, steel, carbon, tantalu tungsten, tungsten carbide, etc. In some cases (carb for example) it has been found that the flow of gas c be stopped, and the discharge will maintain itself vaporization 'of the electrodes.

A discharge characterized by high average electr energy is desired. In normal gas arcs, or discharges, sheath is formed at the end of the are adjacent and par lel to the face of the cathode. In such an arc, the me free path of the electrons is very short as a result the pressure in the arc and the resultant multiple cof sions. Thus, essentially all of the voltage drop of t are occurs across the gap between this sheath and t cathode. The loss of energy, as a result of the ma collisions," results in a low average energy of these ell trons. The electrons that reach the anode are genera non-energetic since they are mostly secondary electrol However, this is not true of the carbon discharge to described. In the carbon arc a sheath is formed betwe the cathode and the arc, and a potential gradientis-ma: tained between the sheath and cathode. The high gradient appears on the outer rim of the cathode whe the intensity of the arc is greatest and the gradientis it at the central portion of the cathode face where the tensity of the arc is not as great. Electrons are acce rated away from the cathode by the aforementioned I tential gradient, and positive ions are accelerated frc the arc to the cathode. In the carbon arc, most of t ions originate at the anode. A large part of the, 1; tential drop occurs along the arc and many energe electrons reach the anode because the arc pressure relatively low, and few collisions occur. Thus, -t average energy of the electrons in the discharge is hi and the mean free path is long. In addition, the avera energy of the ions is higher than usual as can be 8110 by spectrographic analysis, thus further, indicating .tl the potential energy distributions are substantiallyd ferent than those of conventional arcs.

Studies have been made of the carbon discharge determine to what extent it is ionized. For examp spectrographic measurements have been made of d charges of 170, 270 and 425 amperes. A quartz windc was used to permit the study of carbon resonance lin at 2478 A. Reasonably strong spectra of C+ and C were observed; the ratio of C++ to C+ increased wi increased current. No trace of the resonance line neutral carbon was observed, thus indicating that t discharges were more than 99% ionized. -Neutral argt gas was then introduced into the discharge at. both t cathode and anode ends, and spectrographic measui ments made. The pressure was varied from 2X 10 mm. I-Igfto 5 10 mm. Hg. Essentially comple ionizationwas observed as only A+, A++, A+++ and 1 A. lines were detected. The introduction of theg substantially reduced the number of C++ ions detects These results indicate that the introduction of the neuti gas reduces the average energy of the particles in tl discharge.

Referringnowto Fig. 1, both a cathode. 1 and t ade 4 are supported in water cooled brackets 2 and respectively, these brackets being supplied with coolwater through conduits 24 and 25, respectively. The de 4 is provided with an insulated shield 8 mounted insulated bracket and the cathode 1 is provided b insulated shields 6 and 7 mounted on insulated .ckets 16 and 11, respectively. The hole in the cathode connected through a passageway 3 with a gas supply :t tube 23. The magnets 26 and 27 provide a strong gnetic field whose direction is indicated by the arrow The centers of the anode and cathode are aligned mg. the magnetic field, and the corresponding are disme, when struck, is'parallel to the magnetic field. :s have been run in magnetic fields in the range from to 10,000 g'ausses, for example.

the anode 4 is connected by wire 19 to one side of a rce, of R.F. voltage 13, such as used in a conventional ding system. The other side of source 13 is connected awire 18, switch 12 and wire 17 to cathode 1. The vde-cathode path is also electrically energized through e 20, switch 14, wire 21, adjustable source of potential battery 15, and wire 22. The source 15 is used both 1elp initiate an arc discharge between the cathode and dc and also to selectively vary the intensity of the discharge. The are discharge is shown by the broken :s between the cathode and anode. The anode, cath-' anode shield, cathode shields, and brackets 2 and 5 mounted within a vacuum chamber 9 which is evacul by conventional vacuum pumps, not shown.

it operation of Fig. 1, when the anode and cathode :trodes are made of carbon, a carbon discharge may initiated by applying substantially 250 volts DC. and R.F. voltage of substantially the same magnitude to electrodes, and admitting gas to the cathode passage. size of the hole in the cathode is 34 inch, the gas to the cathode is argon, for example, the pressure mm chamber 9 is maintained below 1 micron, and the gnetlefield H ismaintained at approximately 3000 sses. The RF. arc appears first followed by the main :harge'. After the arc is struck, the gas feed and voltage are discontinued. The discharge is sused by carbon ions produced as a result of electron rbardment of the anode and/or other ion-forming ms. The intensity of the discharge may then be varied varying the voltage source 15 between the electrodes. i are is stable at a pressure of 1 micron and at all :sures obtainable below 1 micron. When the arc is ck m a magnetic field of 3000 gausses, in a vacuum 3 l0- mm. Hg, with an arc length of 12 inches, a cathode diameter of A inch, the said are will have lameter of 36 inch and the voltage across the arc 11d bemaintained at approximately 55 volts to pro- :-an arc current of 300 amperes. When the arc th is increased to 5% feet to 6 feet, a voltage of at t 125 volts is required across the arc to produce an current of 300 amperes with the other parameters g the sameas for the 12 inch arc. It can be seen the arc voltage is proportional to the arc length in r to provide an arc current of a predetermined unt. For any configuration there is a critical curbelow which the arc will not continue to run. For pinch arc, this current has been made as low as 50 eres for a few seconds.

he type of discharge formed by the device of Fig. 1 )t a true are in that no stable, homogeneous strucexists. Rather, the discharge constantly undergoes ientation, each section appearing as a filament of arge. The filamentsare in constant motion and lly move across the magnetic field. The life of each tent is limited and new ones are constantly formed. number of filaments'vary in direct proportion to amperage of the discharge. Stable segmented disgesof up' to2000 amperes, having a current density 5,000 amperes per.v square inch, are easily attained.

"additionto "the segmentation of the discharge, the

dynamic conditions thereof induce appreciable impacts and/or thermal gradients at the cathode which cause small charged carbon particles to be torn from the cathode and ejected at high speeds in a wide arc toward the anode. Vaporized carbon from both the cathode and anode absorb gas and thus produce a high pumping capacity for neutral particles. The arc will also act'as an ion pump it proper baffles are used. c I 1 4 There is shown in Fig. 2, one use for an energetic direct-current are. One method of growing a plasma of high energy ions in a thermonuclear machineis that of trapping atomic ions as a result of the dissociation of molecular ions within the machine. This dissociation may be accomplished in several different'ways, one of which is passing the molecular ion beam through an are, such as shown in Fig. 2. Such a method is the subject matter of my co-pending application Serial No. 728,754, entitled Method and Apparatus for Trapping Ions in a Magnetic Field, by John S. Luce, filed April 15,1958. 1

In the device described in that application, high-energy molecular ions are injected into a confining magnetic field perpendicular to the lines of magnetic force. At some point in the orbit of these ions in the magnetic fie1d,.a portion of them are caused to dissociate and/or ionize to form atomic ions. These resultant atomic ions have one-half the momentum of the original mo lecular ions and hence have one-halfpthe radius of curvature in the field. If'the center of the orbits, of these atomic ions coincides with the axis of the magnetic field, the ions will circulate in a ring. If the center of the orbits and the axis of the machine do not coincide, the atomic ion orbit will precess about the point-of origin of the atomic ion. The ions will circulate until a charge exchange'reaction occurs with one of the neutral gas atoms in the system.

The device of Fig- 2 comprises an outer cylindrical shell 39 with joining end walls 42 and '43. End'wall 42 has a circular opening to which a tubular member 46 is afiixed. Member 46 has an end-closure member 48 in which the cathode 30 is fixedly mounted. End wall 43 has a circular opening to which is 'aflixed a tubular member 47. Member 47 has an end'closure 49 in which the anode 31 is fixedly mounted. Outer shell 39 is provided with a circular opening to=which is attached a tubular member 50 which in turn has affixed thereto an end closure member 51. -Fixedly mounted in said member 51 is a tubular member '52 provided with a reduced portion which connectswith an aperture in liner 36. A conventional ion accelerator tube 58 communicates with member 52, and serves to accelerate molecular ions from an external ion source -53 to relatively high energies. The accelerator "tube may be energized by a conventional high voltagegenerator. A suitable high current source of molecular ions may be provided by apparatus such as set forth on page 18 of Nucleonics, vol. 9 (3), 1951; Rev. Sci. Instr., vol. 24, p. 394, 1953, for example, or that described by Von Ardenne, Tabellen der Elektronenphysik, Ionenphysik and Ubermikroskopie," VEB Deutscher Verlog der Wissenschaften, Berlin, 1956 (Duo-Plasmatron). The molecular ions then pass through the aperture in liner 36 and into the path of an arc formedbetween the cathode and anode electrodes 30, 31. This are is a high intensity direct current are such as produced by the device of Fig. 1. .The are formed between said electrodes is substantially enclosed in an inner chamber formed by liner 36 and end walls 56, 57. The walls 56 and 57 have circular openings in axial alignment with the anode and cathode. Surrounding the cathode 30 and anode 31 are suitable tubular baffles 32 and 33, respectively, which extend through the openings in said walls 56 and 57, respectively. Liner 36 has a pair ofcircular'opcnings inlalignment with a pair of circular openings in' outer shell 39. The aligned openings are joined by insulated bushings 54 and 55, respectively. The inner chamber, formed by liner 36 and walls 56 and 57,- is connected to a vacuum through openings 44 and 45 of bushings 54 and 55, respectively. Outer liner 39 also has a pair of additional openings 40 and 41 connected to a vacuum, said openings being connected to an outer chamber located between the shell 39 and inner chamber referred to above. A circular magnetic mirror coil 34 is mounted on apertured wall 37 and is disposed around the outside of inner liner 36 between the ion source tube 52 and bushing 54. Another circular magnetic mirror coil 35 is mounted on apertured wall 38 and is disposed around the outside of inner liner 36 between ion source tube 52 and bushing 55. These mirror coils provide a containing magnetic field whose direction is shown by the arrow H. The magnets 26 and 27 of Fig. 1 are not required when the are striking and maintaining means of Fig. l are used in the device of Fig. 2 since the mag-,

netic mirror coils 34 and 35 take the place of magnets 26 and 27.

In operation of the device set forth in Fig. 2, a high intensity are discharge is initiated between the cathode and anode electrodes by means such as set forth in Fig. l. The inside and outside chambers are evacuated and the pressure of the inside chamber is maintained at approximately mm. Hg, while the outside chamber pressure is maintained at approximately 10- mm. Hg, for example. The mirror coils 34 and 35 have an inside diameter of 17 inches and a spacing between the inner faces of the coils of 18 /2 inches. With these dimensions, a cylinder can be inscribed whose rims just touch the inner edge of the coils and the volume of such a cylinder is then equal to 6.9)(10 cm.. The plasma which is ignited by dissociation of the high energy molecular ions as they pass through the high intensity arc is confined and trapped within the said inscribed volume by the magnetic field H. The gas used for the ion source input is deuterium, for example, and the injection voltage of the molecular ions D is approximately 600 kev., for example, which results in atomic ions D+ of substantially 300 kev. energy. The resultant ring of atomic ions is at least 3 inches wide with a radius of 5.3 inches and a circumference of 33.3 inches. The magnetic field strength at the center of said ring of atomicions is 6880 gausses and at the outer edge of the ring the field strength is 6000 gausses. The current supplied to the magnetic mirror coils is 4250 amperes.

It has been determined that for an arc voltage of 150 volts and an arc current of 300 amperes, there is a 25% breakup of the molecular ions into atomic ions. This percentage increases linearly as the arc current is.increased. It has also been determined that dissociation is inversely proportional to the distance from the anode that the 600 kev. molecular ions are injected into the device.

The use of the carbon arc to create circulating ion beams has certain applications in the particle accelerator art also. For example, the breakup of D ions and trapping of protons from a point 180 from the point of dissociation may be utilized as a method of injecting protons into a proton synchrotron. Moreover,

it has been demonstrated that fifty percent of the power is dissipated in the form of radiation of l ultraviolet light. The arc is a very powerful ioni: agent and reduces charge exchange between trap energetic atomic ions and low energy neutrals by io ing many of the neutrals in the volume. Studies of vacuum properties of the arc indicate not only tha will be useful as an ion pump, but also that the de ited carbon absorbs gases and thereby provides a tional pumping action. This action is strong eno to make it possible to valve off the diffusion put of a facility in which the arc is operating. Probe mt urements and arc current studies have shown that operation of the arc is accompanied by considerz amounts of R.F. oscillation with frequencies at 11 up to tens of megacycles. When used for dissociat of molecular ions the high energy are can also be u for producing large well defined neutral beams. Si the cross section for dissociation decreases slowly the energy increases, this process can be used at desired energy. These neutral particles could be injec into accelerators and then converted into ions.

This invention has been described by way of illus tion rather than limitation, and it should be appai that the invention is equally applicable in fields ot than those described.

What is claimed is:

1. A device for establishing a high-intensity din current carbon are having a current in excess of amperes which comprises a high vacuum enclost Widely spaced carbon anode and cathode electro mounted within said enclosure, means for establish a magnetic field within said enclosure, said magnl field having a direction parallel to the axis of the 1 charge, means for temporarily assisting in the ini tion of an undefined arc discharge directly between s electrodes, and a variable voltage source connected tween said electrodes for also assisting in the initiat and for varying the intensity of said are discharge, s discharge being sustained by the variable voltage sou and the carbon ions and electrons which are relea from the electrodes.

2. A device as set forth in claim 1 in which the me; for assisting in the initiation of an arc includes a son of gas which is fed to the face of the cathode, anc source of R.F. voltage connected between the electro until an arc is struck.

3. A device as set forth in, claim 2 in which the is argon and is fed through a conduit within the catho 4. A device as set forth in claim 3 in which the v uum in the enclosure is maintained at approximat 10" mm. Hg, the spacing between the electrodes is least 6 feet, and the strength of the magnetic field the center of the enclosure is maintained at a selec value in the range from 500 to 10,000 gausses.

References Cited in the file of this patent UNITED STATES PATENTS 2,009,555 I Mathiesen July 30, 19 2,294,498 Heindlhofer Sept. 1, 19 2,728,877 Fischer Dec. 27, 19 2,819,427 Noskowicz Jan. 7, 19 2,826,709 Von Ardenne Mar. 11, 19

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