US3453474A - Plasma arc electrodes - Google Patents

Description

y 1, 1969 G. L. CANN ET AL 3,453,474

PLASMA ARC ELECTRODES Filed April 27. 1966 Sheet of 4 INVENTORS. GORDON L. CANN ROBERT HARDER We M ATTORNEYS July 1, 1969 CANN ET AL 3,453,474

PLASMA ARC ELECTRODES Filed April 27. 1966 Sheet 4? of 4 coaoord ly c rfi u F/G. 2 BY ROBERT LIHARDER 81 a .muzm A T TORNE VS July 1, 1969 5. 1.. CANN ET AL 3,453,474

PLASMA ARC ELECTRODES Filed April 27, 1966 Sheet 3 of 4 o o o o o o o o o o o o o 0 cf 0 o o o OOOOOOOOOOOOOOOOOOO o o o o 57 o o o o o o o o o o o o o o o o o o asasm r wvf MM??? 2 INVENTORS. GORDON L.CANN O O O O O O OOOOOOOOOOO O O O O O O O m ii ATTORNEYS July 1, 1969 CANN ETAL PLASMA ARC ELECTRODES Sheet Filed April 27. 1966 INVENTORS. GORDON L. CANN mvN oww EN [2w (a l T J L T T mm mm www www ROBERT L. ARDER agg ATTORNEYS United States Patent 3,453,474 PLASMA ARC ELECTRODES Gordon L. Cann, Laguna Beach, and Robert L. Harder, Altadena, Califi, assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Continuation-impart of application Ser. No. 458,837, May 20, 1965. This application Apr. 27, 1966, Ser.

Int. Cl. H013 1/50 U.S. Cl. 313-161 3 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part of our copending application bearing Ser. No. 458,837, filed May 20, 1965, and entitled Plasma Arc Electrodes.

This application relates to improved plasma arc electrode assemblies.

Workers in the plasma and other arts have employed electrode assemblies comprising a tapered or pointed cathode electrode surrounded by a concentric annular anode electrode, together with means to introduce a suitable feed gas into the arc region. Such electrode assemblies are useful in connection with plasma torches, plasma containment devices, plasma propulsion devices, and the like. An electrode assembly of this general description is described in copending application Ser. No. 457,414 of the present joint applicant, Gordon L. Cann, filed on May 20, 1965. Difiiculties have been experienced with this type of electrode structure in attempting to increase the size and to extend operation to voltages much in excess of 100 volts or currents much in excess of several hundred amperes. The two major difiiculties encountered are an unstable and destructive cathode arc attachment and excessive erosion of the anode by high energy electrons.

It is accordingly an object of the present invention to provide an arc electrode assembly which is adapted to stable ope-ration at higher powers and with larger dimensions than other devices. This object is achieved in part through the use of buffer electrodes adjacent the cathode and anode.

It is often desired to operate at the highest possible are voltage. Increases in size, current, and magnetic field tend to increase arc voltage, but not very much.

An additional object is therefore the provision of means and methods to increase arc voltage.

A further object of the present invention is to provide an arc electrode assembly adapted to more completely ionize gaseous material fed thereto.

A still further object of the invention is to provide an arc electrode assembly in Which a substantial portion of the arc path is in a region remote from heat absorbing materials and substantially free from unionized gas.

A yet further object of the present invention is to provide an arc electrode assembly adapted to reduce backflow of high energy plasma particles into the electrode structure.

Additional objects, features, and advantages will become apparent on reading the more detailed description which follows.

3,453,474 Patented July 1, 1969 FIG. 1 is a longitudinal cross section of an arc electrode assembly according to the invention;

FIG. 2 is a simplified cross section of a modified electrode assembly;

FIG. 3 is a schematic and simplified cross section of a further modified electrode assembly according to the present invention;

FIG. 4 depicts a modification of the FIG. 3 apparatus; and

FIG. 5 is a schematic cross sectional view of an apparatus in which the electrode assemblies of the invention may be used.

No attempt will be made to describe or number each and every mechanical element which appears in the drawings, as such elements will be obvious to any skilled mechanic or machinist.

Referring to FIG. 1, cathode 10 is a tapered or pointed piece of tungsten and, like all other electrodes in the figure, will normally be axially symmetric. It is mounted in a metal heat sink 14 which, in turn, is mounted at the end of a cathode support and cooling water conduit 21 which is sealed into phenolic support block 18 by seal rings 20. A cathode cooling water inlet 22 is shown at the back of conduit 21. A cathode cooling water outlet will communicate With a cavity 24 in support block 18 that is out of the plane of the drawing and not shown. It will be understood that various other cooling water passages will not appear in the drawing for the same reason. A boron nitrile insulator 26 surrounds cathode 10 but leaves the tip portion exposed. A concentric cathode buffer electrode 28 surrounds cathode 10 and is supported with respect to the cathode and insulated therefrom by insulator 26. As shown, the cathode buffer 28 is tapered internally, and defines a chamber 40 surrounding the tip portion of the cathode and Which is substantially enclosed except for an aperture 30 in the cathode buffer which is coaxial with cathode 10 and positioned slightly in front of the cathode tip. Illustratively, the diameter of this aperture may be 0.1 inch.

Cathode 10 and its heat sink 14 are bored to receive the tubular pressure tap 32 located within the cathode water conduit 21. Cathode 10 contains one or more small channels or passages 34 which connect the pressure tap to the exterior surface of the cathode forward of the cathode insulator 26. The cathode insulator 26 is also provided with a gas passage, or preferably a plurality of circumferentially disposed passages 36 which communicate with the front face of the cathode insulator and are connected to a feed tap 38 in support block 18. Either passages 34 or passages 36 may be used to introduce a fluid to the space adjacent to the cathode, but it is generally preferable to introduce the fluid through passages 36 and to use passages 34 for measuring the pressure adjacent cathode 12.

Tungsten cathode buffer electrode 28 is attached to and is in thermal contact with a hollow heat sink and cooling assembly 50 which is connected to an electrically conductive water inlet tube 54, which can also serve as an electrical connection to the cathode buffer. The corresponding water outlet is not shown.

Cathode buffer electrode 28 is surrounded by a boron nitride insulator 56 and the cathode buffer heat sink 50 is surrounded by a more conventional insulator 58 Which is an extension of the boron nitride insulator 56. Insulators 56 and 58 serve to insulate and support a tungsten anode buffer electrode 60, which is concentrically located about the cathode and cathode buffer, and an anode buffer heat sink and cooling assembly 62 which is fixed thereto. A Water cooled copper anode assembly 70 is mounted on the outside of support block 18 and is electrically insulated from heat sink 50 and 62 by insulators 72 and 7-4. Anode 70 has at its forward end a cylindrical inner surface 76 which illustratively may be 2 inches in diameter and is separated by a small annular space from a cylindrical outer surface of anode buffer 60. Illustratively, the forward surfaces of the cathode buffer 28, insulator 52, anode electrode 60, and anode 70 may lie in a common place as shown. Insulator 74 has a plurality of circumferentially disposed and axially oriented passages 80 which communicate with the annular space 78 defined by anode 70 and anode buffer electrode 60 and which also communicate with an anode gas feed tap 82. There is also provided a radial passage 84 in anode 70 which opens into the annular space 78 and communicates with an anode pressure tap 86.

In most instances it is desirable to operate the electrode assembly in the presence of a magnetic field and accordingly a magnet coil 90 is shown which is insulated from the anode 70 by an insulator 88 which surrounds the anode and also covers the front face thereof. Insulator 86 also prevents arc attachment to the face of the anode, which would cause very rapid erosion. A conventional power supply 92 may be used to operate magnet 90. A water cooling assembly 94 is positioned so as to cool the forward portion of the magnet and also the face of the anode, each of which is likely to be exposed to high temperatures in the operation of the device. A suitable DC power supply 100 and switch 102 are connected between cathode cooling conduit 21 and cathode buffer cooling conduit 54 and a similar DC power supply 104 and switch 106 are connected between cathode cooling conduit 16 and anode assembly 70'.

In operation, the electrode assembly is preferably placed in an evacuated space or chamber, not illustrated. Magnet supply 92 is turned on if a magnetic field is desired and cooling water is supplied under pressure to the various cooling passages. Cathode pressure tap 32 and anode pressure tap 86 are either sealed or connected to pressure gauges, and a feed gas such as hydrogen is introduced through feed tap 38. Power supply 100 is energized and switch 102 is closed in order to start an are between cathode 12 and cathode buffer electrode 28-. After this are is established, feed gas may optionally be introduced through feed tap 38 or inlet tube 82 and power supply 104 is energized and switch 106 closed to draw the are from cathode 12 to anode 70. Thereafter, switch 102 may be opened and power supply 100 may be deenergized. As is known in the art, the power supplies may be adapted to provide higher than normal operating voltages in order to initially strike the arc.

Generally speaking, in the absence of a magnetic field the arc will propagate in a reasonably straight line from the cathode to a localized spot on the anode where destructive erosion will take place. Application of a magnetic field causes the arc attachment to rotate about the anode or extend entirely around the anode and also causes the arc to bend away from the electrodes. The higher the ambient pressure the greater is the magnetic field required for satisfactory operation. Operation may even be extended to atmospheric pressure it the magnetic field is increased to a value in excess of about 30,000 gauss. The magnetic field may also be provided by magnet means distinct from the cathode assembly.

Continual introduction of feed gas through feed tap 38 is desirable in order to maintain a nondestructive plasma forming arc and the introduction of gas through inlet tube of feed tap 82 may optionally be continued. Gas feed rates may typically vary in the range from about .01 to about .1 gram per second.

It is believed that a higher than ambient pressure is formed in cathode chamber 40 surrounding the cathode 12 and that operation of the cathode in this high pressure environment prevents unstable or destructive arc attachment and promotes a stable, dilfuse, nondestructive attachment of the arc to the cathode tip. The electrically floating character of the cathode and anode buffers causes the buffers to operate at an appropriate potential so that the arc extends directly from the cathode to the anode 4 and not to cathode buffer or anode buffer. At the same time, the electrically conductive and thermally conductive character of the cathode buffer facilitates starting the cathode arc and prevents erosion of the cathode buffer under normal conditions. Further, introduction of the feed gas through a confined space surrounding the cathode tip permits the gas to more effectively cool the cathode tip.

The are penetrates at least partly into annular space 78, which is a region of increased pressure, and terminates on the inner surface 76 of anode 70. Annular space 78 is a region of negligible axial electric field and the arc electrons accordingly lose much of their energy to the gas in space 78 before striking anode 70. This obviously minimizes erosion of the anode. The presence of anode buffer electrode 60 adjacent to anode 70 helps to carry away the heat imparted to the gas in space 78, even though the arc does not attach to the anode buffer. The cylindrical geometry of anode 70 and buffer 60 presents a large area for effective heat transfer. Any gas introduced at feed tap 82 will serve to increase the pressure in annular space 78, to flush the electronically heated gas out of the passage, and to provide additional generation of plasma. The cathode and anode buffers need not be insulated from each other and may even comprise a single piece of metal. The use of separate buffer electrodes is not essential, but serves to reduce the likelihood of an arc attaching to the buffer electrodes rather than passing directly between cathode and anode.

The arc electrode assembly of FIGURE 1 was tested in an axial magnetic field of 2000 gauss, using argon as the feed gas. At a steady arc current of 200 amperes the corresponding arc voltage was 150 volts. A conventional electrode assembly described more fully in the copending application 457,414 of Gordon L. Cann, previously alluded to, was operated under the same conditions for comparison but the arc voltage was only 40 volts instead of 150.

FIGURE 2 is a simplified sectional schematic view of a modified form of electrode assembly according to the invention. The cathode buffer electrode 28 and anode buffer electrode 60 of FIG. 1 has been supplemented by a series of alternating electrodes 120 and insulators 122. Subdivision of the cathode anode spacing into a multiplicity of gaps in series further reduces the possibility of inter-electrode arcing in high voltage operation.

A series of passages 124 extend outward from cathode chamber 40, passing through electrodes 120 and insulators 122. These passages do not lie in planes passing through the axis of the device, but are canted to give a tangential velocity to gas issuing therefrom in a direction consistent with the magnetic field. The portion of anode insulator 74 containing passages has been removed and an enlarged inlet tube 82 is connected to a vacuum pump 126. These modifications operate to increase the operating voltage of the device and therefore to increase the temperature of any plasma which is produced.

The operating voltage of a plasma electrode assembly of the type shown in FIGS. 1 or 2 can be given by KT I, w M, MR q In PIH 6 2 Q is the voltage,

T is the plasma temperature adjacent the electrodes,

P is the on-axis gas pressure adjacent the electrodes,

P is the ambient pressure,

a: is the angular rotational velocity of the plasma adjacent the electrodes,

B is the magnetic field,

R is the anode radius,

M, is the atomic (or ionic) mass of the plasma material.

where:

The first term represents essentially a temperature times a pressure gradient, the second term represents a counter EMF generated by the interaction of a plasma rotation and an axial magnetic field, and the third term represents a centrifugal force. At the vicinity of the electrodes, T is relatively low. Therefore, the voltage drop may be raised by increasing the value of to, which appears in the second and third terms of the voltage equation. Injection of gas from passages 124 can give rise to a linear gas velocity near the electrodes of about 2000 meters per second at a radius of 3 cm. At a magnetic field of 3000 gauss this gives rise to a counter EMF term of about 90 volts.

Vacuum pump 126 also helps raise the operating voltage by drawing plasma out of the space in front of the electrode assembly and through annular space 78. This plasma will ordinarily have a high rotational velocity due to the torque provided by the interaction of the radial arc current with the axial magnetic field. As this plasma is drawn out through space 78 it imparts, by viscous drag, some of its angular momentum to the gas immediately adjacent to the electrodes and further increases the rate of rotation of this gas and the counter EMF resulting therefrom.

Vacuum pump 126 has still a further beneficial effect on the operation of the device of FIG. 2. It is desirable to have a relatively high mass flow of gas through cathode chamber 40 and passages 124 in order to assist in cooling the electrodes. However, this gas must be somehow removed from the chamber containing the apparatus in order to permit continuous operation. Much less energy is expended in removing this gas at the relatively high pressure encountered at passage 78 than at the relatively lower pressure encountered in the chamber.

Further difl-lculties arise in attempting to increase the arc voltage to still higher levels, since the arc is formed adjacent to the electrode structure which holds the arc temperature down, and the arc is formed in a region where unionized material is being introduced in order to be ionized in the arc. Both factors tend to limit the maximum attainable arc voltage. The latter problem is particularly troublesome because the presence of unionized material in the arc region causes high energy arc electrons to undergo inelastic collisions with neutral atoms whereby the electron energy is dissipated in converting the atoms into ions. This continuing loss of high energy electrons tends to prevent the arc voltage from exceeding the ionization potential of the particular feed gas by a factor of more than about 6 to 8.

FIGURE 3 depicts in a highly schematic and simplified fashion a yet further embodiment of the present invention designed to overcome these difliculties. As in prior embodiments a central cathode 10 is present and is here surrounded by and recessed within a cathode buffer electrode 28 defining a confined ionizing space 53 extending forward of the tip of cathode 10. Cathode 10 is separated from buffer electrode 28 by insulator 51 which is provided with passages 36 for introduction of ionizable material. Buffer electrode 28 forward of the cathode tip is se mented by insulating rings 28a, [2, c, d, e, and j in order to permit a longitudinal electric ionizing field to extend into the ionizing space '53 and to prevent attachment of the cathode arc to the buffer. As in prior embodiments cooling for cathode and butter electrodes may be provided for but is not explicitly here shown. A much more detailed showing of a structure resembling the structure thus far described is set forth in our application entitled Plasma Arc Electrodes With Buffer Coil, filed approximately simultaneously with the instant application. Surrounding the central cathode 10 and cathode buffer electrode 28, but spaced therefrom is an annular anode 71 that is recessed within axially concentric electrically floating anode buffer electrodes 73 and 75 which define a confined annular ionizing space 83 extending forward of the edge of anode 71. The anode buffer electrodes 73 and 75, like the cathode buffer electrode 28, are segmented by insulating segments 73a through and 75a through i, and for the same reason as previously cited in connection with cathode buffer electrode 28. Between the annular anode 71 and the surrounding annular anode buffer electrodes 73 and 75, insulating rings 77 and 79 are provided. These insulating rings will be seen to include feed passages 81 for purposes of introducing ionizable material into the anode ionizing space 83 just forward of the anode proper.

The various electrodes and segments may lie in planes approximately transverse to the plane of the drawing and the cathode and anode may be at roughly the same lon gitudinal position, as shown. Positioned about the cathode buffer electrode 28 and slightly spaced therefrom appears a buffer magnet coil 55. The coil 55 is thus positioned between cathode buffer electrode 28 and anode buffer electrode 73 although optionally spaced at a greater distance from the latter than from the former. Support means for this coil are not depicted; however, the space surrounding the coil and between the cathode buffer electrode 28 and anode buffer electrode 73 may be left open or occupied by insulating material.

Portions of a magnet coil 57 are shown surrounding the overall electrode assembly. These may be considered to be portions of a magnet such as in FIG. 1 or, alternatively, as part of the magnet 214 that will be referred to later in connection with FIGURE 5. In operation, coil 57 and the buffer magnet coil 55 will be activated and a potential established between cathode 10 and anode 71 in a manner such as has been previously set forth in connection with FIGURES 1 and 2. A power supply 104 and switch 106 are provided for the latter purposes, these elements being similar to correspondingly identified elements in FIGURE 1. In the present embodiment, the major proportion of gas injection takes place at anode feed passages 81. A lesser amount of gas is injected via the cathode feed passages 36 in order to prevent the cathode from eroding and in order to provide a virtual cathode or stream of ionized material at near cathode potential and extending axially downstream of the physical cathode tip, an effect which is useful in connection with apparatus of the type depicted in FIGURE 5 and which is more fully elucidated upon in the applicant, Canns, copending application 457,414, previously alluded to.

Since in the present embodiment ionization takes place primarily at the anode ionization space 83, the full anodecathode potential drop is available to impart energy to the ions in contradistinction to a system of the type wherein ionization occurs principally at the cathode or minimum potential point of the system. In further distinction to a system of the latter type, it has been found that where the principal part of the discharge current is carried as in the present embodimentby ion rather than electron conduction, increased stability in the arc and plasma stream result. The mechanism underlying this effect is not yet fully understood. It should be noted as yet another feature of the present embodiment that the anode ionization space 83 is a relatively enclosed volume and accordingly higher pressures and current densities are available in this region through which the ionizing material from feed passages 81 is forced to flow, thus promoting more complete ionization of the material. The same conditions apply in cathode space 53.

During operation of the electrode assembly the small buffer coil 55 is so energized as to provide a field on its axis which is parallel to the overall magnetic field arising from the magnet coil 57. In the region outside the periphery of coil 55 however, the field provided thereby opposes that of coil 57, and accordingly by suitably adjusting the current to the several coils a saddle point or region 61 of near zero or even reversed field may be established in the total magnetic field located approximately between the coil 55 and the anode buffer 73. Such a region in the field is useful in reducing the backflow of high energy plasma particles into the electrode structure since the plasma flow is preferably parallel to magnetic field lines, and

I thus serves to reduce heat loading and particle loss in the region between coil 55 and buffer 73, and to keep the arc temperature high. It may be noted as well that at points such as 63 which lie between buffer coil 55 and cathode buffer electrode 28, high azimuthal self-magnetic fields resulting from the cathode discharge itself will tend to confine the particles and prevent backflow thereof with resulting energy and particle loss.

Bufier coil 55 keeps the arc discharge away from the electrode structure and reduces heat losses, as described in the preceding paragraph. It thus helps to raise the arc temperature in the vicinity of the electrode structure and thereby raise the arc voltage. It is a further feature of the apparatus of FIGURE 3 that most of the gas is introduced adjacent anode 71 and is very efficiently ionized in the anode ionization space 83. Any residual unionized gas emerging from the ionization space is forced radially outward by the pressure gradient within the discharge, since the neutral particles are incapable of being restrained by any electrostatic or electromagnetic forces, Less gas is introduced at the vicinity of cathode and this gas also is efiiciently ionized in the cathode ionization space 53 so that hardly any unionized gas escapes from space 53. The result of this configuration is that the cathode-anode arc discharge is confined to a path which for the most part is almost completely free of unionized material. The previously described limitations on are potential drop caused by the necessity for ionizing the feed gas now apply only in the immediate regions of the cathode and anode and a high voltage can readily be built up in the remainder of the length of the discharge. The total arc voltage becomes the sum of the high voltage generated in the region of fully ionized plasma plus the lesser voltage drops associated with the cathode and anode electrodes.

FIGURE 4 shows a modified version of the FIG. 3 apparatus. The buffer magnet coil 55 has been removed and a relatively small magnet coil 67 positioned slightly downstream from the electrode assembly where it acts in conjunction with coil 57 to produce strong convergence in the field in the downstream direction. This converging field promotes containment of the annular plasma derived from anode 70 in a manner that is suggested by the displacement of the plasma at region 69 in the diagram. Similar results are thus achieved through the use of coil 67 as are gained by introduction of the buffer coil 55, in that the plasma is prevented from leaking backward and impinging upon the various electrode assembly elements. The converging field resulting from coil 67 also promotes inward motion of the plasma in general, which is deemed desirable. In the embodiment of FIGURE 4 confinement of the cathode jet 85 is, of course, largely achieved by the self-magnetic field associated with this jet.

FIGURE 5 shows a form of plasma containment apparatus in which the present invention may be usefully employed. This application is more fully described in copending application 457,476 of the present joint applicant Gordon L. Cann, filed on Apr. 20, 1965. It includes a chamber 210 which is evacuated by pump 212 and contains hollow magnet coils 214, 216, 218 and 220 which are energized in the same direction by power supplies 222, 224, 226 and 228. Water cooling may be provided for the magnet coils as shown by elements 230, 232 .and 234. Located within coils 214 and 220 are arc electrode assemblies 236 and 238, each of which includes at least a cathode 240 and an anode 242 which are connected to a power supply 248 as well as a gas supply channel 244, which is fed from a source 246 of argon, hydrogen or other ionizable gas. The illustrated apparatus is particularly adapted to form a confined rotating column of high temperature plasma extending from electrode assembly 236 to electrode assembly 238 and having an internal radial electric field. The electrode assemblies of FIGURE 1 or FIGURE 2 are particularly suitable for use as electrotle assemblies 236 and 238 of FIGURE 3. in the illus- 8 trated apparatus, the plasma temperature is partially limited by the electrode arc voltage and the electrode assemblies of the invention therefore result in a desirably higher temperature plasma. The present invention is similarly useful in the containment apparatus described in the copending Cann application 457,414.

Although the invention has been described in terms of detailed preferred embodiments, it will be understood that major changes may be made in apparatus according to the invention without departing from the inventive concept, as long as certain distinctive features are retained. It is believed that these include: (a) the use of a cathode bufier electrode surrounding the cathode and nearly enclosing the cathode tip except for an aperture in front of the cathode tip; (b) the use of a buffer electrode adjacent to the active anode surface, insulated from the anode, and serving to create a confined space in which electrons lose some of their energy before striking the anode; (c) the use of anode gas feed injection adjacent an annular anode recessed within surrounding segmented annular anode buffer electrodes; (d) the use of a magnet coil in the annular space between the aforementioned cathode buffer and the inner of the annular anode buffer electrodes to establish saddle points in the total magnetic field and thereby reduce heat and particle loss in the region between cathode and surrounding anode; (e) aerodynamically spinning the plasma adjacent the electrodes; (f) withdrawing plasma from the anode region; and (g) multiple series connected gaps. These features may be employed in conjunction with each other, or may be employed individually.

While the present invention has been particularly described in terms of specific embodiments thereof it will be understood that in view of the present disclosure numerous modifications thereof and deviations therefrom may now be readily devised by those skilled in the art without yet departing from the present teaching. Accordingly, the present invention is to be broadly construed and limited only by the spirit and scope of the claims now appended hereto.

What is claimed is:

1. A high voltage plasma arc electrode assembly adapted to operate in an evacuated chamber having an axial magnetic field, comprising:

a downstream pointing axial cathode,

a cathode ionizing chamber surrounding said cathode,

and opening downstream, said chamber being penetratable by electric fields,

an annular anode surrounding said cathode and having a circular downstream pointing edge,

an annular ionizing chamber surrounding said anode and opening downstream,

means to form an are between said cathode and anode,

means to introduce a major fiow of ionizable material into said anode ionization chamber and a minor flow of ionizable material into said cathode ionization chamber, whereby most of said ionizable material is ionized in said anode ionization chamber, whereby rnost of said are current is carried by ions accelerated across the full cathode-anode potential drop, and whereby most of said are extends through a region of fully ionized plasma formed in said ion ization chambers, and

means to reduce the backfiow of plasma in the annular region between said cathode and anode comprising a magnetic coil positioned between said cathode and said anode and energized to create an axial magnetic field aiding the axial magnetic field in said evacuated chamber and a return field opposing the axial magnetic field in said chamber.

2. A high voltage plasma arc electrode assembly adapted to operate in an evacuated chamber having an axial magnetic field, comprising:

a downstream pointing axial cathode,

a cathode ionizing chamber surrounding said cathode,

and opening downstream, said chamber being penetratable by electric fields, and said cathode ionizing chamber being defined by a coaxial electrically floating butler electrode surrounding said cathode and said magnetic field on said axis and substantially symmetrical thereabout, said generator comprising (1) a downstream pointing axial cathode,

(2) a cathode ionizing chamber surrounding said cathode and opening downstream, said ionizing electrically insulated therefrom, said annular buffer chamber being penetrable by electric fields, electrode including a portion extending downstream (3) an annular anode surrounding said cathode from said cathode, said portion being segmented by and having a circular downstream pointing alternating electrically insulating and electrically edge, conductive rings, the internal surface of said rings (4) an annular ionizing chamber surrounding said principally defining said chamber, anode and opening downstream,

an annular anode surrounding said cathode and hav- (5) means to form an are between said cathode ing a circular downstream pointing edge, and anode, and

an annular ionizing chamber surroundin said anode (6) means to introduce a major flow of ionizand opening downstream, said annular ionizing able material into said annular ionization chamchamber being defined by a pair of electrically floather and a minor flow of ionizable material into ing annular anode buffer electrodes surrounding said said cathode ionization chamber whereby most anode and electrically insulated therefrom, said anof said ionizable material is ionized in said nular buffer electrodes including portions extending anode ionization chamber where'by most of said downstream of said anode edge, said portions being are current is carried by ions accelerated across segmented by alternating electrically insulating and the full cathode-anode potential drop and whereelectrically conducting rings, the annular space bemost of said are extends through a region of tween said rings of one buffer and said rings of the fully ionized plasma formed in said ionization other buffer serving to principally define said annuchamber, and lar ionization chamber, (e) a backflow prevention means comprising a magmeans to form an are between said cathode and anode, netic coil positioned between said cathode and said means to introduce a major flow of ionizable material (1110116 and enefgilfid to Create an axial magnetic into aid anode ionization chamber and a minor the axial magnetic in said chamber flow of ionizable material into said cathode ioniza- 1I1d return field pp the axial magnetic field tion chamber, whereby most of said ionizable mam sald chamberterial is ionized in said anode ionization chamber, whereby most of said are current is carried by ions accelerated across the full cathode-anode potential References Cited UNITED STATES PATENTS drop, and whereby most of said are extends through 5 3,104,345 9/ 1963 Wilcox et al 315l11 X a region of fully ionized plasma formed in said 3,243,954 4/1966 *Cann 313-231 X ionization chambers. 3,343,027 9/1967 Friihlich 313-111 X 3. Plasma containing apparatus comprising (a) a chamber;

(b) means to evacuate said chamber;

(0) magnetic means to form a longitudinally continuous magnetic field along its axis of said chamber;

((1) at least one plasma arc generator disposed within JAMES W. LAWRENCE, Primary Examiner. 40 R. F. HOSSFELD, Assistant Examiner.

U.S. Cl. X.R. 313-231; 3l5111

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