US3413509A

US3413509A - Electrode structure with buffer coil

Info

Publication number: US3413509A
Application number: US545703A
Authority: US
Inventors: Gordon L Cann; Robert L Harder
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1966-04-27
Filing date: 1966-04-27
Publication date: 1968-11-26
Anticipated expiration: 1985-11-26

Description

Nov. 26, 1968 G. L. CZANN ET AL 3,413,509

ELECTRODE STRUCTURE WITH BUFFER COIL Filed April 27, 1966 4 Sheets-Sheet 1 A T TORNE. KS

ELECTRODE STRUCTURE WITH BUFFER COIL Filed April 27. 1966 4 Sheets-Sheet 2 INVENTORS. 222w '--::ER F/az m q a WQQM ATTORNEYS Nov. 26, 1968 G. L. CANN ET AL I ELECTRODE STRUCTURE WITH BUFFER COIL 4 Sheets-Sheet 5 Filed April 27, 1966 INVENTORS GORDONLCANN oar: RT

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G. L. CANN ET AL ELECTRODE STRUCTURE WITH BUFFER COIL Nov. 26, 1968 4 Sheets-Sheet 4 Filed April 27, 1966 h 2. wmm 1 w; v v H va :1 1: 3w

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INVENTORS GORDON L. CANN BY ROBERT RDER 4 7' TORA/EKS United States Patent 3,413,509 ELECTRODE STRUCTURE WITH BUFFER COIL Gordon L. Cann, Lagnna Beach, Fla., and Robert L.

Harder, Altadena, Califi, assignor 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. No. 545,703

4 Claims. (Cl. 313-161) 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. I

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. 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 difficulties 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 operation 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 are voltage.

A further object of the present invention is to provide an arc electrode assembly adapted to more completely ionize gaseous material fed thereto. Additional objects, features, and advantages will become apparent on reading the more detailed description which follows.

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 simplified cross section of another modified electrode assembly; and

FIG. 4 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 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 16 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 pas- 3,413,509 Patented Nov. 26, 1968 sages will not appear in the drawing for the same reason. A boron nitride 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 also contains one or more small channels on 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. In sulators 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 74. 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 on a common plane 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 nressure tap 86.

In the most instances it is desirable to operate the electrode assembly in the presence of a magnetic field and accordingly a magnet coil 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 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 21 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 closed in order to start an arc between cathode 12 and cathode bufier electrode 28. After this arc is established, feed gas may optionally be introduced through feed tap or inlet tube 82 and power supply 104 is energized and switch 106 closed to draw the arc 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 if the magnetic field is increased to a value in excess of about 30,000 gauss. The magnetic field may also be provided -by magnet rneans distinct from the electrode 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 or feed tap 82 may optionally be continued. Gas feed rates may typically vary in the range from about 0.1 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, diffuse, 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 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 efiectively 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 buflFer electrodes is not essential, but serves to reduce the likelihood of an arc attaching to the butter 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 copending application 457,414 of Gordon L. Cann, filed May 20, 1965, 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 the electrode assembly according to the invention. The cathode buffer electrode 28 and anode butter 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 (P wBR Ma MR e (P1).. 2 e 2 where I 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,

to 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.

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 ad, 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 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 ot 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 eifect 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.

FIGURE 3 is a simplified cross section of a further modified electrode assembly. Corresponding parts are identified by the same reference numerals as in previous embodiments. The assembly is seen to include as in prior embodiments a cathode 10, an anode 70, a cathode heat sink 14, and an inlet 22 for cooling water entering the cathode cavity. In the instant figure a cooling water outlet 23 for water exiting from the cathode cavity also appears. Anode 70, like cathode is a water cooled structure. The coolant outlet 25 appears in the figure; however, the inlet is out of the plane of the drawing, and accordingly is not explicitly shown. A power supply for the cathodeanode circuit is shown at 104 together with its associated switch 106. A boron nitride insulator 26 surrounds the cathode tip and is provided with passages 36 through which ionizable material may be introduced. The feed inlet tube for such ionizable material is once again out of the plane of the drawing and for purposes of simplicity is not here shown.

Cathode 70 in FIG. 3 is seen to be positioned at one end of an axial passage 27 further defined by an electrically floating first buffer electrode 29 and a series of Water cooled buffer segment electrodes 31, 33, and 35. The buffer segment electrode 31 is also separated from first bufi'er electrode 29 by an insulated segment 43 and segment electrodes 31 through are separated from each other by a series of intervening insulators 37, 39 and 41. This construction is employed to define passage 27 because a single electrical insulator would be destroyed by heat. Subdivision of the cathode-anode spacing into a multiplicity of gaps in series, i.e., by insulators 37, 39 and 41, reduces the possibility of cathode-anode surface arcs since each gap adds an increment of voltage sustaining capacity relatively independent of its length. The use of as many alternating electrodes and insulators as possi ble is therefore preferred, but engineering consideration limits the number which may be practically employed.

Unlike prior embodiments buifer electrode 29 is seen to include a buffer magnet coil 45 of very limited axial extension. The diameter of this coil is such as to bring it within the body of buffer 29, the turns being thereby spaced only a relatively small distance from the axis of the overall electrode assembly. The buffer coil 45 is provided with a power supply 47 and is water cooled by coolant introduced through inlet 49. The corresponding cooling outlet is out of the plane of the figure and is accordingly not explicitly shown. The coil 45 is utilized in the present embodiment alone or in addition to larger electrode assembly enveloping magnets such as magnet 90 in FIGURE 1 or magnet 214 in FIGURE 4. Its precise function will be explained in more detail in what ensues; however, in a general sense it may be indicated that this buifer coil serves to establish localized distortions in the gross longitudinal magnetic field that will usually be present where the instant invention is utilized in an environment such as will be discussed in connection with FIGURE 4.

The opposite end of axial passage 27 is defined by the anode electrode 70. In operation switch 106 is closed and an arc is established between cathode 10 and anode 70. Although not explicitly shown here the arc may optionally be established by a procedure similar to that set forth for establishing the arc in FIGURE l-that is to say that a power supply like 100 in FIGURE 1 may be connected between buffer and cathode and transiently utilized so as to bring the arc in stages to its final configuration between cathode and anode.

As previously suggested the electrode assembly of FIGURE '3 will commonly be operated in an environment such as that to be described in connection with FIG- URE 4, wherein large magnets 214 generally surround the assembly 236. A similar relationship is seen in FIG- URE 1 where magnet 90, when operative serves to establish a longitudinal magnet field throughout the active portions of the electrode assembly. Provided accordingly the appropriate power supplied are activated, a 1ongitudinal magnetic field may be illustratively assumed in FIGURE 3 to exist in the vicinity of the arc discharge. In addition to this overall magnetic field which will typically envelop at least all portions of the electrode as sembly to the right of the cathode tip, a localized field may in the instant embodiment be introduced in the vicinity of axial passage 27 by activation of power supply 47 to buffer coil 45. The action of buffer coil 45 under such conditions is such as to intensify the total magnetic field in passage 27 and in particular to produce an increased axial magnetic field in the vicinity of the cathode tip and in the relatively small opening 51 of first buffer electrode 29. This increased magnetic field not only assists in providing stable arc attachment to the cathode tip at the desired low mass flow rates, but acts to confine the arc within the axial passage to thereby prevent arc attachment to the butter 29 or buffer segment electrodes 31 through 35. The configuration of passage 27 itself is furthermore such as to force feed gas introduced through passage 36 to flow through the well-contained are for considerable distances, thereby promoting complete ionization.

Of at least equal importance, the small dimensions of the coil 45 cause the magnetic field contributed thereby to diverge rapidly at greater distances from the cathode 70, thus providing a strong radial component of magnetic field in the region of alternating buifer segment 31 through 35 and insulating segments 37 through 41. This radial component of magnetic field discourages electrical breakdown between adjacent bufier segments since any such breakdown would have to cross magnetic field linessomething which electrical discharges do only with difliculty.

FIGURE 4 shows a form of plasma containment apparatus in which the present invention may be usefully employed. This apparatus is more fully described in the copending application 457,746 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

magnetic coils

214, 216, 218 and 220 which are energized in the same direction by power supplied 222, 224, 226 and 228. Water cooling may be provided for the magnet coils as shown by

element

230, 232 and 234. Located within

coils

214 and 220 are are

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

electrode assemblies

236 and 238 of FIGURE 3. In the illustrated apparatus, the plasma temperature is 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 copending application 457,414 of the present joint applicant, Gordon L. Cann, also filed on Apr. 2C1, 1965.

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 buffer 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 a small butler magnet coil to effect ionization promoting confinement of the arc in an extended axial passage and to provide a strong radial component of magnetic field in portions of the passage to discourage electrical breakdowns thereat, (d) aerodynamically spinning the plasma adjacent the electrodes, (e) withdrawing plasma from the anode region, and (f) multiple series connected gaps. These features are preferably employed in conjunction with each other, but may also be employed individually.

While the present invention has been particularly described in terms of specific embodiments thereof it Wiil 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 are electrode assembly adapted to operate in an evacuated chamber having an axial magnetic field comprising:

(a) an axial cathode tip;

(b) an electrically floating cathode buffer electrode surrounding said cathode tip and defining an axial channel;

() means to introduce gas into said axial channel at said cathode tip;

((1) a series of alternating buffer electrodes and insul ating segments defining an extension of said axial channel;

(e) an anode electrode having an internal aperture and terminating said channel; and,

(f) magnetic coil means surrounding said cathode tip having an inside diameter only slightly larger than said channel diameter and having an axial length small compared to its radial length whereby a strong axial magnetic field is produced in the region of said cathode tip and a diverging magnetic field having a substantial radial component is formed in said channel in the region of said alternating buffer electrodes and insulating segments whereby arcing between said bufier segments is minimized.

2. In a high voltage plasma arc electrode assembly adapted to operate in an evacuated chamber having an axial magnetic field and including an axial cathode tip, an electrically floating cathode buffer electrode surrounding said cathode tip and defining an axial channel, means to introduce gas into said axial channel at said cathode tip, a series of alternating buffer electrodes and insulating segments defining an extension of said axial channel and an anode electrode having an interal aperture and terminating said channel, the improvement comprising:

magnetic coil means surounding said cathode tip having an inside diameter only slightly larger than said channel diameter and having an axial length small compared to its radial length whereby a strong axial magnetic field is produced in the region of said cathode tip and a diverging magnetic field having a substantial radial component is formed in said channel in the region of said alternating buffer electrodes and insulating segments, whereby arcing between said buffer segments is minimized.

3. A plasma containment device comprising:

(a) a chamber;

(b) means to evacuate said chamber;

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

(d) at least one plasma arc generator disposed within said magnetic field on said axis and substantially symmetrical thereabout said generator comprising:

(1) an axial cathode tip;

(2) an electrically floating cathode buffer electrode surrounding said cathode tip and defining an axial channel;

(3) means to introduce gas into said axial channel at said cathode tip;

(4) a series of alternating buffer electrodes and insulating segments defining an extension of said axial channel;

(5) an anode electrode having an internal aperture and terminating said channel; and,

(6) magnetic coil means surrounding said cathode tip having an inside diameter only slightly larger than said channel diameter and having an axial length small compared to its radial length whereby a strong axial magnetic field is produced in the region of said cathode tip and a diverging magnetic field having a substantial radial component is formed in said channel in the region of said alternating buffer electrodes and insulating segments whereby arcing between said bulfer segments is minimized.

4. In a plasma containment apparatus comprising a chamber, means to evacuate said chamber, magnetic means to form a longitudinally continuous magnetic field along an axis of said chamber, at least one plasma arc generator disposed within said magnetic field on said axis and substantially symmetrical thereabout, said generator comprising an axial cathode tip, an electrically floating cathode buffer electrode surrounding said cathode tip and defining an axial channel, means to introduce gas into said axial channel at said cathode tip, a series of alternating bulfer electrodes and insulating segments defining an extension of said axial channel, and an anode electrode having an internal aperture and terminating said channel, the improvement comprising:

magnetic coil means surrounding said cathode tip having an inside diameter only slightly larger than said channel diameter and having an axial length small compared to its radial length whereby a strong axial magnetic field is produced in the region of said cathode tip and a diverging magnetic field having a substantial radial component is formed in said channel in the region ofsaid alternating buffer electrodes and insulating segments, whereby arcing between said buffer segments is minimized.

No references cited.

JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner.