US3449628A - Plasma arc electrodes with anode heat shield - Google Patents

Description

FLASMA ARC ELECTRODES WITH ANODE HEAT SHIELD Filed April 27. 1966 or 6 Sheet June 10; 1969 e. L. CANN ETAL ARDER a ATTORNEY;

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PLASMA ARC ELECTRODES WITH ANODE HEAT SHIELD Filed April 27, 1966 Sheet 2 of s GQR I'J SK E EQ NN F/G. 2 ROBERT L. HARDER ATTORNEYS PLASMA ARC ELECTRODES WITH ANODE HEAT SHIELD Filed April 2'7, 1966 June 10, 1969 G. 1.. CANN ETAL Sheet m UR INVENTORS GORDON L. CANN ROBERT L. HARDER A TTORNEYS PLASMA ARC ELECTRODES WITH ANODE HEAT SHIELD Filed April 27, 1966 June 10, 1969 s. L. CANN ETAL.

Sheet '"INVIJNTURS GORDON L. CANN ROBERT HARDER BY ATTORNEYS June 10, 1969 G. L. CANN ETAL 3,449,628

PLASMA ARC ELE CTRODES WITH ANODE HEAT SHIELD Sheet Filed April 27, 1966 ANODE 8- ANODE SHIELD POWER ABSORPTIO N INPUT POWER KW Filed April 27. 1966 June 10, 1969 G. 1.. CANN ETAL 3,449,628

PLASMA ARC ELECTRODES WITH ANODE HEAT SHIELD Sheet 6 016 FIG 6 INVENTORS' ORDON L.CANN

BERT HARDER W 4 T TORNEVS United States Patent US. Cl. 315-411 8 Claims ABSTRACT OF THE DISCLOSURE A method of and apparatus for electromagnetically containing a body of high temperature plasma are disclosed. The approach disclosed involves extracting from. the plasma a substantial fraction of the heat otherwise absorbed by the anode by providing an electrically floating heat absorbing electrode downstream of and adjacent the anode.

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. Ditficulties have been experienced with this type of electrode structure in high power operations due to the great amounts of heat which must be absorbed by the relatively small anode.

It is accordingly a principal object of the present invention to provide an arc electrode assembly which reduces the anode heat load.

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; and,

FIG. 3 is a schematic cross sectional view of an apparatus in which the electrode assemblies of the invention may be used.

FIG. 4 is a longitudinal cross section of a further modified electrode assembly.

FIG. 5 is a graph showing anode and anode shield heat loads versus input power.

FIG. 6 illustrates one form of anode heat shield.

FIG. 7 shows a different form of heat shield.

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 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 passages 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 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 10.

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 sinks 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 56, 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 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 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 88 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 temperature in the operation of the device. Cooling assembly 94 also functions as an anode heat shield, as will be shown in connection with the description of FIG. 4. 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 closed in order to start an arc between cathode 12 and cathode buffer electrode 28. After this are 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 de-energized. As is known in the art, the power supplied 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 means 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 .01 to about .1 gram per second.

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 interelectrode 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 80 has been removed and an enlarged inlet tube 82 is connected to a vacuum pump 126. These modifications operate to increase the temperature of any plasma which is produced.

FIGURE 3 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, Ser. No. 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 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 electrode assemblies 236 and 238 of FIGURE 3. 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. 20, 1965.

FIGURE 4 shows a further modified electrode structure in conjunction with a plasma containment device similar to but slightly different from that of FIGURE 3. Cathode 10, anode 70, and various other elements are numbered identically with the corresponding elements in FIGURES 1, 2, and 3 No bufier electrodes are employed in this particular design. Anode heat shield 94 is a relatively flat conical structure positioned immediately in front of anode 70 and substantially covering the forward facing surface of the anode. Heat shield 94 is integral with a supporting tube 96 and the two elements jointly surround and protect the entire electrode assembly. Heat shield 94 is separated from anode 70 by a boron nitride insulator 88A, and a phenolic insulator 88B. The shield is also insulated from water-cooled magnet 90 by an asbestos cement insulator 88C and supporting tube 96 is insulated from the electrode assembly by a phenolic insulator 88D. These insulators permit the heat shield and its supporting tube to be electrically floating with respect to the cathode and anode and prevent the cathode-anode arc from attaching to the heat shield.

The illustrated containment device comprises an outer vacuum chamber 154 connected by an insulator 152 to inner vacuum tube which is surrounded by magnets 156. Tube 150 is preferably a water-cooled copper tube or similar element of dissipating large amounts of heat. Only part of the device is shown, as the two halves are mirror images. However, as shown for a similar device in copending 4S7,414previously alluded to--the device may also be operated with a single electrode assembly. Illustratively, tube 150 may be about 30 inches long with 3.9 inch inside diameter.

Anode heat shield 94 reduces the heat load on anode 70 in several ways. It conductively cools the anode and absorbs radiant energy from the hot plasma which would otherwise be absorbed by the anode. It is presently believed that the most important function of the anode shield is to block high-energy plasma ions which would otherwise strike the anode. With the shield in place, it is believed that the arc electrons continue to be collected by the anode, but the plasma ions strike the shield instead, give up their energy, recombine and are exhausted from the system. The practical result is that the large area anode heat shield 94 absorbs much of the heat which would otherwise tend to melt the anode, while the arc is still confined to the inwardly facing surface of the anode.

FIGURE 5 shows the effect of heat shield 94. Curve A shows a measured relationship between the electrical power applied to an arc assembly in the apparatus of FIGURE 4, and the thermal power absorbed by the anode cooling system as determined by the flow rate and temperature rise of the anode cooling system. Curve B shows the thermal power absorption of the anode shield. Curve C shows the combined thermal power absorption of anode and anode shield. It can be seen that the anode heat shield absorbs a progressively higher fraction of the power as the input ower rises.

FIGURE 6 depicts in detail a structure suitable for use as heat shield 94. In order to absorb large amounts of 5 heat and prevent hot spots which might cause arc attachment, the structure shown is provided with an integral cooling system designed to minimize temperature difierentials throughout the body of the shield and particularly at the surface corresponding to 95 in FIG. 4. The structure is seen to include radial passages, such as at 96, extending from the periphery of its disc-like body to an annular chamber 98. The latter is entirely included within the body of the heat shield and closely surrounds opening 99 through which the plasma flow is effected. The shield depicted is formed from a disc-like copper blank by removing a center portion corresponding to opening 99, and boring the several radial passages 96 from the disc periphery to a milled annular groove formed about 99. After this is accomplished, the copper is annealed to develop workability and disc material is spun over the annular groove to a point where the groove is virtually sealed from the disc surface. Final sealing of the groove to form annular chamber 98 is brought about by silver soldering the crevices still remaining between the spun over disc material and the adjacent disc body. Depending on the final configuration desired, the structure may thereafter be spun over to achieve, for example, the slightly conical cross section depicted for the shield 94 in FIGURE 4.

In use, the coolantusually wateris fed to the body of the shield through alternate passages such as passages 96, 100, and so forth. The coolant is seen for example in FIGURE 4, entering one such passage through external duct 108. The coolant returns from the heat shield structure through the remaining and alternating passages present, for example, at 97, 101, and so forth. Thus, in the case of each of the several radial cooling passages, coolant in immediatel adjacent passages will be travelling in opposite directions. In general, the result of introducing coolant in such manner is to effect a flow of the type suggested by the arrows depicted in chamber '98, wherein the coolant is seen to display a tendency to enter through a given passage and subsequently exit smoothly through adjacent passages on alternate sides of that passage through which entry was accomplished. Such action serves in turn to produce a generally uniform cooling action and avoids the high temperature differentials that result in many prior systems which are so designed as to introduce coolant en masse to a first portion of the region to be cooled and extract the coolant in the same manner from a second region of the heat controlled volume.

FIGURE 7 shows a modified version of heat shield 94, particularly adapted for use with high voltage plasma discharges. Shield 94 in this depiction consists of alternating concentric electrically and thermally conductive segments 160 and insulating segments 158. This modification permits a radial electric field to exist in the plasma adjacent the electrode structure and outside the anode radius A solid shield is an equipotential surface and would tend to short circuit an adjacent radial electric field.

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 plasma arc electrode assembly comprising:

(a) a central cathode electrode,

(b) an anode electrode encircling said cathode and electrically insulated therefrom, and

(c) a heat dissipating electrode, said heat dissipating electrode having a central opening aligned with said cathode, said heat dissipating electrode being positioned downstream of, immediately adjacent to and covering substantially all of said anode except the radially inward surface thereof and said heat dissipating electrode being separated from said anode by an insulator and being electrically floating with respect to said anode and cathode.

2. Apparatus according to claim 1 wherein said heat dissipating electrode is water cooled.

3. Apparatus according to claim 1 wherein said heat dissipating electrode is radially symmetric and includes an annular chamber surrounding said central opening and externally terminating radial passages within the body of said heat dissipating electrode extending into said chamberdfor introducing coolant into said heat dissipating electro e.

4. Apparatus according to claim 1 wherein said heat dissipating electrode comprises alternating electrically insulated thermally insulating rings and electrically conductive thermally conductive rings.

5. A plasma arc electrode assembly comprising:

(a) a central cathode electrode;

(b) an electrically insulated cathode buffer electrode substantially enclosing said cathode and having an aperture in line with and in front of said cathode;

(c) an annular anode electrode disposed around said cathode and cathode buffer having an electron receivmg area:

(d) an electrically insulated anode buffer electrode positioned adjacent said electron receiving surface of said anode and defining a channel therewith;

(e) a heat dissipating electrode having a central opening aligned with said cathode, said electrode being immediately adjacent to and covering all of said anode except the radially inward surface thereof, said electrode being separated from said anode by an insulator and being electrically floating with respect to said anode and cathode; and,

(f) means to introduce an ionizable gas between said cathode and said anode.

6. A plasma arc electrode assembly comprising:

(a) a central cathode electrode;

(b) an anode electrode encircling said cathode and electrically insulated therefrom;

(c) means to itnroduce an ionizable gas between said cathode and said anode;

(d) arc power supply means connected between said cathode and said anode;

(e) means to create an axial magnetic field at said electrode assembly; and,

(f) a heat dissipating electrode having a central opening aligned with said cathode, said heat dissipating electrode being positioned downstream of, immediately adjacent to and covering substantially all of said anode except the radially inward surfaces thereof, and said heat dissipating electrode being separated from said anode by an insulator and being electrically floating with respect to said anode and cathode.

7. The method of electromagnetically containing a body of high temperature plasma comprising establishing a longitudinally continuous magnetic field substantially symmetrical about a line, maintaining at least one plasma forming electrical discharge within said magnetic field substantially symmetrical about said line between a central cathode and an encircling anode, and extracting from said plasma a substantial fraction of the heat otherwise absorbed by said anode by covering the axially forward facing downstream surface of said anode with an electrically floating heat absorbing electrode.

8. Plasma containment apparatus comprising:

(a) a chamber;

(b) means to evacuate said chamber;

(c) magnetic means to form a longitudinally continuous magnetic field along a line within said chamber;

((1) a pair of opposed plasma arc generators disposed within said magnetic field on said line and substantially symmetrical thereabout, each generator comprising a central cathode electrode and an anode elec- 7 8 trode encircling said cathode, each said generator inan insulator and being electrically floating with recluding at least one passage terminating between said spect at said anode and cathode.

cathode and anode;

(e) gas supply means to introduce a plasma forming References Cited gas to said passages; 5 UNITED STATES PATENTS power pp y means to n n n arc discharge 3,243,954 4/1966 c 3 between said anode and said cathode; and, 3 315 125 4 1967 p g i 313 231 (g) a heat dissipating electrode positioned at each of said generators, each of said electrodes having a cen- JAMES W. LAWRENCE, Primary Examiner. tral opening aligned with the cathode of said gencr- 10 F HOSSFELD ASH-Stu, Examincl. ator, said electrode being immediately adjacent to and covering substantially all of the anode of said US. Cl. X.R. generator except the radially inward surfaces there- 313 3 39 1 1 231 of, said electrode being separated from said anode by 15

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