US3376459A - Arc plasma generator having a vaporizable containment chamber - Google Patents

Description

WW5 MEEREME SEARQH; W!

April 2, 1968 3,376,459

ARC PLASMA GENERATOR HAVING A VAPORIZABLE CONTAINMENT CHAMBER J7 J. NARBUS ET L Filed April 6, 1965 INVENTORS. GERHARD Fk/ND, JOSEPH J NARBUS, 5Y m M I :7

ATTORNEY United State 3,376,459 ARC PLASMA GENERATOR HAVING A VAlOR- IZABLE CONTAINMENT CHAMBER Joseph J. Narbus, Lindenwold, N1, and Gerhard Frind,

Secane, Pa., assignors to General Electric Company, a

corporation of New York Filed Apr. 6, 1965, Ser. No. 445,989 9 Claims. (Cl. 313-231) ABSTRACT OF THE DISCLOSURE This invention relates to an arc plasma generator and, more particularly, to an arc plasma generator of a simple construction that is capable of producing a highly stable, extremely hot arc plasma that is substantially uncontaminated by electrode decomposition products.

The usual arc plasma generator comprises a pair of spaced-apart electrodes, means for establishing an electric arc in a suitable arcing passageway extending between the electrodes, and means for sending through the arcing passageway a stream of gas that is heated by the arc and exhausted through a suitable exhaust port. The highly heated gas is referred to as an arc plasma.

Typically, the walls of the arcing passage are made of metal sections spaced apart along the length of the passage and insulated from each other. In such constructions it is necessary to provide the metal sections with special cooling means to prevent the metal walls from being vaporized by the are so as to prevent any such metal vapors from entering and contaminating the plasma. Such cooling means for the wall is usually quite involved and expensive. Another feature that has made many prior arc plasma generators unduly expensive and complicated is that the arc plasma has been created by feeding the are with a stream of gas introduced from a source outside the arcing passageway. In addition, the stream has typically been introduced at a plurality of points,.thus further complicating the device.

A plasma generator that embodies the above described features is shown and claimed in US. Patent No. 3,146,- 371 McGinn, assigned to the assignee of the present invention. Another feature of this plasma generator is that a portion of the gas stream introduced from the external source is used for producing a scavenging action that blocks entry of the electrode decomposition products into the plasma stream that is flowing toward the exhaust port. This blocking action greatly reduces contamination of the plasma by electrode decomposition products, but it also involves the complication of requiring an external source for the scavenging gases.

An object of the present invention is to provide an arc plasma generator of a simple construction that can provide an extremely hot arc plasma free from electrode vapors without requiring an external source of gas either for feeding the arc or for scavenging purposes.

Another object is to construct the arc plasma generator in such a manner that the arc plasma exhaust has a high degree of freedom from fluctuations in temperature and velocity.

In carrying out the invention in one form, we provide $3,376,453? Patented Apr. 2, i968 a pair of electrodes between which an arc is adapted to be established. Means defining an arcing passage that extends between the electrodes is provided for receiving the column of the arc, The arcing passage is lined with insulating material that is directly exposed to the arc and is adapted to evolve gases when heated by the arc. These gases are heated to a high temperature by the arc. Ex= tending transversely of the arcing passage at a location intermediate the electrodes is an exhaust passage. Means is provided for forcing most of the gas evolved from the insulating material to flow along said arcing passage toward said exhaust passage and then out through the exhaust passage. Scavenging means is provided for substantially preventing the vapors evolved from each of said electrodes by said are from entering the stream of gases flowing toward the exhaust passage. This scavenging means comprises means for causing a minor portion of the gases evolved from said insulating material by the arc to flow past each of the electrodes in a direction lead ing away from said exhaust passage.

For a better understanding of our invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a cross sectional view showing an arc plasma generator embodying one form of our invention.

FIG. 2 is an enlarged cross sectional view of a portion of FIG. 1.

FIG. 3 is a view taken along the line 33 of FIG. 1.

FIG. 4 illustrates a modified form of the invention viewed along a line such as line 33 of FIG. 1.

Referring now to FIG. 1, there is shown an arc plasma generator 10 that comprises a pair of tubular electrodes 12 between which an electric arc 14 is adapted to be formed. Each of the electrodes 12 is preferably formed of graphite, which is a material having a low rate of arc-erosion. Posi tioned between the electrodes 12 is a block 16 of insulating material having an arcing passage 18 extending therethrough between the electrodes 12. The arcing passage 18 is lined with a tubular liner 20 of insulating material. This liner is directly exposed to the are 14 and is adapted to evolve gases when heated by the arc.

An exhaust port or passage 22 is shown extending transversely of the arcing passage 18 and communicating therewith at a location approximately midway between the two electrodes 12. The transversely-extending exhaust port 22 is also provided with a liner 24, preferably of the same insulating material as the liner 20.

Surrounding each of the tubular electrodes 12 is a cylindrical heat sink 26 of a highly conductive metal such as copper. Preferably, each electrode is press fitted into its surrounding heat sink 26. Each of the illustrated heat sinks 26 comprises two cylindrical members 30 and 32. The cylindrical members are suitably secured to the insulating block 30, as by means of screws 34 extending through aligned openings in these parts and threaded into the block 30. The heat sink members 30* and 32 are respectively formed with internal annular shoulders 40 and 42 that are positioned at opposite ends of the sociated electrode 12. These shoulders 40- and 42 confine the electrode 12 against axial movement.

Each heat sink 26 is capable of quickly absorbing large amounts of heat from its associated electrode member 12, thus helping to limit the temperature of the electrode member. By limiting the temperature of the electrode member 12, the heat sink 12 prevents the adjacent in sulating block 16 from becoming overheated and damaged by the temperature rise of the electrode member. Added protection for insulating block 16 is provided by the shoulder 40 on the heat sink 26. This shoulder 40, which is interposed between the electrode member 12 and the insulating block 16, acts as a heat shield for reducing the through the copper heat sinks 26 between the associated electrode 12 and the associated lead 44.

The are 14 between the electrodes 12 is normally started by relying upon a thin striking wire (not shown) connected between the electrodes. A. high energy source of electric current (not shown) is connected across the leads 44 causing a high current to traverse the striking wire. This high current causes the striking wire to immediately vaporize, thereby establishing the are 14 be tween the electrodes 12. The are terminals are schematically indicated in FIG. 1 as attaching to the cylindrical electrodes 12 about substantially their entire inner periphery. The column of the are 14 extends between the electrodes 12 through the arcing passage 18.

The intense heat of the arc immediately vaporizes a portion of the insulating material forming the liner 20. The resulting gases are quickly heated by the arc to a very high temperature to form an arc plasma, and most of this are plasma is driven at high speed along the arcing passage 18 toward the exhaust port 22. The are plasma is expelled through the exhaust port. 22 at high speed to form the arc plasma jet indicated at 50.

For forcing most of the arc plasma to follow the above described flow path, i.e., along the arcing passage 18 toward the central exhaust port 22 and then out through the exhaust port, we provide plugs 52 at the outer end of each cylindrical electrode 12. Each of these plugs 52 presents a relatively high impedance to the how of arc plasma in an axially outward direction, and thus most of the plasma flows in an opposite direction, i.e., toward the low impedance exhaust port 22.

For maintaining the main arc plasma jet substantially free of contamination from electrode decomposition products, we provide a small venting passage 54 extending through each of the end plugs 52 A minor portion of the arc plasma developed by the arc decomposing the insulating liner 20 is vented through each of these passages 54 via a path that extends axially outward through the bore of the cylindrical electrod 12. '7. his minor ou of the arc plasma carries with. it eier-trode deco pd on products, thus performing a scavenging act" which sub" stantially prevents the electrode decomposition products, or electrode vapors, from entering the stream of arc plasma flowing toward the central exhaust port 22,

The following is a more detailed explanation. of the manner in which this scavenging of the electrode vapors is believed to occur. The are terminal at each electrode heats the inner peripheray of the electrode 12 to a. very high temperature, vaporizing some of the electrode ma= terial, and producing high speed jets of vaporized elec trode material that project away from the hot electrode surface. These jets of electrode vapor, which are schemati= cally depicted at 56 in FIG. 2, tend to leave the electrode surface generally normal thereto. Because the surface on which each arc terminal is rooted is the inner peripheral surface of the electrode 12, the jets tend to leave the elec= trode surface generally normal to the column of the are This is advantageous because a jet projecting in this generally norm-a1 direction is less likely than a downstream-projecting jet to carry electrode vapors into the stream of plasma that is flowing through the arcing passage 18 toward the exhaust port 22. Instead of pro jecting electrode vapors far downstream into the column region of the arc, as would be the case with downstream directed jets, these normally directed jets tend to bounce their electrode vapors off the opposed inner peripheral wall of the electrode 12. To some extent. this tends to confine and collect these electrode vapors inside the bore of the cylindrical electrode 12.

When collected in this manner, the electrode apors can be more easily swept from the arcing passage 18 by the minor flow of arc plasma out the venting passage 54-. This permits us to rely upon a relatively small flow of the useful plasma through the venting passage 54 for effecting the desired scavenging. Being able to limit this flow through venting passage 54 to a low value highly desirable, since this particular flow represents energy lost and not imparted to the main jet 50.

The amount of gas used for such scavenging can be con trolled by controlling the cross-section of the passage 54. We prefer to make these venting passages 54 just large enough to provide a how therethrough that removes sub stantially all of the electrode vapors. This size can be determined experimentally by spectrographically analyz ing the main jet for electrode vapors. The size of the venting passage 54 is increased until the spectrographic analyzer shows a negligible amount of electrode vapors in, the main jet.

There are several related features which contribute to the high efiiciency of our scavenging action. One is that we seal the arcing passage in the region 60 immediately ahead of the electrode 12 and thus prevent any of the hot gases from exhausing transversely of the arcing passage 1-8 in this region. Any gases generated in this region that do not enter the main plasma stream are forced to follow a path through the cyclindrical electrode 12 and out of the passage 54, which is the path for most effective scavenging, as was described hereinabove.

This seal in the region 60 is formed by the shoulder 40 on the heat sink 26 and the close fit that it makes with adjacent parts. In this respect, the shoulder 40 has a planar downstream face 63, and the screws 34 clamp this face 63 tightly against the planar face of insulating block 16, thereby preventing any leakage in a radial direction along this downstream face 63. The upstream face of shoulder 40 is also planar, and the electrode member 12 is clamped against this face by the plug .52. This plug 52 is threaded into the outer shoulder 42 on. the heat sink 26 until it forces the planar downstream surface 64 of the electrode 12 into engagement with the planar upstream surface of shoulder 40.

Another feaure that contributes to our high-efficiency scavenging is that our electrode configuration causes the arc terminal to be generally confined to the inner periphery of the tubular electrode 12. Accordingly, the elec= trode vapor jets are mostly directed radially inward of this surface as was described hereinabove.

The are terminal does not attach to the exposed sur face of shoulder 40 because this surface is slightly re cessed with respect to the arcing passage 18 and also be cause the gas evolved by the immediately-adjacent por tion of the insulating liner 20 develops a slight radiallyinwardly acting force that inhibits the arc terminal. from entering this region. The fact that the shoulder 40 and the recess 65 are quite thin, considered longitudinally of the arcing passage 18, also inhibits the arc terminal from entering this region. For these same reasons. the arc terminal is normally prevented from attaching to the downstream surface 64 of the electrode 12.

It will be apparent that our scavenging action is ob tained in a very simple manner and without the need of any gas derived from an outside source. All of the scavenging gas is obtained from decomposition of the insulating liner 20 by the arc.

The gas for the main jet 50' is likewise derived entirely from decomposition of the insulating liner 20. There is no need to rely on any outside source for this gas. This feature contributes materially to the simplicity and low cost of our arc plasma generator. Another advantage of feeding the arc with wall decomposition products is that we are able to obtain considerably higher temperatures for our plasma jet than can be obtained with the usual plasma generator fed with. gas from an outside source. In this regard, our studies indicate that we can obtain.

temperatures in the range of 30,000 to 50,000 degrees K. A typical maximum temperature range for a plasma generator with metal walls and an outside gas source is about 15,000 to 20,000 degrees K.

One factor that enables us to obtain these very high temperatures is that we can keep the cross-section of our arcing passage 18 very small. Generally speaking and. within limits, the smaller this cross-section, the higher the temperatures obtainable. In the type of plasma genera tort'hat has metal walls, the meal of the walls vaporizes when the cross-section of the arcing passage is reduced to the small values that we may be concerned with, and these metal vapors contaminate the arc plasma. Thus, to avoid such contamination, relatively large cross-section arc passages inust be used with plasma generators of the metal-wall type.

In one specific embodiment of our invention, our arcing passage has inch diameter and a 4 inch length and has been lined with a liner 20 of methyl methacrylate. Power inputs into this generator have been as high as 300 kw. at a current of 700 amperes. In another embodiment, an arcing passage lined with the same material, but one inch in diameter and six inches long has been provided. Power inputs into this generator have been as high as 2000 kw. at 3000 amperes.

In FIG. 4 we have shown an embodiment of our invention which utilizes an arcing passage of a flattened, generally rectangular cross section rather than the cylindrical cross section of FIG. 3. The shorter transverse dimension 1A of such a passageway is controlling in determining the Reynolds number of the fiow in the passage. By keeping this dimension A relatively small and the transverse dimension B relatively large, we can limit the Reynolds number to a sufficientlylow value to inhibit turbulence.

This inhibition of turbulence serves the desirable function of making the arc more stable.

It has been found that our main jet' 50 is exceptionally stable, i.e., free from fluctuations in temperature and velocity. This is believed to be due in large measure to the particular axial flow pattern that is followed by the two main streams of plasma as they travel down the arcing passage toward the exhaust port 22. This axial flow pattern is characterized by a high degree of freedom from transversely-directed components, such as vortextype flow, and this is believed to materially inhibit fluctuations.

Anotherg-factor that inhibits these fluctuations is that the exhaust port 22 is located substantially midway between the electrodes 12. Thus, each of the two main streams flowing toward the exhaust port 22 has substantiallythe same velocity and flow rate, and the streams are therefore able to merge with less turbulence as they exhaust through ports 22.

The composition of the insulating liner 20 will determine the composition of the jet 50. For example, by using a liner of Delrin, which is E. I. du Pont Co.s trademark for a plastic having a formula of (CH O),,, we can obtain a jet consisting of carbon, hydrogen, and oxygen. As another example, by using a liner of methyl methacrylate, (C H O we can obtain a jet consisting of carbon, hydrogen, and oxygen, but in different proportions.

We can also derive our jet from materials normally occurring in the gaseous or liquid states. To do this, we solidify the normally-liquid or normally-gaseous material, converting it into a solid which has the shape of the liner 20.. The resulting solid is then used in the same manner as the above-described liner 20. Examples of materials that can be handled in this manner are water, carbon dioxide, oxygen and nitrogen.

Because of the extremely high temperatures that can be obtained with our plasma generator, our plasma generator can be used to great advantage in spectro-chemical anlysis. One way of doing this is to first pulverize the material that is to be analyzed, and then suitably imbed it in the insulating material of liner 20. When the are is formed, it decomposes this imbedded material along with. the insulating Wall. A spectro-chemical analysis of the extremely hot jet can reveal properties of the test material that would have gone undetected at lower tem peratures.

In connection with spectro-chemical analyzers, it is important to note that the plasma temperature can, by choice of arcing passage diameter, be regulated precisely and repetitively over a Wide range of temperatures. An-- other. significant feature is that with low currents, e.g. 50 amperes or even lower, very high temperatures (of up to 20,000 K.) can be developed, if only the diameter of the arcing passage is made small enough. Since such low currents arecommonly available in laboratories, the versatility of our device in spectro-chemical analysis will be apparent.

Another advantage of our arc plasma generator is that it readily lends itself to operation at high pressures. All thatis required is that the generator be located in a container pressurized to the desired extent. Available evidence indicates that operation at pressures as high as several hundred atmospheres is feasible.

While we have shown and described particular embodi ments of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from our invention in its broader aspects; and we, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An arc plasma generator comprising (a) a pair of electrodes between which an are having its terminals located on said electrodes is adapted to be established,

(b) means defining an arcing passage extending between said electrodes for receiving the column of said arc,

(c) insulating material lining said arcing passage and directly exposed to said are for evolving gases when heated by said are, said gases being heated to a high temperature by said are,

( d) means defining an exhaust passage communicating withsaid arcing passage and extending transversely thereof at a location intermediate said electrodes,

(e) means for forcing most of the gases evolved from said insulating material to flow along said arcing passage toward said exhaust passage and then out through said exhaust passage,

(f) scavenging means for substantially preventing the vapors evolved from each of said electrodes by said are from entering the stream of gases flowing toward said exhaust passage.

(g) said scavenging means comprising means for caus ing a minor portion of the gases evolved from said in sulating material by said are to flow past each of said electrodes in a direction leading away from said exhaust passage.

2. The are plasma generator of claim 1 in which;

(a) each of said electrodes is of a hollow configuration and has an arc terminal-receiving passage ex tending therethrough in general alignment with the portion of said arcing passage adjacent said elec trode, and

(b) said scavenging means comprises means defining a restricted venting passage communicating with each of said arc-terminal receiving passages at the op posite side of said electrode from said arcing passage.

3. The are plasma generator of claim 1 in which:

(a) each of said electrodes is of a hollow configuration and has an arc terminal-receiving passage extending therethrough in general alignment with the portion of said arcing passage adjacent said electrode,

(b) said scavenging means comprises means defining a restricted venting passage communicating with each of said are terminal-recei ing passages at the op-- posite side of said electrode from said arcing passage (c) and sealing means in the region adjacent the up stream surface of each of said hollow electrodes for blocking the escape of gases in a direction. trans versely of, the arcing passage in said regions.

5c The are plasma generator of claim 1 in which said insulating material lining said arcing passage is a material that is a fluid at normal room. temperature but is in a frozen, solid-state condition immediately before said are is established.

St A spectro-chernical analyzer comprising the are plasma generator of claim 1, said insulating liner con taining imbedded particles of the material that is to be analyzed 6 The are plasma generator of claim 1 in which said arcing passage has a transverse cross-sectional configura tion characterized by a relatively small dimension in one direction and a relatively large dimension in a direction transverse to said one direction:

7: The are plasma generator of claim 1 in which? (a) each of said electrodes 18 of a hollow configura= tion and has an arc terminal-receiving passage ex tending therethrough in. general alignment with the portion of said arcing passage adjacent said elec trode,

(b) said scavenging means comprises means defining a restricted venting passage communicating with each of said arc-terminal receiving passages at the op posite side of said electrode from said arcing pas= sage,

(c) each of said electrodes has a downstream surface facing said exhaust passage,

#3 v (d) and means is provided adjacent said downstream surface to normally prevent attachment. of an arc terminal to said downstream surface 8, The arc plasma generator of claim. 1 in which said exhaust passage directly communicates with a space that is at a high pressure relative to normal atmospheric pres sure.

9. An arc plasma generator comprising: (a) a pair of electrodes between which an arc having its terminals located on said electrodes is adapted to be established,

( b) means defining an arcing passage extending be tween said electrodes for receiving the column of: said arc,

(c) insulating material lining said arcing passage and directly exposed to said are for evolving gases when heated by said are, said gases being heated to a high temperature by said arc,

((1) means defining an exhaust passage communicating with said arcing passage and extending transversely thereof at a location intermediate said electrodes,

(e) said insulating material being a material that is a gas at normal room temperature but is in a frozen, solid-state condition immediately before said are is established,

References Cited UNITED STATES PATENTS DAVID L GALVIN, Primary Examiner.

STANLEY D, SCHLOSSER, Examinen

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