US3075065A

US3075065A - Hyperthermal tunnel apparatus and electrical plasma-jet torch incorporated therein

Info

Publication number: US3075065A
Application number: US60364A
Authority: US
Inventors: Adriano C Ducati; Rolf D Buhler
Original assignee: Individual
Current assignee: Individual
Priority date: 1960-10-04
Filing date: 1960-10-04
Publication date: 1963-01-22
Anticipated expiration: 1980-01-22

Description

Sheets 2 Jan. 22', 1963 A. c. DUC l ETAL HYPERTHERMAL TUNNEL AP A ATUS AND ELECTRI PLASMA-JET TORCH INCORPORATED THEREII;

VENTORS. ROLF D. BUHL 1 ER ADRIANO DUCATI ATTORNEY Jan. 22, 1963 A. c. DUCATI ETAL 3,075,065

HYPERTHERMAL TUNNEL APPARATUS AND ELECTRICAL PLASMA-JET TORCH INCORPORATED THEREIN s Sheets-Sheet 3 Filed Oct. 4, 1960 II I I ll QN ll '69 |U0 I Q uumnom 98 V uUmDOm .FZMMKDU n mm Y m w mm R V .C O m m H m R %M 8m H w I WW IE wm NN Eugen m o United States Patent HYPERTHERMAL TUNNEL APPARATUS AND ELECTRICAL PLASMA-JET TORCH INCORPO- RATED THEREIN Adriano C. Ducati, Corona Del Mar, Calif. (R0. Box 1452, Newport Beach, Calif.), and Rolf D. Buhler, 212 Via Koron, Newport Beach, Calif.

Filed Oct. 4, 1960, Ser. No. 60,364 29 Claims. (Cl. 219-75) This invention relates to a hyperthermal tunnel apparatus, and to the electrical plasma-jet torch which forms a major component of such apparatus. The invention also relates to a method of generating plasma, and of effecting aerodynamic testing.

An object of the invention is to provide an electrical plasma-jet torch and method in which the life of the back electrode is greatly increased in comparison to conventional plasma-torch constructions, and in which heat transfer between the gas and back electrode is maximized in order to provide effective pre-heating of the gas and effective cooling of the back electrode by the gas.

A further object is to provide an electrical plasma-jet torch and method in which slight variations in the centering of the back electrode relative to the wall of nozzle passage are rendered relatively unimportant, so that impractically-small machining tolerances are not required.

Another object of the invention is to provide a hyperthermal tunnel apparatus which is characterized by the substantial absence of fluctuations or oscillations in the gas chamber located upstream from the nozzle passage, whereby the apparatus is caused to operate more quietly, smoothly and efliciently than prior-art hyperthermal tunnels of the plasma-torch type.

A further object is to provide a hyperthermal tunnel incorporating means to effect control of the downstream foot point of the electric arc to thus prevent such downstream foot point from uncontrollably reaching the mixing chamber adjacent the throat of the supersonic nozzle.

A further object is to provide a method of generating a stream of high-temperature high-velocity plasma with high efliciency, and of achieving minimum contamination of the plasma by electrode material, maximum life of the back electrode, and minimum interaction between chambers on opposite sides of the gap around the downstream end of the back electrode.

These and other objects and advantages of the invention will be more fully set forth in the following specification and claims, considered in connection with the attached drawings.

In the drawings:

FIGURE 1 is a schematic longitudinal central sectional view of a hyperthermal tunnel apparatus constructed in accordance with a first embodiment of the invention;

FIGURE 2 is a transverse section taken on line 22 of FIGURE 1, and illustrating the tangential introduction of gas into the torch portion of the apparatus;

FIGURE 3 is a greatly enlarged sectional view illustrating the relatively small radial choke gap around the front portion of the back electrode;

FIGURE 4 is a schematic longitudinal central sectional view illustrating a second embodiment of the present invention;

FIGURES 5 and 6 are enlarged fragmentary views illustrating the radial choke gap around the front portion of the back electrode and also illustrating back electrodes which are both water-cooled and not water-cooled; and

FIGURE 7 is a schematic longitudinal central sectional view illustrating a third embodiment of the invention.

Proceeding first to a description of the embodiment of FIGURES 13,'the apparatus is illustrated schematically 3,075,065 Patented Jan. 22, 1963 ICC as comprising an electrical plasma-jet torch 10 which forms the portion of the apparatus to the left of the plane indicated by line X-X in FIGURE 1. The plasma generated in torch 10 flows into a mixing chamber or inlet region 11, where it is thoroughly mixed so that substantially all portions of the plasma have the same temperature. The plasma discharges from the mixing chamber through the throat 12 of a supersonic nozzle 13 into a pressure-resistant tank or vacuum chamber indicated schematically at 14.

The tank 14 is actually a large pressure-resistant vessel the size of which is many times greater, both longitudinally and in diameter, than that of the torch, mixing chamber and nozzle apparatus. Tank 14 has a test section or portion adjacent nozzle 13, and adapted to contain a test object against which plasma is directed from nozzle 13. A pump 16 is connected to tank 14 at a location remote from the supersonic nozzle 13 to effect substantial evacuation of the chamber within the tank, for example to a pressure on the order of a few millimeters of mercury or less. The pump 16 has suflicient capacity to maintain the tank substantially evacuated during continuous operation of the plasma-injector apparatus. Suitable cooling means, not shown, are provided to effect cooling of the gas before introduction thereof into the intake of pump 16.

Electrical plasma-jet torch 10 comprises electricallyconductive metal wall means 17 to define an elongated are or nozzle passage 18 the downstream wall portion 19 of which is substantially cylindrical, being a surface of revolution about the longitudinal axis of the apparatus. Cylindrical wall 19 merges at its end remote from mixing chamber 11 with a generally flared or conical wall portion 21 (FIGURE 3) opening into a pressure chamber 22, such chamber and the flared portion 21 being coaxial with wall 19. The space radially-inwardly from wall 21 (and corresponding walls in subsequent embodiments) is to be regarded as an extension of the arc or nozzle passage 18.

Mounted coaxially in pressure chamber 22 is a generally conical back electrode 23 having a front or working portion 24 located radially-inwardly of the flared wall 21 indicated above. Electrode portion 24 penetrates to the vicinity of the junction between walls 19 and 21, somewhat forwardly of such junction. The exterior surface 26 of portion 24 has substantially the same cone angle as that of wall 21, so that surface 26 and wall 21 are substantially parallel to each other. The back electrode, at least at front portion 24, is formed of a suitable refractory metal such as thoriated tungsten.

At its base, remote from nozzle passage 18, the back electrode 23 is formed with a flange 27 which is in close electrical contact with the generally cup-shaped rear wall 28 of the apparatus. The exterior edge or rim of flange 27 is a cylinder concentric with and spaced radially-inwardly from the cylindrical wall 29 of pressure chamber 22. The radial front wall of flange 27 is spaced sufiiciently far from the working portion 24 of the back electrode that gas flowing tangentially in pressure chamber 22 will be substantially free of turbulence by the time it enters the gap between surfaces 21 and 26.

A suitable gas source, indicated schematically at 31, is connected through a passage or conduit 32 to the portion of pressure chamber 22 radially-outwardly of flange 27, such passage being tangential to the pressure chamber (FIGURE 2) so that gas entering the pressure chamber flows vortically at high velocity. The vortically-flowing gas then flows vortically (helically) through the aboveindicated gap between surfaces 26 and 21, and thereafter flows vortically (helically) in the are or nozzle passage 18 for discharge into mixing chamber 11 and subsequently into vacuum tank or chamber 14.

The portion of back electrode 23 rearwardly of working chamber 33 is formed through which water or other coolant may be passed by means of conduits 34. Correspondingly, the wall means 17 defining passage 18, pressure chamber 22, etc., may be formed with an annular cooling chamber 36 through which Water or other coolant is passed by means of conduits 37. As is shown in FIGURE 1, the cooling chamber 36 may be axially extended to surround the walls defining mixing chamber 11 and the supersonic nozzle 13.

Suitable insulation 38 is provided between the electrically-conductive rear wall 28 and wall means 17, such insulation being shielded from the electric are so that there is no radiation damage to the insulation. A suitable power source 39 adapted to deliver a very high current is connected through

leads

41 and 42 to back electrode 23 and to the wall means 17 which define the nozzle passage 18 and form the front electrode. Source 39 is preferably a source of DC. current capable of delivering many hundreds, thousands or millions of amperes, being normally so connected that the back electrode is negative and the front electrode is positive.

The wall means 17 is also adapted to define the aboveindicated mixing chamber or inlet region 11, such chamber having a diameter much larger than that of nozzle passage 18 or of throat 12. The mixing chamber is illustrated as having a cylindrical wall except for a convergent region adjacent the inlet portion of throat 12. Also, the nozzle passage 18 is shown as having a rounded or beveled portion at the inlet to the mixing chamber. A suitable source 43 of gas is connected through a conduit 44 to mixing chamber 11 in order to introduce therein auxiliary gases or liquids adapted to be mixed with the plasma discharged from nozzle passage 18. Source 43 may also be adapted to introduce suitable powders adapted to react in the mixing chamber, for example in chemical synthesis applications of the apparatus.

The supersonic nozzle 13 has a construction adapted to cause the flow of plasma from mixing chamber 11 into vacuum tank 14 to be supersonic and fully developed. Such flow is normally parallel, so that the shaft of discharging plasma is cylindrical in shape as well as being relatively long. As previously indicated, all operative walls of the supersonic nozzle, mixing chamber, nozzle passage, front and back electrodes, and pressure chamber are surfaces of revolution about the longitudinal axis of the apparatus.

THE SMALL RADIAL CHOKE GAP AROUND- WORKING PORTION 24 OF BACK ELECTRODE 23 It is an important feature of the invention that only a small radial gap 46 (FIGURE 3) is provided around Working portion 24 of back electrode 23, that is to say between surfaces '21 and 26 in the embodiment of FIG- URES l-3. Additionally, it is an important feature of the invention that the pressures in chambers 11 and 22,

as well as the construction of the are or nozzle passage 18, are so correlated to each other and to the radial gap 46 that the flow of gas through such gap is choked. More specifically, such factors are so correlated, as will be discussed subsequently, that the fiow through gap 46 is some.

An important consequence of the above is that there is a very great heat transfer between the gas and the electrode portion 24, which not only provides efificient cooling of such portion but also efiectively pre-heats the gas to result in better arcing in the passage 18 (downstream from the back electrode) and in greatly improved torch efficiency. The degree of heat transfer from electrode portion 24 is so great that the life of the back electrode 23 is many times that of conventional back electrodes in plasma-jet torches. It is emphasized that the maximum heat transfer occurs at the most important point, namely adjacent the extreme forward end of back electrode 23, and that heat transfer from the back electrode is to the gas instead of to the cooling water (which is wasted). It is thus possible to employ less water cooling than in prior-art constructions, and in some instances no water cooling.

The surfaces 21 and 26 are substantially parallel, in addition to being close to each other, so that only a thin layer of gas is passed therebetween from chamber 22 to passage 18. Such gas layer is preferably so thin that substantially all portions thereof absorb heat efliciently, as distinguished from prior-art constructions in which the portions of the gas substantially spaced from the back electrode wall were much cooler than the portions immediately adjacent thereto.

A second important consequence of the close radial spacing of the opposed surfaces at the working portion 24 of back electrode 23 is that, except during starting of the torch, the arc is prevented from being present between surfaces 21 and 26. Instead, the arc (indicated at 47) is caused to be present between the extreme forward end or tip 48 of the back electrode and a portion of the wall 19 a substantial distance downstream from the back electrode. This is a surprising result since it has conventionally been thought that in order to prevent arcing between two electrodes it is advisable to increase, instead of decrease, the distance therebetween. Applicants have discovered, however, that to effectively prevent arcing between the two surfaces it is advisable to decrease the distance therebetween and maintain a high mass flow rate therebetween as will be discussed hereinafter. The result is an eflicient insulation effect in the gap 46, resulting from factors including the fact that the gas is flowing so fast that ionization does not occur until the gas is at a point adjacent the extreme tip 48 of electrode 23.

A further important advantage resulting from the small radial gap, and from other factors discussed herein, is that choked flow is produced in the gap 46. The various factors are so related that flow in gap 46 is choked at least at all times when flow through any portion of apparatus downstream from gap 46 is choked. The gap 46, and the pressure ratios, are such that the flow through gap 46 is sonic as distinguished from either subsonic or supersonic. It follows that a maximum mass flow rate is achieved through gap 46 with consequent maximized heat transfer effect, insulation effect, etc.

Because of the choked flow in gap 46, fluctuations or oscillations present in the gas downstream from choke 46, for example in the mixing chamber 11, do not affect the flow of gas in the pressure chamber 22 or through gap 46. Stated otherwise, the choked flow through gap 46 acts as a barrier which isolates chamber 22 from arc or nozzle passage 18, mixing chamber 11, nozzle 13, etc. This is an important feature since such oscillations or fluctuations have previously resulted in noisy operations, reduced efiiciency, etc.

The exact gap or spacing between the surfaces at choke 46 depends upon a large number of factors including the enthalpies, the desired total mass flow through the apparatus, the type of gas, etc., but should be on the order of one-sixteenth of an inchdown to a few thousandths of an inch or less. The gap size may depend upon machining considerations, that is to say the gap is made just large enough to insure that surfaces 21 and 26 will not touch each other at any point despite the presence of normal machining tolerances. Also, the gap should be made sufficiently large that slight protuberances on surfaces 21 and 26 will not result in bridging of the gap.

It is important that the minimum cross-sectional area of passage 18 downstream from tip 48 should be many times the minimum cross-sectional area of choke gap 46. More specifically, the ratio of the minimum cross-sectional area of passage 18 to the minimum cross-sectional area of choke gap 46 should be at least on the order of five to one. In general, such area ratio should be on the order of the square root of the quotient obtained by dividing the average absolute temperature in passage 18 (downstream from tip 48) by the absolute temperature in chamber 22. Such area ratio thus increases with an increase in current or a decrease in gas flow rate.

THE VORTICAL FLOW OF GAS The vortical gas flow is caused to be such that a gas vortex stabilization action is produced in the nozzle passage 18 to cause the are 47 to be stabilized therein. The vortically-flowing or whirling gas downstream from the back electrode causes the hottest portion of the arc to be near the axis of passage 18 whereas the cooler portion of the gas is adjacent the wall 19, the result being that there is relatively little heat loss to the cooling water in chamber 36.

With relation to the small radial gap around electrode portion 24, the vortical gas flow has been found to make centering of the back electrode relative to wall 21 much less critical than in constructions employing axial gas flow. This is because axially-flowing gas tends to concentrate and flow longitudinally of the back electrode between surfaces 21 and 26 at regions where the spacing therebetween is greatest. It follows that uncontrolled arcing takes place between surfaces 21 and 26, causing burning of back electrode 23 to drastically reduce the life thereof. With vortical flow, on the other hand, the gas is caused to fiow uniformly around the surface 26 regardless of small variations in the distance of surface 21 therefrom. Substantially the same mass flow is thus present between all portions of surfaces 21 and 26 regardless of slight variations in the centering of the back electrode relative to surface 21, so that the above-indicated uncontrolled arcing between such surfaces is prevented. It follows that the plasma torches may be manufactured with small gaps in the absence of fantastically close tolerances such as may not be employed with any degree of economy.

It is emphasized that the vortical flow of gas increases the time required by a particular gas molecule to pass from chamber 22 to the vicinity of tip 48. Each gas molecule is therefore present between the closely-spaced surfaces 21 and 26 for a longer period of time than in torches wherein the flow is purely axial, so that the above indicated pre-heating of the gas and cooling of the electrode tip 24 are enhanced.

The vortically-tlowing gas produces the further benefit that the downstream foot-point where arc 47 contacts wall 19 is rotated about the axis of the torch, instead of burning in at one point and forming a crater.

In order to achieve the above-indicated benefits of vortical flow, it is important that at least the major portion of the gas flow be of the vortical type effected by tangential introduction of gas, in contrast to situations where a small vortical component is superimposed upon a gas flow which is primarily axial. Thus, for example, if chamber 22 were merely filled with gas under high pressure and without vortical flow, and only a small amount of gas were introduced tangentially through conduit 32, the benefits of vortical flow would be reduced or eliminated. It is pointed out that the length of choke gap 46 should not be so great that friction losses therein attenuate the vortex action excessively.

ADDITIONAL IMPORTANT FACTORS An important factor is that the downstream foot point where the electric are 47 enters wall means 17 must be controlled, it being highly undesirable that the foot point uncontrolledly enter the mixing chamber 11 and engage the wall thereof. This is because the plasma in mixing chamber 11 should be electrically neutral and should have a precisely controlled pressure and temperature. If the arc 47 engages the wall of the mixing chamber 11 at various uncontrolled points, the plasma in such chamber is not only prevented from being neutral but is also uncontrolledly influenced by the are. As will be discussed in detail subsequently relative to the embodiment of FIGURE 7, the downstream foot point of are 47 is controlled and is caused to remain in passage 18 by precisely regulating the enthalpy, the pressure in chamber 11, the shape of passage 18, and other factors, in such manner that the gas flow velocity in passage 18 is subsonic.

In order to effect highly efficient heat transfer between the are 47 and the gas in are or nozzle passage 18, such passage should have a relatively small cross-sectional area. The gas is thus caused to fiow adjacent and through the are 47 for efficient heating thereby. Efficient heating is also effected by causing the length of nozzle passage 18 (downstream from the extreme end 48 of back electrode 23) to be a substantial number of times the minimum diameter thereof. Thus, the length of nozzle passage 18 should be at least two times the minimum diameter thereof, and preferably a multiplicity of times the minimum diameter thereof.

Another factor which is highly pertinent to the distance between the surfaces at choke gap 46, and other factors, is the type of gas which is passed through the torch. It is preferred that the gas be one having good insulation characteristics, that is to say which produces a relatively great effect simulating that of insulation. Nitrogen and hydrogen are examples of gases of this type, as are mixtures of nitrogen and hydrogen. On the other hand, gases such as argon require a larger gap since they produce a smaller effect simulating that of insulation. It is important that the gas flowing through choke gap 46 be free of electrically-conductive material such as metal pow der, this being because such powder tends to cause electrical breakdown in the gap and destroy or minimize the insulation effect of the gas.

As previously indicated, the pressures in chamber 22 and in the vacuum chamber 14 are important, as is the pressure in the mixing chamber 11. The pressure ratios should be selected to cause the mass flow through choke gap 46 to be maximum, the flow velocity being sonic, in order to create the above-indicated insulation and other effects. Also, the ratio between the pressure in chamber 22 and that in mixing chamber 11 is so selected in com parison to other factors that the gas flow in passage 18 is caused to be subsonic to thereby prevent the are 47 from being blown uncontrolledly into the mixing chamber.

EMBODIMENT OF FIGURES 4-6 Except as will be described in detail, the embodiment of FIGURES 4-6 is identical to that of FIGURES 1-3, and has been given corresponding reference numerals. It is to be understood that the tank 14 and pump 16 are also employed in the embodiment of FIGURES 4-6, as well as in that of FIGURE 7 to be described hereinafter.

In the embodiment FIGURES 4-6, the conical wall portion 21 of nozzle passages 18 is eliminated and is replaced by a rounded wall 50 which diverges rearwardly into the pressure chamber 22a. The back electrode 51 (FIG- URE 6) of the embodiment of FIGURES 4-6 has a substantially cylindrical forward end portion 52 which extends forwardly into the nozzle passage 18. The choke gap is thus formed at 46a between the external cylindrical surface 53 of electrode portion 52, and the internal cylindrical wall 19 of the nozzle passage 18. Such radial choke gap 46a is shaped as an annulus and has a very small radial dimension as stated in connection with the gap 46 of the previous embodiment.

The portion 52 of the back electrode is shown as having a relatively blunt and generally radial forward surface 54 to which the are 47 strikes. Such blunt forward surface has been found to create a certain degree of turbulence which produces a degree of movement of the rear foot point of arc 47 to thereby minimize burning-in and cratering effects.

The electrode 51 has a radial rear portion 56 corresponding generally to the flange 27 of the previous embodiment. The forward surface of radial portion 56 is fillet shaped at 57, opposite the beveled or rounded surface 50. A convergent region is thus formed between the sur- 7 faces 57 and 50 to elfect feeding of the vortically-flowing gas into the choke gap 46a.

In FIGURES 4 and 6, the wall means 17a defining the are or nozzle passage 18, etc., is not provided with coolant chambers whereas the back electrode 51 is provided with the coolant chamber 33a fed by conduits 34. In certain instances, however, the chamber at 33a may be omitted, as shown in FIGURE 5. In such situations, and also in situations when the chamber 33a is employed, the wall means 17a may be provided with a cooling chamber corresponding generally to that schematically illustrated at 36 in FIGURE 1.

The embodiment of FIGURES 4-6 is, at least in situations where very high currents are employed, inferior to that of FIGURES 1-3. One reason for this is that the embodiment of FIGURES 13 produces the greatest cooling of the back electrode at points adjacent the tip 48, where the current density in the back electrode is greatest.

EMBODIMENT OF FIGURE 7 Except as will be specifically described, the embodiment of FIGURE 7 corresponds to that of FIGURES l-3, and also incorporates the vacuum tank 14 .and pump 16.

According to the embodiment of FIGURE 7, the generally conical surfaces 21a and 260 are formed with different included angles such that they converge in a forward direction. The cross-sectional area of the choke gap thus is reduced, at points progressively downstream from chamber 22a, more rapidly than in the embodiment of FIGURES 1-3. A very rapid gas flow occurs around the extreme forward end 48a of the back electrode 23a, where the heating of the back electrode by the current passing therethrough is the greatest. However, in this embodiment, undesired arcing is more likely to occur around portion 24a at regions adjacent chamber 22a.

The embodiment of FIGURE 7 is formed with a nozzle or are passage 18a which, instead of being cylindrical as in previous embodiments, diverges in a downstream direction so that, in the illustrated form, the wall 19a is conical and has its wide end at the mixing chamber 11 and its narrow end at the tip 48a of the back electrode. The formation of the nozzle passage 18a in such forwardly-divergent manner has been found to cause the are 47 to remain in the passage under the same conditions of electric power, pressure, etc., that would cause the downstream portions of the arc to be blown to a point adjacent throat 12 if the passage 18a were cylindrical.

To maintain all portions of the arc 47 in passage 18a, in this and previous embodiments, the pressures in chamber 22a and 11 are so correlated to each other, to the elec tric power fed to the apparatus, and other factors, that the gas flow in at least the downstream portion of passage 18a is subsonic. Thus, the gas flow increases to sonic in the choke between surfaces 21a and 26a, is subsonic in at least the downstream portions of nozzle passage 18a, is very subsonic in the mixing chamber 11, and increases to fully-developed supersonic flow upon passage through the supersonic nozzle 13. Stated in another way, the pressure in mixing chamber 11 is caused to be sufficiently high to prevent the gas flow in at least the downstream portion of passage 18a from being sonic or supersonic.

It is to be understood that the embodiments of FIG- URES 1-6 may also be formed with the forwardly-divergent arc or nozzle passage. In all of such constructions, the cross-sectional area of the nozzle passage (numbered 18a in FIGURE 7), at points adjacent chamber 11, should be much greater than the cross-sectional area of throat 12.

Instead of (or in addition to) causing the are or nozzle passage to diverge forwardly, the diameter thereof adjacent but spaced somewhat from chamber 11 may he suddenly increased or stepped. The arc then strikes to the relatively large-diameter wall of the nozzle passage,

at points immediately adjacent (but not in) the chamber 11.

In all embodiments, it is desirable that the wall means 17, 17a or 1712 be divided into two mutually-insulated portions. The insulator for accomplishing this result should lie in a plane perpendicular to the axis of the apparatus and located at the junction between the arc passage and the mixing chamber.

DESCRIPTION OF METHOD, AND SUMMARY OF OPERATION In all embodiments of the invention, an arc may be initiated in the choke gap by injecting therein ionized gas, namely by means of the arc-starting apparatus described in co-pending patent application Serial No. 790,692, filed February 2, 1959, for Apparatus and Method for Initiating an Electrical Discharge. The starting apparatus projects through the wall means 17 and terminates in an explosion chamber which communicates through a part in surface 21 (FIGURE 3) with the choke gap 46 at a point adjacent the working portion 24 of the back electrode. Once an arc has been initiated, the vortically flowing gas moves the arc downstream and causes it to stretch out along the passage 18 to assume the position indicated at 47.

As previously stated, the pressure ratios and other factors are so calculated that gas flow through the choke gap 46 is sonic, at least adjacent the extreme forward end of the back electrode, which flow in combination with the close spacing between the walls near the tip of the back electrode create the previously-described benefits of high heat transfer, long electrode life, high efiiciency, prevention of arcing across the choke gap, etc. The pressure ratios are also so selected that the arc 47 will not be blown uncontrolledly into the mixing chamber 11 for striking at some random point of the wall thereof, or even of the wall of the supersonic nozzle 13.

As an example, reference is made to the embodiment of FIGURES l3. The nozzle passage 18 (downstream from tip 48) may have a diameter of 1 inch, and may have a length (downstream from tip 48) of 4 inches. The distance between surfaces 21 and 26 is inch. The inner diameter of pressure chamber 22 is 2 /2 inches, the diameter of throat 12 is 1 inch, and the diameter of mixing chamber 11 is 2 /2 inches. Power is fed to the apparatus from the current source 39, the current being 2,000 amperes DC. at a voltage of volts, with the back electrode negative and the wall means 17 positive. The pressure in chamber 22 is 1 atmosphere absolute, and that in mixing chamber 11 is on the order of /2 atmosphere or lower. These pressures are total pressures. The tank 14 is evacuated to a pressure less than 10 mm. mercury by the pump 16, the latter communicating with the end of the vessel remote from nozzle 13.

The gas passed through the torch may be nitrogen, and oxygen may be introduced from source 43 into mixing chamber 11 in such proportion that the outflowing plasma has a composition simulating that of atmospheric air. A test object is mounted in the evacuated chamber 14 adacent the supersonic nozzle 13, so that the plasma will be directed against the test object as desired. As previously stated, various chemicals may be introduced through conduit 44 from source 43, or from other sources of liquids, gases, powders, etc., for mixing and melting in the chamber 11 to provide various results such as chemical synthesis.

It is to be understood that the plasma-jet torch portion of each apparatus, that is to say the portion upstream from mixing chamber 11 (to the left of plane XX in FIGURE 1, and corresponding planes in FIGURES 4 and 7), may be used separately from the remaining (downstream) portions of the apparatus. The water chamber 36 would then, of course, have a complete radial wall adacent the downstream end of passage 18.

Various embodiments of the present invention, in addition to what has been illustrated and described in detail, may be employed without departing from the scope of the accompanying claims.

We claim:

1. An electrical plasma-jet torch, which comprises wall means having an arc passage therethrough, the wall of said arc passage being a surface of revolution about a central axis, a protuberant back electrode having an exterior surface shaped as a surface of revolution about said central axis, said exterior surface of said back electrode being disposed radially inwardly from said passage wall at least at the forward tip portion of said back electrode, the minimum spacing between said passage wall and said back electrode surface at said tip portion being sufficiently small in relation to the mass flow of gas through the torch to form a choke gap around said tip portion, front-electrode means provided in said torch downstream from said tip portion, current-supply means to effect flow of a high current through said back electrode and through said front electrode means to maintain a high-current electric arc in said arc passage forwardly of said tip portion, and means to effect sufficient mass fiow of gas through all portions of said choke gap and then through said are passage that said flow is choked in said choke gap, said gas being pre-heated in said choke gap and effecting cooling of said tip portion of said back electrode, said gas then being heated by said arc in said arc passage to form plasma.

2. The invention as claimed in claim 1, in which the minimum spacing between said passage wall and said back electrode surface at said tip portion is on the order of one-sixteenth inch and smaller.

3. The invention as claimed in claim 2, in which the minimum cross-sectional area of said are passage downstream from said back electrode is at least on the order of five times the minimum cross-sectional area of said choke gap.

4. The invention as claimed in claim 1, in which said wall means is formed of electrically-conductive material at least at said passage wall in the vicinity of said tip portion and downstream therefrom to form said frontelectrode means, and in which said current-supply means is connected to said back electrode and to said electrically-conductive material for maintenance of an arc therebetween, said are being prevented by said gas from traversing said choke gap.

5. An electrical plasma-jet torch, which comprises wall means having an arc passage therethrough, the wall of said are passage being a surface of revolution about a central axis, a protuberant back electrode having an exterior surface shaped as a surface of revolution about said central axis, said exterior surface of said back electrode being disposed radially inwardly from said passage wall at least at the forward tip portion of said back electrode, the minimum spacing between said passage wall and said back electrode surface at said tip portion being sufficiently small in relation to the mass flow of gas through the torch to form a choke gap around said tip portion, current-supply means to effect flow of a high current through said back electrode to maintain a highcurrent electric arc in said passage forwardly of said tip portion, and means to effect vortical flow of gas through said choke gap and then through said passage, said flow being vortical about said central axis, the mass flow of said gas being sufiicient that said flow is choked in said choke gap, said gas being pre-heated in said choke gap and effecting cooling of said tip portion of said back electrode, said gas then being heated by said are in said passage to form plasma.

6. The invention as claimed in claim 5, in which the minimum spacing between said passage wall and said back electrode surface at said tip portion is on the order of one-sixteenth inch and smaller.

7. The invention as claimed in claim 6, in which the minimum cross-sectional area of said are passage downstream from said back electrode is at least on the order of five times the minimum cross-sectional area of said choke gap.

8. The invention as claimed in claim 5, in which front electrode means are provided at said passage wall downstream from said tip portion of said back electrode, and in which said current-supply means is connected to said back electrode and said front electrode means for maintenance of an arc therebetween in said passage.

9. The invention as claimed in claim 8, in which said wall means is formed of electrically-conductive material at least at said passage wall in the Vicinity of said tip portion and downstream therefrom to form said front-elec trode means, and in which said current-supply means is connected to said back electrode and to said electricallyconductive material for maintenance of an arc therebetween, said arc being prevented by said gas from traversing said choke gap.

10. An electrical plasma-jet torch, which comprises wall means having an arc passage therethrough, the wall of said passage being a surface of revolution about a central axis, a protuberant back electrode having an exterior surface shaped as a surface of revolution about said central axis, said exterior surface of said back electrode being disposed radially inwardly from said passage wall at least at the forward tip portion of said back electrode, the minimum spacing between said passage wall and said back electrode surface at said tip portion being sufficiently small in relation to the mass flow of gas through the torch to form a choke gap around said tip portion, said passage wall being constructed to prevent choked gas flow in said passage downstream from said tip portion unless gas flow in said choke gap is also choked, current-supply means to effect flow of a high current through said back electrode to maintain a high-current electric arc in said passage forwardly of said tip portion, and means to effect vortical flow of gas about said central axis through all portions of said choke gap and then through said passage at a massfiow rate sufficient that said flow is choked in said choke gap, said gas being pre-heated in said choke gap and ef- =fecting cooling of said tip portion of said back electrode, said gas then being heated by said are in said passage to form plasma.

11. The invention as claimed in claim 11, in which the minimum cross-sectional area of said passage downstream from said tip portion of said back electrode is many times the minimum cross-sectional area, of said choke gap.

12. An electrical plasma-jet torch, which comprises wall means having an arc passage therethrough, the wall of said passage being a surface of revolution about a central axis, a protuberant back electrode having an exterior surface shaped as a surface of revolution about said central axis, said exterior surface of said back electrode being spaced radially inwardly from said passage Wall at least at the forward tip portion of said back electrode, the spacing between said passage wall and said back electrode surface at said tip portion being small whereby a choke gap is formed therebetween, front electrode means provided in said torch at said passage downstream from said tip portion, means to effect flow of a high current through said back electrode and through said front electrode means to maintain a high-current electric arc therebetween in said passage forwardly of said tip portion, and means to effect flow of gas at sonic velocity through said choke gap, said gas then flowing through said passage for heating by said are to form plasma.

13. The invention as claimed in claim 12, in which said wall of said passage is so shaped that said gas flows at sub-sonic velocity downstream from said tip portion.

14. An electrical plasma-jet torch, which comprises nozzle electrode means formed of metal and having an elongated nozzle or arc passage therethrough, an elongated metal back electrode insulated from said nozzle electrode means, said back electrode having at least its forward end portion inserted into said nozzle passage, the

surface of said forward end portion being spaced radially inwardly from said passage wall a slight distance to form a choke gap around said forward end portion, currentsupply means to effect fiow of a high current through said nozzle electrode means and said back electrode to maintain an arc therebetween in said nozzle passage forwardly of said back electrode, and means to effect vortical flow of gas around said back electrode in said choke gap and then effect flow of said gas through said nozzle passage forwardly of said back electrode for conversion by said are into plasma, said gas-flow means being adapted to effect suflicient mass flow of gas in said choke gap that said flow is choked therein.

15. The invention as claimed in claim 14, in which said space between said surface and wall at said choke gap is on the order of one-sixteenth inch and less.

16. The invention as claimed in claim 14, in which said current-supply means is a DO source having its positive terminal connected to said nozzle electrode means, and its negative terminal connected to said back electrode, and in which means additional to said gas How are provided to cool at least one of said back electrode and said nozzle electrode means.

17. The invention as claimed in claim 14, in which the length of said nozzle passage downstream from the forward end of said back electrode is at least a plurality of times the minimum diameter of said nozzle passage, and in which the minimum cross-sectional area of said nozzle passage downstream from said forward end of said back electrode is many times the minimum cross-sectional area of said choke gap.

18. The invention as claimed in claim 14, in which said Wall of said nozzle passage is divergent in a direction forwardly from said choke gap.

19. The invention as claimed in claim 14, in which opposed portions of said nozzle passage wall and of said surface are generally conical.

20. The invention as claimed in claim 19, in which the cone angle of said nozzle passage wall is substantially greater than that of said surface.

21. The invention as claimed in claim 14, in which opposed portions of said nozzle passage wall and of said surface are generally cylindrical,

22. The invention as claimed in claim 14, in which said nozzle passage wall has a substantially conical upstream portion converging forwardly to a substantially cylindrical downstream portion coaxial therewith, and in which said forward end portion of said back electrode is substantially conical and is disposed radially-inwardly of said upstream portion of said nozzle passage wall, said forward end portion converging forwardly to a tip located forwardly of the junction between said conical and cylindrical portions of said nozzle passage wall.

23. The invention as claimed in claim 14, in which the extreme forward end of said back electrode is relatively blunt.

24. A method of generating electrical plasma, comprising providing a back electrode having at least its forward end portion inserted into a nozzle passage and disposed sufficiently close to the wall of said nozzle passage to form a small choke gap radially around said forward end por' tion, the distance between opposed surfaces of said back electrode and wall at said choke gap being caused to be on the order of one-sixteenth of an inch and less, maintaining a high-current electric arc in said passage forwardly of said back electrode and by passing current through said back electrode, and passing gas forwardly through said choke gap and passage at a sufiiciently high mass-flow rate to effectively cool said forward end portion and heat said gas prior to additional heating thereof in said passage by said arc.

25. The invention as claimed in claim 24, in which said method includes the step of selecting said gas from a group consisting of hydrogen and nitrogen and mixtures thereof, said gas being free of conductive powder.

26. The invention as claimed in claim 24, in which said method includes the gs tep of maintaining said are in said passage only, impressing a negative DC. voltage on said back electrode, and impressing a positive DC. voltage on an electrically-condugtive portion of said nozzle passage wall downstream froin said back electrode.

27. A method of; generating electrical plasma, comprising providing a back electrode having at least its forward end portion inserted into a nozzle passage and disposed sufiiciently close to the wall of said nozzle passage to form a small choke gap radially around said forward end portion, maintaining a highcurrent electric arc in said passage forwardly of said back electrode and by passing current through said back electrode, and effecting forward flow of gas vortically around said back electrode and through said choke gap at a sufiiciently high mass-flow rate to effectively cool said forward end portion and heat said gas prior to additional heating thereof in said passage by said arc.

28. The invention as claimed in claim 27, in which said method comprises causing said choke gap to be sulficiently small that the distance between opposed surfaces of said back electrode and wall at said choke gap is on the order of one-sixteenth of an inch and less.

29. A method of generating electrical plasma, comprising providing electrically-conductive metal wall means to define an elongated nozzle passage, providing a back electrode having at least its forward end portion inserted into said nozzle passage and disposed sufficiently close to the wall thereof to form a small choke gap having a radial dimension on the order of one-sixteenth of an inch and less, maintaining a high-current electric arc in said passage forwardly of said back electrode by passing a current through said back electrode, and effecting forward flow of gas vortically around said back electrode and through said choke gap at a sufiiciently high mass-flow rate to effectively cool said forward end portion and heat said gas prior to additional heating thereof in said passage by said are.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Research & Devolpment, Jan. 1960,

pp. 5-9-11-12 and 15.

Claims

1. AN ELECTRICAL PLASMA-JET TORCH, WHICH COMPRISES WALL MEANS HAVING AN ARC PASSAGE THERETHROUGH, THE WALL OF SAID ARC PASSAGE BEING A SURFACE OF REVOLUTION ABOUT A CENTRAL AXIS, A PROTUBERANT BACK ELECTRODE HAVING AN EXTERIOR SURFACE SHAPED AS A SURFACE OF REVOLUTION ABOUT SAID CENTRAL AXIS, SAID EXTERIOR SURFACE OF SAID BACK ELECTRODE BEING DISPOSED RADIALLY INWARDLY FROM SAID PASSAGE WALL AT LEAST AT THE FORWARD TIP PORTION OF SAID BACK ELECTRODE, THE MINIMUM SPACING BETWEEN SAID PASSAGE WALL AND SAID BACK ELECTRODE SURFACE AT SAID TIP PORTION BEING SUFFICIENTLY SMALL IN RELATION TO THE MASS FLOW OF GAS THROUGH THE TORCH TO FORM A CHOKE GAP AROUND SAID TIP PORTION, FRONT-ELECTRODE MEANS PROVIDED IN SAID TORCH DOWNSTREAM FROM SAID TIP PORTION, CURRENT-SUPPLY MEANS