US3614376A

US3614376A - Plasma torch

Info

Publication number: US3614376A
Application number: US847948A
Authority: US
Inventors: Tosikatu Manabe; Tetsuo Geiyo; Yasuzi Hamura
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1968-08-07
Filing date: 1969-08-06
Publication date: 1971-10-19
Anticipated expiration: 1988-10-19
Also published as: DE1940040B2; DE1940040A1

Abstract

A plasma torch provided with a cathode-supporting structure having a recess formed at one end thereof opposed to a nozzle, said recess having its bottom surface carrying a cathode tip attached thereto and having its peripheral surface formed with shielding gas injection holes opening into said recess tangentially to the periphery thereof, wherein said shielding gas injection holes are located at a distance from the electron emission surface of said cathode tip and the diameter of said recess at its opening end is smaller than the diameter of said recess at the portion thereof where said shielding gas injection holes are formed.

Description

inventors Tosilratu Manabe l-lachioji-shi; Tetsuo Gejyo, Tokyo; Yasuzi Hamura, Tokyo, all of Japan Appl. No. Filed Patented Assignee 847,948 Aug. 6, 1969 0a. 19, 1971 Hitachi, Ltd.

Tokyo, Japan Priority Japan Aug. 7, 1968 PLASMA TORCH 11 Claims, 12 Drawing Figs.

US. Cl.

Int. Cl.

Field of Search 219/121 P, v 219/75 B23k9/00 COOL/N6 77 mm? 4% 645 [56] References Cited UNITED STATES PATENTS 3,027,446 3/ I962 Browning 219/75 3,118,046 1/1964 Harrington 2l9/75 3,131,288 4/1964 Browning 219/75 X Primary Examiner-J. V. Truhe Assistant Examiner-C. L. Albritton A!l0rneyCraig, Antonneli and Hill ABSTRACT: A plasma torch provided with a cathode-supporting structure having a recess formed at one end thereof opposed to a nozzle, said recess having its bottom surface carrying a cathode tip attached thereto and having its peripheral surface formed with shielding gas injection holes opening into said recess tangentially to the periphery thereof, wherein said shielding gas injection holes are located at a distance from the electron emission surface of said cathode tip and the diameter of said recess at its opening end is smaller than the diameter of said recess at the portion thereof where said shielding gas injection holes are formed.

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f SHEET u UF 4 ARC GAS we WATER mm? v INVENTORS rosrKATq MANABE' srsuo GEJYo d ynSu zr HAMuRA W, W q 1m ATTORNEYS PLASMA TORCH BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a plasma jet torch, and more particularly to the construction of such a torch in which the resistivity of the cathode and nozzle can be improved when active gas is used as operating gas.

DESCRIPTION OF THE PRIOR ART A plasma jet flame has been extensively used in various fields of art such as welding, cutting and fusing of various workpieces, coating of metallic or ceramic material by spraying, chemical reactions, sources of light and heat, and so on.

The plasma jet flame utilized in these fields is of a relatively great capacity and the electrodes in the torch for producing such a plasma flame are exposed to a very high temperature during the operation. Especially the are spot of the cathode and its neighboring portion are not seldom exposed to a high temperature above thousands of degrees centigrade.

In order to prevent the oxidation of the electrodes, nonoxidizable gases such as argon, nitrogen, hydrogen and the like are often used as the operating gas for producing a plasma jet flame. These nonoxidizable gases, however, are usually expensive, and especially they are economically disadvantageous if the plasma jet flame resulting from them is used in such operations as welding, cutting and fusing which require a great deal of operating gas.

For this reason, attempts have recently been made to employ an oxidizable gas such as air or the like as the operating gas. One'of these attempts has proposed to construct the electrodes, especially the cathode, with a material which is highly resistive to oxidation even at a high temperature, and another attempt is that a small quantity of inert gas be used to shield the cathode surface and thereby protect the cathode against the operating gas which comprises air or other active gas. There is known an excellent example of the latter attempt as shown in FIG. 1 of the accompanying drawings.

In the illustrated example of the prior art, current is supplied from a DC source through a lead wire 11 to a cathode l2. Electrons emitted from the cathode 12 pass through a passageway 23 to strike against the inner wall of a passageway 24 formed in and anode 13. The anode 13 is connected with the power source through a lead wire 14 to close the circuit. The cathode I2 is supported by a supporting member 15 which is secured to a nonconductive body 16 by means of screws. Between the cathode 14 and a noule 17 there is interposed a spacer 18 formed of heat resistive oxide. The nozzle 17 is maintained in electrically isolated state during the operation of the torch, and it has the passageway 23 formed therein as described. The spacer 18 of oxide has a stepped passageway 25 formed therethrough. One of the ends of the passageway 25 which is adjacent to the cathode 12 has a reduced diameter, and the other end which opens into the passageway 23 has an enlarged diameter.

Gas inlet pipes

29 and 30 for introducing different gases are provided to the body 16 at its rear end portion, and

gas injection holes

26 and 27 communicating with the respective

gas inlet pipes

29 and 30 are provided to introduce different gases into the stepped passageway 25. As shown in FIG. 2, which is a cross-sectional view taken along line II-ll of FIG. 1, the gas injection holes 26 are formed tangentially to the periphery of the cathode end face and open into the narrow portion of the passageway in the same plane as the cathode end face 20. The gas injection holes 27 are similarly formed tangentially to the periphery of the passageway 25 but open into the large-diametered portion of the passageway 25.

ln the above-described plasma torch according to the prior art, inert gas such as argon or the like is supplied through the gas inlet pipe 29 and gas injection holes 26 onto the end face of the cathode. This inert gas forms a high-speed helical flow over the cathode end face so as to act as a shield for therefor.

Active operating gas such as air or the like is introduced through the other gas inlet pipe 30 and injected through the gas injection holes into the passageway 25, where the active gas forms a helical flow in the same direction as the aforementioned inert gas flow, and the active and inert gases merge together in the passageway 23 within the nozzle to be discharged through the opening formed at the fore end of the torch.

In the conventional plasma torch of the described construction, the operating gas formed into a powerful helical flow has a low pressure in its axial center portion, and this low-pressure center portion allows arc discharge to be effected therethrough for a ready ignition, as well as reduces the damage caused to the inner wall of the torch. Cool inert gas is always blown through the injection holes 26 to the end face 20 of the cathode during the operation and this inert gas reduces to some extent the tendency of the active gas to disperse toward the cathode end face 20. This leads to a great advantage that the cathode end face can be efficiently protected by a relatively small quantity of inert gas.

Nevertheless, in such a conventional plasma jet torch, the checking of the tendency of the active gas to disperse toward the electron emission surface of the cathode or the stabilization of the arc discharge is not sufficient to allow the torch to be operated for a sufficient length of time. Also, the quantity of the expensive inert gas required for the shielding gas is still insufiiciently small.

When active gas is employed as the operating gas, the protection of the nozzle portion may be accomplished by providing a good cooling of the same portion, but there is known a more complete protection of the nozzle portion, which is achieved by flowing water through the nozzle in such a manner that a helical stream of water is formed along the inner surface of the nozzle, as is disclosed in US. Pat. No. 2,906,858 in the name of H. S. Morton, Jr. According to this method, the inner surface of the nozzle can be protected substantially perfectly against the corrosion due to active gas and against the heat from the plasma jet flame.

SUMMARY OF THE INVENTION The present invention intends to provide a plasma torch in which a small quantity of inert gas or neutral gas may be used to shield the cathode while active gas such as air or the like may be used as the major operating gas, and the invention lies especially in the structural improvements relating to the portion of the plasma torch which is adjacent to the cathode.

It is an object of the present invention to provide an improved plasma torch which ensures less consumption of the cathode and enables air or other active gas to be used as the are gas.

It is another object of the present invention to provide an improved plasma torch in which the cathode surface can be protected by a smaller quantity of inert gas than required in the prior art plasma torch.

According to the present invention the plasma torch comprises a cylindrical body, a nozzle formed at the fore end of said body, a cathode-supporting structure supporting a cathode at one end and fixed to the other end of said body in electrically insulated relationship therewith, and in such a manner that said cathode is located at a predetermined distance from said noule, means for supplying operating gas into said nozzle so that a helical flow of said operating gas may be formed within said body and injected through the outlet opening of said nozzle, means for applying voltage to produce arc discharge between said cathode and said nozzle or a suitable conductor member located in front of said nozzle, a recess formed at the fore end of said cathode-supporting structure, said recess having its bottom surface flush with the electron emission surface of said cathode, shielding gas injection holes formed at intervals along the periphery of said recess nearer to its end opening toward said nozzle, said shielding gas injection holes opening into said recess tangentially to the periphery thereof, and means for supplying shielding gas to said injection holes, the diameter of said recess at said opening end thereof being smaller than the diameter of said recess at the portion thereof where said shielding gas injection holes are formed.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described more fully with respect to the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of the plasma torch according to the prior art;

FIG. 2 is a cross-sectional view taken along line Il-II of FIG. 1;

FIG. 3 is a longitudinal sectional view showing an example ofthe plasma torch according to the present invention;

FIG. 4 is a cross-sectional view taken along line lV-IV of FIG. 3;

FIG. 5 is a cross-sectional view taken along line V-V OF FIG. 3;

FIGS. 6 to 8 are longitudinal sectional views of various modified forms of the cathode-supporting structure according to the present invention;

FIG. 9 is a photographic view showing a longitudinal section of the cathode tip and its neighboring portion and for illustrating the degree of consumption of the cathode tip during the operation of the plasma torch according to the present invention;

FIG. 10 is a similar photographic view for illustrating the degree of consumption of the cathode tip during the operation of the prior art plasma torch;

FIG. 11 is a graph for comparatively showing the consumption rate of the cathode tip in the plasma torch according to the present invention and that in the plasma torch according to the prior art; and

FIG. 12 is a fragmentary view showing, in longitudinal section, the fore end portion of the torch according to another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference has previously been made to FIGS. 1 and 2 showing the prior art plasma torch.

Referring to FIGS. 3 to 5, there is shown an example of the plasma torch constructed according to the present invention. The plasma torch comprises a body 32 provided with a nozzle portion 31 at one end thereof. Within the body 32 there is fixedly provided a cathode-supporting structure 33 with an insulating cylindrical member 48 interposed between the body 32 and the cathode-supporting structure 33. At the fore end of the structure 33 facing the nozzle 31 there is formed a recess 34 in the form of a truncated cone having its opening end smaller in diameter than the opposite bottom end. Embedded inwardly of the bottom surface of the recess 34 is a cathode tip 36 in such a manner that at least the center portion of the bottom surface of the recess forms an electron emission surface 35. At the rear end of the cathode-supporting structure 33 are provided an inlet pipe 37 for introducing shielding gas and an inlet 38 for introducing coolant water, the pipe 37 and inlet 38 being respectively communicated with a shielding gas passageway 39 and a coolant water passageway 40 both provided within the cathode supporting structure 33. The shielding gas passageway 39 has shielding gas injection holes 41 which open into the recess 34. These shielding gas injection holes 41 are formed so as to open tangentially of the sidewall of the recess 34 and spaced apart a predetermined distance from the bottom of the recess 34, as shown in FIG. 5, so that a powerful helical flow of shielding gas may be provided in the recess 34 when the shielding gas in injected thereinto.

An inlet pipe 42 for introducing operating gas is provided on the outer surface of body 32, and the inlet pipe 42 is communicated with an annular passageway 43 for operating gas formed in the body 32 at a distance from the opening end of the recess 34 toward the nozzle 31. In communication with the annular passageway 43 there are provided operating gas injection holes 45 which open into a cavity 44 formed in the body 32. The operating gas injection holes 45 open tangentially of Ill) the inner sidewall of the body 32, as shown in FIG. 4, so that a helical flow of operating gas in the same direction as the helical flow of shielding gas may be provided in the cavity 44 forwardly of the recess 34 when the operating gas is injected thereinto. Axially formed in the wall of the body 32 is another passageway 47 having one end thereof communicated with the coolant water passageway 40 through a hole 49 formed in the insulating cylindrical member 48. The other end of the passageway 50 provided so as to surround the inner wall of the nozzle 31 formed at the fore end of the body 32. A coolant water exhaust pipe 51 communicating with the circulation passageway 50 is provided outwardly of the body 32 at the fore end thereof.

Ifthe above-described plasma torch of the present invention is used as a nontransfer type one, the coolant water inlet 38 provided at the rear end of the cathode-supporting structure 33 will also serve as a terminal for connection with a negative power source and the coolant exhaust pipe 51 provided at the fore end will also serve as a terminal for connection with a positive power source. When coolant water is flowed through the coolant water system and at the same time the coolant water inlet and outlet pipes are connected to the respective power sources with the operating gas flowing through the operating gas inlet pipe 42, then there is caused an arc discharge between the cathode 35 and the inner surface of the nozzle 31 as previously described, whereby the operating gas is heated and ionized and intensely converged to form a plasma jet flame 52 which is injected through the nozzle 31.

When the plasma torch is used as a transfer type one, are discharge is caused between a suitable conductor (not shown) disposed in front of the nozzle 31 and the cathode 35.

If an active gas such as air or the like is used as the operating gas with this torch, an inert gas such as argon is introduced into the recess 34 through the shielding gas injection holes 41 so that a helical flow of the shielding gas may be formed within the recess 34.

It will be appreciated from the foregoing that the plasma torch according to the present invention provides much more effective protection of the cathode against the corrosion resulting from the active gas than the conventional plasma torch. This is accomplished primarily because a predetermined distance provided between the shielding gas injection holes 41 and the electron emission surface 35 serves to prevent the helical flow of shielding gas from being disturbed even if some degree of unsmoothness is initially present or later caused to be present in the cathode tip 36.

If use is made, for example, of air as the operating gas and argon as the shielding gas in the conventional plasma torch as shown in FIG. I to cause a plasma jet flame, the consumption of the cathode tip will be such that the powerful helical flows of operating gas and shielding gas would cause the are spot to be fixed substantially in the center of the electron emission surface, with an unfavorable result that a degree of recess is produced in that portion of the electron emission surface which is exposed to the are spot and that a degree of bulge is formed around such a recess.

In the case where the shielding gas is injected so as to contact the end face of the cathode as is the case with the conventional torch, the recess and bulge formed as'described just above in the electron emission surface will greatly disturb the helical flow of the shielding gas, which in turn will cause the are spot to restlessly move around on the electron emission surface of the cathode. Such restless movement of the are spot will lend itself to increase the information of recess and bulge on the electron emission surface, resulting in a greater disturbance of the helical flow of the shielding gas and accordingly in a greater tendency of the inert gas to disperse onto the electron emission surface. Thus, the cathode suffers from significantly great consumption due to the inert gas.

In contrast, with the plasma torch of FIG. 3 constructed according to the present invention, the provision of a space between the electron emission surface of the cathode and the shielding gas injections holes substantially prevents the helical flow of shielding gas from being disturbed even if any recess and bulge are initially present or later caused to be present in the electron emission surface of the cathode, and thereby minimizes the tendency of such recess and bulge to be enlarged on the electron emission surface as in the case of the prior art plasma torch. Also, the amount of the inert gas being dispersed onto the electron emission surface of the cathode may be reduced to an extremely small value.

The second reason for the excellent protection of the cathode provided in accordance with the present invention is that the smaller diameter of the recess 34 at the opening end thereof than the diameter of the recess 34 at the portion thereof where the shielding gas injection holes are formed enables the convergence of the shielding gas to be accomplished more effectively than in the prior art plasma torch, and this leads to a greater stabilization of arc discharge.

This may be further explained. A flow of fluid has a nature that it flows along the wall surface with which it is in contact. Therefore, if the diameter of the recess at the opening end thereof is substantially equal to the diameter of the recess at the portion where the shielding gas injection holes are formed, as in the case of the prior art plasma torch of FIG. 1, then the shield gas at the opening end of the recess tends to disperse peripherally of the end face of the cathode-supporting structure and the shielding gas will sharply decrease its shielding effect in the outside of the recess.

In contrast, according to the present invention, the reduced diameter of the recess at the opening end thereof serves to decrease the tendency of the shielding gas to disperse peripherally of the end face of the cathode-supporting structure and enhance the helical flow rate of the shielding gas at the opening end of the recess. This provides a greater convergence of the shielding gas and enables the are spot to be fixed at a predetermined point on the electron emission surface of the cathode to thereby provide a highly stable discharge. Furthermore, the increased helical flow rate of the shielding gas extremely minimizes the dispersion of the active gas onto the electron emission surface of the cathode and this also greatly contributes to the protection of the cathode.

Y Turning to FIGS. 6, 7 and 8, there are shown, in longitudinal section, various forms of the fore end portion of the cathodesupporting structure according to the present invention.

In the example shown in FIG. 3, the recess 34 formed at the fore end of the cathode supporting structure is in the form of a truncated cone having its diameter linearly progressively decreased from the bottom to the opening end thereof, whereas the recess 34 may take other various configurations.

For example, as illustrated in FIGS. 6 and 7, the recess 34 may have a sidewall 53 formed in an inwardly or outwardly curved shape. Alternatively, as shown in FIG. 8, the recess 34 may have an opening end of the same diameter as the opposite end but the opening end may have an apertured plate 54 attached thereto, the plate 54 being formed with a small diametered aperture centrally thereof. These alternative forms of the recess lead to substantially the same function and effect as achieved in the FIG. 3 example.

FIG. 9 is a longitudinal sectional view of the fore end portion of the cathode supporting structure according to the present invention, and FIG. 10 is a similar view of the similar portion according to the prior art, both for illustrating the consumption of the cathode.

The operating conditions for the respective plasma torches are given in table 1 below:

TABLE I Present invention (FIG. 9) Prior art (FIG. 10)

Are current 300A. 300A. Shielding gas N, 7 L./min. Ar l4 Llmin. Operating gar Air 63 l.lmin. Air 54 l./min. Operation hours l2 hrs. l2 hrs.

The material used for each cathode tip is tungsten containing 2 percent thorium and both torches are of the same construction in the other portions than the part shown by the photographs of FIGS. 9 and 10.

As will be apparent from the above table, the present invention employed nitrogen as the shielding gas and its flow rate was half that in the prior art torch while the flow rate of air as the operating gas was 10 percent greater than that in the prior art. In spite of such worse operating conditions than those for the prior art device, the consumption of the cathode tip after 12 hours of operation was much less in the torch of the present invention (FIG. 9) than in the prior art torch (FIG. 10).

FIG. 11 illustrates the relation between the consumption of the cathode and the operation time in the torches of FIG. 9 (present invention) and FIG. 1 (prior art), the former being represented by curve A and the latter by curve B.

The operating conditions for each torch are: shielding gas N 10 l./min., operating gas air 60 l./min., and arc current 300 A. As will be seen from the result shown in FIG. 11, the consumption rate of the cathode tip in the plasma torch of the present invention is about one-sixth of that in the conventional plasma torch.

The life of the entire torch is substantially governed by the life of the cathode. Therefore, the foregoing fact means that the present invention can increase the life of the torch by six times that provided by the conventional construction.

Furthermore, according to the present invention, a helical stream of water may be flowed along the inner surface of the noule and injected through the opening of the nozzle is such a manner that the plasma flame is surrounded by a film of water, whereby the inner wall of the nozzle may be protected against damage by the film of water, and other notable effects may also be provided as will be described later.

FIG. 12 is a schematic view showing, in longitudinal section, the nozzle portion in an example of the plasma torch according to the present invention, in which a helical stream of water is flowed along the inner wall surface of the nozzle as mentioned above.

The device of FIG. 12 includes an inlet pipe 61 provided outwardly of the body 32 for introducing water therethrough into the nozzle, an annular water circulation passageway 62 formed in the body 32 and communicated with the water inlet pipe 61 through a passageway 63, and water injection holes 64 opening into the nozzle 31 tangentially to the inner periphery thereof.

In the shown device, water is forced into the circulation passageway 62 via inlet pipe 61 and then injected through the injection holes 64 to provide a helical stream of water 65 which flows in close contact with the inner side of the nozzle. When reaches the nozzle outlet, this stream of water is discharged in the form of an outwardly diverged stream of water 66, together with the plasma flame 52.

The arrangement shown in FIG. 12 provides the following advantages:

1. The inner surface of the nozzle can be effectively cooled without arc discharge being hampered, and the nozzle can be protected against any thermal or chemical damage due to the very hot plasma flame, and thereby a semipermanent life of the nozzle can be provided.

2. The outer periphery of the plasma flame is intensely cooled and thereby provided with a greater pinch effect which enables the plasma flame to have a higher energy density.

3. Poisonous N0, gas resulting from the use of air or like fluid containing oxygen and/or nitrogen as the operating gas can be effectively removed. Oxygen or nitrogen heated within the plasma flame, when it leaves the nozzle, contacts the called shoulder defamation which might arise at the point of the material subjected to the severing. If a plasma flame is used to cut a metallic member by fusing, the portion of the metallic member adjacent to the severed point is also heated to a very high temperature so that the metal in the cross section is melted to form a rounded end face or what is called shoulder deformation." According to the prior art such shoulder deformation" has been very difiicult to prevent, whereas this can be readily prevented by the water stream injected together with the plasma flame as shown in FIG. 12 and thereby a sharp cross section may be obtained in the severed workpieces.

As has hitherto been discussed, the plasma torch of the present invention may be effectively operated for sufiiciently long hours using a small quantity of inert gas or neutral gas such as nitrogen as the shielding gas while using air or other active gas as the operating gas. This greatly reduces the economical limitation of the operating gas which has been a great problem in using the plasma jet flame for the working of various workpieces or for the source of light, heat, etc. Thus, the present invention provides more extensive application of the plasma jet flame and its industrial value is of great significance.

We claim:

1. A plasma torch having an electrode-supporting structure provided with a recess formed at one end thereof opposite to a nozzle, said recess having an opening end of smaller diameter than the opposite bottom and and having its bottom surface flush with the electron emission surface of the electrode, and at least one shielding gas injection hole opening into said recess tangentially at the side of said recess located at a distance from said surface of said electrode tip.

2. A plasma torch comprising a cylindrical body,

a nozzle formed at the fore end of said body,

an electrode-supporting structure supporting an electrode at one end and fixed to the other end of said body in electrically insulated relationship therewith, means for supplying operating gas into said nozzle so that a helical flow of said operating gas may be formed within said body and injected through an outlet opening of said nozzle,

means for applying voltage to produce arc discharge between said electrode and a suitable conductor member located in front of said electrode,

a recess formed at the fore end of said electrode-supporting structure opposite to said nozzle,

said recess having an opening end of smaller diameter than the opposite bottom end and having its bottom surface flush with the electron emission surface of the electrode, and i at least one shielding gas passage opening tangentially through the wall of said recess at the side of said recess and located at a distance from said surface of the electrode.

3. A plasma torch according to claim 2, wherein an orifice having a diameter smaller than that of said recess is provided at the end of said electrode-supporting structure where said recess is formed, so that the diameter of said recess at said opening end is smaller than the diameter of said recess at any other portion thereof.

4. A plasma torch according to claim 2, wherein means for providing a helical stream of water is provided in the inner surface of said nozzle between the outlet opening of the nozzle and the operating gas injection holes.

5. A plasma torch according to claim 2, wherein said recess has a taper so that the diameter of the recess is continuously reduced from said bottom surface toward said opening end.

6. A plasma torch according to claim 5, wherein the sidewall of said recess consists of a conical surface so that the diameter of the recess is continuously reduced from said bottom surface toward said opening end.

7. A plasma torch according to claim 5, wherein the sidewall of said recess has the shape of a hyperboloid of revolution so that the diameter of said recess is continuously reduced from said bottom surface toward said opening end.

8. A plasma torch according to claim 1, wherein said recess has a taper so that the diameter of the recess is continuously reduced from said bottom surface toward said opening end.

9. A plasma torch according to claim 8, wherein the sidewall of said recess consists of a conical surface so that the diameter of the recess is cotinuously reduced from said bottom surface toward said opening end.

10. A plasma torch according to claim 8, wherein the sidewall of said recess has the shape of a hyperboloid of revolution so that the diameter of said recess is continuously reduced from said bottom surface toward said opening end.

11. A plasma torch according to claim 1, wherein an orifice having a diameter smaller than that of said recess is provided at the end of said electrode-supporting structure where said recess is formed, so that the diameter of said recess at said opening end is smaller than the diameter of said recess at any other portion thereof.

Claims

2. A plasma torch comprising a cylindrical body, a nozzle formed at the fore end of said body, an electrode-supporting structure supporting an electrode at one end and fixed to the other end of said body in electrically insulated relationship therewith, means for supplying operating gas into said nozzle so that a helical flow of said operating gas may be formed within said body and injected through an outlet opening of said nozzle, means for applying voltage to produce arc discharge between said electrode and a suitable conductor member located in front of said electrode, a recess formed at the fore end of said electrode-supporting structure opposite to said nozzle, said recess having an opening end of smaller diameter than the opposite bottom end and having its bottom surface flush with the electron emission surface of the electrode, and at least one shielding gas passage opening tangentially through the wall of said recess at the side of said recess and located at a distance from said surface of the electrode.