Abstract:
The invention relates to an inductive plasma torch comprising: a cylindrical metal containment cage ( 1 ); a metal element solidly connected to the containment cage ( 1 ), extending radially from the periphery of one end thereof; and an inductor ( 5 ) surrounding the containment cage ( 1 ). The aforementioned containment cage ( 1 ) and element are divided along axial planes into regularly distributed sectors, and the sectors are rigidly connected to one another alternately by: a portion of the containment cage ( 1 ) on the side opposite the element, or by a portion of the element on the side opposite the containment cage ( 1 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a National Stage of PCT International Application Serial Number PCT/FR2012/050295, filed Feb. 10, 2012, which claims priority under 35 U.S.C. §119 of French Patent Application Serial Number 11/51130, filed Feb. 11, 2011, the disclosures of which are incorporated by reference herein. 
     BACKGROUND 
     The present invention relates to inductive plasma torches. 
     DISCUSSION OF THE RELATED ART 
     In an inductive plasma torch, a plasma gas is injected into a confinement cage where it is submitted to an A.C. magnetic field which ionizes the gas to form a plasma. 
     The confinement cage of an inductive plasma torch should have several characteristics:
         tightness to gases,   letting through the magnetic field, and   resisting very high temperatures since the temperature at the heart of the plasma may reach values on the order of 7,000° C.       

     Cold confinement cages formed of sectorized conductive cylinders cooled by the circulation of a liquid are currently used. 
       FIG. 1  is a perspective view cut along a vertical plane of an inductive plasma torch with a cold confinement cage of the type described in U.S. Pat. No. 5,877,471. 
     Confinement cage  1  is formed of multiple separated parallel metal tubes  2 , arranged to define together a hollow cylinder. Tubes  2  extend between a bottom part  3  on the upper side and a cap  4  on the lower side. The upper portion of confinement cage  1  is surrounded with an inductive winding  5 . A gas injector  7  penetrates into confinement cage  1  through cap  4  all the way to inductive winding  5 . Bottom part  3  is pierced with a flame outlet opening. The assembly is rigidified by bars  8  connecting bottom part  3  and cap  4  outside of confinement cage  1 . 
     Confinement cage  1  is made tight by a sheath made of an insulating material, not shown in  FIG. 1 , surrounding the assembly of tubes  2 . 
     Inductive winding  5  is hollow, and a cooling liquid flows inside of it. Tubes  2  also conduct a cooling liquid injected and discharged from cap  4 . 
     When an A.C. current flows through inductor  5 , this creates an axial A.C. magnetic field intended to ionize the plasma gas injected into confinement cage  1  to form a plasma. The magnetic field is capable of creating eddy currents in the various conductive materials forming the torch. Such currents have two adverse effects. They heat up the conductors by Joule effect and induce an attenuation of the axial magnetic field. The fact for the confinement cage to be made of separate parallel tubes is equivalent to a sectorization of this cage, which results in that the magnetic field can cross it with some attenuation while eddy currents cannot flow around this cage. A problem is however posed regarding the bottom part and the cap, and especially the bottom part arranged around a region taken to a very high temperature in operation by the torch flame. A problem of interference caused by the A.C. magnetic field radiated outside of the torch is also posed. 
     SUMMARY 
     An object of an embodiment of the present invention is to provide an inductive plasma torch having all its elements properly cooled down. 
     Another object of an embodiment of the present invention is to provide an inductive plasma torch having a simple manufacturing and assembly. 
     Another object of an embodiment of the present invention is to provide an inductive plasma source. 
     Another object of an embodiment of the present invention is to provide an inductive plasma source having an improved electric efficiency. 
     Another object of an embodiment of the present invention is to provide an inductive plasma source capable of operating in the presence of a high-temperature radiating medium in front of this torch. 
     Another object of an embodiment of the present invention is to provide an inductive plasma source provided with a protection against parasitic radiations from the magnetic field. 
     Another object of an embodiment of the present invention is to provide an inductive plasma torch of small volume. 
     Thus, an embodiment of the present invention provides an inductive plasma torch comprising a cylindrical metal confinement cage, a metal element rigidly attached to the confinement cage, radially extending, outwards, from the periphery of an end thereof, and an inductor surrounding the confinement cage, wherein the confinement cage and said element are divided along axial planes into regularly distributed sectors, and wherein the sectors are alternately attached to a portion of the confinement cage on the side opposite to the element and to a portion of said element on the side opposite to the confinement cage. 
     According to an embodiment of the present invention, said element is a laterally—extending bottom part. 
     According to an embodiment of the present invention, said element comprises an external cylindrical cage, concentric to the confinement cage and attached thereto by the bottom part. 
     According to an embodiment of the present invention, the confinement cage and said element are crossed by ducts. 
     According to an embodiment of the present invention, the confinement cage and said element are made of copper. 
     An embodiment of the present invention provides a method for manufacturing an inductive plasma torch, wherein a block of metallic material, comprising a first cylinder and an element rigidly attached to the first cylinder by one end thereof radially extending outwards from the periphery of an end of the first cylinder, is formed, and wherein axial slots are formed to define sectors in said block, each slot crossing the element or the first cylinder and the cutting being alternately interrupted a short distance from an edge of the element opposite to the first cylinder and at a short distance from an edge of the first cylinder opposite to the element. 
     According to an embodiment of the present invention, said element is a laterally—extending bottom part. 
     According to an embodiment of the present invention, said element is a second cylinder concentric to and rigidly attached to the first cylinder by a bottom part. 
     According to an embodiment of the present invention, said block is formed by milling. 
     According to an embodiment of the present invention, the conductive material is copper. 
     According to an embodiment of the present invention, ducts are formed across the thickness of the cylinder and of said element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
         FIG. 1  is a perspective view cut along a vertical plane of an inductive plasma torch with a cold confinement cage of the type described in U.S. Pat. No. 5,877,471, 
         FIG. 2A  is a perspective view cut along a vertical plane of an inductive plasma torch according to an embodiment of the present invention, 
         FIG. 2B  is a perspective view illustrating three adjacent sectors of the confinement cage and of the bottom part of the plasma torch of  FIG. 2A , 
         FIG. 3A  is a perspective view cut along a vertical plane of an inductive plasma torch according to another embodiment of the present invention, and 
         FIG. 3B  is a perspective view illustrating three adjacent sectors of the confinement cage, of the external cage, and of the bottom part of the plasma torch of  FIG. 3A . 
     
    
    
     The same reference numerals designate the same elements in the various drawings. 
     DETAILED DESCRIPTION 
       FIG. 2A  illustrates an embodiment of an inductive plasma torch with cold walls. The plasma torch comprises a cylinder-shaped metal confinement cage  1 . The confinement cage is rigidly attached to a cooled metal bottom part  10  laterally extending outwards from the periphery of the upper end of confinement cage  1  (on the flame outlet side), such a bottom part being used as a thermal shield relative to a hot medium, for example, a melt of a molten material, receiving the torch flame. A cap  4  is assembled on the lower side. Confinement cage  1  and bottom part  10  form a single element which is divided into sectors by axial slots. The slots are interrupted so that the sectors are alternately attached by junction regions  11  extending between sectorized portions next to confinement cage  1  on the side opposite to bottom part  10  and by junction regions  12  extending between sectorized portions next to bottom part  10  on the side opposite to confinement cage  1 . An inductive winding  5  arranged on the side of bottom part  10  surrounds confinement cage  1 . A gas injector  7 , not integrally shown, penetrates into confinement cage  1  through cap  4  all the way to inductive winding  5 . 
       FIG. 2B  is a perspective view illustrating in enlarged fashion three adjacent sectors of the confinement cage and of the bottom part of the inductive plasma torch of  FIG. 2A .  FIG. 2B  especially illustrates internal ducts enabling a cooling fluid to flow within the thickness of the metal forming confinement cage  1  and bottom part  10 . 
     A pair of adjacent sectors  101  and  102  attached to each other by a junction  12  located at the end of bottom part  10  opposite to confinement cage  1  and attached to the neighboring sectors by junctions  11  located at the lower end of confinement cage  1  is considered. A duct  30  comprises five duct sections  30 - 1  to  30 - 5  formed inside of the walls of confinement cage  1  and of bottom part  10 . Each section communicates with the next section. Section  30 - 1  extends vertically from an opening  32  in the lower portion of confinement cage  1  of sector  101  all the way to a region  30 - a  located in bottom part  10  of sector  101 . Section  30 - 2  extends radially in bottom part  10  of sector  101  from region  30 - a  to a region  30 - b  located at the end of bottom part  10  of sector  101  opposite to confinement cage  1 . Section  30 - 3  extends in bottom part  10  from region  30 - b  to a region  30 - c  located in bottom part  10  of section  102  and symmetrical to region  30 - b  of sector  101 . Section  30 - 4  extends radially in bottom part  10  of sector  102  from region  30 - c  to a region  30 - d  located at the level of confinement cage  1  of sector  102 . Section  30 - 5  extends vertically in confinement cage  1  of sector  102  from sector  30 - d  to an opening  33  in the lower portion of confinement cage  1  of sector  102 . 
       FIG. 3A  illustrates another embodiment of an inductive plasma torch. The inductive plasma torch comprises a cylinder-shaped confinement cage  1  and an external cage  9  having the shape of a coaxial cylinder. Confinement cage  1  and external cage  9  are connected on the upper side (on the flame outlet side) by a bottom part  10 . A cap  4  is assembled on the lower side. Confinement cage  1 , external cage  9 , and bottom part  10  form a single metallic element, for example, made of copper, which is divided into sectors by axial slots. The slots are interrupted so that the sectors are attached, on the side opposite to the bottom, alternately by junction regions  11  extending between sectorized portions next to confinement cage  1  and by junction regions  13  extending between sectorized portions next to external cage  9 . An inductive winding  5  arranged on the side of bottom part  10  surrounds confinement cage  1 . A gas injector  7 , not integrally shown, penetrates into confinement cage  1  through cap  4  all the way to inductive winding  5 . The external cage aims at limiting electromagnetic radiations emitted towards the outside. 
       FIG. 3B  is a perspective view illustrating in enlarged fashion three adjacent sectors of the confinement cage, of the external cage, and of the bottom part of the inductive plasma torch of  FIG. 3A . The central portion of confinement cage  1  is stripped open for clarity.  FIG. 3B  especially illustrates internal ducts enabling a cooling fluid to flow within the thickness of the metal forming confinement cage  1 , external cage  9 , and bottom part  10 . 
     A pair of adjacent sectors  101  and  102  attached to each other by a junction  13  located at the lower end of external cage  9  and attached to the neighboring sectors by junctions  11  located at the lower end of confinement cage  1  is considered. A duct  30  comprises seven duct sections  30 - 1 ,  30 - 2 ,  30 - 6  to  30 - 8 ,  30 - 4  and  30 - 5  formed inside of the walls of confinement cage  1 , of external cage  9 , and of bottom part  10 . Each section communicates with the next section. Section  30 - 1  extends vertically from an opening  32  in the lower portion of confinement cage  1  of sector  101  all the way to a region  30 - a  located in bottom part  10  of sector  101 . Section  30 - 2  extends radially in bottom part  10  of sector  101  from region  30 - a  to a region  30 - b  located at the level of the external cage of sector  101 . Section  30 - 6  extends vertically in the external cage of sector  101  from region  30 - b  to a region  30 - e . Section  30 - 7  extends horizontally in the external cage from region  30 - e  in sector  101  to a region  30 - f  in sector  102 . Section  30 - 8  extends vertically in the external cage of sector  102  from region  30 - f  to a region  30 - c  located in bottom part  10  of sector  102 . Section  30 - 4  extends radially in bottom part  10  of sector  102  from region  30 - c  to a region  30 - d  located at the level of confinement cage  1  of sector  102 . Section  30 - 5  extends vertically in confinement cage  1  of sector  102  from end  30 - d  to an opening  33  in the lower portion of confinement cage  1  of sector  102 . 
     In  FIGS. 2B and 3B , the cooling fluid is injected into ducts  30  through openings  32  and discharged through openings  33 . 
     The duct sections are for example formed by drilling. They are closed by the inserting of plugs and/or by soldering at the drilling openings at the locations where duct  30  should not be open. 
     It should be understood that the duct structures and shapes illustrated in  FIGS. 2B and 3B  are possible embodiments only. Many other structures may be provided. In particular, several ducts per sector may be provided. 
     The confinement cage, the bottom and preferably the external cage, when provided, are made tight by filling the space between sectors with a thermal insulator. 
     The manufacturing of such plasma torches is simple since the confinement cage, the bottom part and, if present, the external cage, form one and the same element. Such an element may be formed by molding, machining, or by welding of different sub-elements. To manufacture this one-piece assembly, it may be started from a copper block which is milled to define the bottom part, the confinement cylinder, and possibly the external cylinder. Once this block has been formed, simple sawing operations will provide a division into sectors. Of course, this is likely to have various alterations. For example, the cylinder(s) and the bottom part may be manufactured separately and welded or assembled in another way and slotted to achieve a division into sectors while maintaining the coherence of the assembly. 
     An advantage of the torch structures described herein is their easy mounting. Indeed, the internal cage, the bottom part and possibly the external cage forms a one-piece assembly which is thus easy to mount. 
     Another advantage is the fact that there is a single cooling circuit. 
     The plasma torch comprising a sectorized external cage is particularly compact. Indeed, the inductor is located in a cold area and protected from dust of the outer environment, the dimensions of the plasma torch may be decreased without fearing breakdowns due to the strong A.C. currents flowing through the inductor. Conversely, for a given torch volume, the torch structure comprising a sectorized external cage described herein may be associated with an A.C. current generator more powerful than in the case of prior structures. For example, for the above-mentioned dimensions, the generator power is limited to 200 kW for a torch structure equivalent to that described in  FIG. 1 , to be compared with 350 kW for the torch structure comprising a sectorized one-piece assembly. 
     In an embodiment, the confinement cage, the bottom part and, if provided, the external cage, are made of copper. The cap is made of a fluorinated polymer such as PTFE GF25, better known as Teflon. The outer diameter of the external cage is 210 mm, the inner diameter of the confinement cage is 50 mm, and the external diameter of the inductive winding is 110 mm. The height of the confinement cage and of the external cage is 290 mm. The thickness of the confinement cage is 10 mm. The injector penetrates into the confinement cage all the way to a distance of 70 mm away from the bottom part. The inductive winding starts 30 mm away from the bottom part and ends 110 mm away from the bottom part. The confinement cage, the bottom part and, according to the embodiment, the external cage, are divided into  12  regularly-distributed sectors. The sectorization, when it is continued to the lower edge of the confinement cage, is interrupted 20 mm away from the bottom of the external cage or from the bottom part according to the embodiment. The spacing between sectors is 1.5 mm. The diameter of the ducts in the confinement cage, the bottom part and, according to the embodiment, the external cage, is 3 mm. 
     Specific embodiments of the present invention have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, the shape and the dimensions of the confinement cage, the shape and the dimensions of the external cage, the cooling circuit, the nature of the material forming the confinement cage, the external cage, or the bottom part, and the method for making the confinement cage, the bottom part, and the external cage tight, will be selected by those skilled in the art according to the desired performances of the plasma torch. 
     The number of sectors may be selected by those skilled in the art to optimize the features of the torch, and especially to promote the propagation of the magnetic field towards the inside of the structure and limit its propagation towards the outside of the structure when the plasma torch is provided with a sectorized external cage. 
     In an embodiment, the thickness of the external cage may be chosen to be greater than that of the confinement cage. 
     Different usual alternative embodiments of plasma torches have not been described herein. In particular, a ring made of a refractory material forming a thermal shield protecting the bottom part against the thermal radiation generated by the material heated by the plasma torch may be added to the bottom part on its outer side.