Patent Publication Number: US-6215091-B1

Title: Plasma torch

Description:
CROSS REFERENCE TO RELATED ART 
     This application claims priority of Korean patent application No. 1998-20509 filed on Jun. 3, 1998, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The present invention relates to a plasma torch, more particularly, to a plasma torch that has high efficiency, the input power of which is more than 1 megawatt (MW), and the cathode of which has long life span. 
     2) Background of the Invention 
     in “UIE ARC PLASMA REVIEW 1998” which illustrates many kinds of plasma torches, among them two representative plasma torches will be explained. 
     FIG. 2 shows one of the transferred-solid type plasma torches of the prior art, a Daido Steel brand plasma torch. This is a scaled-up version of the torch design used for cutting and welding. The long tungsten cathode is recessed behind the copper nozzle and operates in the transferred mode with argon gas. The torch is available in sizes up to about 1 MW 
     FIG. 3 shows one of the non-transferred hollow type plasma torches of the prior art, an SKF brand, a SKF plasma torch. This design is segmented with a fixed length arc column with magnetic field coils at each end. The torch has two equal diameter copper electrodes, the capped rear electrode being connected negative. So the magnetic field makes a rotating arc root in the ups electrode and this results in long cathode life. The insulated segments between the electrodes stretch the length of the arc column to a larger length that will develop a higher arc voltage. Torches with power ratings from 100 kW to 8 MW are available. The SKF torch has a field coil for rotating the arc foot of the upstream electrode and thus can make more than 1 MW output. However efficiency of the torch is lower than the transferred type since temperature of the plasma rapidly reduces as it departs from the nozzle. To exchange the cathode, the whole assembly surrounding the cathode must be removed, which results in high exchange cost. 
     SUMMARY OF THE INVENTION 
     Therefore, it is one of the objects of the present invention to provide a plasma torch with a long life cathode and whose output is more than 1 MW. 
     It is another object of the present invention is to provide a plasma torch whose cathode can be easily exchanged. 
     It is still another object of the present invention is to provide a plasma torch whose cathode region has low pressure, which helps ignition at lower voltage. 
     To accomplish these objects, the present invention provides a plasma torch including 
     a body having a gas supplier for supplying plasma gas; 
     a cathode supporter for supporting the button-cathode; 
     a button-cathode for generating plasma arc, assembled to the front end of the cathode supporter by a bolting means; and 
     a hollow-cathode which surrounds the button-cathode, has a predetermined space from the button-cathode, is assembled to the cathode supporter, and is made of material with higher work function than the material for the button-cathode. 
     Preferably, the present invention also provides a plasma torch with multiple-solenoid coil surrounding the cathode. A moving peak current with flat current is applied to the multiple-solenoid coil in order to move arc foot on the surface of the button-cathode back and forth, which can prolong cathode life. 
     The button-cathode and the hollow-cathode are assembled by a bolting means, which enables easy exchange of the button-cathode which generates plasma arc and experiences wear. 
     The button-cathode of the invention is positioned so as to be surrounded by the hollow-cathode, which makes the pressure of the surrounding area of the button-cathode low, and thus makes ignition at lower voltage and enables stable flame of the plasma arc column generated at the button-cathode. 
     The plasma torch of the present invention has a hollow-cathode surrounding a hot button type cathode, so it can be called a hollow hot button-cathode torch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of this invention, and many of the attendant advantages thereof, will be apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same and similar components, wherein: 
     FIG. 1 is a cross sectional view of the plasma torch according to the invention; 
     FIG. 2 is a schematic cross sectional view of a Daido Steel manufactured plasma torch; and 
     FIG. 3 is a schematic cross sectional view of a SKF manufactured plasma torch. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the invention will be explained with reference to the accompanying drawing, FIG.  1 . 
     The torch according to the invention is essentially structured to have two electrodes, cathode  20  and anode  10 , a gas inlet  30 , a first multiple-solenoid coil  71  for rotating arc root of the cathode  20 , and a second multiple-solenoid coil  72  for rotating arc root of the anode  10 . In an other embodiment of the invention, in transferred type torch the anode  10  is not essential and can be omitted. Cathode  20  has a stick-shaped button-cathode  22  and a hollow type copper cathode  23 , both of which are assembled to a cathode support  24  that is made of copper. 
     The copper cathode support  24  has a plurality of cooling holes for cooling cathode  20 . The straight cooling holes  50  are disposed in diverse directions to increase cooling efficiency and it is preferable to be formed in a helical pattern following in the longitudinal direction of the cathode support  24 . When the gas is supplied through the gas inlet  30 , the gas forms turbulence while passing the cooling holes  50 . The heat transfer between cathode  20  and gas increases whole heat efficiency of the torch. 
     The front portion  27  of the cathode support  24  has two divided parts, upper and lower parts  27   a  and  27   b,  and a hole  26  for the button-cathode  22 . The button-cathode  22  is assembled between the two parts  27   a  and  27   b  of the cathode support  24  through the hole  26  by a bolt  25 . The cylindrical-shaped hollow-cathode  23  has teeth  29  in its interior surface, which mesh with the teeth  29   a  formed in the outer surface of the front portion  27  of the cathode support  24 . Thus between the button-cathode  22  and the hollow-cathode  23  a space A can be defined. And since a gap between the button-cathode  22 , the hollow-cathode  23  and the cathode support  24  may cause contact resistance, it is preferable that the button-cathode  22  and the hollow-cathode  23  should be tightly engaged to the cathode support  24 . 
     The button-cathode  22  is preferably made of thoriated tungsten which has small work function and thus has an easy emission of thermionic electrons, which results in a small sputtering cross section for avoidance of wear of the hollow-cathode. The thoriated tungsten can be replaced by Hafnium(Hf), other high melting point metals or metals containing thorium. To dope tungsten with 1-3 wt % thorium will result in lower work function. Since the work functions of the two metals, copper and thoriated tungsten, are different from each other, button-cathode  22  emits almost all thermionic electrons and the hollow-cathode  23  does not wear. 
     The anode  10  positioned in front of the cathode  20  is made of OFHC(oxygen free high-conductive copper). OFHC is copper whose oxygen is removed to increase electric conductivity, and it is often used as electric conducting material for radio frequency or microwave. The anode  10  can be made of copper alloy with zirconium or chromium. 
     The first of the multiple-solenoid coils  71  surrounding body  3  in the position of the hot button-cathode  22  is made of copper. If only one coil of the multiple-solenoid coils  71  receives high peak-current compared to the flat current in the other coils, there exists high magnetic flux peak on the surface of the cathode  22 . And after that, if high peak-current moves to the next coil successively, the peak point of magnetic flux moves axially on the surface of the cathode  22 . This makes the trap of arc roots and movement of arc roots in both axial and azimuthal direction on the surface of the cathode by Lorentz force. Therefore, the button-cathode  22  does not wear locally, but wears equally throughout the cathode  22 , which helps the life of the cathode  22  to be prolonged. 
     The second the multiple-solenoid coil  72  surrounding the body  3  of the torch in the position of the anode  10  is also made of copper. And if this multiple-solenoid coil  72  receives current in the same way as the first multiple-solenoid coil  71 , there also are the trap of arc roots and movement of arc roots in both axial and azimuthal direction on the surface of the anode by Lorentz force. Therefore the plasma arc can be focused and concentrated well, and the temperature in the arc becomes uniform. Also heat stress to the anode  10  will be reduced, which increases the life span of the anode  10 . 
     The body  3  of the torch is essentially made of stainless steel, which is excellent in mechanical strength, in enduring corrosion, and in transmissivity of magnetic field, and has lower thermal conductivity than copper, which can reduce heat loss to the outside of the torch. 
     The outer surface of the body  3  is surrounded by cooling passage  40 , which protects the body from being overheated, and through which air, cooling oil, or water can flow. The cooling passage  40  is formed with double jacket. The body  3  is sealed or closed by a disk-shaped rear portion  60  of insulating material or Teflon. A stainless steel plug  61 , that is electrical feed through with vacuum tight, is secured to rear center of the rear portion  60  where the cathode support  24  is screwed and passes through center of the plug  61 . The rear portion  60  is fixed to the body  3  by stainless steel flange  40   a  and bolts  63 . And to prevent gas leakage O-ring or copper gasket (not shown) is provided between flange  40   a  and rear portion  60 . 
     Now the operation for the torch of this embodiment will be explained. 
     Gas flows into the body  3  through the gas inlet  30 , and the gas pressure around the space A adjacent to the button cathode  22  is lowered, which can be explained by Bernoulli principle. Since the gas pressure around the button-cathode  22  is lowered, the plasma arc can be ignited at lower voltage and is stable, which effect can be explained by Paschen curve. 
     Since magnetic flux in the axial direction of the cathode  22  and to-and-fro motion of the high magnetic flux peak points can make arc root rotate and move forward and backward along the button-cathode  22 . Thus the button-cathode  22  wears out equally throughout the whole cathode  22 , which increases life span of the button-cathode  22 . 
     Also due to the second multiple-solenoid coil  72 , the plasma arc can be focused and concentrated well, and the temperature in the arc becomes uniform, which increases the life span of the anode  10 . 
     When the protruded portion  22   a  of the button-cathode  22  wears out, disengaging the bolt  25  and being reassembled with the hollow-cathode  23  can move the button-cathode  22  forward. Alternatively, the button-cathode  22  can be exchanged by following the same method. 
     The torch of the invention has following advantages: 
     1) since gas pressure around cathode region A can be lowered, which helps the lowering of ignition voltage, the torch can be applied to a more stable operation; 
     2) since the multiple-solenoid coil around the upstream button-cathode  22  can make the upstream button-cathode  22  wear equally, the life span of the button-cathode  22  can be increased; and 
     3) since operating simple engaging means, such as bolts can do exchange of the upstream electrode, it does not take much time. 
     The torch according to the invention can be applied to a transferred type or non-transferred type torch according to the treated material. And it can be used in plasma spray coating, plasma melting and reduction for metal or non-metal, incineration process for non resoluble material, heat pyrolysis and solidification for nuclear reactor waste, decommissioning of nuclear power plants. 
     The output of the torch can range from small power below 1 MW to high power over 1 MW. 
     The arrangement described above is only illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the present invention.