Abstract:
A plasma generator for three phase mains alternating current operation has three plasma generation tubes interconnected with a nozzle, each plasma generation tube having a plasma initiator for forming a plasma into an electrode ring, the electrode ring including substantially tangential gas introduction orifices which cause gas entering the electrode ring to helically rotate. Each of the electrode rings is coupled to a unique one of the three phases of AC voltage supply, such that when the initiator plasma is introduced into one of the electrode rings, a plasma discharge occurs with a path from the electrode ring, through the plasma generation tube, and to a different electrode ring. Each electrode ring has gas introduced in a helically rotating manner such that the erosion of the surface of the electrode ring is uniform over the entire surface, and minimally erosive in a single arc attachment spot, since the arc spot is constantly moving as provided by the helical trajectory of the gas entering the electrode.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the field of plasma gas generators, and particularly plasma gas generators continuously producing a source of plasma and operating on polyphase mains alternating current (AC). 
   BACKGROUND OF THE INVENTION 
   Plasma generators form high energy plasma gas, which is then used for a variety of application, including plasma-jet cutting, coatings, hard-facing, vitrification of radioactive materials, disinfection of waste, and many other applications. Industrial plasma generation systems may consume large amounts of power on the order of mega-watts (MW), and for these systems, it is desired that the plasma generator be simple and reliable. One problem of particular interest in high power plasma generation systems is extending the life of the electrode at the plasma-conductive interface. The plasma attachment region, known as the arc spot, of the electrode may preferentially erode compared to the other regions of the electrode, resulting in premature replacement of the electrode. In general, it is desired to have an electrode with large exposed surface area that is suitable for some form of liquid cooling. 
   PATENT PRIOR ART 
   U.S. Pat. No. 5,801,489 describes a plasma generator operating directly from three phase mains power and utilizing an ionized gas which is introduced proximal to electrodes connected to the three phases of main power, thereby forming plasma between the electrodes. The electrodes achieve distributed wear patterns because the plasma self-induced magnetic field, also known as the rail gun effect, causes the plasma arc to move along the electrode from a position of short arc length to a position of long arc length. While this results in a uniform electrode wear on the working area of the electrode, one disadvantage is that the end to end plasma arc length varies by more than a ratio of 3:1 from initiation to termination. It is desired to have a plasma arc which is of comparatively constant length and density. Another problem of this system is that as the plasma arc travels down the extent tubular electrode, radial variations in the arc spot may be minimal, leading to path erosion along the electrode which may be worn excessively in a single path compared to other regions. 
   U.S. Pat. No. 3,140,421 describes a polyphase plasma generator having linearly arranged electrodes, whereby a plasma is formed between two adjacent electrodes and swept down a plasma tube to an exit aperture. The generator has no provision for uniform electrode wear. 
   U.S. Pat. No. 3,953,705 describes a plasma generator for operation with direct current, where the plasma generator has a sequential series of plasma cavities, each with a plasma entrance and exit, with air introduced in each cavity and having a circumferential velocity to prevent the plasma from eroding the plasma channel as it is transported from one electrode to another. 
   U.S. Pat. No. 4,013,867 describes a plasma generator for three phase power, where the generator has a plurality of plasma tubes connected with a common chamber, whereby the plasma initially forms across an annular gap, after which a plasma gas is introduced in the gap and travels down the plasma tubes. The plasma is centered in each plasma tube using an effect of an external magnetic field source, shown as a solenoidal coil. 
   OBJECTS OF THE INVENTION 
   A first object of the invention is a plasma generator suitable for coupling to polyphase mains power and operating at the mains line frequency, where the plasma generator has a plurality of plasma sources interconnected through a plasma nozzle, each plasma source having a plasma initiator which couples plasma into the area of an annular electrode which is coupled to one of the phases of the mains power, and the annular electrode is separated from the nozzle by a plasma channel. 
   A second object of the invention is a uniform wear annular electrode centered on an axis and having an extent, the annular electrode for use in a plasma generator accepting a gas for forming a plasma, the plasma forming an arc spot on the annular electrode, the annular electrode having a plurality of apertures positioned to form circumferential gas flow over the extent of the annular electrode, and also beyond the extent of the annular electrode, such that the introduction of the plasma gas causes the arc spot to rotate circumferentially about the electrode, and also over the extent of the electrode. 
   A third object of the invention is an annular electrode for use in a plasma generator, the plasma having an arc spot on the surface of the annular electrode, the annular electrode also having a plurality of gas introduction ports over the extent of the annular electrode, at least one of which is adjacent to a first end of the annular electrode, and another which is adjacent to a second end of the annular electrode and opposite the first end, such that by controlling flow or pressure to first end or second end, the plasma arc spot may rotated circumferentially, and also varied continuously from the first end to the second end. 
   A fourth object of the invention is a plasma generator having a plurality of annular electrodes, each annular electrode coupled to a mains voltage phase at a mains frequency, the annular electrode having an axis which defines an extent of the annular electrode, and an annular electrode inner surface which includes apertures for the introduction of gas and circulation of the gas and a plasma formed from the gas in a circumferential direction about the axis, and also varying over the extent of the annular electrode, such that the plasma location may be temporally varied circumferentially and over the extent of the electrode to cause uniform arc spot electrode erosion over time. 
   SUMMARY OF THE INVENTION 
   A plasma generator has three elongate plasma sources connected together by a nozzle. Each elongate plasma source has an initiator end for generating an initial plasma, an annular electrode having an inner surface into which the initial plasma may form, both of which are positioned about a plasma source axis, the electrode having and a plurality of tangential gas introduction apertures for causing a formed plasma to rotate circumferentially and axially after formation. The annular electrode is followed by a plasma channel coupled to the nozzle, such that when the initiators of each plasma source is ionizing the gas, each of the three annular electrodes are excited by an electrical voltage that is substantially 120 degrees out of phase with any other electrode, thereby resulting in the formation of a plasma from one annular electrode, through the plasma channel to the nozzle, thereafter through a different plasma channel and to the related annular electrode. Through the introduction of the gas using circumferential apertures, the plasma arc attachment to the annular electrode rotates from the applied force of the introduced gas, thereby preventing spot wear on the annular electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a simplified projection view of the plasma generator. 
       FIG. 2  shows a projection view of the plasma generator from the nozzle end. 
       FIG. 3  shows angled section A-A of the plasma generator of  FIG. 2 . 
       FIG. 4  shows the plasma arc initiator and annular electrode. 
       FIG. 5  shows section B-B and section D-D of  FIG. 4 . 
       FIG. 6  shows section C-C of  FIG. 4 . 
       FIG. 7  shows a control system for the plasma generator. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a simplified projection view of a plasma generator  100 , which comprises nozzle  150  having exit aperture  152  for the emission of plasma formed in a plurality of plasma tubes  124 - 1 ,  124 - 2 , and  124 - 3 . The individual elements are shown in an exploded view, however the plasma generator is a closed system whereby native gas enters at inlets  120 - 1 ,  122 - 1 , etc., for each plasma source, and exits as a fully formed plasma at nozzle exit aperture  152 . Each plasma tube such as  124 - 1  has an inner plasma channel for the passage of plasma through a plasma forming extent  112 - 1  with one end of the plasma channel attached to nozzle  150  and the other end attached to annular electrode  118 - 1 , which in turn is coupled to one phase of the secondary winding of three phase transformer  146 . The annular electrode  118 - 1  has an inner surface, optionally with a diameter in the range of 30 mm to 300 mm, and the region where a plasma interacts with the annular electrode inner surface is known as a plasma arc spot  116 - 1 . In device use, the high current density combined with the high plasma temperature causes erosion of the annular electrode  118 - 1  if the arc spot  116 - 1  is stationary. The erosion may be controlled and electrode temperature reduced if the arc attachment spot  116 - 1  moves over the inner conductor surface circumferentially and over the extent of the electrode along the center axis, where the electrode extent may optionally be in the range 30 mm-300 mm. The circumferential movement is generated by the introduction of gas through apertures in the annular electrode, where the apertures are tangential to a circle having a center on the electrode axis and a radius less than the inner surface of the annular electrode, as will be described later. Additionally, gas inlets  120 - 1  and  122 - 1  are located at opposite extents of the annular electrode, and the gas inlets are separately controllable, such that in addition to the circumferential movement of the arc spot on the surface of the electrode, the arc spot may be moved axially along the extent of the electrode, thereby distributing the energy of the arc spot over a comparatively large electrode surface in a controllable manner. Additionally, the circumferential movement of the introduced gas over the electrode surface provides a cooling effect which reduces the surface temperature of the annular electrode. Adjacent to the annular electrode  118 - 1  is a plasma initiator  108 - 1  for forming an initial plasma near the inner surface of the annular electrode  118 - 1 . In one embodiment of the invention, the plasma initiator  108 - 1  has an inner electrode  102 - 1  and an outer electrode  104 - 1  driven by an initiator voltage  106 - 1 , which may be a continuous high frequency voltage sufficient to maintain a continuous source of initiation plasma, where the initiation plasma has an electron density n e  from 10 12 /cm 3  to 10 14 /cm 3 . The construction of other plasma tubes  124 - 2  and  124 - 3 , as well as annular electrodes  118 - 2  and  118 - 3  with gas inlets  120 - 2 ,  122 - 2 ,  120 - 3 , and  122 - 3 , and plasma initiators related to each electrode are substantially as described for the first plasma tube  124 - 1 . 
   Each electrode  118 - 1 ,  118 - 2 ,  118 - 3  is driven by a different phase of three phase transformer  146 , which may be at a voltage in the range of 400 to 10,000 volts RMS (root mean squared) and at the frequency of the mains voltage  148 . Transformer  146  is shown as a three phase delta-delta transformer, although it could be a wye-delta, or any combination, as is known in the prior art of three phase power. As the applied voltage is a sinusoidal alternating current voltage, the plasma that is formed is making and breaking in each plasma tube at the line voltage frequency. Also, the plasma initiation voltage to cause breakdown is lower than the voltage required to maintain the plasma. For these two reasons, it is useful to provide some sort of current limiting impedance for each electrode to limit the plasma current and thereby establish the current density of the plasma, and this function is performed by current limiter  154 , shown as series inductors applied on each branch of transformer  146 , although it is also possible to place the current limiters on the individual leads on the secondary of the output transformer  146  where the currents may be lower but operating voltage higher. The current limiting inductors  154  may also include adjustable taps so that the current limit may be set manually or automatically. 
     FIG. 2  shows the projection view from the plasma nozzle  150  exit aperture  152 , including the plasma tubes  124 - 1 ,  124 - 2 , and  124 - 3 . Annular electrode housings  202 - 1 ,  202 - 2 , and  202 - 3  and associated gas inlets  204 - 1 ,  206 - 1 ,  204 - 2 ,  206 - 2 ,  204 - 3 , and  206 - 3  are also shown, and these structures may be seen in section view from angular section A-A of  FIG. 2 , which is shown in  FIG. 3 . 
     FIG. 3  shows the angled section view A-A through  FIG. 2 , and includes two of the plasma tubes  124 - 1  and  124 - 2  and related structures, which are each placed symmetrically about respective electrode axis  310 - 1  and  310 - 2 . Nozzle  150  has cooling fluid passage  302 , exit aperture  152  and a nozzle mixing chamber which is common to plasma tubes  124 - 1  and  124 - 2 , which also have related liquid cooling jackets  308 - 1  and  380 - 2  with respective coolant inlets  306 - 1  and  306 - 2 , as well as coolant exhaust ports (not shown). Examining a single plasma tube, electrode, and related structure, plasma tube  124 - 1  and annular electrode  118 - 1  are cylindrical and positioned about electrode axis  310 - 1 , and the annular electrode  118 - 1  includes rear plasma gas inlet  206 - 1  and front plasma gas inlet  204 - 1 , where the structure of electrode  118 - 1  causes the plasma gas to be introduced into the annular electrode with a flow tangential to a circle having a center on the axis  310 - 1  and a radius within the electrode  118 - 1  inner surface, as will be seen in other views. The apertures in the annular electrode which introduce the plasma gas may be in the range of 0.5 mm to 2 mm, or any diameter required to generate circumferential gas rotation inside the annular electrode. Adjacent to the annular electrode  118 - 1  is the plasma initiator, which comprises outer electrode  104 - 1 , inner electrode  102 - 1 , and an annular insulator  314 - 1  positioned between them. When a plasma initiation voltage is provided between the inner electrode  102 - 1  and outer electrode  104 - 1 , an initiating plasma arc  312 - 1  is formed which enters the extent of annular electrode  118 - 1 . The structures of second plasma tube  124 - 2 , electrode  118 - 2 , and initiator formed by outer electrode  104 - 2 , inner electrode  102 - 2  with annular insulator  314 - 2  cause plasma arc  312 - 2  to form in the other annular electrode  118 - 2 , resulting in the formation of primary plasma arc  140  from one annular electrode  118 - 1  to another annular electrode  118 - 2 . Isolators  312 - 1  and  312 - 2  isolate the initiator structures from the conductive electrodes  118 - 1  and  118 - 2 , and isolators  316 - 1  and  316 - 2  isolate the annular electrode  118 - 1  and  118 - 2  from the structures surrounding plasma tubes  124 - 1  and  124 - 2 . Additionally, cooling ports  302 - 1  allow for the flow of coolant through the plasma initiators, which typically require on the order of 0.1% of the main plasma power developed from one annular electrode such as  118 - 1  to another such as  118 - 2 . The bulk of the cooling requirements are handled by electrode coolant ports  304 - 1  and  304 - 2  and plasma tube coolant ports  306 - 1  and  306 - 2  as shown. 
     FIG. 4  shows a detailed section view of the initiator and annular electrode for a single plasma tube, including inner electrode  102 - 1 , outer electrode  104 - 1 , and initiator plasma arc  312 - 1  which is formed between the inner and outer initiator electrodes and into the extent of annular electrode  118 - 1 . The annular electrode  118 - 1  also has coolant port  304 - 1  coupled to coolant passage  402 - 1  for removing heat formed in the annular electrode. The annular electrode  118 - 1  also has plasma gas apertures  406 - 1  as well as front circumferential gas passage  410 - 1  and rear circumferential gas passage  408 - 1 . 
     FIG. 5  shows a shared cross section view for B-B and D-D of  FIG. 4 , with both views perpendicular to the center axis  310 - 1 , showing the circumferential gas passages  502 - 1 . The circumferential apertures  502 - 1  cause the gas entering port  206 - 1  (for section B-B) and port  204 - 1  (for section D-D) to swirl in the same direction at both ends and also in the middle of the electrode as shown in  FIG. 6 . Although shown in the example as clockwise circumferential movement, the particular direction of rotation is unimportant. The swirling plasma gas causes the arc spot which tends to start near the initiator and attach to an adjacent point of the electrode to be swept circumferentially around the center axis  310 - 1  and down the extent of the annular electrode along axis  310 - 1 . In the best mode of the invention, the gas pressures at ports  206 - 1  and  204 - 1  are controlled such that the plasma initiation and helical arc movement occurs through the entire electrode extent over a single plasma arc formation and extinguishment cycle, which is derived from the mains AC frequency. 
     FIG. 6  shows annular electrode in section view C-C of  FIG. 4 , and includes the plasma gas port  506 - 1 , which couples the gas through port  404 - 1  and into the annular electrode  118 - 1  via circumferential apertures  406 - 1 , which may be tangent to reference circle  502 - 1  which has a center on the central axis  310 - 1  and a radius which is less than the inner radius of electrode  118 - 1 . Cooling ports  304 - 1  and  504 - 1  couple cooling fluid to cooling jacket  402 - 1 . 
     FIG. 7  shows the system operational diagram for all of the components of the plasma generation system. The plasma initiators of plasma generation system  100  are powered by system controller  702 , which also measures the applied voltages via voltage sensors  710 ,  712 ,  714  and current sensors  704 ,  706 ,  708  from the output of transformer  146 , as current limited by limiter  154 . The application of mains voltage to the current limiter  154  and transformer  146  is controlled by first contactor  706  and second contactor  718 , which are also controlled by controller  702 . The use of inductors to limit current results in a strong power factor shift, which is compensated by capacitor bank  720 , which serves to add capacitive reactive current to the mains current flow to offset the inductive reactive current of the current limiter  154 . The low voltage initiators fed by injector power supply  724  are current limited by initiator current limit inductor  726  for the case where the source  724  is an AC transformer coupled to the mains, or alternatively it may not be required if source  724  is a continuous HF source. The initiator voltages are sensed by sensors  728 ,  732 , and  736 , while the related currents are read by current sensors  730 ,  734 ,  738 . The other inputs read by the controller  702  include coolant flow  762 , temperature  752 , and gas inlet flow sensor  758 . Coolant valve  750  and gas valve  756  turn the respective flows on and off as part of the operational sequence. There are many safety interlocks and the like which may be practiced in the present invention. One sequence of operation is as follows: 
   1) Verify inlet coolant temperature and flow (open valve  750 , measure temperature  764 ); 
   2) Upon satisfactory coolant temperature and verified flow, open gas valve  756  and regulate pressure  758 ; 
   3) Apply voltage to plasma initiators via supply  724 , measure and control initiator voltages ( 728 ,  732 ,  736 ) and currents ( 730 ,  734 ,  738 ) applied to injectors; 
   4) Apply secondary voltages to annular electrodes via contactors  716 ,  718   
   5) control cooling water and gas flows during plasma production 
   6) Orderly shutdown: remove annular electrode power, remove gas flow, remove plasma initiator power, wait for plasma areas to cool down, remove water flow. 
   There are many variations of the present invention which may be practiced, and the particular variations mentioned herein are for illustration only, and are not intended to limit the invention. 
   The plasma gasses which may be introduced into the annular electrode apertures and ports include individually or in combination: air, carbon dioxide (CO 2 ), carbon monoxide (CO), chlorine gas (Cl), Fluorine (F), Nitrogen (N 2 ), Argon (Ar), Helium (He), Hydrogen gas (H 2 ), and their related compounds, and water vapor. 
   The annular electrode may be formed from any of the following metals individually or in combination: alloys of iron (Fe) and/or copper (Cu) optionally with additives of rare earth metals, or Tungsten (W) optionally with any of the rare earth metals, including Lanthanum (La), Thorium (Th), or Yttrium (Y). 
   The current limiting function provided by inductor  154  of  FIG. 1  and  FIG. 7  may include separate inductors placed in series with each transformer primary or secondary winding, or it may comprise any alternate means for limiting current to the plasma arc, including transformer windings which are loosely coupled to the core, and have a self inductance which is suitable to limit current to the desired level, or alternatively the current limiter may comprise a ballast resistance, although inductive throttles are preferred as they provide instant reaction to plasma instability. 
   The circumferential plasma gas introduction on the inner surfaces of the annular electrode may be accomplished via apertures in the size range 0.1 mm to 0.2 mm, or any range larger or smaller than this. The apertures can have central bores which are tangential to a circle inside the inner radius of the annular electrode, or they may comprise any arrangement of apertures which cause circumferential force to be applied to the plasma arc spot. Additionally, the plasma initiation may be accomplished by an electrical arc triggered initiation, as shown in  FIG. 4 , or it may be a continuous supply of plasma furnished to the region of the annular electrode, or any other means known in the prior art. 
   While the invention is shown for three phases, the invention may be practiced without upper or lower limit to the number of phases or angular separation by having the number of plasma tubes with associated annular electrode equals the number of phases, and connecting each plasma tube to a unique electrode.