Abstract:
A miniature microwave plasma torch apparatus ( 10 ) is described. The microwave plasma torch apparatus ( 10 ) is used for a variety of applications where rapid heating of a small amount of material is needed. The miniature microwave plasma torch apparatus ( 10 ) operates near or at atmospheric pressure for use in materials processing. The apparatus ( 10 ) provides a wide range of flow rates so that discharge properties vary from diffusional flow of radicals for gentle surface processing to high velocity, approaching supersonic, torch discharges for cutting and welding applications. The miniature microwave plasma torch apparatus ( 10 ) also has a very small materials processing spot size.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to provisional Patent Application Ser. No. 60/560,145 filed Apr. 7, 2004. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   REFERENCE TO A “COMPUTER LISTING APPENDIX SUBMITTED ON A COMPACT DISC” 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   This invention relates to a microwave plasma torch apparatus which enables the production of a miniature plasma. The present invention also relates to a method for the use of the apparatus. 
   (2) Description of Related Art 
   Microwave plasma discharge source design and applications have been well developed over the past three decades. Various microwave plasma sources have been demonstrated with many gases over an operating range of pressures from 0.1 mTorr to several atmospheres, input powers from one watt to six kilowatts, and discharge diameters from 0.2 mm to 25 cm. Many basic microwave coupling and discharge structures have been investigated including quartz dome confined disk shaped plasmas at the end of resonant cavities, atmospheric microwave discharge jets/torch, stripline applicators for miniature microwave discharge generation and tubes through waveguides. 
   Some related art include: J. Asmussen, “Electron cyclotron resonance microwave discharges for etching and thin-film deposition,”  J. Vac. Sci. Technol . A, vol. 7, pp 883-893 (1989). J. Asmussen, J. Hopwood, and F. C. Sze, “A 915 MHz/2.45 GHz ECR plasma source for large area ion beam and plasma processing,”  Rev. Sci. Inst ., vol. 6 pp 250-252 (1990). J. Asmussen, T. Grotjohn, P. Mak and M. Perrin, “The design and application of electron cyclotron resonance discharges,”  Invited Paper  for the 25 th  anniversary edition of the IEEE Trans. on Plasma Science, 25, 1196-1221 (1997). T. A. Grotjohn, A. Wijaya and J. Asmussen, Microwave Microstripline Circuits for the Creation and Maintenance of Mini and Micro Microwave Discharges, U.S. Pat. No. 6,759,808 issued Jul. 6, 2004. T. A. Grotjohn, J. Asmussen and J. Narendra, “Microstripline applicator for generating microwave plasma discharges,” 2003 NSF Design, Manufacturing and Industrial Innovation Conference Proceedings, Birmingham, Ala. 2003. J. Hopwood, D. K. Reinhard and J. Asmussen, “Experimental conditions for uniform anisotropic etching of silicon with a microwave electron cyclotron resonance plasma system,”  J. Vac. Sci. Technol . B., vol. 6 1896-1899 (1988). J. Hopwood, D. K. Reinhard and J. Asmussen, “Charged particle densities and energy distributions in a multipolar electron cyclotron resonant plasma etching source,”  J. Vac. Sci. Technol . A, vol. 8, pp 3103-3112 (1990). G. King, F. C. Sze, P. Mak, T. A. Grotjohn and J. Asmussen “Ion and neutral energies in a multipolar electron cyclotron resonance plasma source,”  J. Vac. Sci. Technol . A, vol. 10, pp 1265-1269 (1992). P. Mak, G. King, T. A. Grotjohn, and J. Asmussen, “Investigation of the influence of electromagnetic excitation on election cyclotron resonance discharge properties,”  J. Vac. Sci. Technol . A, vol 10, pp 1281-1287 (1992). S. Whitehair, J. Asmussen and S. Nakanishi, “Microwave electrothermal thruster performance in helium gas,”  J. of Propulsion and Power , vol 3, pp. 136-144 (1987). 
   In a microwave torch, gas flows through a nozzle structure and microwave energy is introduced to create the discharge. Atmospheric pressure or near atmospheric pressure microwave plasma torches have application in the general areas of cutting, welding, toxic materials destruction, plasma-assisted CVD, plasma-assisted etching, surface treatment and materials heating. A variety of materials can be processed including metals, fiberglass, ceramics, and textiles. 
   In general, plasma torches can produce higher gas temperatures and/or higher reactive species than simple combustion processes. Microwave powered plasma torches have an advantage over transferred arc plasma sources in that the material being cut or processed does not need to be a metal. Hence, they can perform plasma cutting or processing on non-conducting materials such as ceramics and fiberglass, or on multi-layer materials. Also the ability to control the power level and flow rate yields a wide range of processing conditions. Applications range from gentle surface treatment for use in surface sterilization to intense torches for cutting very high temperature materials. Microwave sources/torches also have the advantage compared to DC electrode based systems that they can operate readily with reactive gases, such as oxygen, without rapid electrode erosion problems. The specific related art is cited at the end of the specification. 
   The ability of microwave sources/torches to process a wide variety of materials is similar to laser based processes such as cutting and welding. The trade-off between plasma torches and lasers is that laser cutting, for example, gives a cut width of 0.01-0.05 mm, and known microwave torches are limited to cut widths of several 100&#39;s of microns. However, an important advantage of the microwave plasma torch is that its capital investment can be considerably less than the equivalent laser processing technology. Plasma torches can also be combined with laser technology to produce a hybrid cutting tool. In these hybrid tools the plasma jet has a laser beam propagating down the center of the jet. Because the plasma jet is heating the material, almost all the laser energy can be applied to the energy needed for high-precision deep cuts. This hybrid torch/laser technology thus is able to use a less costly laser system while still achieving the processing rate of a more costly laser system. There is a need for miniature microwave torches. 
   U.S. Pat. No. 4,611,108 to Leprince et al. discloses a microwave plasma torch excited by means of microwave energy delivered by a rectangular cross-section waveguide which is transversed by a delivery tube extending to the discharge outlet. A rectangular piston in the waveguide is capable of displacement in a sliding motion to form a short-circuit and is one of the factors to permit impedance-matching of the system. The delivery tube is hollow to carry a gas to the end which narrows to a discharge outlet having a 2 mm internal diameter. There is no liquid cooling system. 
   U.S. Pat. No. 6,184,982 to Karanassios discloses an in-torch vaporization sample introduction system for sample introduction into a spectrometer. The inductively-coupled plasma device (Fassel-type torch) includes a plasma with a central channel and load coil, fed by Argon though outer and intermediate feed channels in an enlarged gas tube. 
   U.S. Pat. No. 6,213,049 to Yang discloses a nozzle-injector for plasma deposition of thin-film coatings. The nozzle-injector utilizes an arc torch as the plasma generator and releases the plasma into a vacuum chamber reactor towards a substrate to be coated. 
   U.S. Pat. No. 6,218,640 to Selitser discloses a method of processing a semiconductor device using an inductive plasma torch. An inductive coil is used to apply an oscillating magnetic field to a plasma confinement tube to ignite and sustain a plasma. 
   U.S. Pat. No. 6,397,776 to Yang et al. discloses an expanding thermal plasma system for large area chemical vapor deposition. Each of a plurality of plasma generating means produce an expanding plume of plasma which impinge on a substrate within a deposition chamber for the purpose of producing a coating on the substrate. 
   While the related art teach microwave plasma torches, there still exists a need for a miniature microwave torch which provides a small plasma discharge. 
   OBJECTS 
   An object of the present invention is to provide a miniature microwave plasma torch apparatus that operates near or at atmospheric pressure for use in materials processing. The miniature plasma torch can also be applied to plasma torch spectroscopy applications. The apparatus provides a wide range of flow rates so that discharge properties vary from diffusional flow of radicals for gentle surface processing to high velocity, approaching supersonic, torch discharges for cutting and welding applications. It is particularly an object of the present invention to provide a miniature microwave plasma torch with a materials processing spot size of about 0.25 mm to a few mm&#39;s. Another object is to provide a hybrid microwave plasma torch/laser apparatus for materials processing. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a side view of the miniature coaxial microwave torch apparatus  10  of the present invention. 
       FIG. 2  is a side view of the microwave torch apparatus  10  held by an operator. 
       FIG. 3  illustrates a cutaway perspective view and  FIG. 4  illustrates a cutaway side view the microwave torch apparatus  10  in operation. A plasma discharge  40  is shown at the nozzle  35  of the microwave torch apparatus  10 . The flow rate is 200 sccm argon. The microwave power absorbed in the apparatus is 15 to 20 W (Watts). The plasma discharge  40  that extends from the torch apparatus  10  is 400-700 μm (micrometer) in diameter and 3-4 mm (millimeter) long. 
       FIG. 5  is a graph showing the power (W) required as a function of flow rate of argon (sccm). The power is that absorbed in the coaxial apparatus  10  and the discharge. The minimum power occurred at 200 sccm for this apparatus  10 . 
       FIG. 6  is a graph showing plasma discharge length in millimeters as a function of argon flow rate (sccm). 
       FIG. 7  is a graph showing power density (W/cm 3 ) as a function of argon flow rate (sccm). 
       FIG. 8  is a cross-section of one embodiment of the microwave torch apparatus  10  illustrated in  FIGS. 1 to 4 . 
       FIG. 8A  is a cross-section taken along line  8 A- 8 A,  FIG. 8B  is a cross-section taken along line  8 B- 8 B, and  FIG. 8C  is a cross-section taken along line  8 C- 8 C of the microwave torch apparatus  10  of  FIG. 8 . 
       FIG. 9  is an end view of the nozzle  35  of the microwave torch apparatus  10  showing orifice  36 .  FIG. 9A  is a cross-section of the nozzle  35  along line  9 A- 9 A of  FIG. 9 . 
       FIG. 10  is an end view of the first spacer  11  of the microwave torch apparatus  10 .  FIG. 10A  is a cross-section of the first spacer  11  along line  10 A- 10 A of  FIG. 10 . 
       FIG. 11  is a side view of the flange tube  16  and the outer conductor  17  of the microwave torch apparatus  10 . 
       FIG. 12  is a perspective view of the cone shaped coaxial taper  12  of the microwave torch apparatus  10 .  FIG. 12A  is a side view of the cone shaped coaxial taper  12 .  FIG. 12B  is a cross-section of the cone shaped coaxial taper  12  taken along line  12 B- 12 B of  FIG. 12A . 
       FIG. 13  is a perspective view of the finger coupling  13  of the microwave torch apparatus  10 .  FIG. 13A  is an end view of the finger coupling  13 .  FIG. 13B  is a side view of the finger coupling  13 .  FIG. 13C  is a cross-section of the finger coupling  13  along line  13 C- 13 C of  FIG. 13B .  FIG. 13D  is a cross-section of the finger coupling  13  along line  13 D- 13 D of  FIG. 13B . 
       FIG. 14  is a perspective view of the adjustable coupling flange  14  of the microwave torch apparatus  10 .  FIG. 14A  is an end view of the adjustable coupling flange  14 .  FIG. 14B  is a cross-section of the adjustable coupling flange  14  taken along line  14 B- 14 B of  FIG. 14A . 
       FIG. 15  is a perspective view of the fixed coupling flange  15  of the microwave torch apparatus  10 .  FIG. 15A  is an end view of the fixed coupling flange  15 .  FIG. 15B  is a cross-section of the fixed coupling flange  15  taken along line  15 B- 15 B of  FIG. 15A . 
       FIG. 16  is a perspective view of the flange tube  16  of the microwave torch apparatus  10 .  FIG. 16A  is an end view of the flange tube  16 .  FIG. 16B  is a side view of the flange tube  16 .  FIG. 16C  is a cross-section of the flange tube  16  taken along line  16 C- 16 C of  FIG. 16B . 
       FIG. 17  is a perspective view of the outer conductor  17  of the microwave torch apparatus  10 .  FIG. 17A  is an end view of the outer conductor  17 .  FIG. 17B  is a side view of the outer conductor  17 .  FIG. 17C  is a cross-section of the outer conductor  17  taken along line  17 C- 17 C of  FIG. 17A . 
       FIG. 18  is a perspective view of the second spacer  18  of the microwave torch apparatus  10 .  FIG. 18A  is an end view of the second spacer  18 .  FIG. 18B  is a cross-section of the second spacer  18  taken along line  18 B- 18 B of  FIG. 18A . 
       FIG. 19  is a perspective view of the core  19  of the microwave torch apparatus  10 .  FIG. 19A  is a side view of the core  19 .  FIG. 19B  is a cross-section of the core  19  taken along line  19 B- 19 B of  FIG. 19A . 
       FIG. 20  is a perspective view of the rod  20  of the microwave torch apparatus  10 .  FIG. 20A  is a side view of the rod  20 .  FIG. 20B  is a cross-section of the rod  20  taken along line  20 B- 20 B of  FIG. 20 . 
       FIG. 21  is a cross-sectional view of a tuning portion of the microwave torch apparatus  10 . 
       FIG. 22  is a cross-sectional view of the tuning stub  21  of the microwave torch apparatus  10 . 
       FIG. 23  is a cross-sectional view of the outer conductor  17  of the microwave torch apparatus  10 .  FIG. 23A  is a cross-sectional view of the adapter  23  showing first thread  23 A and second thread  23 B, and  FIG. 23B  is a cross-sectional view of the nut  24 . 
       FIG. 24  is a cross-sectional view of the holder  33  of the microwave torch apparatus  10 .  FIG. 24A  is a cross-sectional view of the stub cap  32 . 
       FIG. 25  is an end view of the first finger stock snap ring  25  of the microwave torch apparatus  10 .  FIG. 25A  is a cross-sectional view of the first finger stock snap ring  25  of the microwave torch apparatus  10 .  FIG. 25B  is an end view of the second finger stock snap ring  26 .  FIG. 25C  is a cross-sectional view of the second finger stock snap ring  26 . 
       FIG. 26  is an end view of the third spacer  27 .  FIG. 26A  is a cross-sectional view of the third spacer  27  taken along line  26 A- 26 A of  FIG. 26 . 
       FIG. 27  is a cross-sectional view of the inner conductor  28 .  FIG. 27A  is a cross-sectional view of the inner conductor  28  and inner conductor cap  29  taken along line  27 B- 27 B of  FIG. 27 . 
       FIG. 28  is a cross-sectional view showing the assembled inner conductor  28  and nozzle  35 . 
       FIG. 29  is an end view of the inner conductor cap  29  of the microwave torch apparatus  10 .  FIG. 29A  is a cross-sectional view of the inner conductor cap  29  taken along line  29 A- 29 A of  FIG. 29 . 
       FIG. 30  is a cross-sectional view of the outer conductor  17  and the inner conductor  28  illustrating the microwave power inlet, water cooling inlet and gas inlet of the microwave torch apparatus  10 . 
       FIG. 31  is a cross-sectional view illustrating the assembled microwave power inlet portion of the microwave torch apparatus  10 . 
       FIG. 32A  is a drawing schematically illustrating the microwave torch apparatus  10  when microwave energy is provided which shows the field lines between the inner conductor  28  and outer conductor  17 .  FIG. 32B  is a drawing illustrating the apparatus  10  when microwave energy is provided which shows the field lines between the inner conductor  28  and outer conductor  17  during operation generating a plasma discharge  40 .  FIG. 32C  is a cross-sectional view along line  32 C- 32 C of  FIG. 32A  showing the field lines between the inner conductor  28  and outer conductor  17 . 
   

   SUMMARY OF THE INVENTION 
   The present invention provides a microwave plasma torch (discharge) apparatus which comprises: an elongate tuneable microwave applicator with opposed ends comprising inner and outer conductive tubular members defining the microwave applicator, the applicator having an inlet port for the microwaves and the inner tubular member defining an inwardly tapering nozzle support having an opening on one of the ends of the applicator; a first conduit member mounted through the applicator secured in the opening in the nozzle support for supplying a gas through an orifice isolated at the end of the first conduit member for generating the plasma torch (discharge) produced by the microwaves in the applicator; a second conduit member mounted adjacent to the first conduit member for supplying a cooling fluid which cools the inner tubular member, the nozzle and the first conduit member while the torch (discharge) is operating. 
   In further embodiments the orifice in the first conduit member has a diameter of less than about 1 mm. Preferably, the applicator is tuneable by a (tuning stub  21 ) between the tubular members. In further embodiments the inner tubular member is slidably mounted through a support inside the outer tubular member. In still further embodiments the inner tubular member is supported inside the outer tubular member by non-conductive spacers. In further embodiments the outer tubular member has an inner diameter of about 1.27 cm or less and the inner tubular member has an outside diameter of less than about 0.5 cm and the first and second conduit members are each less than about 0.25 cm in outside diameter. In preferred embodiments the torch (plasma) formed by the plasma at the orifice has a diameter of less than about 1 mm. In further embodiments the microwave energy inlet port is provided intermediate the ends of the applicator. In further embodiments a first conduit member is mounted through the applicator at the end opposite the end of the nozzle support. 
   The present invention provides a method for treating a substrate which comprises: (a) providing a microwave plasma torch (discharge) apparatus which comprises: an elongate tuneable microwave applicator with opposed ends comprising inner and outer conductive tubular members defining the microwave applicator, the applicator having an inlet port for the microwaves and the inner tubular member defining an inwardly tapering nozzle support having an opening on one of the ends of the applicator; a first conduit member mounted through the applicator secured in the opening in the nozzle support for supplying a gas through an orifice in the first conduit member for generating the plasma torch (discharge) produced by the microwaves in the applicator; a second conduit member mounted adjacent to the first conduit member for supplying a cooling fluid which cools the inner tubular member, the nozzle, and the first conduit member, while the torch is operating, adjacent to the substrate and generating the plasma so that the substrate can be acted upon by the plasma; and (b) generating the plasma to produce the plasma torch (discharge) at the orifice to treat the substrate while providing the cooling fluid in the second conduit. 
   In further embodiments the inlet port is provided intermediate the ends of the applicator. In further embodiments the first conduit member is mounted through the applicator from an opposite of the ends of the applicator from the nozzle support. In preferred embodiments, the applicator is tunable by varying the position of the inner tubular member 
   DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention relates to a microwave plasma torch apparatus which comprises: (a) an elongate tuneable microwave applicator with opposed ends comprising inner and outer conductive tubular members defining the microwave applicator, the applicator having an inlet port for the microwaves and the inner tubular member defining an inwardly tapering nozzle support having an opening on one of the ends of the applicator; (b) a first conduit member mounted through the applicator secured in the opening in the nozzle support for supplying a gas through an orifice in the first conduit member for generating the plasma torch (flame discharge) produced by the microwaves in the applicator; (c) a second conduit member mounted adjacent to the first conduit member for supplying a cooling fluid which cools the inner tubular member, the nozzle and the first conduit member while the torch is operating. 
   The present invention also relates to a method for treating a substrate which comprises: (a) providing a microwave plasma torch apparatus which comprises: an elongate tuneable microwave applicator with opposed ends comprising inner and outer conductive tubular members defining the microwave applicator, the applicator having an inlet port for the microwaves and the inner tubular member defining an inwardly tapering nozzle support having an opening on one of the ends of the applicator; a first conduit member mounted through the applicator secured in the opening in the nozzle support for supplying a gas through an orifice in the first conduit member for generating the plasma torch (discharge) produced by the microwaves in the applicator; a second conduit member mounted adjacent to the first conduit member for supplying a cooling fluid which cools the inner tubular member, the nozzle and the first conduit member, while the torch is operating, adjacent to the substrate and generating the plasma so that the substrate can be acted upon by the plasma; and (b) generating the plasma to produce the torch (discharge) at the orifice to treat the substrate while providing the cooling fluid in the second conduit. 
   The present invention provides a compact, atmospheric pressure, miniature microwave plasma torch apparatus. The torch has a minimal size, an efficient microwave power utilization and a versatility that allows easy adaptation to various applications. One specific application is a hybrid microwave plasma torch/laser system for materials processing. Markets for the miniature torch include the cutting of non-metallic materials. Metal can be readily cut using transfer arc plasma sources. The non-metallic materials here include ceramics, fiberglass, textiles, and multi-layer materials that could contain metals, ceramics and fiberglass. A second market for the microwave torch is the processing of materials, which in addition to cutting, includes welding and cleaning with reactive gases. This includes the use of oxygen torches, fluorine containing torches, nitrogen containing torches. Another is the destruction of various volatile organic compounds using the high processing temperature of microwave torches. Other possible markets related to the cleaning, decontamination, surface processing of materials using a low gas temperature, atmospheric pressure plasma source of reactive radicals. Additionally, the microwave plasma torch technology developed can be applied to a wider variety of applications beyond materials processing including (1) destruction of volatile organics (VOC&#39;s), freon compounds, and other toxic gases, (2) sterilization of surfaces, and (3) pretreatment of fuels for improving burning efficiency and cleanliness. A compact microwave torch can be used to cut multi-layer materials rapidly. 
   The second aspect of this invention is to use the microwave plasma torch apparatus to assist with laser processing. This is a hybrid materials processing technology that combines laser cutting/welding with microwave torch technology. The basic idea of using a hybrid technology is that in some applications the microwave torch technology can be used to heat the material being cut/processed so that the laser can be used more efficiently to perform the cutting without most of the energy going to just heat up the material. Microwave torches are used in emission spectroscopy systems for the plasma excitation source. 
   One embodiment of the present invention is illustrated in detail in the Figures.  FIG. 1  illustrates the miniature coaxial microwave torch apparatus  10  and  FIG. 2  illustrates how the microwave torch apparatus  10  is typically manipulated by an operator. The microwave torch apparatus  10  is comprised of an inner conductor  28  as the inner conductive tubular member coaxially mounted through an outer conductor  17  as the outer conductive tubular member. At a first end  28 A of the inner conductor  28  a gas feed tube  30  provides a gas to a nozzle  35 , as an inwardly tapering portion of the inner tubular member, coaxially mounted on a second end  28 B of the inner conductor  28  where a plasma discharge is formed. A cooling fluid is provided to cool the apparatus  10  in a cooling fluid tube  31  which circulates in the inner conductor  28  before exiting at a cooling fluid output port  34 . Microwave energy is provided into a cone shaped coaxial taper  12  which channels the microwave energy into the outer conductor  17  through a flange tube  16 , which is affixed to the cone shaped coaxial taper  12  by means of adjustable coupling flange  14  and fixed coupling flange  15 , and finally to the nozzle  35  adjacent to a first end  17 C of the outer conductor  17 . In use,  FIGS. 3 and 4  illustrate a plasma discharge  40  formed at the nozzle of the microwave torch apparatus  10 . The plasma discharge  40  shown in  FIGS. 3 and 4  is formed when argon is provided at a flow rate of 200 sccm. The microwave power absorbed in the apparatus  10  when operated in this manner is 15 to 20 W. The resulting plasma discharge size is 400-700 μm (micrometer) in diameter and 3-4 mm (millimeter) long. 
   An assembly of the parts is illustrated in  FIG. 8 . The cross-section of  FIG. 8  shows the nozzle  35  from which the plasma is generated mounted on inner conductor  28  with a first spacer  11  supporting an inner conductor  28  inside the outer conductor  17 . The first spacer  11  can be constructed of any suitable non-conductive material, preferably fluoropolymer resin such as TEFLON® brand fluoropolymer resin (Dupont, Wilmington, Del.). Input power is fed into an opening  17 B (See  FIG. 17 ) in the side of the outer conductor  17  by means of outer conductor flange tube  16  (See  FIG. 11 ,  FIG. 16 ). An adjustable coupling flange  14  and fixed coupling flange  15  (See  FIG. 14 .  FIG. 15 . respectively) are utilized to attach a cone shaped taper  12  (See  FIG. 12 ) to the flange tube  16 . A core  19  (See  FIG. 19 ), preferably constructed of brass, is mounted inside taper  12  and passes through the centers of adjustable coupling flange  14  and fixed coupling flange  15 . The brass core  19  connects to rod  20  (See  FIG. 20 ) which is mounted inside flange tube  16 . The other end of the rod  20  is attached to finger coupling  13 . Rod  20  is centered inside flange tube  16  by means of a second spacer  18  (See  FIG. 18 ). Second spacer  18  can also be constructed of any suitable material, preferably fluoropolymer resin such as TEFLON® brand fluoropolymer resin (Dupont, Wilmington, Del.). The brass core  19  inside of taper  12  which connects to rod  20  and is located inside of the flange tube  16 , couples microwave energy to the inner conductor flange tube  16 . 
     FIG. 8A  to  FIG. 31  show assembly and detailed drawings of various portions of the preferred apparatus  10  of the present invention.  FIG. 8A  illustrates a cross-section taken along line  8 A- 8 A ( FIG. 8 ) near the nozzle  35  end of the apparatus  10 . The relation of nozzle  35  and the outer conductor  17  can be seen and is spaced by cavity  22 . It can be seen in  FIG. 9  and  FIG. 9A  that the nozzle  35  narrows to an orifice  36  as an opening from which gas can emerge to ignite the plasma discharge  40  ( FIGS. 3 and 4 ). The cross-section of the nozzle  35  in  FIG. 9A  illustrates the orifice  36  from which the gas escapes at a first end and at an opposed end of the nozzle  35  is a narrow portion  35 A which fits snugly in the second end  28 B (See  FIG. 1 ) of the inner conductor  28  as seen in  FIG. 28 .  FIG. 10  is an end view and  FIG. 10A  is a cross-section of the first spacer  11  showing an inner surface  11 A which fits around the inner conductor  28  and thereby holds both the inner conductor  28  and nozzle  35  in the proper position relative to outer conductor  17 .  FIG. 17C  shows the cross-section of the outer conductor  17  showing an inner surface  17 A, which is snugly fit with respect to the outer surface  11 B ( FIG. 10A ) of the first spacer  11 . 
   As described previously, the opening  17 B in the side of the outer conductor  17  is covered with the flange tube  16 , illustrated in  FIGS. 16 through 16C , which is affixed to the outer conductor  17  at the curved end  16 B of the flange and projects from a side of the outer conductor  17 .  FIG. 11  is a side view of the assembled flange tube  16  and outer conductor  17  of the microwave torch apparatus  10 . Fitting snugly within the flange  16 , as best seen in  FIG. 8B , is the second spacer  18 . The second spacer  18  is shown in detail in  FIGS. 18 ,  18 A and  18 B, and has four holes  18 C running a length of the second spacer  18  opening between the cavity  22  in the outer conductor  17  and a cavity  12 D in the cone shaped coaxial taper  12 , when the cone shaped coaxial taper  12  is secured against the flat end  16 C of the flange  16 . The outer surface  18 A of the second spacer  18  fits against inner surface  16 A of the flange  16 , while the inner surface  18 B of second spacer  18  fits over the rod  20  to hold it in place. The rod  20  ( FIGS. 20 ,  20 A,  20 B) runs from a first end  20 A, where it is attached to the finger coupling  13  ( FIG. 13 ) having laterally projecting fingers  13 A which encircle and grip the inner conductor  28 , to a second end  20 B having external threads  20 C. External threads  20 C secure rod  20  to an internal thread  19 C in a first end  19 A of core  19 , seen in  FIGS. 19 ,  19 A and  19 B. The core  19  narrows down from a first end  19 A to a narrow portion  19 D at a second end  19 B. 
   The core  19  ( FIGS. 19 ,  19 A,  19 B) is thereby attached and extends longitudinally along a central axis of the cone shaped coaxial taper  12  which is affixed at a first end  12 A of the cone shaped coaxial taper  12  ( FIGS. 12 ,  12 A,  12 B) to the flat end  16 C of the flange  16 . A depression  14 A of the adjustable coupling flange  14  fits over a rim  12 B of the taper  12  and the adjustable coupling flange  14  ( FIGS. 14 ,  14 A,  14 B) is secured to the fixed coupling flange  15  so as to hold the rim  12 B in a depression  15 A in fixed coupling flange  15  ( FIGS. 15 ,  15 A,  15 B). Adjustable coupling flange  14  is secured to the flat end  16 C of the flange  16  by means of four bolts  37  passing through four holes  14 B in the adjustable coupling flange  14  and four holes  15 B in fixed coupling flange  15 . The fixed coupling flange  15  is affixed to the flat end  16 C of the flange  16  by a weld or other means known in the art. The assembled parts of the microwave input portion are best seen in cross-section in  FIG. 31 . Microwave power is supplied through a coupler (not shown) to an opening at a second end  12 C of the cone shaped coaxial taper  12  to supply microwaves into the cavity  12 D in the taper and thereby into the cavity  22  in the microwave torch apparatus  10  as illustrated by the arrow labeled “microwave power” in  FIG. 30 . The microwave power is channeled to the cavity between inner conductor  28  and outer conductor  17  as seen illustrated in  FIG. 8C  in a cross-section of a central portion of the microwave torch apparatus  10 . 
     FIG. 21  illustrates a cross-sectional view of an assembled tuning portion of the microwave torch apparatus  10  which allows a user to tune the microwaves in the cavity  22 . A tuning stub  21  ( FIGS. 8 ,  21  and  22 ) is mounted between the inner conductor  28  and outer conductor  17  for adjusting the microwave standing wave between the outer conductor  17  and inner conductor  28  to couple microwave energy to the plasma discharge  40 . A stub cap  32  is used to adjust the tuning stub  21 . The tuning stub  21  is mounted by means of adapter  23 , which is secured within outer conductor  17 . The adapter  23  has a first thread  23 A on an outer surface of a first end  23 C of the adapter  23  which is interlocked in a thread  17 E in the inner surface  17 A at a second end  17 D of the outer conductor  17 . The adapter  23  supports a nut  24  ( FIG. 23B ) by means of a second thread  23 B on an outer surface of a second end of the adapter  23  which interlocks with internal thread  24 A within the nut  24 . The rim  24 B of the nut  24  grips a rim  33 A on a first end  33 B of the holder  33  to lock holder  33  in place, while allowing rotational freedom. 
     FIG. 24  illustrates the holder  33  in detail. The external surface of the holder  33  can be knurled to assist gripping when tuning the apparatus  10 . An internal thread  33 C runs on an internal surface of the holder  33  from first end  33 B to second end  33 D. The internal thread  33 C interlocks with an external thread  21 A, as can be seen in  FIG. 22 , of the tuning stub  21 . The external thread  21 A extends from a first end  21 B of the tuning stub  21  towards a rim  21 C disposed at a second end  21 D of the tuning stub  21 , which allows a user to advance the rim  21  C from a position resting against the first end  23 C of the adapter  23  inwardly into the cavity  22  between outer conductor  17  and inner conductor  28  so as to tune the microwave torch apparatus  10 . Disposed around the rim  21 C is a first finger stock snap ring  25  ( FIG. 25 ,  25 A) and disposed within the second end  21 D beneath the rim  21 C is a second finger stock snap ring  26  ( FIG. 25B ,  25 C) so as to be mounted on the tuning stub  21  in contact with the outer conductor  17  and the inner conductor  28 , respectively. Finally, in the inner surface  21 E at the first end  21 B of the tuning stub  21  is an internal thread  21 F. An external thread  32 B ( FIG. 24A ) on a first end of the stub cap  32  is threaded into the internal thread  21 F of the tuning stub  21  ( FIGS. 21 and 22 ). On a second end of the stub cap  21  is a rim  32 A ( FIG. 24A ). 
   As can be seen in  FIG. 22 , a third spacer  27  ( FIG. 26 ,  26 A) is mounted within an inner surface  21 E of the tuning stub  21 , the third spacer  27  held in place by snap fit ring  38 . The third spacer  27  fits over a sleeve (not shown) which surrounds the inner conductor  28 . The third spacer  27  can be constructed of any suitable material, preferably fluoropolymer resin such as TEFLON® brand fluoropolymer resin (Dupont, Wilmington, Del.). 
   This assembly allows for independent adjustment of the tuning stub  21  and the inner conductor  28 . In this way the microwaves can be tuned to approximately an impedance matched condition. The gas is fed through gas feed tube  30  into the nozzle  35  to exit the nozzle at orifice  36 . Due to the taper on the nozzle  35  between the inner conductor  28  and outer conductor  17  the microwaves are confined to the cavity  22  and a very small plasma discharge  40  is created near the orifice  36  ( FIGS. 3 and 4 ). 
   An inner conductor cap  29 , illustrated in  FIG. 29  and  FIG. 29A , supports a first conduit member which is the gas feed tube  30  and a second conduit member which is the cooling fluid tube  31 . A narrow portion  29 B at a first end of the cap  29  secures into an inner surface at the first end  28 A of the inner conductor  28  and is soldered in place. At a second end of the cap  29  a projecting rim  29 A rests against the first end  28 A of the inner conductor  28 . The cooling fluid tube  31  passes through a second hole  29 D of the cap  29  to supply a cooling fluid to the inside of the nozzle  35  at the second end  28 B of the inner conductor  28  where it then can freely flow back down within a cavity  28 D to the first end  28 A of the inner conductor  28  to exit out of the cooling fluid output port  34  disposed near the first end  28 A of the inner conductor  28 . The gas feed tube  30  passes through a first hole  29 C in the cap  29  to supply gas to the orifice  36  of the nozzle  35  at the opposing end of the microwave torch apparatus.  FIGS. 8 ,  28  and  30  show the details of the assemblies. 
   The Figures thus show a miniature coaxial microwave torch apparatus  10  with an inner conductor  28  and outer conductor  17 . In a preferred embodiment, the overall apparatus  10  diameter is 12.5 mm (½ inch) and the outer conductor internal diameter is 11.1 mm. The gas is flowed through a 0.4 mm orifice  36  in the nozzle  35  supplied down the center of the apparatus in a gas feed tube  30  passing through an inner conductor  28 , the inner conductor  28  having an outer diameter of 4.75 mm. The axial position of the inner conductor  28  is adjustable to change the position of the orifice  36  where the plasma discharge is generated and maintained. The inner conductor  28  is water-cooled, having the cooling fluid flowing through the space in the inner conductor  28  before exiting at the cooling fluid exit port  34 . The characteristic impedance of the coaxial apparatus is 50 Ω. The power lines are at 50 Ω impedance. There are matched standing waves at resonance. 
   The behavior of the microwave electric fields in the microwave plasma torch apparatus  10  is shown in  FIGS. 32A ,  32 B and  32 C.  FIGS. 32A and 32C  show the field without the plasma discharge. In the coaxial region between the inner conductor  28  and outer conductor  17  the microwave electric fields are in a radial direction. The microwave energy propagates as a transverse electromagnetic (TEM) wave from the microwave power input coupling structure down to the nozzle  35 . The size reduction of the inner conductor at the nozzle end focuses the microwave electric field. This focusing is important for focusing and maintaining the discharge at the end of the inner conductor. The electric field lines that originate at the inner conductor  28  terminate on the outer conductor  17 . 
   When a plasma discharge  40  is present as illustrated in  FIG. 32B , the discharge  40  is an electrical conductor, therefore currents can flow in the discharge. The discharge acts, in effect, like an extension of the inner conductor. The microwave electric field lines adjust to extend from both the inner conductor  28  and plasma discharge  40  to the outer conductor  17  as shown in the right portion of  FIGS. 32B and 32C . A key feature of the microwave torch apparatus  10  is that the microwave electric fields exist between the inner conductor  28  and the outer conductor  17 . The result of this field arrangement is that the plasma discharge can interact with non-conductors, that is the workpiece being processed by the plasma does not need to be an electrical conductor as is necessary with DC torches. 
   The two adjustments for tuning in the torch apparatus  10  are the position of the tuning stub  21  and the position of the inner conductor  28  as shown in  FIG. 8 . The position of the inner conductor  28  is determined by manually sliding the inner conductor  28  through the outer portion of the torch consisting of tuning stub  21 , stub cap  32 , holder  33 , nut  24 , and outer conductor  17 . Friction against the first spacer  11  and inner conductor finger coupling  13  holds the inner conductor  28  at a fixed position. The position of the tuning stub  21  is determined by holder  33 . The turning of holder  33  moves the tuning stub  21  via the intermeshed internal thread  33 C of the holder  33  and external thread  21 A of the tuning stub  21 . The first end  33 B of holder  33  is held at a fixed vertical position (as oriented in the Figures) between nut  24  and adapter  23 . During assembly adapter  23  is held in place by threading it into the outer conductor  17  with first thread  23 A of adaptor  23 . Then the holder  33  slides on. Next, nut  24  threads on adapter  23  via second thread  23 B on the adaptor  23  to secured the holder  33  to outer conductor  17 . Due to this, the holder  33  is free to rotate about the axis, but it is held from moving up and down by nut  24  and adapter  23 . 
   EXAMPLE 
   A very useable miniature plasma was created by the microwave plasma torch of the present invention. The minimum required power (W), resulting plasma discharge length (mm) and the power density (W/cm 3 ) of the plasma at the nozzle  35  during operation of the microwave torch apparatus  10  were studied at various argon flow rates (sccm). The plasma discharge  40  as illustrated in  FIGS. 3 and 4  is formed when argon is provided at a flow rate of 200 sccm, and the microwave power absorbed in the apparatus  10  when operated in this manner is 15 to 20 W (Watts) as is shown in the graph of  FIG. 5 .  FIG. 5  is a graph showing the power required as a function of the flow rate of argon. The power is that absorbed in the coaxial apparatus  10  and the discharge. The minimum power occurred at 200 sccm for this apparatus  10 . 
   The resulting plasma discharge size when operated at 200 sccm is 400-700 μm (micrometer) in diameter and 3-4 mm (millimeter) long. This plasma discharge length is shown as one of the data points in  FIG. 6 .  FIG. 6  is a graph showing plasma discharge length in millimeters as a function of flow rate (sccm). The power absorbed in the apparatus and discharge was 30 W (except at 700 sccm where the power absorbed was 37 watts). The plasma discharge was about 400-700 μm in diameter. As can be seen, there is an optimum flow rate for this apparatus  10  as a function of discharge length of about 200 sccm of argon which coincides with that seen in  FIG. 5 . 
     FIG. 7  is a graph showing power density (W/cm 3 ) as a function of argon flow rate. Power densities are calculated assuming that all of the power absorbed in the apparatus  10  and discharge goes to the discharge. The discharge diameter is assumed to be about 700 μm for this calculation. The calculated power density is lowest at about 200 sccm which appears to be the optimum for the particular apparatus  10  illustrated in  FIG. 1  and  FIG. 2 . 
   While the present invention is described herein with reference to illustrated embodiments, it should be understood that the invention is not limited hereto. Those having ordinary skill in the art and access to the teachings herein will recognize additional modifications and embodiments within the scope thereof. Therefore, the present invention is limited only by the claims attached herein.