Abstract:
A microwave resonant cavity is provided. The microwave resonant cavity includes: a sidewall having a generally cylindrical hollow shape; a gas flow tube disposed inside the sidewall and having a longitudinal axis substantially parallel to a longitudinal axis of the sidewall; a plurality of microwave waveguides, each microwave waveguide having a longitudinal axis substantially perpendicular to the longitudinal axis of the sidewall and having a distal end secured to the sidewall and aligned with a corresponding one of a plurality of holes formed on the sidewall; a top plate secured to one end of the sidewall; and a sliding short circuit having: a disk slidably mounted between the sidewall and the gas flow tube; and at least one bar disposed inside the sidewall and arranged parallel to the longitudinal axis of the sidewall.

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is a national Stage Patent Application of PCT International Patent Application No. PCT/US2012/043482, which was filed on Jun. 21, 2012 under 35 U.S.C. §371 and claims priority of both U.S. Patent Application No. 61/500,624, filed on Jun. 24, 2011 and U.S. patent application Ser. No. 13/529,110, filed on Jun. 21, 2012, which are all hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to plasma generators, and more particularly to systems having a resonant cavity for generating a plasma therein. 
     In recent years, microwave technology has been applied to generate various types of plasma. In some applications, igniting and sustaining plasma requires a high power microwave generator. The existing microwave techniques are not suitable, or at best, highly inefficient due to one or more of the following drawbacks. First, the existing systems lack proper scalability, where scalability refers to the ability of a system to handle varying amounts of microwave input power in a graceful manner or its ability to be enlarged/reduced to accommodate the variation of the input power. For instance, the required microwave input power may vary depending on the types, pressure, and flow rates of the gas to be converted into plasma. Second, the economics of scale for a magnetron increases rapidly as the output power increases. For instance, the price of a 10 KW magnetron is much higher than the price of ten 1 KW magnetrons. Thus, there is a need for a plasma generating system that has high scalability and is cheaper than currently available plasma generating systems without compromising the output power. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present disclosure, a microwave resonant cavity includes: a sidewall having a generally cylindrical hollow shape and formed of a material opaque to a microwave; a gas flow tube disposed inside the sidewall, formed of a material transparent to a microwave, and having a longitudinal axis substantially parallel to a longitudinal axis of the sidewall; a plurality of microwave waveguides, each said microwave waveguide having a longitudinal axis substantially perpendicular to the longitudinal axis of the sidewall and having a distal end secured to the sidewall and aligned with a corresponding one of a plurality of holes formed on the sidewall; a top plate formed of a material opaque to a microwave and secured to one end of the sidewall; and a sliding short circuit. The sliding circuit includes: a disk formed of a material opaque to a microwave and slidably mounted between the sidewall and the gas flow tube, the disk having an outer rim snuggly fit into the sidewall and a hole into which the gas flow tube being snuggly fit; and at least one bar disposed inside the sidewall and arranged parallel to the longitudinal axis of the sidewall. By moving the bar along the longitudinal direction of the sidewall, the space defined by the top plate, sidewall, and the disk is adjusted to form a microwave resonant cavity inside the gas flow tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a plasma generating system in accordance with one embodiment of the present invention. 
         FIG. 2  is an exploded perspective view of the microwave resonant cavity in  FIG. 1 . 
         FIGS. 3A-3C  are top views of alternative embodiments of the microwave resonant cavity in  FIG. 2 . 
         FIGS. 4A-4C  are perspective views of alternative embodiments of the microwave resonant cavity in  FIG. 1 . 
         FIG. 5  is a schematic cross sectional view of an alternative embodiment of the microwave resonant cavity in  FIG. 2 . 
         FIG. 6  is a schematic cross sectional view of an alternative embodiment of the microwave resonant cavity in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic diagram of a system  10  for generating microwave plasma in accordance with one embodiment of the present invention. As illustrated, the system  10  may include: a microwave resonant cavity  26 ; microwave supply units  11   a - 11   c  for providing microwaves to the microwave resonant cavity  26 ; and waveguides  24   a - 24   c  for transmitting microwaves from the microwave supply units  11   a - 11   c  to the microwave resonant cavity  26 , where the microwave resonant cavity  26  receives a gas and/or gas mixture from a gas tank  28  or another source such as flue gas. 
     The microwave supply unit  11   a  provides microwaves to the microwave resonant cavity  26  and may include: a microwave generator  12   a  for generating microwaves; a power supply  13   a  for supplying power to the microwave generator  12   a ; and an isolator  15   a  having a dummy load  16   a  for dissipating reflected microwaves that propagate toward the microwave generator  12   a  and a circulator  18   a  for directing the reflected microwaves to the dummy load  16   a.    
     In one embodiment, the microwave supply unit  11   a  further includes a coupler  20   a  for measuring fluxes of the microwaves; and a tuner  22   a  for reducing the microwaves reflected from the microwave resonant cavity  26 . The components of the microwave supply unit  11   a  shown in  FIG. 1  are well known and are listed herein for exemplary purposes only. Also, it is possible to replace the microwave supply unit  11   a  with a system having the capability to provide microwaves to the microwave resonant cavity  26  without deviating from the present invention. A phase shifter may be mounted between the isolator  15   a  and the coupler  20   a.    
     The microwave supply units  11   b  and  11   c  are shown to have similar components as the microwave supply units  11   a . However, it is noted that the microwave supply units  11   b  and  11   c  may have components different from those of the unit  11   a , insofar as they can generate and deliver microwaves to the waveguides  24   b  and  24   c , respectively. 
       FIG. 2  is an exploded perspective view of the microwave resonant cavity  26  in  FIG. 1 . As depicted, the microwave resonant cavity (shortly, cavity hereinafter)  26  includes a top plate  41  having an inlet port  51  for receiving gas  53  from the gas tank  28 ; a bottom plate  43  having an outlet port (or, outlet hole)  44  for discharging gas therethrough; and a sidewall  42  connected to the distal ends of the waveguides  24   a - 24   c . The distal end of the waveguide  24   a  is secured to the sidewall  42  so that the microwave energy flowing through the proximal end  40   a  of the waveguide  24   a  enters into the sidewall  42 . Likewise, the microwave energy flowing through the proximal ends  40   b  and  40   c  of the waveguides  24   b  and  24   c  enters the sidewall  42 . The top plate  41 , sidewall  42 , and bottom plate  43  may be formed of any suitable material, such as metal, that is opaque to the microwave. The cavity  26  also includes a gas flow tube  46  that is transparent to the microwave and preferably formed of quartz. 
     The top and bottom ends of the gas flow tube  46  are sealed to the top plate  41  and the bottom plates  43  of the cavity  26 , respectively, so that the gas entered into the tube  46  through the inlet port  51  is excited into plasma and exits through the outlet port  44  of the bottom plate  43 . The microwave energy received through the waveguides  24   a - 24   c  excites the gas into plasma when the gas flows through the gas flow tube  46 . 
     The cavity  26  may also include a sliding short  48  having a disk  49  and bars  50 . The disk  49  is dimensioned to slidably fit into the space between the inner surface of the sidewall  42  and the outer surface of the gas flow tube  46 , and formed of material opaque to the microwave, preferably metal. During operation, the microwaves discharged from the distal ends of the waveguides  24   a - 24   c  form an interference pattern in the gas flow tube  46 . As the user slides the bars  50  up and down along the longitudinal direction  56  of the cavity  26 , the distance between the disk  49  and the top plate  41  is changed so that the interference generates a peak amplitude region in the gas flow tube  46 , i.e., the impedance matching may be obtained by adjusting the location of the disk  49  relative to the top plate  41 . It is noted that the bars may be attached to a suitable tuning mechanism, such as a micrometer fixed to the outer surface of the bottom plate  43  so that the user can tune the impedance at high precision Optionally, a motor attached to the bars  50  may be used for an automated control. 
     It is noted that the microwaves generated by the three microwave supply units  11   a - 11   c  are combined in the gas flow tube  46 . As such, if the microwave supply units are identical, the maximum intensity of microwave field within the gas flow tube  46  would be the same as the intensity generated by one microwave supply unit that has the output power three times as large as the microwave supply unit  11   a . This feature provides two advantages; scalability and cost reduction in manufacturing a microwave supply unit. The operator of the system  10  may selectively turn on the microwave supply units  11   a - 11   c  so that the intensity of the microwave field in the gas flow tube  46  may be varied. For instance, the microwave intensity for igniting the plasma in the gas flow tube  46  may vary depending on the types of gas  53 . The operator may optimize the microwave intensity in the gas flow tube  46  by selectively turning on the microwave supply units  11   a - 11   c . It is noted that the system  10  has only three microwave supply unit. However, it should be apparent to those of ordinary skill in the art that the system may include any other suitable number of microwave supply units. 
     The price of the microwave generator  12   a , especially the magnetron, increases rapidly as its power output increases. For instance, the price of ten magnetrons of the commercially available microwave oven is considerably lower than that of one high power magnetron which has an output power ten times that of the microwave oven. Thus, the multiple microwave generators feature of the system  10  allows the designer to build a low cost microwave generating system without compromising the total maximum power. 
       FIGS. 3A-3C  are top views of alternative embodiments  60 ,  70 , and  80  of the cavity sidewall  42  in  FIG. 2 . As depicted, the sidewall may have a suitable polygonal shape, such as rectangle, hexagon, or octagon, where a waveguide may be fixed to each side of the polygon. The phases of the microwaves exiting from two adjacent waveguides may be differentiated so that the interference between the microwaves generates the maximum intensity in the gas flow tubes  62 ,  72 , and  82 . It is noted that gas flow tubes  62 ,  72 , and  82  may have other suitable cross sectional geometry, such as rectangle, hexagon, or octagon. It is further noted that the angle θ (shown in  FIG. 1 ) between two adjacent waveguides may be adjusted to optimize the interference between two microwaves. 
       FIG. 4A  is a perspective view of an alternative embodiment  100  of the cavity  26  in  FIG. 1 . For brevity, only the sidewall and waveguides are show in FIG.  4 A. As depicted, the cavity  100  is similar to the cavity  26  in  FIG. 1 , with the difference that the waveguides  102   a - 102   c  are e-plane waveguides. 
       FIGS. 4B and 4C  are perspective views of alternative embodiments  114  and  124  of the cavity  26  in  FIG. 1 . As depicted, the cavities  114  and  124  are similar to the cavity  26 , with the differences that the locations of the waveguides  112   a - 112   c  and  122   a - 122   c  relative to the sidewalls of the cavities  114  and  124  are different. The locations of the waveguides are determined to optimize the interference pattern in the gas flow tubes (not shown in  FIGS. 4B-4C  for brevity) disposed within the cavities  114  and  124 . 
       FIG. 5  is a schematic cross sectional view of an alternative embodiment  200  of the microwave resonant cavity  26  in  FIG. 2 . As depicted, the cavity  200  includes a top plate  241  having an inlet hole  243  for receiving gas from the gas tank  28  (not shown in  FIG. 5 ); a sidewall  242  connected to the distal ends of the waveguides  224   a - 224   b ; a gas flow tube  246  having a bottom hole  244  for discharging gas therethrough; and sliding short circuit  248  having a disk  249  and bars  250 . Since the materials and functions of the components of the cavity  200  are similar to those of their counterparts of the cavity  26 , the detailed description is not repeated. The difference between the cavities  26  and  200  is that the cavity  200  does not have a bottom plate while the cavity  26  includes the bottom plate  43 . 
       FIG. 6  is a schematic cross sectional view of an alternative embodiment  300  of the cavity  26  in  FIG. 2 . As depicted, the cavity  300  is similar to the cavity  26 , with the difference that the top and bottom portions of the gas flow tube  346  protrude outside the top plate  341  and the bottom plate  343 , respectively. The gas flow tube  346  includes a top hole  343  and a bottom hole  344  for receiving and discharging the gas therethrough. Alternatively, the gas flow tube  360  may have a gas inlet port  360  in place of the hole  343 , where the inlet port  360  is angled with respect to the longitudinal axis of the gas flow tube  346  to impart swirling motion to the injected gas. 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.