Patent Publication Number: US-9433071-B2

Title: Dielectric barrier discharge plasma generator

Description:
FIELD OF THE INVENTION 
     The invention relates to dielectric barrier discharge (DBD) plasma generation devices. In particular, the invention relates to a DBD plasma generator that controllably produces uniform, non-equilibrium plasma and discharges a plasma plume at atmospheric pressure. 
     BACKGROUND OF THE INVENTION 
     Non-thermal, non-equilibrium DBD plasma plumes/jets are known in the prior art and have many medical and engineering applications including wound healing, wound sterilization, blood coagulation, scar treatment, surface decontamination, surface treatment, and plasma sterilization. The plume of non-equilibrium DBD plasma generators is discharged in open air and does not require any special plasma enclosure. Therefore, the plume can be located at any distance from the application target without interference with the generator structure. Furthermore, the risk of contamination from contact with or adherence between the application target and plasma enclosure is eliminated. 
     The precise chemical/biological reaction mechanism between the plasma plume and the target, which produces the aforementioned beneficial effects, is still under investigation. Several theories have been proposed. 
     According to one theory, the presence of various gases along with moisture in the air produce several chemically reactive species in the plasma plume that react with the target. Chen&#39;s work shows that the plasma effluent of the plume carries an abundance of reactive atomic oxygen (RAO), which is the catalyst for plasma medical effects. As RAO reacts with H 2 O in blood, it produces H 2 O 2 . Some of the H 2 O 2  is decomposed to oxygen, which dissolves into tissue to increase oxygen tension. H 2 O 2  also triggers fibroblast growth factor, platelet derived growth factor and other factors to induce reactions such as inflammation and angiogeneis. As a result, the healing process is improved and healing time is reduced. Chen C., Air Plasma Effects on Bleeding Control and Wound Healing, PhD Thesis, Department of Electrical Engineering, Polytechnic Institute of NYU, June 2011, UMI Number: 3457994. 
     According to another theory, radicals in plasma support the endogenous radical-mediated defenses and healing mechanisms of tissue and derive the formation of cell mediators such as nitric oxide. For example, Laroussi et al. concluded that for non-equilibrium, atmospheric air plasmas, oxygen-based and nitrogen-based reactive species played the most important role in the bacterial inactivation process Lederer E., Plasma Blows Wounds Clean, http://news.doccheck.com/com/article/211278-plasma-blows-wounds-clean/. 
     According to Soffels et al., plasma releases controllable amounts of short-lived reactive oxygen (ROS) and nitrogen (RNS) species that address only the target areas in the tissue. Each of these species has different physiological functions such as antibacterial, pro-apoptotic, pro-inflammatory (ROS), or anti-inflammatory and pro-apoptotic (RNS). External administration of ROS or RNS by plasma locally reinforces the natural physiological processes. Stoffels E., Roke A. J. M., Deelman L. E., Delayed Effects of Cold Atmospheric Plasma on Vascular Cells, Plasma Processes and Polymers, No. 5, 2008, 599-605. 
     Regardless of the mechanism, it has been experimentally confirmed that plasma treatment conditions can be tuned to achieve many desired medical effects, especially in medical sterilization and treatment of different types of skin diseases. Plasma treatment conditions may be tuned by, for example, varying the treatment conditions and/or plasma characteristics including the degree of ionization, electron&#39;s temperature, gas temperature, input power (voltage) of the generator, input gas composition, exposure time to the plasma plume, and distance between the plasma plume and the target. 
     In general, prior art plasma generators use two electrodes, such as parallel, metallic plates, separated by a dielectric material. Typically, the electrodes are fixed relative to one another, which stagnant configuration produces the same plume characteristics for a give set of input values. It would be desirable to provide a plasma generator having one or more electrodes that are movable, which relative movement provides another means of changing or tuning the characteristics of the plasma plume. 
     Many prior art plasma generators also require high input power, complex heavy-duty pulse generators, amplifiers, or complicated RF generators in order to create the plasma-generating electric field. Such electrical requirements greatly inhibit the portability of such devices and significantly add to the cost of production. Therefore, it would be desirable to provide a plasma generator that has basic components and low power requirements so that the device can operate portably with a low voltage battery source. 
     SUMMARY OF THE INVENTION 
     The present invention provides devices for producing uniform, non-equilibrium plasma and discharging a plasma plume at atmospheric pressure. The devices include means for adjusting properties of the plasma plume exiting therefrom including one or more of the following: gas temperature; length; size; degree of ionization or relative presence of various radicals; and uniformity of plasma. Because the plasma plume can be adjusted, the device has broad medical applications including sterilization, wound healing, inactivation of bacteria, surgery, and surface treatment and engineering applications including ozone generation. 
     In a preferred embodiment, the device comprises a dielectric barrier discharge plasma generator that is capable of producing an adjustable plasma plume in open air at atmospheric pressure. Preferably, the plasma generator can produce a relatively long plasma plume using several different source gases including helium, argon, and nitrogen. Because the device produces a plasma plume in open air at atmospheric pressure, it can be operated without vacuum systems surrounding the target site. Open air operation also produces many radicals and ion species that are important for several medical applications. 
     Because the device produces uniform, non-equilibrium (cold) plasma in preferred embodiments, the device can be used for applications where high-temperature, high-pressure plasma discharges are inappropriate. For example, in medical applications, thermal diffusion to tissue adjacent the target can be eliminated and damage limited by adjusting the gas flow rate and the gas temperatures of the exiting plume. 
     In other preferred embodiments, the device is small and portable. Due to its small size, the device produces a plasma plume that is localized and precise, and does not damage the area surrounding the target. The device includes a probe that can be held in a single hand and easily manipulated by the operator. The associated accessories, such as the power source and gas source, can fit on a movable cart, or be incorporated within the probe, so that the system is portable. 
     In another preferred embodiment, the device has low power requirements and does not require heavy-duty pulse generators, amplifiers, or complicated RF generators. The device can be operated with a low voltage DC power such as a 12 volt battery. The frequency of the output voltage may be about 1 kHz to about 100 kHz. This low power requirement ensures that the plasma plume can be safely placed in direct contact with living tissue and delicate surfaces including living flesh, skin, and wounds. The plasma device is essentially electrically neutral since the plasma plume induces electrical currents in the target on a microamp level. 
     In an additional preferred embodiment, the device uses low gas flow rates, preferably less than 1.0 standard liters per minute (SLPM), which minimizes the device&#39;s operating cost. The device&#39;s low pressure requirements also eliminates damage to exposed delicate tissues, which may be caused by over-pressurization of the gas plume contacting the exposed surface. 
     In yet another preferred embodiment, the device includes nozzle means for projecting the plasma plume from the tip of the hand-held probe in either the radial or axial direction. This feature gives the operator greater maneuverability in small spaces such as surgery and dentistry. 
     Similar to most DBD generators, the device produces plasma by applying an electric field between two electrodes. In an additional preferred embodiment, one electrode has means for generating a plurality of separate, high-intensity electric fields along at least a portion of its surface. These multiple electric fields break down and create a controllable, uniform plasma inside the plasma generator that is expelled through an exit port into open air. This construction requires far less power than prior art plasma generators. In one preferred embodiment, the electric field generating means comprises a plurality of equally-spaced protrusions electrically-connected to and transversely-extending from at least a portion of the inner electrode base. The protrusions may comprise wire bristles having a cross-sectional area that is much less than the cross-sectional area of the inner electrode base. 
     In still another preferred embodiment, one electrode is movable relative to the other so that the location of plasma generation within a dielectric tube can be changed. Movement of the electrode changes the characteristics of the plume including generation of various radicals and species in the plasma plume. 
     In one preferred embodiment the device comprises a gas source, a power source, and a plasma generator probe having a central axis, a proximal end, and a distal end from which the plume of plasma is discharged. The probe includes an elongate housing, an elongate, dielectric ionization conduit, an elongate inner electrode, and an outer electrode that is slidably arranged on the outer surface of the ionization conduit and electrically connected to the power source. The electrodes are constructed and arranged so that movement of the outer electrode relative to the inner electrode changes at least one property of the plasma plume. 
     The housing has a central axis, an open distal end and a proximal end. The ionization conduit has a central axis arranged in coaxial relationship within the housing. The ionization conduit has a port at an open discharge end proximate the open discharge end of the housing, and a proximal end arranged in sealed fluid communication with the plasma gas source. 
     The inner electrode extends within the ionization conduit, and has a distal end proximate the distal end of the housing and a proximal end electrically connected to the power source. The distal portion of the inner electrode has a construction that is different than a proximal portion, and is located proximate the ionization conduit port. 
     In this preferred embodiment, the central electrode has an elongate base extending generally parallel to the central axis of the ionization tube and has a plurality of bristles electrically-connected to and transversely-extending from at least a portion of the electrode base. The bristles have a cross-sectional area that is much less than the cross sectional area of the electrode base. The length of the bristle portion is greater than the axial length of the outer electrode. 
     The length of the bristles ranges from about 200 microns to 1 mm and the density of the bristles along the base ranges from about 10 bristles/mm to about 20 bristles/mm. In this embodiment, the bristles are integrally formed with the electrode base. However, in other embodiments, the bristles and base are separate, electrically-connected elements and may be made from different electrically-conductive materials. In one preferred embodiment, the bristles are spaced equally from one another along the length and around the perimeter of the pin portion. 
     The outer electrode can be slid axially between a first limit position aligned with the inner electrode distal portion and a second limit position aligned with the inner electrode proximal portion.  15 . In one preferred embodiment, the outer electrode can slide along the entire length of the bristle portion. 
     In one preferred embodiment, the housing and dielectric ionization conduit comprise cylindrical tubes having a generally concentric arrangement. The outer electrode comprises an annular ring having an inner diameter larger than the outer diameter of the ionization tube and an outer diameter smaller than the inner diameter of the housing tube. The radial distance between the inner electrode and the inner surface of the outer electrode is between about 5 to 10 mm. The axial length of the annular ring is about 1 to 15 mm. 
     In another preferred embodiment, the device includes a diverter nozzle connected to the open distal end of the housing that changes the flow direction of the plasma plume. Alternatively, or additionally, the diverter also divides the plume into more than one flow direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a plasma generating device in accordance with a preferred embodiment of the invention; 
         FIG. 2  is an electrical schematic of the power circuit of the device show in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of a plasma generating probe in accordance with another preferred embodiment of the invention; 
         FIG. 4  is another cross-sectional view of the plasma generating probe of  FIG. 3 ; 
         FIG. 5  is an exploded assembly view of the main components of the plasma generating probe of  FIG. 3 ; 
         FIG. 6  is a side elevation of the main components of  FIG. 5  shown in an assembled condition; 
         FIG. 7  is a chart comparing plasma plume length as a function of applied voltage of the power source; 
         FIG. 8  is a chart comparing plume gas temperature as a function of distance from the nozzle exit; 
         FIG. 9  is a chart showing the emission spectra of one plasma plume generated by the apparatus of  FIG. 3 ; 
         FIG. 10  is a schematic illustration of a plasma generating device in accordance with an additional preferred embodiment of the invention; 
         FIG. 11  is a cross-section of a plasma generating probe in accordance with yet another preferred embodiment of the invention; 
         FIG. 12  is another cross-section of the plasma generating probe of  FIG. 11 ; and, 
         FIG. 13  is an enlarged, fragmentary view of the distal portion of the inner electrode showing the radially-protruding bristles. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S) 
     For the purpose of illustrating the invention, several embodiments of the invention are shown in the accompanying drawings. However, it should be understood by those of ordinary skill in the art that the invention is not limited to the precise arrangements and instrumentalities shown therein and described below. Throughout the specification, like reference numerals are used to designate like elements. Throughout the specification, as used in connection with various elements and portions of elements, the terms “distal” and “proximal” refer to their spatial relationship relative to the end of the generator probe into which gas is input and opposite the end from which plasma is discharged. The term “plume temperature” means the temperature of the gas within the plume. 
     An apparatus for generating a uniform, non-equilibrium plasma plume in accordance with a preferred embodiment of the invention is schematically illustrated in  FIG. 1 . The apparatus, designated generally by reference numeral  10 , comprises a dielectric barrier discharge (DBD) plasma generator probe  12 , a plasma gas source  14 , and an electric power source  16 . The plasma plume  8  is discharged from the probe  12  at atmospheric pressure. The generator probe  12  can be held in one hand and easily manipulated relative to the treatment target. 
     Referring to  FIG. 1 , the plasma generator probe includes an elongate housing  18  having a central axis, a proximal end wall  18   a  and a distal end wall  18   b  from which the plasma plume  8  is discharged. The plasma plume  8  can be characterized by measuring the plume temperature, plume length, and emission spectra as a function of input power, axial position of the outer annular electrode, the type of gas, and the gas flow rate through the generator probe  12 . 
     The housing  18  may be made of any material having sufficient rigidity to support the probe&#39;s internal components and be hand held by the operator. The housing  18  should also preferably be made of an insulating material. For example, the housing  18  may be made from a thermoplastic used to make precision parts requiring high stiffness, low friction and excellent dimensional stability such as polyoxymethylene. The distal end wall  18   b  of the housing  18  includes an exit port  20  through which the plasma plume  8  is expelled. The proximal end wall  18   a  of the housing  18  includes sealed apertures  22 ,  24  through which electrical connector cables  26 ,  27  extend, and a port  28  through which a gas supply tube  30  extends. The housing  18  is otherwise sealed. 
     An elongate, dielectric ionization conduit  32  is arranged in a generally coaxial relationship with the housing  18 . The conduit  32  has a port  34  at a distal discharge end  32   b , which tapers in the form of a concentrating nozzle. The port  34  in the conduit aligns with the exit port  20  in the distal end of the housing  18 . The proximal end  32   a  of the conduit is connected in sealed fluid communication with the gas supply tube  30 . The ionization conduit is made from a dielectric material such as glass or machinable ceramic that can withstand high temperatures. 
     An elongate inner electrode  36  extends within the ionization conduit  32 . The distal end  36   b  of the electrode  36  is positioned near the distal port  34  in the ionization conduit  32 . The proximal end  36   a  of the electrode  36  is located near the proximal end  18   a  of the housing  18  and connects to the power supply  16  via a connector cable  27 . 
     The inner electrode  36  has an elongate base  38  extending generally parallel to the central axis of the ionization conduit  32 . The inner electrode  36  also includes means for generating a plurality of separate, high-intensity electric fields along the length and around the perimeter of the electrode base  38 . The generating means may comprise a plurality of electrically-conductive bristles  40  fixed to and extending radially from the electrode base  38 . The electrode base  38  and bristles  40  may be made from an electrically conductive material such as copper, stainless steel, or aluminum. The base  38  and bristles  40  preferably are, but need not be, made from the same electrically-conductive material. The base  38  need not be integrally formed with the bristles  40  so long as they are connected in electrical conductivity. Preferably, the cross-sectional area of the bristles  40  is less than the cross-sectional area of the base  38 . 
     In the preferred embodiment shown in  FIG. 1 , the bristles  40  are equally spaced both axially and radially on the electrode base  38  and along the entire length of the base  38  located within the ionization conduit  32 . However, in other preferred embodiments, the bristles  40  may be provided on less than the entire length of the base  38  without departing from the scope of the invention. Furthermore, the bristles  40  may be spaced unequally but in a defined pattern along the axial length of the base  38 . For example, the spacing between bristles  40  may increase/decrease exponentially or factorially along the base  38  length. In other less preferred embodiments, the bristles  40  are randomly spaced along the base length. The size (length or cross section) and shape of the bristles  40  may also vary along the length of the inner electrode. 
     The size of the bristles  40  may vary depending on the intended application. In preferred embodiments, the length of the pins may range from 200 microns to 1 mm, and preferably be less than 1 mm. The diameter of the bristles may also range from 1 mm to a few mm. 
     The number of bristles per unit length of inner electrode, i.e., density, may vary depending on the intended application. For example, the density of the bristles  40  may vary from a few per mm to several dozen per mm along the inner electrode base. Embodiments with higher pin density will have more uniform plasma production in the region between the outer and inner electrode. 
     The total number of bristles, and the length of electrode base  38  connected to bristles  40 , may also vary depending on the intended application. For example, in the embodiment shown in  FIG. 1 , the entire length of inner electrode contained within the ionization conduit  32  is connected to bristles  40 . However, in the embodiments shown in  FIGS. 3-6 and 11-12 , the bristles  140  are only connected to a distal portion of the inner electrode base. 
     An outer electrode  42  is slideably arranged on the outer surface of the ionization conduit  32  and connected to the power source  16  by a connector cable  26 . The outer electrode  42  has an inner shape and dimension that compliments and is slightly larger than the outer shape and dimension of the ionization conduit  32 . The outer electrode  42  also has an outer shape and dimension that compliments and is slightly smaller than the shape and inner dimension of the housing  18 . These complimenting shapes and sizes allow the outer electrode to slide axially along the length of the ionization conduit  32 . 
     The outer electrode  42  may be made of an electrically-conductive material such as stainless steel, copper, or aluminum. The outer electrode may, but need not be, made from the same electrically-conductive material as the inner electrode  36 . 
     In the embodiment shown in  FIG. 1 , the power source  16  comprises a remote device wired to the generator probe  12 . However, in other preferred embodiments, the power supply may be attached to or incorporated into the probe housing  18 . 
     A schematic diagram of the power circuit of a preferred embodiment is shown in  FIG. 2 . The power supply comprises a low-voltage, direct current battery  44 , a DC/AC converter  46 , and a ballast resistor  48 . The power supply preferably produces AC voltages from 1 kV to about 12 kV and may vary in frequency from about 1 kHz to about 100 KHz. The ballast resistor value may range from about 10 k Ohm to about 100 k Ohm. In a preferred embodiment, the battery comprises a common 12 volt battery. The AC power supply may have means to control the frequency and amplitude of the voltage signal. In another embodiment, the AC power supply includes means to vary the frequency and the amplitude of the signal independently. 
     In one preferred embodiment, the gas source  14  comprises a pressurized tank of a nitrogen, helium, argon or other gas known for producing plasma. The gas source  14  preferably includes a valve(s) and gas flow meter(s) to monitor and regulate the pressure and flow rate of gas through the generator probe  12 . The pressurized gas may also comprise air; however, as discussed below, the input power required to ionize air is much higher than for ionizing argon, helium or nitrogen. 
     The gas pressure may be adjusted to achieve low gas flow rates and to avoid over-pressurization, i.e., plasma pressure/velocity that damages the target, especially in medical applications. For example, for very sensitive applications, the gas flow rate can be adjusted to about 1 SLPM to about 5 SLPM. For other less sensitive applications, the gas flow rate may be adjusted up to 15 SLPM or higher. 
     When the power source  16  is energized, a voltage differential is created between the inner  36  and outer  42  electrodes. The electrical discharge between the inner and outer electrodes creates a uniform and controllable DBD plasma discharge in the ionization conduit  32 , which is expelled from the exit port  20  in the housing  18 . The plasma is non-equilibrium and weekly ionized. The plasma created in the ionization conduit is not an arc plasma, which is usually rendered as an equilibrium plasma having very high gas and electron temperatures (ranging from 0.5 eV to several electron volts). Instead, as voltage is applied across the electrodes, streamers momentarily initiate at the tip of each bristle  40  on the inner electrode  36 . The streamers propagate towards the dielectric surface, i.e., inner surface of the ionization conduit  32 . Due to charging of the dielectric surface, streamers do not have sufficient time to convert into arcs. Since the charge is not removed by any conductor, the current ceases and a new breakdown occurs at the tip of bristles  40 , thereby sustaining the plasma inside the ionization conduit  32 . 
     The input power required to create the uniform and controllable DBD plasma discharge in the ionization conduit  32  varies depending on the input gas. For common plasma producing gases such as nitrogen, helium, and argon at very low pressures, the input power requirement is very low, e.g., up to tens of Watts; however, if air is used to produce the plasma plume, the input power requirement is much higher, e.g., up to hundreds of Watts. 
     Plasma production within the ionization conduit  32  occurs in the region of overlap (axial alignment) between the inner  36  and outer  42  electrodes. Plasma production does not initiate on any of the bristles  40  that are non-overlapping with the outer electrode  42 . Because the axial location of the outer electrode  42  can be adjusted relative to the inner electrode  38 , the location within the ionization conduit at which plasma is produced can also be adjusted. By varying the plasma production location, at least one property of the exiting plasma plume can be adjusted. By changing the axial location along the inner electrode  36  at which ionization occurs, the plume temperature, length, and degree of ionization of the exiting plasma plume  8  can be adjusted and controlled to suit a particular application. For example, when the outer electrode  42  is positioned very close to the exit port  20  in the housing  18 , a very intense, relatively-high temperature plasma plume is produced. Conversely, when the outer electrode  42  is positioned far away from the exit port  20 , a less intense, lower temperature plasma plume exits the probe  12 . The properties of the plasma plume can also be adjusted and controlled by varying the gas type, the gas flow rate through the ionization conduit  32 , and input voltage. 
     An apparatus for generating a uniform, non-equilibrium plasma plume from a gas source and power source in accordance with another embodiment of the invention is shown in  FIGS. 3-6 . The apparatus comprises a dielectric barrier discharge plasma generator probe  112  having a construction similar to the probe  12  disclosed above. The probe  112  connects to a plasma gas source and an electric power source such as the gas source  14  and power source  16  disclosed above. The plasma plume  8  is discharged from the probe  112  at atmospheric pressure. 
     Referring to  FIGS. 3-6 , the plasma generator probe  110  includes an elongate, tubular housing  118  having a central axis, a proximal end wall  118   a  and a distal end wall  118   b  from which the plasma plume is discharged. In this embodiment, the housing  118  is made of polyoxymethylene, which has high stiffness, low friction and excellent dimensional stability. The distal end wall  118   b  of the housing  118  includes an exit port  120  through which the plasma plume  8  is expelled. The proximal end wall  118   a  of the housing  18  includes an aperture  122  through which an electrical connector cable  126  extends, and a port  128  through which a Y-shaped gas supply connector  130  extends. The housing  118  is sealed around the cable  126  and Y-connector  130 . 
     The Y-connector  130  has a central axis and aperture  130   a  extending through a threaded trunk portion  130   b , which then splits into a threaded branch portion  130   c  and a barbed branch portion  130   d . A rib  131  traverses the central aperture proximate the open end of the trunk portion  130   b  as best seen in  FIG. 5 . The rib  131  includes a central axial bore  133  slightly larger than the outer diameter of the central electrode  136 . The rib  131  is narrow enough so that the central aperture  130   a  is not completely blocked as seen in  FIG. 5 . The Y-connector  130  has a central, hexagonally-shaped shoulder portion  130   e , which abuts and seals to the proximal end plate  118   a  as best seen in  FIGS. 3 and 4 . The threaded branch portion  130   c  connects to the gas source such as  14  via a flexible gas line (not shown). An O-ring  135  and cap  137  seal the proximal end of barbed branch portion  130   d  around the central, inner electrode  136 . 
     The threaded trunk portion  130   b  of the Y-connector  130  cooperatively engages the proximal end  139   b  of an ionization tube mount  139 . The tube mount  139  has a central axis and aperture  139   a , a proximal female threaded portion  139   b , a hexagonally-shaped shoulder portion  139   c , and a distal male threaded portion  139   d . As best seen in  FIGS. 3 and 4 , the Y-connector  130  and tube mount  139  clamp to opposed sides of the proximal end wall  118   a  of the housing  118 . 
     An elongate, dielectric ionization tube  132  is mounted in the distal end  139   d  of the tube mount  139  in a generally coaxial relationship with the housing  118 . The ionization tube  132  is made from blown glass. The ionization tube  132  has a generally-cylindrical shape, an open proximal end  132   a , and an exit port  134  at a distal discharge end  132   b , which tapers in the form of a concentrating nozzle. The port  134  in the conduit aligns with the exit port  120  in the distal end of the housing  118 . 
     The proximal end  132   a  of the ionization tube  132  is connected in sealed fluid communication with the gas connector  130  by the tube mount  139 . Referring to  FIGS. 3-4 , the outer diameter of the ionization tube  132  is smaller than the inner diameter of the tube mount  139 . The proximal end  132   a  of the ionization tube  132  inserts into the distal end  139   d  of the tube mount  139  and is held in place by a compression fitting. In this embodiment, the compression fitting comprises an O-ring  141 , a compression ring  143 , and a compression nut  145  having female threads that cooperatively engage the distal male threaded portion  139   d  of the tube mount  139 . The O-ring  141  surrounds and seals the outer surface of the ionization tube  132  when compressed by the compression ring  143  and nut  145  against the end surface of the distal portion  139   d  of the tube mount  139 . 
     The distal end  132   b  of the ionization tube  132  is supported by the housing  118 . In this embodiment, the tapered, nozzle end of the ionization tube sits in an annular pocket  147  that is adjacent and coaxial with the exit port  120 . 
     An elongate inner electrode  136  is mounted by the Y-connector  130  in a generally coaxial relationship within the housing  118 . The proximal end  136   a  of the inner electrode  136  extends completely through the Y-connector  130  and connects to the power supply  16  via a connector cable (not shown). The distal end  136   b  of the inner electrode  136  is positioned proximal the exit port  134  in the ionization tube  132 . 
     The inner electrode  136  has an elongate base  138  extending generally parallel to the central axis of the ionization tube  132 . The distal portion  136   b  of the inner electrode  136  has a plurality of electrically-conductive bristles  140  fixed to and extending radially from the electrode base  138 . The electrode base  138  and bristles  140  are made from stainless steel. In this embodiment, the electrode comprises a modified hand-held cleaning and deburring tube brush comprising a single spiral of bristles  140  twisted between two wires that form the base  138 . 
     In this embodiment, the base  138  and bristles  140  are formed from round wire. The diameter of the bristles is about 0.003 in. while the base diameter is about 0.094 in. 
     In this preferred embodiment shown in  FIGS. 3-6 , the bristles  140  are equally spaced both axially and radially on the electrode base  138 . The bristles  140  are formed on only a distal portion  136   b  of the base measuring about 1 in. while the total inner electrode length is about 4 in. 
     An outer electrode  142  is slideably arranged on the outer surface of the ionization tube  132  and connected to the power source  16  by a connector cable  126 . In this preferred embodiment, the outer electrode  142  comprises an annular ring having an inner bore  142   a  that is slightly larger than the outer diameter of the ionization tube  132 , and an outer diameter that is smaller than the inner diameter of the housing  118 . These complimenting shapes and sizes allow the outer electrode  142  to slide axially along the length of the ionization conduit  132 . 
     In this preferred embodiment, the outer electrode  142  is made from stainless steel and has a length of about 0.645 in. A radial, threaded bore  147  receives a screw  149  that attaches the connector cable  126  in electrical connectivity to the outer electrode  142 . As best seen in  FIG. 4 , the cable  126  has sufficient flexibility and slack to allow the outer electrode to translate about 1-2 inches. 
     The properties of the plasma plume can be adjusted and controlled by varying the gas type, the gas flow rate through the ionization conduit  32 , the input voltage, and the location of the outer electrode  142  relative to the bristles  140  on the inner electrode. For example, the graph of  FIG. 7  shows how the plume length can be varied by varying the input voltage. In this preferred embodiment, the plume length increases as the applied voltage increases; however, after a certain input voltage, the plasma plume becomes more intense but does not increase in length. 
     Similar to its dependency on input voltage, the plume length generally increases as the input pressure increases; however, after a certain input pressure, the plume length starts shortening. It is theorized that this effect is caused by turbulence within the ionization conduit at high flow rates. It is also theorized that the recombination rate for the charged radicals within the plasma is also dependent on the gas flow rate, applied voltage, and the axial distance traversed by the plasma within the ionization tube  132 . 
     In this preferred embodiment, the plume temperature is within acceptable and desired ranges for medical applications. The graph of  FIG. 8  shows the plume temperature measured at various positions along its length. The distances are measured from the exit distal port  20 . The applied voltage was ˜8 kV with ballast resistor of 25 kΩ with argon at 0.8-1 SLPM. Argon gas was input at a flow rate of about 1 SLPM. The data shows, in general, that the plume temperature remains within an acceptable range necessary for medical applications including, but not limited to, wound treatment, sterilization, and blood coagulation. However, it should be appreciated that the data of  FIG. 8  does not represent the full operating temperature range for the device. The temperature of the plasma jet can be varied by adjusting the operating parameters discussed above. 
     In this preferred embodiment, the plume also contains radicals that are desirable for medical applications. The graph of  FIG. 9  shows the spectral features of the plasma plume as measured proximate the distal port  120 . Spectral features variation along the plume axial direction is not shown in  FIG. 9 . In this embodiment, the supply gas was argon. An Ocean Optics HR 4000CG-UV-NIR spectrometer was used to capture the spectral features of the plasma plume. A multimode optical fiber with a collimating lens mounted on its input end was used to capture the light from the plume. The output end of the fiber was directly connected to the spectrometer.  FIG. 9  shows various argon lines that have been identified from the NIST data base. NIST Atomic Spectra Database: http://physics.nist.gov/cgi-bin/ASD/lines1.pl. It was found that the presence and the intensity of various lines in the spectral signature shown in  FIG. 9  was heavily dependent on the axial position of the outer annular electrode, the applied voltage across the electrodes, the gas type, and the gas flow rate. The control of various radicals in the plasma jet generated by this device is important for medical applications where it has been shown that plasma releases controllable amounts of short-lived reactive oxygen (ROS) and nitrogen (RNS) species that address only the target areas in the tissue. Each of these species has different physiological functions. For example, ROS has antibacterial, pro-apoptotic, and pro-inflammatory properties. RNS has anti-inflammatory and pro-apoptotic properties. 
     It should be appreciated by those of ordinary skill in the art that the results shown in the graphs of  FIGS. 7-9  are included only for the purpose of illustrating operation of the generator at certain operating conditions. The results do not, by any means, represent the full operating range for various parameters including gas flow rates, diameter of the exit nozzle, axial position of the outer annular electrode with respect to the nozzle exit, input power, electrode composition, and dielectric composition. 
     The plasma generating device described above also produces a large volume of ozone, which volume or percentage depends on the gas flow rates and the applied voltages across the electrodes. The presence of ozone can be increased or decreased by adding a small fraction of oxygen or air in the mainstream gas used in the system. Ozone plays a part of a cleaning/serializing agent in medical applications and its control gives an additional benefit in these applications. Running the plasma only on oxygen or air can turn it into an ozone generator that may have many applications in engineering including surgical equipment sterilization. 
     In this preferred embodiment, the proximal wall  118   a  of the housing  118  is not integrally formed with the main body of the housing  118 . Instead, it has a shoulder that can be inserted into the end cavity of the main housing and held therein by friction. Alternatively, the proximal wall  118   a  of the housing  118  could be removably fixed to the end of the main housing body with other known fastening means. In these preferred embodiments, the axial position of the outer electrode is adjusted by removing the main outer housing body, manually sliding the outer electrode to the desired axial location, and then re-installing the main body of the housing. 
     An apparatus for generating a uniform, non-equilibrium plasma plume in accordance with another preferred embodiment of the invention is schematically illustrated in  FIG. 10 . The apparatus, designated generally by reference numeral  210 , comprises a plasma generator probe  212 , a plasma gas source  214 , and an electric power source  216 . The plasma plume  8  is discharged from the probe  212  at atmospheric pressure. The generator probe  212  can be held in one hand and easily manipulated relative to the treatment target. 
     The generator probe  212  comprises a DBD plasma generator probe having a construction similar to the probes  12  and  112  disclosed above. However, in this embodiment, the probe  210  includes a nozzle  251  connected to the exit port  220  that changes the direction of the plasma plume  8  and/or bifurcates the plasma plume  8 . 
     Referring to  FIG. 10 , the plasma generator probe  210  includes an elongate, tubular housing  218  having a central axis, a proximal end wall  218   a  and a distal end wall  218   b  to which the deflector nozzle  251  is attached. The distal end wall  218   b  includes an exit port  220  through which the plasma plume  8  flows into the nozzle  251 . The proximal end wall  218   a  of the housing  218  includes apertures through which electrical connector cables  226 ,  227  extend, and a port through which a gas supply connector  230  extends. The housing  218  is sealed around the cables  226 , 227  and gas supply connector  230 . 
     A primary dielectric ionization conduit  232  is arranged in a generally coaxial relationship with the housing  218 . The conduit  232  has a port  234  at a distal discharge end  232   b , which connects to the secondary ionization conduit  253  within the deflector nozzle  251 . The proximal end  232   a  of the conduit  232  is connected in sealed fluid communication with the gas supply tube  230 . 
     An inner electrode  236  extends within the primary ionization conduit  232 . The distal end  236   b  of the electrode  236  is positioned near the distal port  234  and connects to the secondary inner electrode  253  (described below). The proximal end  236   a  of the primary inner electrode  236  is located near the proximal end  218   a  of the housing  218  and connects to the power supply  16  via a connector cable  226 . 
     The inner electrode  236  has an elongate base  238  extending generally parallel to the central axis of the ionization conduit  232  and a plurality of electrically-conductive bristles  240  fixed to and extending radially from the electrode base  238 . In the embodiment shown in  FIG. 10 , the bristles  240  are equally spaced both axially and radially on the electrode base  238  and along the entire length of the base  238  located within the ionization conduit  232 . However, in contrast with the embodiment shown in  FIG. 1 , all of the bristles  240  do not have equal lengths. In this embodiment, the bristles at the proximal end of the inner electrode  236  are longer than the bristles near the distal end. 
     An outer electrode  242  is slideably arranged on the outer surface of the ionization conduit  232  and connected to the power source  16  by a connector cable  226 . The outer electrode  242  has an inner shape and dimension that compliments and is slightly larger than the outer shape and dimension of the ionization conduit  232 . The outer electrode  242  also has an outer shape and dimension that compliments and is slightly smaller than the shape and inner dimension of the housing  218 . These complimenting shapes and sizes allow the outer electrode to slide axially along the length of the ionization conduit  232 . 
     In the preferred embodiment shown in  FIG. 10 , the deflector nozzle  251  is attached to the distal end of the housing  218 . The nozzle  251  acts as an extension of the ionization conduit  232  and changes the direction of the plasma plume  8  compared to the embodiments disclosed above. In this embodiment, the nozzle re-directs the plasma plume approximately 90 degrees relative to the longitudinal axis of the primary ionization conduit  232 . In this embodiment, the nozzle  251  also bifurcates the plasma plume  8 ; however, in other embodiments the nozzle  251  re-directs the plume  8  without bifurcating or otherwise dividing the plume  8 . 
     The nozzle  251  includes an elongate housing  257  having a central axis and opposed end walls  257   a ,  257   b , each of which includes an exit port  259 ,  260  through which the plasma plume  8  is expelled. A secondary dielectric ionization conduit  253  is arranged in a generally coaxial relationship with the nozzle housing  257 . The conduit  253  has ports at each end, which align with the exit ports  259 ,  260  in the housing  257 . The nozzle  251  also has a port  263  in the side wall, which connects to the exit port  220  of the primary housing  218 . Alignment of the ports  220  and  263  connects the primary ionization conduit  232  and secondary ionization conduit  253  in sealed fluid communication. 
     An inner electrode  254  extends within the nozzle ionization conduit  253 . The electrode  254  has a “T” shape with a trunk end  254   a , which is connected to the distal end of the primary inner electrode  236 , and two branch ends  254   b ,  254   c  which are located proximate the exit ports  259 ,  260  in the nozzle  251 . The inner electrode  254  has an elongate base  267  extending generally parallel to the central axis of the ionization conduit  253 , and a plurality of electrically-conductive bristles  269  fixed to and extending radially from the electrode base  267 . 
     A pair of outer electrodes  265 ,  266  are slideably arranged on the outer surface of the ionization conduit  253  and connected to the power source  16 . The outer electrodes  265 ,  266  have an inner shape and dimension that compliments and is slightly larger than the outer shape and dimension of the ionization conduit  253 . The outer electrodes  265 ,  266  also have an outer shape and dimension that compliments and is slightly smaller than the shape and inner dimension of the housing  257 . These complimenting shapes and sizes allow the outer electrode to slide axially along the length of the ionization conduit  253 . 
     In the embodiment shown in  FIG. 10 , the inner electrodes  236 ,  254  and outer electrodes  242 ,  265 ,  266  are connected in series to the same power source  16 . However, in other embodiments, the electrodes of the primary ionization tube and nozzle may be connected in parallel to the same power source, or connected to different power sources. In yet other embodiments, the separate powers sources include means for controlling electrical input parameters including voltage, frequency, etc., for even more tuning control of the plasma plume. 
     In this preferred embodiment, the nozzle  251  can be rotated about the central axis of the primary housing  218  so that the plume  8  exits at any desired angle. This feature is particularly useful for medical applications where, for example, the target area is located within a small cavity that restricts the degree to which the housing may be tilted. 
     An apparatus for generating a uniform, non-equilibrium plasma plume from a gas source and power source in accordance with yet another embodiment of the invention is shown in  FIGS. 11-12 . The apparatus comprises a dielectric barrier discharge plasma generator probe  312  having a construction the same as the probe  312  illustrated and describe above with respect to  FIGS. 3-6  except with the modifications described below. In this embodiment, the probe  312  includes means for manually adjusting the axial position of the outer electrode without disassembling the housing as described above with respect to the embodiment shown in  FIGS. 3-6 . 
     In this preferred embodiment, the gas connector  330 , tube mount  339 , O-ring  341 , O-ring compression ring  343 , compression nut  345 , end cap  337 , ionization conduit  332 , inner electrode  336 , and outer electrode  342  have the same construction as the gas connector  130 , tube mount  139 , O-ring  141 , O-ring compression ring  143 , compression nut  145 , end cap  137 , ionization conduit  132 , inner electrode  136 , and outer electrode  142 . 
     The housing  318  has a construction similar to the housing  118  of the embodiment show in  FIGS. 3-6 ; however, in this embodiment, the housing includes a longitudinal slot  350  in the radial wall of the housing  318 . A thumb tab  352  is slideably mounted within the slot  350 . The outer surface of the thumb tab  352  has a shape that compliments the thumb of an operator including contoured fore  352   a  and aft  352   b  surfaces divided by an elevated shoulder  352   c . The inner surface  352   d  of the thumb tab  352  is connected to the outer electrode  342  with screws  354  or other means. The dimensions of the tab  352  and slot  350  are constructed to provide a resistive force that can be overcome by an average operator but will hold the outer electrode in place during normal use. 
     The length of the slot  350  preferably extends along the entire length of the inner electrode  336  that is connected to bristles  340 . This construction allows the operator to manually slide the outer electrode  342  to any position in axial alignment with any portion of the bristled inner electrode  336 . As described above, such movement of the outer electrode  342  will change the characteristics of the plasma plume. For a set of gas and power input parameters, the operator can fine tune the plasma plume during treatment by simply sliding the thumb tab fore and aft. 
     The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described herein, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. For example, the generator probe may have two flat plate electrodes separated by a flat dielectric material. In this embodiment, any shape of dielectric tubes and any shape of electrodes may be incorporated in the probe provided one of the electrodes has very protuberances or bristles on which the electric field will concentrate to create tiny streamers that do not turn into arcs.