FIELD OF THE INVENTION [002] This invention relates to methods and apparatus for forming plasma jets, and in particular for igniting, modulating, sustaining, and using such jets.

BACKGROUND OF THE INVENTION [003] Plasma jets are known. It is also known that plasmas can be ignited by subjecting a gas to a sufficient amount of microwave radiation. Plasma ignition, however, is usually easier at gas pressures substantially less than atmospheric pressure. However, vacuum equipment, which can be used to lower the gas pressure, can be expensive, slow, and energy-consuming. Moreover, the use of such equipment can limit manufacturing flexibility.

BRIEF SUMMARY OF A FEW ASPECTS OF THE INVENTION [004] A method is provided for forming a plasma jet at an aperture, which can be formed in a wall of a cavity, using a plasma catalyst. The method can include (1) flowing a first gas into the cavity, (2) forming a plasma in the cavity by subjecting the first gas to electromagnetic radiation having a frequency less than about 333 GHz in the presence of the plasma catalyst, and (3) allowing at least a portion of the plasma to pass out of the cavity through the aperture such that a plasma jet is formed outside the cavity proximate to the aperture.

[005] Another method is provided for forming a plasma jet. Again, the jet is formed at an aperture formed in a wall of a cavity. The method can include (1) flowing a first gas into the cavity, (2) forming a plasma from the first gas in the cavity, (3) allowing at least a portion of the plasma to pass out of the cavity through the aperture such that a plasma jet is formed outside the cavity proximate to the aperture, and (4) flowing a second gas outside the cavity proximate to the plasma jet.

[006] Yet another method is provided for forming a plasma jet. The method includes (1) flowing a gas into the cavity, (2) forming a plasma in the cavity from the gas, (3) allowing at least a portion of the plasma to pass out of the cavity through the aperture such that a plasma jet is formed outside the cavity proximate to the aperture, and (4) directing a laser beam proximate to the plasma jet such that the plasma jet and the laser beam are directed at substantially the same location.

[007] Still another method is provided for forming a plasma jet. The jet is formed at an aperture formed in a wall of a cavity, which includes a first surface proximate to the aperture. The first surface can be electrically conductive and substantially thermally resistant. The cavity can also include another electrically conductive surface that faces the first surface. The method can include (1) flowing a gas into the cavity, (2) forming a plasma from the gas in the cavity, (3) allowing at least a portion of the plasma to pass out of the cavity through the aperture such that a plasma jet is formed outside the cavity proximate to the aperture, and (4) applying an electric potential between the first surface and the second surface, such that the plasma concentrates closer to the first surface than the second surface in the cavity.

[008] A plasma catalyst for initiating, modulating, and sustaining a plasma is also provided. The catalyst can be passive or active. A passive plasma catalyst can include any object capable of inducing a plasma by deforming a local electric field (including an electromagnetic field) consistent with this invention, without necessarily adding additional energy. An active plasma catalyst, on the other hand, is any particle or high energy wave packet capable of transferring a sufficient amount of energy to a gaseous atom or molecule to remove at least one electron from the gaseous atom or molecule in the presence of electromagnetic radiation. In both cases, a plasma catalyst can improve, or relax, the environmental conditions required to ignite a plasma.

[009] Additional plasma catalysts, and methods and apparatus for igniting, modulating, and sustaining plasma jets consistent with this invention are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS [010] Further aspects of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: [011] FIG. 1 shows a schematic diagram of an illustrative plasma system consistent with this invention; [012] FIG. 1A shows an illustrative embodiment of a portion of a plasma system for adding a powder plasma catalyst to a plasma cavity for igniting, modulating, or sustaining a plasma in a cavity consistent with this invention; [013] FIG. 2 shows an illustrative plasma catalyst fiber with at least one component having a concentration gradient along its length consistent with this invention; [014] FIG. 3 shows an illustrative plasma catalyst fiber with multiple components at a ratio that varies along its length consistent with this invention; [015] FIG. 4 shows another illustrative plasma catalyst fiber that includes a core underlayer and a coating consistent with this invention; [016] FIG. 5 shows a cross-sectional view of the plasma catalyst fiber of FIG. 4, taken from line 5-5 of FIG. 4 consistent with this invention; [017] FIG. 6 shows an illustrative embodiment of another portion of a plasma system including an elongated plasma catalyst that extends through ignition port consistent with this invention; [018] FIG. 7 shows an illustrative embodiment of an elongated plasma catalyst that can be used in the system of FIG. 6 consistent with this invention; [019] FIG. 8 shows another illustrative embodiment of an elongated plasma catalyst that can be used in the system of FIG. 6 consistent with this invention; [020] FIG. 9 shows an illustrative embodiment of a portion of a plasma system for directing radiation into a radiation chamber consistent with this invention; [021] FIG. 10 shows a simplified cross-sectional view an illustrative plasma-jet apparatus consistent with this invention; [022] FIG. 11 shows a nozzle portion of an illustrative plasma-jet apparatus consistent with this invention; [023] FIG. 12 shows a front view of illustrative nozzle portion consistent with this invention; [024] FIG. 13 shows a front view of another illustrative nozzle portion consistent with this invention; [025] FIG. 14 shows another illustrative embodiment of a plasma-jet apparatus consistent with this invention that includes at least one laser consistent with this invention; [026] FIG. 15 shows a front view of another illustrative nozzle portion of a plasma-jet apparatus with multiple solid-state lasers positioned circumferentially about plasma-jet aperture consistent with this invention; [027] FIG. 16 shows a cross-sectional view of the nozzle portion shown in FIG. 15, taken along line 16-16 of FIG. 15, during operation consistent with this invention ; and [028] FIG. 17 shows still another illustrative plasma-jet apparatus consistent with this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS [029] This invention may relate to methods and apparatus for initiating, modulating, and sustaining plasmas and plasma jets for a variety of applications, including heat-treating, synthesizing and depositing carbides, nitrides, borides, oxides, and other materials, coating, doping, carburizing, nitriding, and carbonitriding, sintering, multi-part processing, joining, sintering, decrystallizing, making and operating furnaces, gas exhaust-treating, waste-treating, incinerating, scrubbing, ashing, growing carbon structures, generating hydrogen and other gases, plasma-assisted processing in manufacturing lines, sterilizing, cleaning, etc.

[030] This invention can be used, for example, for controllably generating heat and for plasma-assisted processing to lower energy costs and increase heat- treatment efficiency and plasma-assisted manufacturing flexibility.

[031] The following commonly owned, concurrently filed U. S. patent applications are hereby incorporated by reference in their entireties: U. S. Patent Application No. 10/, (Atty. Docket No. 1837.0008), No. 10/, (Atty.

Docket No. 1837.0009), No. 10/, (Atty. Docket No. 1837.0010), No. 10/, (Atty. Docket No. 1837.0011), No. 10/, (Atty. Docket No. 1837.0012), No. 10/, (Atty. Docket No. 1837.0013), No. 10/, (Atty. Docket No. 1837.0015), No. 10/, (Atty. Docket No. 1837.0016), No. 10/, (Atty. Docket No. 1837.0017), No. 10/, (Atty. Docket No. 1837.0018), No. 10/, (Atty. Docket No. 1837.0020), No. 10/, (Atty. Docket No. 1837.0021), No. 10/, (Atty. Docket No. 1837.0023), No. 10/, (Atty. Docket No. 1837.0024), No. 10/, (Atty. Docket No. 1837.0026), No. 10/, (Atty. Docket No. 1837.0027), No. 10/ (Atty. Docket No. 1837.0028), No. 10/, (Atty. Docket No. 1837.0029), No. 10/, (Atty. Docket No. 1837.0030), No. 10/, (Atty. Docket No. 1837.0032), and No. 10/, (Atty. Docket No. 1837.0033).

[032] Illustrative Plasma-Jet System [033] FIG. 1 shows illustrative plasma-jet system 10 consistent with one aspect of this invention. In this embodiment, cavity 12 is formed in a vessel located inside radiation chamber (i. e., applicator) 14. In another embodiment (not shown), the vessel and radiation chamber 14 are the same, thereby eliminating the need for two separate components. The vessel in which cavity 12 is formed can include one or more radiation-transmissive thermally-insulating layers to improve the thermal insulation properties of the vessel without significantly shielding cavity 12 from the radiation.

[034] In one embodiment, cavity 12 can be formed in a vessel made of ceramic. Due to the extremely high temperatures that can be achieved with plasmas consistent with this invention, a ceramic capable of operating at about 3,000 degrees Fahrenheit can be used. The ceramic material can include, by weight, 29.8% silica, 68.2% alumina, 0.4% ferric oxide, 1 % titania, 0. 1% time, 0.1 % magnesia, 0.4% alkalies, which is sold under Model No. LW-30 by New Castle Refractories Company, of New Castle, Pennsylvania. It will be appreciated by those of ordinary skill in the art, however, that other materials, such as quartz, and those different from the one described above, can also be used consistent with the invention.

[035] In one successful experiment, a plasma was formed in a partially open cavity inside a first brick and topped with a second brick. The cavity had dimensions of about 2 inches by about 2 inches by about 1.5 inches. At least two holes were also provided in the brick in communication with the cavity: one for viewing the plasma and at least one hole for providing the gas. The size of the cavity can depend on the desired plasma process being performed. Also, the cavity should at least be configured to prevent the plasma from rising/floating away from the primary processing region.

[036] Cavity 12 can be connected to one or more gas sources 24 (e. g. , a source of argon, nitrogen, hydrogen, xenon, krypton) by line 20 and control valve 22, which may be powered by power supply 28. Line 20 may be tubing (e. g., between about 1/16 inch and about 1/4 inch, such as about 1/8"). Also, if desired, a vacuum pump can be connected to the chamber to remove any fumes that may be generated during plasma processing.

[037] A radiation leak detector (not shown) was installed near source 26 and waveguide 30 and connected to a safety interlock system to automatically turn <BR> <BR> off the radiation (e. g. , microwave) power supply if a leak above a predefined safety<BR> limit, such as one specified by the FCC and/or OSHA (e. g. , 5 mW/cm2), was detected.

[038] Radiation source 26, which may be powered by electrical power supply 28, directs radiation energy into chamber 14 through one or more waveguides 30. It will be appreciated by those of ordinary skill in the art that source 26 can be connected directly to cavity 12 or chamber 14, thereby eliminating waveguide 30. The radiation energy entering cavity 12 can be used to ignite a plasma within the cavity. This plasma can be substantially sustained and confined to the cavity by coupling additional radiation with the catalyst.

[039] Radiation energy can be supplied through circulator 32 and tuner 34 (e. g. , 3-stub tuner). Tuner 34 can be used to minimize the reflected power as a function of changing ignition or processing conditions, especially after the plasma has formed because microwave power, for example, will be strongly absorbed by the plasma.

[040] As explained more fully below, the location of radiation-transmissive cavity 12 in chamber 14 may not be critical if chamber 14 supports multiple modes, and especially when the modes are continually or periodically mixed. As also explained more fully below, motor 36 can be connected to mode-mixer 38 for making the time-averaged radiation energy distribution substantially uniform <BR> <BR> throughout chamber 14. Furthermore, window 40 (e. g. , a quartz window) can be disposed in one wall of chamber 14 adjacent to cavity 12, permitting temperature <BR> <BR> sensor 42 (e. g. , an optical pyrometer) to be used to view a process inside cavity 12.

In one embodiment, the optical pyrometer output can increase from zero volts as the temperature rises to within the tracking range.

[041] Sensor 42 can develop output signals as a function of the temperature or any other monitorable condition associated with a work piece (not shown) adjacent cavity 12 and provide the signals to controller 44. Dual temperature sensing and heating, as well as automated cooling rate and gas flow controls can also be used. Controller 44 in turn can be used to control operation of power supply 28, which can have one output connected to source 26 as described above and another output connected to valve 22 to control gas flow into cavity 12.

[042] The invention has been practiced with equal success employing microwave sources at both 915 MHz and 2.45 GHz provided by Communications and Power Industries (CPI), although radiation having any frequency less than about 333 GHz can be used. The 2.45 GHz system provided continuously variable microwave power from about 0.5 kilowatts to about 5.0 kilowatts. A 3-stub tuner allowed impedance matching for maximum power transfer and a dual directional coupler was used to measure forward and reflected powers (not shown). Also, optical pyrometers were used for remote sensing of the work piece temperature.

[043] As mentioned above, radiation having any frequency less than about 333 GHz can be used consistent with this invention. For example, frequencies, such as power line frequencies (about 50 Hz to about 60 Hz), can be used, although the pressure of the gas from which the plasma is formed may be lowered to assist with plasma ignition. Also, any radio frequency or microwave frequency can be used consistent with this invention, including frequencies greater than about 100 kHz. In most cases, the gas pressure for such relatively high frequencies need not be lowered to ignite, modulate, or sustain a plasma, thereby enabling many plasma-processes to occur at atmospheric pressures and above.

[044] The equipment was computer controlled using LabView 6i software, which provided real-time temperature monitoring and microwave power control.

Noise was reduced by using shift registers to generate sliding averages of suitable number of data points. Also, the number of stored data points in the array were limited to improve speed and computational efficiency. The pyrometer measured the temperature of a sensitive area of about 1 cm2, which was used to calculate an average temperature. The pyrometer sensed radiant intensities at two wavelengths and fit those intensities using Planck's law to determine the temperature. It will be appreciated, however, that other devices and methods for monitoring and controlling temperature are also available and can be used consistent with this invention. Control software that can be used consistent with this invention is described, for example, in commonly owned, concurrently filed U. S. Patent Application No. 10/, (Attorney Docket No. 01837.0033), which is hereby incorporated by reference in its entirety.

[045] Chamber 14 had several glass-covered viewing ports with radiation shields and one quartz window for pyrometer access. Several ports for connection to a vacuum pump and a gas source were also provided, although not necessarily used.

[046] System 10 also included a closed-loop deionized water cooling system (not shown) with an external heat exchanger cooled by tap water, During operation, the deionized water first cooled the magnetron, then the load-dump in the circulator (used to protect the magnetron), and finally the radiation chamber through water channels welded on the outer surface of the chamber.

[047] Plasma Catalysts [048] A plasma catalyst consistent with this invention can include one or more different materials and may be either passive or active. A plasma catalyst can be used, among other things, to ignite, modulate, and/or sustain a plasma at a gas pressure that is less than, equal to, or greater than atmospheric pressure.

[049] One method of forming a plasma consistent with this invention can include subjecting a gas in a cavity to electromagnetic radiation having a frequency less than about 333 GHz in the presence of a passive plasma catalyst. A passive plasma catalyst consistent with this invention can include any object capable of <BR> <BR> inducing a plasma by deforming a local electric field (e. g. , an electromagnetic field) consistent with this invention, without necessarily adding additional energy through the catalyst, such as by applying an electric voltage to create a spark.

[050] A passive plasma catalyst consistent with this invention can also be a nano-particle or a nano-tube. As used herein, the term"nano-particle"can include any particle having a maximum physical dimension less than about 100 nm that is at least electrically semi-conductive. Also, both single-walled and multi- walled carbon nanotubes, doped and undoped, can be particularly effective for igniting plasmas consistent with this invention because of their exceptional electrical conductivity and elongated shape. The nanotubes can have any convenient length and can be a powder fixed to a substrate. If fixed, the nanotubes can be oriented randomly on the surface of the substrate or fixed to the substrate <BR> <BR> (e. g. , at some predetermined orientation) while the plasma is ignited or sustained.

[051] A passive plasma catalyst can also be a powder consistent with this invention, and need not comprise nano-particles or nano-tubes. It can be formed, for example, from fibers, dust particles, flakes, sheets, etc. When in powder form, the catalyst can be suspended, at least temporarily, in a gas. By suspending the powder in the gas, the powder can be quickly dispersed throughout the cavity and more easily consumed, if desired.

[052] In one embodiment, the powder catalyst can be carried into the cavity and at least temporarily suspended with a carrier gas. The carrier gas can be the same or different from the gas that forms the plasma. Also, the powder can be added to the gas prior to being introduced to the cavity. For example, as shown in FIG. 1A, radiation source 52 can supply radiation to radiation cavity 55, in which plasma cavity 60 is placed. Powder source 65 provides catalytic powder 70 into gas stream 75. In an alternative embodiment, powder 70 can be first added to <BR> <BR> cavity 60 in bulk (e. g. , in a pile) and then distributed in the cavity in any number of ways, including flowing a gas through or over the bulk powder. In addition, the powder can be added to the gas for igniting, modulating, or sustaining a plasma by moving, conveying, drizzling, sprinkling, blowing, or otherwise, feeding the powder into or within the cavity.

[053] In one experiment, a plasma was ignited in a cavity by placing a pile of carbon fiber powder in a copper pipe that extended into the cavity. Although sufficient radiation was directed into the cavity, the copper pipe shielded the powder from the radiation and no plasma ignition took place. However, once a carrier gas began flowing through the pipe, forcing the powder out of the pipe and into the cavity, and thereby subjecting the powder to the radiation, a plasma was nearly instantaneously ignited in the cavity.

[054] A powder plasma catalyst consistent with this invention can be substantially non-combustible, thus it need not contain oxygen or burn in the presence of oxygen. Thus, as mentioned above, the catalyst can include a metal, carbon, a carbon-based alloy, a carbon-based composite, an electrically conductive polymer, a conductive silicone elastomer, a polymer nanocomposite, an organic- inorganic composite, and any combination thereof.

[055] Also, powder catalysts can be substantially uniformly distributed in the plasma cavity (e. g. , when suspended in a gas), and plasma ignition can be precisely controlled within the cavity. Uniform ignition can be important in certain applications, including those applications requiring brief plasma exposures, such as in the form of one or more bursts. Still, a certain amount of time can be required for a powder catalyst to distribute itself throughout a cavity, especially in complicated, multi-chamber cavities. Therefore, consistent with another aspect of this invention, a powder catalyst can be introduced into the cavity through a plurality of ignition ports to more rapidly obtain a more uniform catalyst distribution therein (see below).

[056] In addition to powder, a passive plasma catalyst consistent with this invention can include, for example, one or more microscopic or macroscopic fibers, sheets, needles, threads, strands, filaments, yarns, twines, shavings, slivers, chips, woven fabrics, tape, whiskers, or any combination thereof. In these cases, the plasma catalyst can have at least one portion with one physical dimension substantially larger than another physical dimension. For example, the ratio between at least two orthogonal dimensions should be at least about 1: 2, but could be greater than about 1: 5, or even greater than about 1: 10.

[057] Thus, a passive plasma catalyst can include at least one portion of material that is relatively thin compared to its length. A bundle of catalysts (e. g., fibers) may also be used and can include, for example, a section of graphite tape.

In one experiment, a section of tape having approximately thirty thousand strands of graphite fiber, each about 2-3 microns in diameter, was successfully used. The number of fibers in and the length of a bundle are not critical to igniting, modulating, or sustaining the plasma. For example, satisfactory results have been obtained using a section of graphite tape about one-quarter inch long. One type of carbon fiber that has been successfully used consistent with this invention is sold under the trademark MagnamiteO, Model No. AS4C-GP3K, by the Hexcel Corporation, of Anderson, South Carolina. Also, silicon-carbide fibers have been successfully used.

[058] A passive plasma catalyst consistent with another aspect of this invention can include one or more portions that are, for example, substantially spherical, annular, pyramidal, cubic, planar, cylindrical, rectangular or elongated.

[059] The passive plasma catalysts discussed above include at least one material that is at least electrically semi-conductive. In one embodiment, the material can be highly conductive. For example, a passive plasma catalyst consistent with this invention can include a metal, an inorganic material, carbon, a carbon-based alloy, a carbon-based composite, an electrically conductive polymer, a conductive silicone elastomer, a polymer nanocomposite, an organic-inorganic composite, or any combination thereof. Some of the possible inorganic materials that can be included in the plasma catalyst include carbon, silicon carbide, molybdenum, platinum, tantalum, tungsten, carbon nitride, and aluminum, although other electrically conductive inorganic materials are believed to work just as well.

[060] In addition to one or more electrically conductive materials, a passive plasma catalyst consistent with this invention can include one or more additives (which need not be electrically conductive). As used herein, the additive can include any material that a user wishes to add to the plasma. For example, in doping semiconductors and other materials, one or more dopants can be added to the plasma through the catalyst. See, e. g. , commonly owned, concurrently filed U. S. Patent Application No. 10/,, (Attorney Docket No. 1837.0026) which is hereby incorporated by reference in its entirety. The catalyst can include the dopant itself, or it can include a precursor material that, upon decomposition, can form the dopant. Thus, the plasma catalyst can include one or more additives and one or more electrically conductive materials in any desirable ratio, depending on the ultimate desired composition of the plasma and the process using the plasma.

[061] The ratio of the electrically conductive components to the additives in a passive plasma catalyst can vary over time while being consumed. For example, during ignition, the plasma catalyst could desirably include a relatively large percentage of electrically conductive components to improve the ignition conditions. On the other hand, if used while sustaining the plasma, the catalyst could include a relatively large percentage of additives. It will be appreciated by those of ordinary skill in the art that the component ratio of the plasma catalyst used to ignite and sustain the plasma could be the same.

[062] A predetermined ratio profile can be used to simplify many plasma processes. In many conventional plasma processes, the components within the plasma are added as necessary, but such addition normally requires programmable equipment to add the components according to a predetermined schdule.

However, consistent with this invention, the ratio of components in the catalyst can be varied, and thus the ratio of components in the plasma itself can be automatically varied. That is, the ratio of components in the plasma at any particular time can depend on which of the catalyst portions is currently being consumed by the plasma. Thus, the catalyst component ratio can be different at different locations within the catalyst. And, the current ratio of components in a plasma can depend on the portions of the catalyst currently and/or previously consumed, especially when the flow rate of a gas passing through the plasma chamber is relatively slow.

[063] A passive plasma catalyst consistent with this invention can be homogeneous, inhomogeneous, or graded. Also, the plasma catalyst component ratio can vary continuously or discontinuously throughout the catalyst. For example, in FIG. 2, the ratio can vary smoothly forming a gradient along a length of catalyst 100. Catalyst 100 can include a strand of material that includes a relatively low concentration of a component at section 105 and a continuously increasing concentration toward section 110.

[064] Alternatively, as shown in FIG. 3, the ratio can vary discontinuously in each portion of catalyst 120, which includes, for example, alternating sections 125 and 130 having different concentrations. It will be appreciated that catalyst 120 can have more than two section types. Thus, the catalytic component ratio being consumed by the plasma can vary in any predetermined fashion. In one embodiment, when the plasma is monitored and a particular additive is detected, further processing can be automatically commenced or terminated.

[065] Another way to vary the ratio of components in a sustained plasma is by introducing multiple catalysts having different component ratios at different times or different rates. For example, multiple catalysts can be introduced at approximately the same location or at different locations within the cavity. When introduced at different locations, the plasma formed in the cavity can have a component concentration gradient determined by the locations of the various catalysts. Thus, an automated system can include a device by which a consumable plasma catalyst is mechanically inserted before and/or during plasma igniting, modulating, and/or sustaining.

[066] A passive plasma catalyst consistent with this invention can also be coated. In one embodiment, a catalyst can include a substantially non-electrically conductive coating deposited on the surface of a substantially electrically conductive material. Alternatively, the catalyst can include a substantially electrically conductive coating deposited on the surface of a substantially electrically non-conductive material. FIGS. 4 and 5, for example, show fiber 140, which includes underlayer 145 and coating 150. In one embodiment, a plasma catalyst including a carbon core is coated with nickel to prevent oxidation of the carbon.

[067] A single plasma catalyst can also include multiple coatings. If the coatings are consumed during contact with the plasma, the coatings could be introduced into the plasma sequentially, from the outer coating to the innermost coating, thereby creating a time-release mechanism. Thus, a coated plasma catalyst can include any number of materials, as long as a portion of the catalyst is at least electrically semi-conductive.

[068] Consistent with another embodiment of this invention, a plasma catalyst can be located entirely within a radiation chamber to substantially reduce or prevent radiation energy leakage. In this way, the plasma catalyst does not electrically or magnetically couple with the vessel containing the cavity or to any electrically conductive object outside the cavity. This can help prevent sparking at the ignition port and radiation from leaking outside the chamber during the ignition and possibly later if the plasma is sustained. In one embodiment, the catalyst can be located at a tip of a substantially electrically non-conductive extender that extends through an ignition port.

[069] FIG. 6, for example, shows radiation chamber 160 in which plasma cavity 165 can be placed. Plasma catalyst 170 is elongated and extends through ignition port 175. As shown in FIG. 7, and consistent with this invention, catalyst 170 can include electrically conductive distal portion 180 (which can be placed in chamber 160) and electrically non-conductive portion 185 (which can be placed substantially outside chamber 160, but can extend somewhat inside of chamber 160). This configuration helps prevent an electrical connection (e. g., sparking) from forming between distal portion 180 and chamber 160.

[070] In another embodiment, shown in FIG. 8, the catalyst can be formed from a plurality of electrically conductive segments 190 separated by and mechanically connected to a plurality of electrically non-conductive segments 195.

In this embodiment, the catalyst can extend through the ignition port between a point inside the cavity and another point outside the cavity, but the electrically discontinuous profile significantly prevents sparking and energy leakage.

[071] Another method of forming a plasma consistent with this invention includes subjecting a gas in a cavity to electromagnetic radiation having a frequency less than about 333 GHz in the presence of an active plasma catalyst, which generates or includes at least one ionizing particle.

[072] An active plasma catalyst consistent with this invention can be any particle or high energy wave packet capable of transferring a sufficient amount of energy to a gaseous atom or molecule to remove at least one electron from the gaseous atom or molecule in the presence of electromagnetic radiation.

Depending on the source, the ionizing particles can be directed into the cavity in the form of a focused or collimated beam, or they may be sprayed, spewed, sputtered, or otherwise introduced.

[073] For example, FIG. 9 shows radiation source 200 directing radiation into radiation chamber 205. Plasma cavity 210 is positioned inside of chamber 205 and may permit a gas to flow therethrough via ports 215 and 216. Source 220 directs ionizing particles 225 into cavity 210. Source 220 can be protected, for example, by a metallic screen which allows the ionizing particles to pass through but shields source 220 from radiation. If necessary, source 220 can be water- cooled.

[074] Examples of ionizing particles consistent with this invention can include x-ray particles, gamma ray particles, alpha particles, beta particles, neutrons, protons, and any combination thereof. Thus, an ionizing particle catalyst can be charged (e. g. , an ion from an ion source) or uncharged and can be the product of a radioactive fission process. In one embodiment, the vessel in which the plasma cavity is formed could be entirely or partially transmissive to the ionizing particle catalyst. Thus, when a radioactive fission source is located outside the cavity, the source can direct the fission products through the vessel to ignite the plasma. The radioactive fission source can be located inside the radiation chamber <BR> <BR> to substantially prevent the fission products (i. e. , the ionizing particle catalyst) from creating a safety hazard.

[075] In another embodiment, the ionizing particle can be a free electron, but it need not be emitted in a radioactive decay process. For example, the electron can be introduced into the cavity by energizing the electron source (such as a metal), such that the electrons have sufficient energy to escape from the source. The electron source can be located inside the cavity, adjacent the cavity, or even in the cavity wall. It will be appreciated by those of ordinary skill in the art that the any combination of electron sources is possible. A common way to produce electrons is to heat a metal, and these electrons can be further accelerated by applying an electric field.

[076] In addition to electrons, free energetic protons can also be used to catalyze a plasma. In one embodiment, a free proton can be generated by ionizing hydrogen and, optionally, accelerated with an electric field.

[077] Multi-mode Radiation Cavities [078] A radiation waveguide, cavity, or chamber can be designed to support or facilitate propagation of at least one electromagnetic radiation mode. As used herein, the term"mode"refers to a particular pattern of any standing or propagating electromagnetic wave that satisfies Maxwell's equations and the applicable boundary conditions (e. g. , of the cavity). In a waveguide or cavity, the mode can be any one of the various possible patterns of propagating or standing electromagnetic fields. Each mode is characterized by its frequency and polarization of the electric field and/or the magnetic field vectors. The electromagnetic field pattern of a mode depends on the frequency, refractive indices or dielectric constants, and waveguide or cavity geometry.

[079] A transverse electric (TE) mode is one whose electric field vector is normal to the direction of propagation. Similarly, a transverse magnetic (TM) mode is one whose magnetic field vector is normal to the direction of propagation. A transverse electric and magnetic (TEM) mode is one whose electric and magnetic field vectors are both normal to the direction of propagation. A hollow metallic waveguide does not typically support a normal TEM mode of radiation propagation.

Even though radiation appears to travel along the length of a waveguide, it may do so only by reflecting off the inner walls of the waveguide at some angle. Hence, <BR> <BR> depending upon the propagation mode, the radiation (e. g. , microwave) may have either some electric field component or some magnetic field component along the axis of the waveguide (often referred to as the z-axis).

[080] The actual field distribution inside a cavity or waveguide is a superposition of the modes therein. Each of the modes can be identified with one or more subscripts (e. g., TE10 ("tee ee one zero"). The subscripts normally specify how many"half waves"at the guide wavelength are contained in the x and y directions. It will be appreciated by those skilled in the art that the guide wavelength can be different from the free space wavelength because radiation propagates inside the waveguide by reflecting at some angle from the inner walls of the waveguide. In some cases, a third subscript can be added to define the number of half waves in the standing wave pattern along the z-axis.

[081] For a given radiation frequency, the size of the waveguide can be selected to be small enough so that it can support a single propagation mode. In such a case, the system is called a single-mode system (i. e. , a single-mode applicator). The TE10 mode is usually dominant in a rectangular single-mode waveguide.

[082] As the size of the waveguide (or the cavity to which the waveguide is connected) increases, the waveguide or applicator can sometimes support additional higher order modes forming a multi-mode system. When many modes are capable of being supported simultaneously, the system is often referred to as highly moded.

[083] A simple, single-mode system has a field distribution that includes at least one maximum and/or minimum. The magnitude of a maximum largely depends on the amount of radiation supplied to the system. Thus, the field distribution of a single mode system is strongly varying and substantially non- uniform.

[084] Unlike a single-mode cavity, a multi-mode cavity can support several propagation modes simultaneously, which, when superimposed, results in a complex field distribution pattern. In such a pattern, the fields tend to spatially smear and, thus, the field distribution usually does not show the same types of strong minima and maxima field values within the cavity. In addition, as explained more fully below, a mode-mixer can be used to"stir"or"redistribute"modes (e. g., by mechanical movement of a radiation reflector). This redistribution desirably provides a more uniform time-averaged field distribution within the cavity.

[085] A multi-mode cavity consistent with this invention can support at least two modes, and may support many more than two modes. Each mode has a maximum electric field vector. Although there may be two or more modes, one mode may be dominant and has a maximum electric field vector magnitude that is larger than the other modes. As used herein, a multi-mode cavity may be any cavity in which the ratio between the first and second mode magnitudes is less than about 1: 10, or less than about 1: 5, or even less than about 1: 2. It will be appreciated by those of ordinary skill in the art that the smaller the ratio, the more distributed the electric field energy between the modes, and hence the more distributed the radiation energy is in the cavity.

[086] The distribution of plasma within a processing cavity may strongly depend on the distribution of the applied radiation. For example, in a pure single mode system, there may only be a single location at which the electric field is a maximum. Therefore, a strong plasma may only form at that single location. In many applications, such a strongly localized plasma could undesirably lead to non- uniform plasma treatment or heating (i. e., localized overheating and underheating).

[087] Whether or not a single or multi-mode cavity is used consistent with this invention, it will be appreciated by those of ordinary skill in the art that the cavity in which the plasma is formed can be completely closed or partially open.

For example, in certain applications, such as in plasma-assisted furnaces, the cavity could be entirely closed. See, for example, U. S. Patent Application No. 10/, (Atty. Docket No. 1837.0020), which is hereby fully incorporated herein by reference. In other applications, however, it may be desirable to flow a gas through the cavity, and therefore the cavity must be open to some degree. In this way, the flow, type, and pressure of the flowing gas can be varied over time.

This may be desirable because certain gases with lower ionization potentials, such as argon, are easier to ignite but may have other undesirable properties during subsequent plasma processing.

[088] Mode-mixing [089] For some plasma-jet applications, a cavity containing a uniform plasma is desirable. However, because microwave radiation can have a relatively long wavelength (e. g., several tens of centimeters), obtaining a uniform distribution can be difficult to achieve. As a result, consistent with one aspect of this invention, the radiation modes in a multi-mode cavity can be mixed, or redistributed, over a period of time. Because the field distribution within the cavity must satisfy all of the boundary conditions set by the inner surface of the cavity, those field distributions can be changed by changing the position of any portion of that inner surface.

[090] In one embodiment consistent with this invention, a movable reflective surface can be located inside the radiation cavity. The shape and motion of the reflective surface should, when combined, change the inner surface of the cavity during motion. For example, an"L"shaped metallic object (i. e.,"mode- mixer") when rotated about any axis will change the location or the orientation of the reflective surfaces in the cavity and therefore change the radiation distribution therein. Any other asymmetrically shaped object can also be used (when rotated), but symmetrically shaped objects can also work, as long as the relative motion (e. g. , rotation, translation, or a combination of both) causes some change in the location or orientation of the reflective surfaces. In one embodiment, a mode-mixer can be a cylinder that is rotatable about an axis that is not the cylinder's longitudinal axis.

[091] Each mode of a multi-mode cavity may have at least one maximum electric field vector, but each of these vectors could occur periodically across the inner dimension of the cavity. Normally, these maxima are fixed, assuming that the frequency of the radiation does not change. However, by moving a mode-mixer such that it interacts with the radiation, it is possible to move the positions of the maxima. For example, mode-mixer 38 can be used to optimize the field distribution within cavity 14 such that the plasma ignition conditions and/or the plasma sustaining conditions are optimized. Thus, once a plasma is excited, the position of the mode-mixer can be changed to move the position of the maxima for a uniform time-averaged plasma process (e. g. , heating).

[092] Thus, consistent with this invention, mode-mixing can be useful during plasma ignition. For example, when an electrically conductive fiber is used as a plasma catalyst, it is known that the fiber's orientation can strongly affect the minimum plasma-ignition conditions. It has been reported, for example, that when such a fiber is oriented at an angle that is greater than 60 degrees to the electric field, the catalyst does little to improve, or relax, these conditions. By moving a reflective surface either in or near the cavity, however, the electric field distribution can be significantly changed.

[093] Mode-mixing can also be achieved by launching the radiation into the applicator chamber through, for example, a rotating waveguide joint that can be mounted inside the applicator chamber. The rotary joint can be mechanically <BR> <BR> moved (e. g. , rotated) to effectively launch the radiation in different directions in the radiation chamber. As a result, a changing field pattern can be generated inside the applicator chamber.

[094] Mode-mixing can also be achieved by launching radiation in the radiation chamber through a flexible waveguide. In one embodiment, the waveguide can be mounted inside the chamber. In another embodiment, the waveguide can extend into the chamber. The position of the end portion of the <BR> <BR> flexible waveguide can be continually or periodically moved (e. g. , bent) in any<BR> suitable manner to launch the radiation (e. g. , microwave radiation) into the chamber at different directions and/or locations. This movement can also result in mode- mixing and facilitate more uniform plasma processing (e. g. , heating) on a time- averaged basis. Alternatively, this movement can be used to optimize the location of a plasma for ignition or other plasma-assisted process.

[095] If the flexible waveguide is rectangular, a simple twisting of the open end of the waveguide will rotate the orientation of the electric and the magnetic field vectors in the radiation inside the applicator chamber. Then, a periodic twisting of the waveguide can result in mode-mixing as well as rotating the electric field, which can be used to assist ignition, modulation, or sustaining of a plasma.

[096] Thus, even if the initial orientation of the catalyst is perpendicular to the electric field, the redirection of the electric field vectors can change the ineffective orientation to a more effective one. Those skilled in the art will appreciate that mode-mixing can be continuous, periodic, or preprogrammed.

[097] In addition to plasma ignition, mode-mixing can be useful during subsequent plasma processing to reduce or create (e. g. , tune) "hot spots"in the chamber. When a microwave cavity only supports a small number of modes (e. g., less than 5), one or more localized electric field maxima can lead to"hot spots" (e. g. , within cavity 12). In one embodiment, these hot spots could be configured to coincide with one or more separate, but simultaneous, plasma ignitions or processing events. Thus, the plasma catalyst can be located at one or more of those ignition or subsequent processing positions.

[098] Multi-location ignition [099] A plasma can be ignited using multiple plasma catalysts at different locations. In one embodiment, multiple fibers can be used to ignite the plasma at different points within the cavity. Such multi-point ignition can be especially beneficial when a uniform plasma ignition is desired. For example, when a plasma is modulated at a high frequency (i. e. , tens of Hertz and higher), or ignited in a relatively large volume, or both, substantially uniform instantaneous striking and restriking of the plasma can be improved. Alternatively, when plasma catalysts are used at multiple points, they can be used to sequentially ignite a plasma at different locations within a plasma chamber by selectively introducing the catalyst at those different locations. In this way, a plasma ignition gradient can be controllably formed within the cavity, if desired.

[0100] Also, in a multi-mode cavity, random distribution of the catalyst throughout multiple locations in the cavity increases the likelihood that at least one of the fibers, or any other passive plasma catalyst consistent with this invention, is optimally oriented with the electric field lines. Still, even where the catalyst is not optimally oriented (not substantially aligned with the electric field lines), the ignition conditions are improved.

[0101] Furthermore, because a catalytic powder can be suspended in a gas, it is believed that each powder particle may have the effect of being placed at a different physical location within the cavity, thereby improving ignition uniformity within the cavity.

[0102] Dual-Cavity Plasma Ignitinq/Sustaining [0103] A dual-cavity arrangement can be used to ignite and sustain a plasma consistent with this invention. In one embodiment, a system includes at least a first ignition cavity and a second cavity in fluid communication with the first cavity. To ignite a plasma, a gas in the first ignition cavity can be subjected to electromagnetic radiation having a frequency less than about 333 GHz, optionally in the presence of a plasma catalyst. In this way, the proximity of the first and second cavities may permit a plasma formed in the first cavity to ignite a plasma in the second cavity, which may be sustained with additional electromagnetic radiation.

[0104] In one embodiment of this invention, the first cavity can be very small and designed primarily, or solely for plasma ignition. In this way, very little microwave energy may be required to ignite the plasma, permitting easier ignition, especially when a plasma catalyst is used consistent with this invention.

[0105] In one embodiment, the first cavity may be a substantially single mode cavity and the second cavity is a multi-mode cavity. When the first ignition cavity only supports a single mode, the electric field distribution may strongly vary within the cavity, forming one or more precisely located electric field maxima. Such maxima are normally the first locations at which plasmas ignite, making them ideal points for placing plasma catalysts. It will be appreciated, however, that when a plasma catalyst is used, it need not be placed in the electric field maximum and, many cases, need not be oriented in any particular direction.

[0106] Plasma Jets [0107] Consistent with this invention, methods and apparatus for forming plasma jets are provided. Generally, plasma jets can be formed by generating plasmas in cavities and then permitting portions of the plasmas to exit the cavities through one or more apertures. Apparatus consistent with this invention need not include electrodes.

[0108] FIG. 10 shows illustrative plasma-jet apparatus 300 consistent with this invention. Apparatus 300 can include cavity 305, which can have at least one <BR> <BR> aperture 310 formed in cavity wall 312, plasma catalyst 315 (e. g. , positioned inside or near cavity 305), electromagnetic radiation source 320 for directing electromagnetic radiation into cavity 305, and gas source 325 for directing a gas into cavity 305 through gas port 330.

[0109] A method of forming plasma jet 350 can include flowing a gas from source 325 into cavity 305, forming plasma 340 in cavity 305 by subjecting the gas to electromagnetic radiation provided by radiation source 320 in the presence of a plasma catalyst, and allowing at least a portion of plasma 340 to pass out of cavity 305 through aperture 310 such that plasma jet 350 is formed outside cavity 305 proximate to aperture 310. The radiation can have any frequency less than about 333 GHz. It will be appreciated that by using a plasma catalyst consistent with this invention, plasma formation can occur at pressures, for example, at or above atmospheric pressure.

[0110] Cavity 305, gas source 325, radiation source 320, and plasma catalyst 315 can be similar to any of the cavities, sources, and catalysts already described above with respect to FIGS. 1-9. Also, the formation of a plasma, including the igniting, modulating, and sustaining of such a plasma within a cavity using a plasma catalyst has also been described hereinabove. Therefore, any of these previously described components and processes can be used to form a plasma jet consistent with this invention, and they will not be described again here.

[0111] As shown in FIG. 10, plasma catalyst 315 can be mounted to support structure 335 for positioning catalyst 315 at any position and/or orientation within cavity 305. Structure 335 can be removable, disposable, and/or replaceable, if desired. Thus, structure 335 can be in the form of a replaceable cartridge that contains or supports plasma catalyst 315. Structure 335 can be made from any material capable of withstanding the heat produced by plasma 340 within cavity 305, although structure 335 can be cooled with any convenient means.

[0112] It will be appreciated by those skilled in the art that structure 335 need not include an elongated portion, as shown in FIG. 10, and can be in any convenient form (e. g. , a plug having a catalytic surface, a cartridge that contains a<BR> catalyst, etc. ). When structure 335 extends through cavity wall 312, the portion of structure 335 at wall 312 can be substantially radiation-opaque to prevent radiation directed into cavity 305 by radiation source 320 from leaking outside cavity 305.

[0113] FIG. 11 shows a nozzle portion of another illustrative plasma-jet apparatus consistent with this invention. The apparatus can include cavity 355 in which plasma 357 is formed. Plasma 357 can form from a first gas supplied by a gas source (not shown). Also, a wall of cavity 355 can include at least one aperture 360 where plasma jet 370 can be formed. Apparatus 350 can also include gas guide 365, which can direct a second gas in the vicinity of aperture 360 for any purpose, including the prevention of oxidation at or near plasma jet 370.

[0114] Like apparatus 300, the apparatus shown in FIG. 11 can also include an electromagnetic radiation source (not shown) for directing electromagnetic radiation into cavity 355 and a plasma catalyst (not shown), which can, for example, be positioned inside or near cavity 355. It will be appreciated, however, that the use of electromagnetic radiation to form a plasma within cavity 355 is just one method of plasma formation and that any plasma formation method can be used including, for example, direct induction, in which an electric field is applied by ramping down the current in a solenoid.

[0115] Thus, consistent with this invention, an illustrative method for forming plasma jet 370 at jet aperture 360 is provided. The method can include flowing the first gas into cavity 355, forming plasma 357 from the first gas in cavity 355, allowing at least a portion of plasma 357 to pass out of cavity 355 through aperture 360 such that plasma jet 370 is formed outside cavity 355 proximate to aperture 360, and flowing second gas 375 outside cavity 355 proximate to plasma jet 370.

[0116] Second gas 375 can flow through second aperture 380 adjacent jet aperture 360. In one embodiment, second aperture 380 can partially or completely surround jet aperture 360. Thus, as shown in FIG. 11, second aperture 380 can be substantially annular, and may be concentric with jet aperture 360. It will be appreciated, however, that apertures 360 and 380 can have any convenient shape, depending on the plasma-process to be performed outside cavity 355.

[0117] For example, FIG. 12 shows a front view of illustrative nozzle portion 381 (e. g. , taken along line 12-12 of FIG. 11). As shown in FIG. 12, jet aperture 382 can be located inside of annular second aperture 384, and these apertures can be substantially concentric. Similarly, FIG. 13 shows a front view of another illustrative nozzle portion 391 (e. g. , taken along line 13-13 of FIG. 11). As shown in FIG. 13, rectangular jet aperture 392 can be located inside of rectangular second aperture 394.

[0118] Returning to FIG. 11, it will be appreciated that during plasma-jet formation, second gas 375 can form a barrier between jet 370 and air outside cavity 355 to substantially prevent oxidation from occurring at or near plasma jet 370. Alternatively, second gas 375 can react with the plasma outside of cavity 355 to take part in any plasma jet-assisted process, such as a coating or joining process.

[0119] Once again, cavity 355, as well as gas sources, radiation sources, and plasma catalysts can be similar to any of the cavities, sources, and catalysts already described above. Also, the formation of a plasma, including the igniting, modulating, and sustaining of such a plasma, within a cavity using a plasma catalyst has also been described hereinabove. Therefore, these and other components and processes can be used to form a plasma jet consistent with this invention, but they will not be described again here.

[0120] FIG. 14 shows another illustrative embodiment of plasma-jet apparatus 400 consistent with this invention that includes at least one laser. It will be appreciated that a laser beam can be directed toward a work piece at any convenient time, including before, during, or after the formation of a plasma.

Apparatus 400 can include cavity 405, which includes at least one aperture 410 formed in cavity wall 412, electromagnetic radiation source 420, which can direct electromagnetic radiation into cavity 405, gas source 425, which can direct a gas into cavity 405 through gas port 430, and laser 435, which can direct laser beam 450 toward work piece 440.

[0121] It will be appreciated that the configuration shown in FIG. 14 is only illustrative and that other configurations can be used consistent with this invention.

For example, radiation source 420 and laser 435 can both be directed into a cavity that is different from cavity 405, but which permits both the electromagnetic radiation from source 420 and laser beam 450 to pass into cavity 405, through aperture 410, and toward work piece 440. Similarly, as described more below, laser 435 could be mounted outside of cavity 405, near aperture 410, and directed toward work piece 440, if desired.

[0122] Also, as described above, plasma 455 can be formed using any known method, such as direct induction. Thus, in this case, radiation source 420 would be unnecessary and could be replaced with a solenoid and an electric current source.

[0123] A method using apparatus 400 for forming and directing a plasma jet in combination with a laser beam is also provided. The method can include flowing a gas supplied from gas source 425 into cavity 405, forming plasma 455 in cavity 405 from the gas, allowing at least a portion of plasma 455 to pass out of cavity 405 through aperture 410 such that plasma jet 460 is formed outside cavity 405 proximate to aperture 410, and directing laser beam 450 proximate to plasma jet 460 such that plasma jet 460 and laser beam 450 are directed at substantially the same location, such as point 470 on work piece 440.

[0124] As shown in FIG. 14, and consistent with one embodiment of this invention, cavity 405 includes wall (or barrier) 475 that is substantially transmissive to laser beam 450. In one embodiment, wall 475 can include a transmissive portion and a non-transmissive portion. Thus, the laser beam can be directed first through wall 475 and then through aperture 410. Although not shown in FIG. 14, it will be appreciated that laser beam 450 can be directed and reflected appropriately using one or more reflective surfaces along the beam path. These surfaces can be placed in or out of cavity 405 and be used to direct laser beam 450 in any desired direction. Moreover, the reflective surfaces can be used to change the direction of laser beam 450 before, during, or after use of plasma jet 460. In this case, a controller can be used to control the orientation of these surfaces.

[0125] As mentioned above, one or more laser beams can be directed from at least one position outside the cavity and proximate the aperture. For example, FIG. 15 shows a front view of illustrative nozzle portion 500 of a plasma jet apparatus consistent with this invention with multiple solid-state lasers 505 positioned circumferentially about plasma jet aperture 510. Lasers 505 can be mounted, for example, directly or indirectly on housing 515 in which the plasma cavity is formed. FIG. 16 shows a cross-sectional view of the nozzle portion shown in FIG. 15, taken along line 16-16 of FIG. 15, during operation. As shown in FIG.

16, plasma jet 520 has point 525 located along jet 520 at working distance 530 and laser beams 508 can be directed toward point 525. Alternatively, one or more lasers can be located remotely from aperture 510 and directed appropriately.

[0126] Returning to FIG. 14, for example, a method of using a plasma jet consistent with this invention is provided. The method can include monitoring a processing status of work piece 440 located outside cavity 405. The status can be a function of exposure to plasma jet 460. Then, when the status is a predetermined status, the method can include directing laser beam 450 toward work piece 440.

The status could be, for example, a temperature associated with the work piece, a thickness of a coating on the work piece, etc. If the plasma jet is used to deposit a coating on a substrate, the jet could contain one or more coating materials. In this case, the status could also be, for example, the composition of the coating or even a plasma-jet exposure time. Once the coating is deposited, the coating can be further heated by absorption of the laser beam to improve the adhesion or quality of the coating. Alternatively, the surface of the substrate can be preheated by the laser before the plasma jet is even ignited.

[0127] Voltage source 485 of FIG. 14 can apply an electric bias between work piece 440 and at least a portion of cavity wall 412. Alternatively, source 485 can apply an electric bias between work piece 440 and electrode 490, which can be located between cavity 405 and work piece 440. In this way, the vessel in which cavity 405 is formed need not be electrified.

[0128] Although a cavity can be used to confine a plasma consistent with this invention, an electric field and/or a magnetic field can also be applied to further confine or otherwise modify the plasma within the cavity. This additional confinement and/or modification can be used to prevent a plasma from heating certain inner surface (s) of the cavity wall, and to increase the energy of a plasma- jet output.

[0129] FIG. 17 shows illustrative apparatus 550 for forming a plasma jet consistent with this invention. Apparatus 550 can include vessel 557, in which cavity 555 can be formed, and a gas source (not shown) for directing a gas into cavity 555. Cavity 555 can include at least one aperture 560 formed in cavity wall 565. An electromagnetic radiation source for directing electromagnetic radiation into cavity 555 and a plasma catalyst for relaxing the plasma ignition, modulation, and sustaining conditions can also be included, although they are not necessary, nor are they shown in FIG. 17 for illustrative simplicity.

[0130] Consistent with this invention, cavity 555 can include electrically conductive and substantially thermally resistant inner surface 570, which can be proximate to aperture 560, electrically conductive surface 575, which faces surface 570, and voltage source 580, which can apply a potential difference between surfaces 570 and 575. Magnetic field H can also be applied to the plasma by passing an electric current through coil winding 576, which can be external or internal to vessel 557.

[0131] A method for forming plasma jet 585 at aperture 560 can also be provided. The method can include (1) flowing a gas into cavity 555, (2) forming plasma 590 from the gas in cavity 555, (3) allowing at least a portion of plasma 590 to pass out of cavity 555 through aperture 560 such that plasma jet 585 is formed outside cavity 555 proximate to aperture 560, and (4) applying an electric potential between surfaces 570 and 575 and/or passing an electric current through coil 576.

[0132] Application of an electric potential between surfaces 570 and 575 using voltage source 580 can cause plasma 590 to accelerate charged particles towards aperture 560. Surfaces 570 and 575 can be disposed on, or be integral with, vessel 557. Alternatively, surfaces 570 and 575 can be separate from the internal surface of vessel 557. In this case, these surfaces can be plates or screens that are suspended or otherwise mounted in cavity 555. Alternatively, surfaces 570 and 575 can be discs or rings or any other part having a convenient shape configured for use in plasma cavity 555.

[0133] As mentioned above, magnetic field H can be applied to plasma 590 by passing an electric current through coil 576. The magnetic field can exert a deflecting force on the charged particles that move perpendicular to the magnetic field. Consequently, charged particles in the plasma will be substantially prevented from moving radially outward (i. e. , perpendicular to longitudinal axis 577 of coil 576). As a result, the inner surface of cavity 555 close to coil 576 will be heated less. In addition, because the plasma will tend to move toward axis 577, a hotter, more concentrated plasma can form along longitudinal axis 577 of cavity 555 and a more efficient plasma jet can be formed at aperture 560.

[0134] In addition, the electric current can flow through coil 576, or a potential can be applied by source 580 between surfaces 570 and 575 or both, during any time period, including before the formation of plasma 590, during the formation of plasma 590, and after the formation of plasma 590. It is believed that the principal benefit may result when the potential is applied while the plasma is formed (that is, while the plasma is being modulated or sustained) in cavity 555.

Also, the magnetic field can be applied at any time, including before, during, or after plasma formation. As a result, one or more plasma characteristics (e. g., physical <BR> <BR> shape, density, etc. ) can be varied by flowing an electric current through coil 576 or by applying a potential between surfaces 570 and 575.

[0135] The potential applied by voltage source 580 can cause surface 570 to be more positive or more negative than surface 575. In one embodiment, positively changed ions of atoms and molecules within plasma 590 can be attracted toward surface 570 by applying a relatively negative potential to surface 570.

Because the positive ions, which are attracted by negative surface 570, will transfer at least some of the kinetic energy to surface 570, surface 570 can be made from a material that can withstand relatively high temperatures (e. g. , 1,000 degrees Fahrenheit and above). In one embodiment, that surface can include molybdenum, which is also electrically conductive.

[0136] Surface 570 can include two or more layers. The outer layer, which faces or contacts plasma 590 during operation, can be selected to withstand very high temperatures (although not necessarily electrically conductive). The under layer, then, can be electrically conductive, but not necessarily capable of withstanding very high temperatures. Additional layers can be used as well to enhance its heat-resistance and/or its electrical conductivity.

[0137] An electric potential can also be applied between vessel 557 and a work piece located outside cavity 555 to accelerate plasma 590 through aperture 560 toward the surface (not shown). When a sufficient electric current flows through the work piece, the temperature of the work piece can be increased through a resistive heating as well as from the increased kinetic energy of the charged particles striking the work piece.

[0138] In the foregoing described embodiments, various features are grouped together in a single embodiment for purposes of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the following claims are hereby incorporated into this Detailed Description of Embodiments, with each claim standing on its own as a separate preferred embodiment of the invention.