FIELD OF THE INVENTION [002] This invention relates to methods and apparatus for plasma-assisted heating, and in particular to plasma furnaces.

BACKGROUND OF THE INVENTION [003] It is known that furnaces can be powered by electricity or gas. Gas furnaces burn gas and pollute the environment. Electric furnaces, while more environmentally friendly, are normally incapable of generating heat quickly and effectively.

[004] It is also known that furnaces can be powered with plasma. For example, graphite electrode-type plasma furnaces have been reported for use as an incinerator. Also, plasma furnaces that use multiple helium plasma torches to melt and process materials have also been reported. Moreover, a plasma can be ignited by subjecting a gas to a sufficient amount of microwave radiation at reduced pressure, but vacuum equipment can be expensive, slow, and energy-consuming.

Moreover, the use of such equipment can limit the applications of such furnaces.

BRIEF SUMMARY OF A FEW ASPECTS OF THE INVENTION [005] Consistent with the present invention, plasma heating apparatus and methods are provided. In one embodiment, a heating apparatus including a cavity can have at least one heat-conductive wall. The heating apparatus can further include at least one electromagnetic radiation source configured to irradiate the gas into the cavity and form the plasma in the cavity. The heating apparatus can further include at least one heating region located external to the cavity and adjacent the thermally conductive wall for receiving energy generated by the plasma.

[006] In another embodiment consistent with this invention, a heating method can include directing radiation into a cavity containing a gas to form a plasma in the cavity. The method can further include modulating or sustaining the plasma in the cavity for a period of time sufficient to heat at least one thermally conductive cavity wall. The method can further include radiating energy through the at least one cavity wall to heat matter on an opposite side of the wall.

[007] Plasma catalysts, and methods and apparatus for igniting, modulating, and sustaining a plasma consistent with this invention are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS [008] 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: [009] FIG. 1 shows a schematic diagram of an illustrative apparatus consistent with this invention; [010] FIG. 2 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; [011] FIG. 3 shows an illustrative plasma catalyst fiber with at least one component having a concentration gradient along its length consistent with this invention; [012] FIG. 4 shows an illustrative plasma catalyst fiber with multiple components at a ratio that varies along its length consistent with this invention; [013] FIG. 5A shows another illustrative plasma catalyst fiber that includes a core underlayer and a coating consistent with this invention; [014] FIG. 5B shows a cross-sectional view of the plasma catalyst fiber of FIG. 5A, taken from line 5B-5B of FIG. 5A, consistent with this invention; [015] 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; [016] 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; [017] 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; [018] FIG. 9 shows an illustrative embodiment of a portion of a plasma system for directing radiation into a radiation chamber consistent with this invention; [019] FIG. 10 shows a cross-sectional view of illustrative plasma-assisted heating apparatus consistent with this invention; [020] FIG. 11 shows a flow-chart of an illustrative method for plasma- assisted heating consistent with this invention; [021] FIG. 12 shows a cross-sectional view of another illustrative plasma- assisted heating apparatus consistent with this invention; [022] FIG. 13 shows a cross-sectional perspective view of another illustrative plasma-assisted heating apparatus consistent with this invention; [023] FIG. 14 shows a cross-sectional perspective view of yet another illustrative plasma-assisted heating apparatus consistent with this invention; [024] FIG. 15 shows a cross-sectional view of an illustrative plasma- assisted melting furnace consistent with this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS [025] Consistent with the present invention, systems and methods for plasma-assisted heating are provided. As described more fully below, any type of matter (e. g., solid, fluid, or gas) can be heated by modulating or sustaining a plasma in a cavity, optionally with a plasma catalyst. In one embodiment, the cavity may have a radiation-transmissive wall and a thermally conductive wall. A plasma can be formed in the cavity by irradiating a gas located in the cavity with electromagnetic radiation. As the temperature of the plasma rises, radiative energy absorbed by the plasma can be transferred, in the form of thermal energy, to the matter (in an adjacent chamber, for example) through the thermally conductive wall. In one embodiment, a radiation source, such as a microwave radiation source, may direct radiation at the gas.

[026] It will be appreciated that multiple radiation sources may be used consistent with this invention, such as described in commonly owned, concurrently filed U. S. Patent Application No. 10/, (Attorney Docket No. 1837.0018), which is hereby incorporated by reference in its entirety. Further, phase and/or frequency of the radiation sources may be varied as described in commonly owned, concurrently filed U. S. Patent Application No. 10/, (Attorney Docket No. 1837.0010), which is also incorporated by reference in its entirety. Thus, this invention can be used for controllably generating heat and for plasma-assisted processing with lower energy costs and increased efficiency.

[027] Plasma catalysts for initiating, modulating, and sustaining a plasma are also provided. A 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 (e. g. , an electromagnetic field) consistent with this invention without necessarily adding additional energy through the catalyst, such as by applying a voltage to create a spark. An active plasma catalyst, on the other hand, may 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.

[028] 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. 1837.0021), No. 10/, (Atty. Docket No. 1837.0023), No. 10/, (Atty. Docket No. 1837.0024), No. 10/, (Atty. Docket No. 1837.0025), 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).

[029] Illustrative Plasma Furnace [030] FIG. 1 shows illustrative plasma system 10 consistent with one aspect of this invention. In this embodiment, cavity 12 is formed in a vessel that is positioned inside radiation chamber (i. e., applicator) 14. In another embodiment (not shown), vessel 12 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 insulating layers to improve its thermal insulation properties without significantly shielding cavity 12 from the radiation.

[031] In one embodiment, cavity 12 is 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. It will also be appreciated that because the furnace operating temperature can be different for different types of applications, the material used to make the vessel may only need to withstand temperatures substantially below 3,000 degrees Fahrenheit, such as about 2,500 degrees, or about 1,000 degrees Fahrenheit, or even lower.

[032] In one embodiment, 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, for some applications, the cavity can be configured to prevent the plasma from rising/floating away from the primary heating region.

[033] 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 or any other supply. Line 20 may be tubing (e. g. , between about 1/16 inch and about % inch, such as about 1/8"), but can be any channel or device capable of supplying a gas to cavity 12.

Also, if desired, a vacuum pump (not shown) can be connected to chamber 14 to remove any undesirable fumes that may be generated during plasma processing.

[034] A radiation leak detector (not shown) can be 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.

[035] In one embodiment, the radiation apparatus may include radiation source 26 for directing radiation into the cavity. The radiation apparatus may further include other radiation sources (not shown) for directing additional radiation into the cavity. Radiation source 26, which may be powered by electrical power supply 28, can direct radiation 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 chamber 14 or cavity 12, thereby eliminating waveguides 30.

The radiation energy entering cavity 12 can be used to ignite a plasma within the cavity. This plasma can be modulated or substantially sustained and confined to the cavity by coupling additional radiation with the catalyst.

[036] Radiation source 26 may be a magnetron, a klystron, a gyrotron, a traveling-wave tube amplifier, or any other device capable of generating radiation.

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 over a broad range of pressures, including atmospheric pressure and above.

[037] For example, the invention may be practiced by 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. A 3-stub tuner may allow impedance matching for maximum power transfer and a dual directional coupler may be used to measure forward and reflected powers.

[038] Radiation energy can be supplied by radiation source 26 through <BR> <BR> 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 before the plasma has formed because microwave power, for example, will be strongly absorbed by the plasma, although the use of a circulator and a tuner is optional.

[039] 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 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.

[040] Sensor 42 can develop output signals as a function of the temperature or any other monitorable condition associated with a work piece (not shown) within cavity 12 and provide the signals to controller 44. Dual temperature sensing and heating, as well as automated cooling gas flow control, 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.

[041] The equipment may be computer controlled using LabView 6i software, which may provide real-time temperature monitoring and microwave power control. Noise may be reduced by using sliding averages of suitable number of data points. Also, by using shift registers and buffer sizing the number of stored data points in the array may be limited to improve speed and computational efficiency. The pyrometer may measure the temperature of a sensitive area of about 1 cm2, which may be used to calculate an average temperature. The pyrometer may sense 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. 1837.0033), which is hereby incorporated by reference in its entirety.

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

[043] The exemplary furnace may also include a closed-loop deionized water cooling system (not shown) with an external heat exchanger cooled by tap water. During operation, the deionized water may first cool 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.

[044] Plasma Catalysts [045] 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.

[046] 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 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.

[047] 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 (e. g. , at some predetermined orientation) while the plasma is ignited or sustained.

[048] 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.

[049] In one embodiment, a 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. 2, radiation source 52 can supply radiation to radiation cavity 55, in which plasma cavity 60 is placed. Powder source 65 can provide catalytic powder 70 into gas stream 75. In an alternative embodiment, powder 70 can be first added to 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.

[050] 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.

[051] 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.

[052] 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).

[053] 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.

[054] 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 Magnamite@, Model No. AS4C-GP3K, by the Hexcel Corporation, of Anderson, South Carolina. Also, silicon-carbide fibers have been successfully used.

[055] 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.

[056] 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.

[057] 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.

[058] 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 modulating or 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, modulate, and/or sustain the plasma could be the same.

[059] 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 schedule. 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.

[060] 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. 3, 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.

[061] Alternatively, as shown in FIG. 4, 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.

[062] 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.

[063] 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. 5A and 5B, 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.

[064] 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.

[065] Consistent with another embodiment of this invention, a plasma catalyst can be located entirely within a radiation cavity 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 prevents sparking at the ignition port and prevents radiation from leaking outside the cavity 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.

[066] FIG. 6, for example, shows radiation chamber 160 in which plasma cavity 165 is 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 is placed in chamber 160) and electrically non-conductive portion 185 (which is placed substantially outside chamber 160). This configuration prevents an electrical connection (e. g. , sparking) between distal portion 180 and chamber 160.

[067] 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.

[068] 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.

[069] 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.

[070] For example, FIG. 9 shows radiation source 200 directing radiation into radiation chamber 205. Plasma cavity 210 can be positioned inside of chamber 205 and may permit a gas to flow therethrough via its gas ports. 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.

[071] 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 to substantially prevent the fission products (i. e. , the ionizing particle catalyst) from creating a safety hazard.

[072] 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.

[073] 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.

[074] Multi-mode Radiation Cavities [075] 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 <BR> <BR> 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.

[076] 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).

[077] 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.

[078] 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.

[079] 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.

[080] 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.

[081] 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.

[082] 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.

[083] 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).

[084] 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. 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 that facilitate plasma formation, such as argon, are easier to ignite but may not be needed during subsequent plasma processing.

[085] Mode-mixing [086] For many applications, a cavity containing a uniform plasma is desirable. However, because 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.

[087] 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.

[088] 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 12 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).

[089] 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° 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.

[090] 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 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.

[091] 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 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.

[092] 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.

[093] 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.

[094] In addition to plasma ignition, mode-mixing can be useful during subsequent plasma-assisted processing (e. g. , heating) to reduce or create (e. g.,<BR> tune) "hot spots"in the chamber. When a 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 or heating positions.

[095] Multi-location Ignition [096] 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.

[097] 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.

[098] 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.

[099] Dual-Cavity Plasma lgniting/Sustaining [0100] A dual-cavity arrangement can be used to ignite and sustain a plasma consistent with this invention. In one embodiment, a system can include 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.

[0101] 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 radiation energy may be required to ignite the plasma, permitting easier ignition, especially when a plasma catalyst is used consistent with this invention.

[0102] 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.

[0103] Illustrative Plasma-Assisted Heating [0104] Methods and apparatus for plasma-assisted heating may be provided consistent with this invention. A plasma catalyst can be used to facilitate the igniting, modulating, or sustaining of the plasma at gas pressures below, at, or above atmospheric pressure.

[0105] FIG. 10 shows a cross-sectional view of illustrative plasma-assisted heating apparatus 300 consistent with this invention. Apparatus 300 can include cavity 305 for containing a gas, at least one radiation source 310, and heating region 315. Radiation source 310 can be configured to irradiate a gas located in cavity 305 and to ignite, modulate, and/or sustain a plasma from the gas in cavity 305. Cavity 305 can have at least one thermally conductive wall 320 for conducting the energy absorbed by the plasma in cavity 305 to a location outside cavity 305. Heating region 315 can be located external to cavity 305 (as shown in FIG. 10) and adjacent to wall 320. A radiation-transmissive window 328 can be used to prevent the plasma from moving from cavity 305 to waveguide 325.

[0106] Radiation source 310 can be, for example, a magnetron, a klystron, a gyrotron, a traveling wave tube, a solid state microwave source, or any other radiation source capable of supplying electromagnetic radiation having a frequency less than about 333 GHz. As shown in FIG. 10, radiation source 310 can direct radiation through waveguide 325. Alternatively, radiation source can be coupled to cavity 305 using a coaxial cable selected to propagate the desired type of radiation.

In another embodiment, radiation source 310 can simply direct radiation toward cavity 305 without any physical connection.

[0107] For example, as shown in FIG. 1, radiation source 26 directs radiation into chamber 14 and that radiation passes through the vessel containing cavity 12. In this case, the cavity can have a wall portion that is at least partially radiation-transmissive (e. g. , ceramic, quartz, etc.). It will be appreciated that any other technique for directing radiation into a plasma cavity can also be used consistent with this invention.

[0108] When radiation source 310 is turned on, or activated, care should be taken to prevent radiation from reflecting from an inner surface of cavity 305 and reentering source 310, which could damage it. One way to prevent such damage is to use a plasma catalyst consistent with this invention. As described above, the use of a plasma catalyst can cause a plasma to ignite nearly instantaneously when radiation first enters cavity 305. Because plasma strongly absorbs electromagnetic radiation, the existence of a plasma in cavity 305 can substantially prevent radiation from reflecting back into radiation source 310 at the early stages of plasma ignition. An isolator (not shown in FIG. 10) can also be located between cavity 305 and source 310 to further prevent potentially damaging radiation from passing back to radiation source 310, if desired.

[0109] In addition, one or more radiation sources can be used to direct radiation into cavity 305 consistent with this invention. In this case, radiation generated by one radiation source can be undesirably directed into another radiation source, which can also lead to the same type of damage due to reflected power. Once again, the use of a plasma catalyst consistent with this invention can essentially eliminate this danger because nearly all radiation will be strongly absorbed once the plasma is formed. For this reason, and as described in commonly owned, concurrently filed Kumar U. S. Patent Application No. 10/, (Attorney Docket No. 1837.0018), which is hereby incorporated by reference in its entirety, one of the sources can be activated before the others to ignite the plasma, and then subsequent source can be activated. Alternatively, a first source can be cross-polarized with a second source.

[0110] As explained more fully above, a plasma catalyst can be located at any operational position. For example, a plasma catalyst can be located in the plasma cavity. In this case, the catalyst could be subject to ongoing plasma exposure during the use of the furnace, which could undesirably degrade the catalyst over time. Therefore, a plasma catalyst can also be placed proximate to the cavity, including in a wall of cavity 305 or adjacent to such a wall. For example, in the case of passive plasma catalyst, the catalyst only needs to be placed close enough to cavity 305 such that it deforms the electromagnetic fields in the cavity.

In the case of an active catalyst, the source can be placed outside the cavity, but the ionizing particles can be directed into the plasma cavity.

[0111] As shown in FIG. 10, cavity 305 can be completely closed, or sealed. Thus, once a sufficient amount of gas in placed in cavity 305, the same gas can be used to repeatedly form a plasma within cavity 305. Alternatively, cavity 305 can include one or more gas ports (not shown) for flowing gas into and out of cavity 305 during plasma generation. Although in this configuration the cavity would not be completely sealed, it could still substantially confine a plasma and be used to generate heat consistent with this invention. Moreover, gas flow can be used, if desired to control the temperature of the plasma. That is, higher gas flow rates could be used to cool, or at least substantially slow down the rate of heating, of a plasma-assisted furnace consistent with this invention.

[0112] When cavity 305 is sealed, plasma catalyst 327, as well as the gas, can be located in cavity 305. In this case, the plasma catalyst can be coated with a thermal-protective layer. Alternatively, the catalyst can be moved from an optimum position in the cavity for igniting the plasma to a less optimum position, where the plasma would be less likely to degrade or consume the catalyst during operation.

In another embodiment, the plasma catalyst can be fixed in the cavity, but a mode- mixer can be used to change the field distribution within the cavity. For example, the distribution can be originally tuned such that a hot spot is located at the plasma catalyst but then subsequently tuned such that the hot spot is located adjacent to wall 320.

[0113] As shown in FIG. 10, heat conducting region 315 can include a thermally conductive conduit for channeling fluid. The fluid can be, for example, a liquid or a gas, or a combination of both. Water, oil, or any other fluid capable of absorbing energy, either by thermal conduction or radiative absorption, can be used. Alternatively, heating region 315 can be configured to contain a eatable solid. Solids, such as iron, can be useful for storing energy in the form of thermal energy. In addition, a furnace consistent with this invention can be used to heat any other type of solid, such as a semiconductor, to perform any type of processing requiring heat.

[0114] FIG. 11 shows a flow-chart of an illustrative method for plasma- assisted heating consistent with this invention. In step 350, radiation can be directed into a cavity containing a gas to ignite a plasma in the cavity. In step 355, the plasma can be modulated or sustained in the cavity for a period of time sufficient to heat at least one thermally conductive cavity wall. And, in step 360, heat can be conducted or radiated through the wall to transfer thermal energy to matter on an opposite side of the wall.

[0115] As previously explained with respect to apparatus 300, a plasma can be ignited in step 350 by subjecting a gas in the cavity to the radiation in the presence of a plasma catalyst, which may be active or passive. As also previously explained, the plasma can be modulated or sustained by directing radiation into the cavity from at least one radiation source. When multiple radiation sources are used, a first source can direct radiation into the cavity before any of the other sources to ensure that a radiation absorbing plasma has been formed. This can help prevent radiation from reflecting or propagating into these radiation sources and damaging them.

[0116] FIG. 12 shows a cross-sectional view of another illustrative plasma- assisted heating apparatus 370. In this example, cavity 375 can include two functional walls : radiation-transmissive wall 322 and thermally conductive wall 324.

As previously explained, radiation-transmissive wall 322 can be formed from any material capable of substantially transmitting radiation at the frequency provided by one or more radiation sources. Some of the radiation-transmissive materials that can be used consistent with this invention include, for example, quartz, Al203, and many ceramics.

[0117] Based on the mode pattern of the radiation within the outer walls of apparatus 370, walls 322 and 324 can be configured to modulate or sustain a plasma in some regions, and to prevent formation of a plasma in other regions within cavity 375. For example, in one embodiment, plasma formation can be maximized near wall 324, such that a maximum amount of energy can be transferred to heating region 330, which can be located adjacent to heat- conductive wall 324. As explained above, a heating region consistent with this invention can be used to heat a fluid or a solid, depending on the application. In this case, region 330 is partially bound by outer wall 326. Walls 324 and 326, when combined, can form a channel through which a fluid can flow and be heated.

[0118] As shown in FIG. 12, wall 324 can be curved to increase its surface area, which can increase the rate that energy passes through wall 324. It will be appreciated that other shapes can also be used. It will also be appreciated that a single furnace could have multiple plasma cavities, and each of those cavities can have the same or different shape.

[0119] Also, although not shown in FIG. 12, a plasma catalyst can be used with plasma apparatus 370, and any other plasma-assisted furnace consistent with this invention, to facilitate igniting, modulating, and/or sustaining a plasma. As previously explained, the use of a plasma catalyst can relax the conditions required to form a plasma, which can make the plasma more controllable, even at pressures at or above atmospheric pressure.

[0120] It will be appreciated that such catalysts may be particularly useful due to their substantially continuous catalyzing effect, as opposed to spark plugs, for example, which only spark periodically. Continuous catalysis can be especially useful during periodic processes that require repeated striking and restriking of a plasma.

[0121] FIG. 13 shows a cross-sectional perspective view of another illustrative plasma-assisted heating apparatus 400. In this apparatus, plasma cavity 405 is cylindrical, although it will be appreciated that the shape can also be rectangular, or any other convenient shape capable of supporting at least one mode of radiation. Cavity 405 can be formed in tube 410 that is at least thermally conductive. Radiation can be directed into cavity 405 in any convenient manner, such as by a waveguide or a coaxial cable. As shown in FIG. 13, coaxial cable 415 can be used to direct radiation into cavity 405. Plasma tube 410 can be open or closed. If open, the gas can flow longitudinally.

[0122] Plasma tube 410 can be located inside of outer tube 420. In this way, heating region 430 can be located between tube 410 and 420 and a fluid can be heated while flowing through this region. It will be appreciated that the shape of outer tube 420 need not be cylindrical, but can be rectangular or any other convenient shape.

[0123] FIG. 14 shows a cross-sectional perspective view of yet another illustrative plasma-assisted heating apparatus 450. In this apparatus, plasma cavity 480 can be an annular space between two electrically conducting tubes (e. g. , in a co-axial waveguide). Tubes 460 and 470 are shown to be cylindrical, for example, but the shape can also be rectangular or any other convenient shape.

Thus, cavity 480 can be formed between tube 460, which can be thermally conductive, and tube 470, which can be electrically conductive and surrounded by an insulating jacket (not shown).

[0124] During operation, radiation can be directed into cavity 480 in any convenient manner, such as by a waveguide or a coaxial cable. As shown in FIG.

14, coaxial cable 465 can be used to direct radiation into cavity 480. Alternatively, the radiation can be fed coaxially through a radiation-transmissive window (not shown). Also, heating region 455 can be located inside tube 460 and a fluid can be heated while flowing through this region.

[0125] It will also be appreciated that although tubes 410 and 420 of FIG. 13 and tubes 460 and 470 of FIG. 14 are shown as straight, they need not be.

For example, the tubes can have a spiral, serpentine, or any other convenient form. It will also be appreciated that the positions of heating regions 430 and 480 can be reversed with plasma cavities 405 and 455, respectively.

[0126] FIG. 15 shows a cross-sectional view of illustrative plasma-assisted melting furnace consistent with this invention. Furnace 500 can include melting column 505, surrounding plasma cavity 510, and electromagnetic radiation source (not shown) for directing electromagnetic radiation 515 having a frequency less than about 333 GHz into plasma cavity 510. Additional structural elements (shown and not shown) are described below in conjunction with a method for melting objects, such as metal ore and solid waste.

[0127] In one embodiment, an illustrative method for melting metal ore can include adding metal ore 502 to a melting region (e. g., melting column 505), <BR> <BR> forming a plasma in cavity 510 by subjecting a gas (e. g. , argon) to electromagnetic<BR> radiation 515 in the presence of plasma catalyst 520 (e. g. , carbon fiber), wherein cavity 510 can have thermally conductive wall 525, sustaining the plasma in cavity 510 such that energy from the plasma passes through wall 525 into melting column 505 and melts metal ore 502 into molten metal 530 and slag 535, and then collecting molten metal 530.

[0128] Plasma catalyst 520 can be an active plasma catalyst and a passive plasma catalyst. Because these such catalysts have been described in detail above, they are not described here again, although it will be understood that any of the previously mentioned catalysts, and any of the associated methods, can be used to melt objects, such as metal ore or solid waste, consistent with this invention.

[0129] As shown in FIG. 15, melting column 505 can be defined by conductive wall 525 of inner tube 540 and cavity 510 can be defined between inner tube 540 and an outer tube 545. Inner tube 540 can be located substantially inside outer tube 550, if desired. When the tubes are oriented substantially vertically, two or more gases can be used to preferentially form a plasma closer to inner tube <BR> <BR> 540. In one embodiment, a first gas (e. g. , argon), which relatively easily forms a plasma, can be flowed into cavity 510 near the inner tube 540 so that the gas flows close to wall 525 and forms a plasma there. A second gas (not shown) that does not easily form a plasma can also be flowed into cavity 510 near outer tube 545 during the plasma sustaining. It will be appreciated that additional embodiments for preferentially forming a plasma near wall 525 can involve application of electric and magnetic fields.

[0130] During operation, melting column 505 can be substantially vertical.

Then, metal ore can be added to top end 555 of melting column 505 and molten metal 530 can be collected at bottom end 560 of melting column 505.

Radiation 515 can be directed into cavity 510 through coaxial waveguide 565, which can be separated from cavity 510 by radiation-transmissive plate 570. In one embodiment, inner tube 525 can have an outer diameter and outer tube 545 can have an inner diameter, such that the ratio of the inner diameter to the outer diameter is between about 2.5 and about 3.0, including about 2.72. Depending on the metal being melted, treated, or extracted, suitable reactant 575 (e. g. , a reducing agent, such as air plus coke, or carbon monoxide) can be added to the components. This addition can be used, for example, to separate metal from other related compounds.

[01311 It will be appreciated that a movable conductive plate can be placed at one or both of the axial ends of cavity 510 to adjust the length of cavity 510. By adjusting this length, the electromagnetic radiation standing wave pattern can be shifted. In one embodiment, the amount of shifting can be at least about a quarter of a wavelength.

[0132] The spatial dependence of electromagnetic radiation absorption can also be adjusted by applying an electric potential (not shown) to inner tube 540, for example. The applied potential can attract the plasma and increase its concentration at the surface of wall 525, thereby increasing the rate at which energy passes through conductive wall 525.

[0133] In another embodiment, conductive wall 525 can include multiple apertures (not shown) formed between cavity 510 and melting column 505. The apertures can allow plasma formed in cavity 510 to flow into column 505 to heat the objects located there. As described more fully above, the plasma can be used to convey one or more reactants into column 505. To prevent molten metal and other undesirable liquids and gases from flowing from column 505 into cavity 510, the longitudinal axes of the apertures can be directed in a downward direction.

That is, the end of each aperture that opens into cavity 510 can be higher than the end of the same aperature that opens into column 505.

[0134] Additional plasma-assisted melting methods are also provided consistent with this invention. For example, in one embodiment, a plasma can be formed in a cavity by subjecting a first gas to electromagnetic radiation having a frequency less than about 333 GHz in the presence of a plasma catalyst. Then, a second gas (which can be the same as the first gas) can be heated with the plasma. Solid metal ore can be added to a melting vessel and the heated gas can be directed toward the metal ore to at least melt the metal ore into a molten metal and a slag. Next, the molten metal can be collected and used for any purpose.

[0135] In another example, a plasma can be formed in a cavity by subjecting a gas to electromagnetic radiation having a frequency less than about 333 GHz in the presence of a plasma catalyst. Then, metal ore can be conveyed through the plasma until the metal ore melts and the molten metal is collected. In one embodiment, the ore can be conveyed on a heat-resistant conveyor and the plasma can be formed beneath the conveyor. This conveyor configuration can take advantage of the plasma's natural tendancy to rise in the cavity.

[0136] 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.