FIELD OF THE INVENTION [002] This invention relates to methods and apparatus for plasma-assisted processing of multiple parts and, particularly, to processing multiple parts using various types of cavities and electromagnetic radiation supply configurations.

BACKGROUND [003] It is known that a plasma can be ignited by subjecting a gas to a sufficient amount of electromagnetic radiation. It is also known that radiation- induced plasmas may be used to process (e. g. , join, heat-treat, etc. ) parts within a plasma-processing cavity. When substantially simultaneous or sequential plasma- processing of multiple parts are desired, spatially uniform plasma ignition and processing can be difficult. Moreover, igniting and sustaining plasmas, however, can be slow, expensive, and energy-consuming, especially when pressures less than atmospheric pressure are used. Therefore, conventional plasma-assisted processing of multiple parts can limit manufacturing flexibility.

BRIEF SUMMARY OF A FEW ASPECTS OF THE INVENTION [004] Methods and apparatus for plasma-assisted processing of multiple parts are provided. In one embodiment consistent with this invention, a method can be provided for processing multiple parts using a plasma induced by electromagnetic radiation. The method can include placing a plurality of parts to be processed in a plurality of sub-regions of a cavity formed within a processing vessel, introducing a gas into the vessel, exposing the gas to electromagnetic radiation to form a plasma in the vessel, and modulating or sustaining the plasma in each of the sub-regions so that each of the plurality of parts is simultaneously exposed to plasma.

[005] In another embodiment consistent with this invention, an apparatus is provided for processing multiple parts using a radiation-induced plasma. The apparatus can include a vessel in which a cavity is formed, wherein the cavity has a plurality of sub-regions each for containing at least one part to be processed. At least one electromagnetic radiation source can be configured to direct the radiation into the vessel, such that the radiation and the gas cooperate to form a plasma in at least two of the sub-regions, the sub-regions being configured to enable each of the plurality of parts to be subjected to the plasma in a substantially simultaneous fashion. The apparatus can also include a conduit for supplying the gas to the vessel.

[006] In yet another embodiment consistent with this invention, each of the sub-regions can be defined by at least one internal cavity wall. In addition, at least one plasma catalyst can be located in one or more of the sub-regions.

[007] A plasma catalyst for initiating, modulating, and sustaining a plasma consistent with this invention 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.

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

Additional cavity shapes, and methods and apparatus for plasma-processing of multiple parts using such cavity shapes, are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS [009] 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: [010] FIG. 1 shows a schematic diagram of an illustrative plasma system consistent with this invention; [011] 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; [012] FIG. 3 shows an illustrative plasma catalyst fiber with at least one component having a concentration gradient along its length consistent with this invention; [013] FIG. 4 shows an illustrative plasma catalyst fiber with multiple components at a ratio that varies along its length consistent with this invention; [014] FIG. 5 shows another illustrative plasma catalyst fiber that includes a core underlayer and a coating consistent with this invention; [015] FIG. 6 shows a cross-sectional view of the plasma catalyst fiber of FIG. 5, taken from line 6--6 of FIG. 5, consistent with this invention; [016] FIG. 7 shows a cross-sectional view of an illustrative embodiment of another plasma-processing system, including an elongated plasma catalyst that extends through an ignition port consistent with this invention; [017] FIG. 8 shows an illustrative embodiment of an elongated plasma catalyst that can be used in the system of FIG. 7 consistent with this invention; [018] FIG. 9 shows another illustrative embodiment of an elongated plasma catalyst that can be used in the system of FIG. 7 consistent with this invention; [019] FIG. 10 shows a side cross-sectional view of an illustrative embodiment of a portion of a plasma-processing system for directing ionizing radiation into a plasma cavity consistent with this invention; [020] FIG. 11 shows a plan cross-sectional view of a plasma-processing cavity formed in a vessel, as well as multiple parts that are to be processed, consistent with this invention; [021] FIG. 12 shows a cross-sectional view of the plasma-processing cavity of FIG. 11, taken along line 12-12 of FIG. 11, consistent with this invention; [022] FIG. 13 shows a cross-sectional view of an illustrative cavity with multiple sub-regions formed in a vessel, including parts that are to be processed, consistent with this invention; [023] FIG. 14 shows a cross-sectional view of another illustrative cavity with multiple sub-regions using a single radiation source consistent with this invention; [024] FIG. 15 shows a cross-sectional view of yet another illustrative cavity with multiple sub-regions formed in a vessel using separate, devoted radiation sources and a voltage supply consistent with this invention; and [025] FIG. 16 shows a flow-chart illustrating an illustrative plasma-assisted multi-part processing method consistent with this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS [026] This invention may relate to methods and apparatus for initiating, modulating, and sustaining plasmas and for plasma-assisted processing of multiple parts for a variety of applications, including, for example, heat-treating, synthesizing and depositing carbides, nitrides, borides, oxides, and other materials ; doping, carburizing, nitriding, carbonitriding, sintering, joining, decrystallizing, ashing, sterilizing, cleaning, etc.

[027] 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. 1837.0017), 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. 1837.0025), 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).

[028] Illustrative Plasma System [029] FIG. 1 shows illustrative plasma-assisted processing system 10 consistent with one aspect of this invention. In this embodiment, plasma- processing cavity 12 can be formed in vessel 13 that is positioned inside radiation chamber (i. e. , applicator) 14 and multiple parts 11 can be located at least partially within cavity 12. In another embodiment (not shown), vessel 13 and radiation chamber 14 are the same, thereby eliminating the need for two separate components.

[030] Vessel 13 can include one or more radiation-transmissive insulating layers to improve its thermal insulation properties without significantly shielding cavity 12 from the radiation used to form a plasma. As explained more fully below, more than one cavity can be formed in vessel 13. In one embodiment, multiple cavities can be formed in vessel 13 and those cavities can be in fluid communication with one another. At least one portion of each of parts 11 can be placed in cavity 12. Alternatively, multiple vessels can be placed in chamber 14 and each of the vessels can have multiple cavities, if desired.

[031] As used herein, a plasma-processing cavity is any localized volume capable of igniting, modulating, and/or sustaining a plasma consistent with this invention. It will be appreciated that a cavity consistent with this invention need'not be completely closed, and may indeed be open. It is known that a plasma can be ignited by subjecting a gas to a sufficient amount of radiation. The plasma may then be modulated or sustained by direct absorption of the radiation, but may be assisted by a plasma catalyst as well.

[032] 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, for example, 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% lime, 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 operating temperature can be different for different processing processes, the material used to make the vessel may only need to withstand temperatures substantially below 3,000 degree Fahrenheit, such as 2,500 or about 1,000 degrees Fahrenheit, or even lower.

[033] In one experiment, a catalyzed processing 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 processing being performed. Also, the cavity can be configured to prevent the plasma from rising/floating away from the primary processing region.

[034] As shown in FIG. 1, for example, 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 or any other device capable of delivering a gas.

In one embodiment, the diameter of the tube is sufficiently small to prevent <BR> <BR> radiation leakage (e. g. , between about 1/16 inch and about % inch, such as about 1/8"). Also, if desired, a vacuum pump (not shown) can be connected to the chamber to remove any undesirable fumes that may be generated during processing. Although not shown in FIG. 1, cavity 12 and chamber 14 can have one or more separate gas ports for removing gas.

[035] 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.

[036] Radiation source 26, which may be powered by electrical power supply 28, can direct 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 with the assistance of a plasma catalyst. This plasma can be substantially sustained and confined to the cavity by coupling additional radiation with the catalyst.

[037] Radiation energy can be supplied through optional circulator 32 and <BR> <BR> 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 radiation power, for example, will be strongly absorbed by the plasma.

[038] As explained more fully below, the location of 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 throughout chamber 14. <BR> <BR> <P>Furthermore, as shown in FIG. 1, for example, 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.

[039] Sensor 42 can develop output signals as a function of the temperature or any other monitorable condition associated with work piece 11 within 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. Although not shown in FIG. 1, chamber 14 can have a separate gas port for removing gas.

[040] 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 (not shown) was used to measure forward and reflected powers. Also, optical pyrometers were used for remote sensing of the sample temperature.

[041] 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 processing 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.

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

Noise was reduced by using sliding averages of suitable number of data points.

Also, to improve speed and computational efficiency, the number of stored data points in the buffer array was limited by using shift registers and buffer sizing. 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.

[043] 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. For example, control software that can be used consistent with this invention is described 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.

[044] 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.

[045] 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.

[046] Plasma Catalysts [047] 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.

[048] One method of forming a processing 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.

[049] A passive plasma catalyst consistent with this invention can be, for example, 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 nano-tubes, doped and undoped, can be particularly effective for igniting plasmas consistent with this invention because of their exceptional electrical conductivity and elongated shape. The nano-tubes can have any convenient length and can be a powder fixed to a substrate. If fixed, the nano- tubes can be oriented randomly on the surface of the substrate or fixed to the <BR> <BR> substrate (e. g. , at some predetermined orientation) while the plasma is ignited or sustained.

[050] 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.

[051] 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. 2, radiation source 52 can supply radiation to radiation chamber 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 (eg., in a pile) and then distributed in cavity 60 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.

[052] In one experiment, a processing 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. Such instantaneous ignition can substantially eliminate potentially damaging radiation that could otherwise reflect back into the radiation source.

[053] 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 nano-composite, an organic- inorganic composite, and any combination thereof.

[054] 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 consistent with this invention. 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).

[055] 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.

[056] 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.

[057] 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.

[058] 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 electrically 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 nano-composite, 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.

[059] 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. (Atty. 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.

[060] 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 that may be desirable in a processing process. It will be appreciated by those of ordinary skill in the art that the component ratio of the plasma catalyst used to ignite and, later, to sustain the plasma, could be the same.

[061] 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.

[062] 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.

[063] 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.

[064] 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.

[065] 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. 5 and 6, 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.

[066] 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.

[067] 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 can prevent 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.

[068] FIG. 7, for example, shows radiation chamber 160 in which plasma- processing cavity 165 is placed. Plasma catalyst 170 can be elongated and extend through ignition port 175. As shown in FIG. 8, 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, but can extend somewhat into chamber 160).

This configuration can prevent an electrical connection (e. g. , sparking) between distal portion 180 and chamber 160.

[069] In another embodiment, shown in FIG. 9, 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 can prevent sparking and energy leakage.

[070] 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.

[071] 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.

[072] For example, FIG. 10 shows radiation source 200 directing radiation into plasma cavity 210, which can be positioned inside of chamber 205. Plasma cavity 210 may permit a gas to flow through it via ports 215 and 216, if desired.

Source 220 can direct 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.

[073] 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.

[074] 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.

[075] 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.

[076] Multi-mode Radiation Cavities [077] 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.

[078] 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, 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).

[079] 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.

[080] 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.

[081] 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.

[082] 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.

[083] 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.

[084] 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.

[085] 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).

[086] 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, commonly owned, concurrently filed U. S. Patent Application No. 10/, (Attorney Docket No. 1837.0020), which is 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.

[087] Mode-mixing [088] 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 substantially uniform plasma distribution can be difficult to achieve. As a result, consistent with one aspect of this invention, the radiation modes in a multi-mode processing 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.

[089] 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 location or orientation of the reflective surfaces. In one embodiment, a mode-mixer can be a cylinder that is rotable about an axis that is not the cylinder's longitudinal axis.

[090] 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 of FIG. 1 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).

[091] 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.

[092] 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.

[093] 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.

[094] 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.

[095] 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.

[096] 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 processing 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" <BR> <BR> (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.

[097] Multi-location ignition [098] 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.

[099] Also, in a multi-mode joining 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.

[0100] 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.

[0101] Dual-Cavity Plasma Ignitinq/Sustaining [0102] A dual-cavity arrangement can be used to ignite and sustain a processing plasma consistent with this invention. In one embodiment, a system includes at least an ignition cavity and a second cavity in fluid communication with the ignition cavity. To ignite a plasma, a gas in the 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.

[0103] In one embodiment of this invention, the ignition 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.

[0104] In one embodiment, the ignition cavity may be a substantially single mode cavity and the second cavity a multi-mode cavity. When the 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.

[0105] Illustrative Plasma-Assisted Processing of Multiple Parts [0106] As used herein, the term"plasma-assisted processing", or simply "plasma-processing,"refers to any operation, or combination of operations, that involves the use of a plasma. Thus, a plasma-process can include, for example, heat-treating, synthesizing and depositing carbides, nitrides, borides, oxides, and other materials, doping, carburizing, nitriding, carbonitriding, sintering, joining, decrystallizing, ashing, sterilizing, cleaning, etc.

[0107] In one embodiment consistent with this invention, substantially simultaneous or sequential plasma-processing of multiple parts can be performed by placing those parts in different sub-regions of the same cavity. Each sub-region may be defined by one or more internal cavity walls, although such walls are optional. One radiation source, or several radiation sources combined, can supply radiation to each of the sub-regions to form a processing plasma in each sub- region. At least one plasma catalyst (as described above) may also be placed within or near at least one, and optionally each, of the sub-regions to assist in the igniting, modulating, or sustaining of a plasma there, if desired. The shape of each of the sub-regions and the location of the parts within the sub-regions may be adapted to achieve any desirable plasma distribution for any particular plasma- assisted process. For example, a substantially uniform plasma or a strongly varying plasma distribution can be formed. As explained more fully above, a mode- mixer can be used to"stir"or"redistribute"modes to provide a more uniform time- averaged field distribution (and therefore plasma distribution) within the cavity.

[0108] In another embodiment, each sub-region may have at least one separate, devoted radiation source. The radiation can be provided to the individual <BR> <BR> sub-regions through an appropriately shaped waveguide (e. g. , a horn that connects the radiation source (s) to the individual cavities formed in a large ceramic block) or supplied directly without the use of a waveguide.

[0109] FIG. 11 shows a cross-sectional view of illustrative plasma- processing cavity 360 formed in vessel 313 consistent with this invention.

Vessel 313 can be radiation transmissive when located in a radiation chamber (such as chamber 14 of FIG. 1) or radiation-opaque, thereby eliminating the need to place vessel 313 in another chamber. Vessel 313 can include a plurality of apertures 340,342, and 344 through which parts 310,312, and 314 can extend, respectively. Parts 310,312, and 314 can be joined to parts 320,322, and 324, respectively, for example, as shown in FIG. 11. In this case, parts 320,322, and 324 are located entirely within cavity 360, but portions of these parts may also extend outside of cavity 360. Parts 310,312, and 314 can also be placed entirely within cavity 360, if desired.

[0110] It will be appreciated that the number of apertures and the number of parts extending therethrough can be more or less than three, and that these numbers need not be the same. That is, multiple parts can simultaneously extend through a single aperture, if desired. It will further be appreciated that parts 310, 312, and 314 need not be configured along a single wall of vessel 313 and may be configured in any convenient manner that permits the desired plasma-process to occur.

[0111] Parts 310,312, 314,320, 322, and 324 can be supported by one or more inner surfaces of cavity 360. For example, FIG. 12 shows a cross-sectional view of vessel 313 and parts 320,322, and 324, taken along line 12-12 of FIG. 11.

As shown in FIG. 12, parts 320,322, and 324 can be supported on inner surface 335 in recesses 336,337, and 338. Although these recesses are separate, they need not be and, in fact, the surface need not be recessed at all. In this case, the recesses permit parts 310,312, and 314 to contact parts 320,322, and 324, respectively, at predetermined locations. Alternatively, recesses can be replaced with raised portions (not shown) or simply be flush with the rest of surface 335, if desired. In yet another embodiment, one or more additional mounting structures can be used to support the parts in any convenient position and orientation. As described more fully in commonly owned, concurrently filed U. S. Patent Application 10/, (Attorney Docket No. 1837.0028), which is incorporated by reference in its entirety, these recesses or raised portions can also be used to selectively form a plasma, or prevent formation of a plasma, within cavity 360.

[0112] A plasma can be formed in cavity 360 by subjecting a gas to electromagnetic radiation. As shown schematically in FIG. 11, gas may be supplied and removed through one or more gas ports and, as shown schematically in FIG.

12, the radiation can be supplied by radiation source 326 through horn-shaped radiation waveguide 330 or a coaxial cable (not shown). Alternatively, waveguide 330 can be eliminated if source 326 is mounted directly to vessel 313. In this case, radiation source 326 can supply radiation, and is used to modulate or sustain a plasma for processing all of parts 310,312, 314,320, 322, and 324. Alternatively, multiple radiation sources (not shown) can be used to direct radiation into cavity 360. As explained more fully below, each of the sources can also be used to modulate or sustain a plasma for one or more respective parts.

[0113] As described more fully above, a plasma catalyst can be used to ignite, modulate, and/or sustain a plasma within cavity 360 at a gas pressure that is less than, equal to, or greater than atmospheric pressure. The plasma catalyst can be active or passive.

[0114] FIG. 13 shows another illustrative embodiment of a plasma- processing cavity with multiple internal walls 430 and 432 forming separate sub- regions 460,462, and 464 for processing multiple parts 410,412, 414,420, 422, and 424. For illustrative simplicity, FIG. 13 does not show a radiation source.

However, one or more radiation sources can be used to supply radiation to each of the sub-regions. Also, although separate gas ports are shown for each sub-region, it will be appreciated that one or more common gas ports can be used to supply a gas to all of the sub-regions and that each of those sub-regions need not be completely separated--that is, the sub-regions may be in fluid communication with each other (see, for example, FIG. 14).

[0115] Vessel 413 can be radiation transmissive when located in a radiation chamber (such as chamber 14 of FIG. 1) or radiation-opaque, thereby eliminating the need to place vessel 413 in another chamber. Vessel 413 can include a plurality of apertures 440,442, and 444 through which parts 410,412, and 414 can extend, respectively. Parts 410,412, and 414 can be joined to parts 420,422, and 424, for example, as shown in FIG. 13. In this case, parts 420,422, and 424 can be located entirely within sub-regions 460,462, and 464, respectively, but portions of these parts may also extend outside of these sub-regions. Parts 410,412, and 414 can also be placed entirely within sub-regions 460,462, and 464, if desired.

[0116] It will be appreciated that the number of apertures (or the number of sub-regions) and the number of parts extending therethrough can be more or less than three and that these numbers need not be the same. That is, multiple parts can be located within a single sub-region and simultaneously extend through a single aperture, if desired. It will further be appreciated that parts 410,412, 414, 420,422, and 424 need not be configured along a single wall of vessel 413 and may be configured in any convenient manner that permits the desired plasma- process to occur. Also, parts 410,412, 414,420, 422, and 424 can be supported by one or more inner surfaces of sub-regions 460,462, and 464.

[0117] A plasma can be formed in sub-regions 460,462, and 464 by subjecting a gas to electromagnetic radiation. As shown schematically in FIG. 13, gas may be supplied and removed through one or more gas ports and, as shown schematically in FIG. 14, the radiation can be supplied by radiation source 426 through radiation waveguide 430 with multiple branches 470,472, and 474. It will be appreciated that source 426 can selectively or sequentially supply radiation to respective sub-regions 460,462, and 464.

[0118] For example, one or more control valves (not shown) may be installed in respective channels 470,472, and 474. These control valves can partially or entirely block the radiation through respective channels 470,472, and 474, if desired, in any type of timing sequence. It will be appreciated that these valves may be powered by a power supply (such as power supply 28 of FIG. 1). It will also be appreciated that a controller (such as controller44 of FIG. 1) may control the operation of the control valves. It will further be appreciated that by controlling the operation of the control valves, selective, simultaneous, or sequential plasma processing within respective sub-regions may be achieved. Alternatively, a single distributor or multiplexer can be used before branches 470,472, and 474 to distribute radiation into the various cavities as needed.

[0119] Although FIG. 14 schematically shows a single radiation source supplying radiation to each of the sub-regions through separate channels 470,472, and 474, it will be appreciated that the source can also be used to supply radiation to all of the sub-regions without separate channels, such as when vessel 413 is radiation-transmissive and located in a radiation chamber. Alternatively, multiple radiation sources (not shown) can be used to direct radiation into individual sub- regions. Each of the sources can also be used to modulate or sustain a plasma for one or more of the respective sub-regions.

[0120] As described more fully above, a plasma catalyst can be used to ignite, modulate, and/or sustain a plasma within sub-regions at a gas pressure that is less than, equal to, or greater than atmospheric pressure. The plasma catalyst can be active or passive.

[0121] FIG. 15 shows a cross-sectional view of yet another illustrative embodiment of a plasma-processing cavity with multiple internal walls forming separate sub-regions 560,562, and 564 for processing multiple parts 510,512, and 514. In this case, multiple radiation sources 526,527, and 528 can be used to direct radiation into sub-regions 560,562, and 564 through horn-shaped waveguides 530,532, and 534, respectively. Although FIG. 15 schematically shows each of the multiple radiation sources supplying radiation to one respective sub-region through separate waveguides 530,532, and 534, these waveguides can be eliminated if sources 526,527, and 528 are mounted directly to vessel 513.

[0122] Although FIG. 15 schematically shows each of the multiple radiation sources supplying radiation to one sub-region, it will be appreciated that each of the sources can be used to modulate or sustain a plasma for more than one sub- regions (See, e. g., FIG. 14). Alternatively, more than one radiation sources can be combined, and can supply radiation to one cavity or sub-region. It will also be appreciated that radiation can be directed into respective sub-regions 560,562, and 564 substantially simultaneously. It will further be appreciated that radiation can be directed into respective sub-regions 560,562, and 564 sequentially, if desired, using one or more control signals (not shown).

[0123] An electric bias may be applied to the any of the parts during a plasma-assisted processing consistent with the invention. For example, FIG. 15 shows how voltage supply 580 may apply an electric bias to parts 510,512, and 514. Such a bias may facilitate heating of the parts as well as promote deposition by accelerating charged particles in the plasma toward the parts, which may encourage uniform processing. The bias applied to the parts may be, for example, AC, DC, pulsed, continuous, or periodic or preprogrammed. The magnitude of the bias may be selected according to the particular application. For example, the magnitude of the voltage may range from about 0.1 volts to about 100 volts, or even several hundred or thousands of volts, depending on the desired rate of attraction of the charged particles. Further, the bias may be positive or negative, or alternate therebetween. It will be appreciated that the parts may be placed on an electrically conductive plate (not shown) and a potential bias may be applied to the plate during a plasma-assisted process consistent with the invention.

[0124] FIG. 16 shows a flow chart of an illustrative plasma-assisted multi- part process method consistent with this invention. The process can include, for example, heat-treating, synthesizing and depositing carbides, nitrides, borides, oxides, and other materials, doping, carburizing, nitriding, carbonitriding, sintering, joining, decrystallizing, ashing, sterilizing, cleaning, etc. It will be appreciated that a plurality of different types of heat treatments can be conducted substantially simultaneously.

[0125] In step 600, multiple parts can be placed in or near a plurality of sub- regions of a cavity formed within a processing vessel. As explained more fully above, an electric bias may be applied to the any of the parts during a plasma- assisted processing consistent with the invention. Each sub-region may be defined by one or more internal cavity walls, although such walls are optional. The walls may be layers of a material that are substantially radiation transmissive or opaque.

At least one plasma catalyst (as described above) may also be placed within (or near) at least one, and optionally each, of the sub-regions to assist in the igniting, modulating, or sustaining of a plasma there, if desired.

[0126] In step 610, a gas can be introduced into the vessel, and therefore into the sub-regions, through one or more gas inlets. The gas can be introduced through at least one gas inlet. Each sub-region may have at least one separate gas inlet and each gas inlet may be connected to a gas flow controller, such that each of the sub-regions has an independently controllable amount of gas flowing therethrough. Alternatively, the gas may flow into a processing cavity through the same aperture in which a part is located.

[0127] In step 620, the gas can be exposed to electromagnetic radiation to form a plasma in the vessel and therefore the sub-regions. One radiation source, or several radiation sources combined, can supply radiation to each of the sub- regions to form a processing plasma in each sub-region. Alternatively, each sub- region may have at least one separate, devoted radiation source. As explained more fully above, a mode-mixer can be used to"stir"or"redistribute"modes to provide a more uniform time-averaged radiation distribution (and therefore plasma distribution) within the cavity and any sub-regions. Plasma formation can be prevented by shielding a portion of the part's surface with a substantially radiation opaque material, if desired.

[0128] In step 630, the plasma can be sustained in each of the sub-regions so that each of the plurality of parts can be exposed to a plasma. The plasma can be sustained in the cavity or sub-regions by continued absorption of radiation until the plasma-assisted process is over or until a predetermined temperature indicative of a particular process status is attained.

[0129] 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.