CROSS-REFERENCE OF RELATED APPLICATIONS 0001 Priority is claimed to U.S. Provisional Patent Application No. 60/378,693, filed May 8, 2002, No. 60/430,677, filed Dec. 4, 2002, and No. 60/435,278, filed Dec. 23, 2002, all of which are fully incorporated herein by reference.

FIELD OF THE INVENTION 0002 This invention relates to methods and apparatus for plasma-assisted joining two or more objects together and, particularly, to joining one or more portions of such objects in various types of cavities using a plasma catalyst.

BACKGROUND 0003 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 join parts. Igniting and sustaining plasmas, however, can be slow, expensive, and energy-consuming, especially when reduced pressures are needed to ignite the plasma. Therefore, use of conventional plasma-assisted joining can limit joining flexibility.

BRIEF SUMMARY OF A FEW ASPECTS OF THE INVENTION 0004 Methods and apparatus for plasma assisted joining of one or more parts are provided. In one embodiment consistent with this invention, a method is provided for joining at least two parts using a plasma induced by electromagnetic radiation having a wavelength of . The method can include placing at least first and second joining areas in proximity to one another in a cavity, forming a plasma in the cavity by subjecting a gas to radiation in the presence of a plasma catalyst, and sustaining the plasma at least until the first and second joining areas are joined by directing the radiation into the cavity. 0005 In another embodiment consistent with this invention, at least two parts to be joined (e.g., brazed) to each other may be placed in a cavity formed in a vessel. A plasma catalyst may be placed in or near the cavity. A brazing material (e.g., ring) may at least partially encircle one of the parts and abut the surface of the other part. In this embodiment, the brazing results from at least partial melting the brazing ring using a plasma induced by radiation. 0006 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. 0007 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 selectively plasma-joining in general, are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS 0008 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: 0009 FIG. 1 shows a schematic diagram of an illustrative plasma system consistent with this invention; 0010 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; 0011 FIG. 3 shows an illustrative plasma catalyst fiber with at least one component having a concentration gradient along its length consistent with this invention; 0012 FIG. 4 shows an illustrative plasma catalyst fiber with multiple components at a ratio that varies along its length consistent with this invention; 0013 FIG. 5 shows another illustrative plasma catalyst fiber that includes a core underlayer and a coating consistent with this invention; 0014 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; 0015 FIG. 7 shows a cross-sectional view of an illustrative embodiment of another plasma-joining system, including an elongated plasma catalyst that extends through an ignition port consistent with this invention; 0016 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; 0017 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; 0018 FIG. 10 shows a side cross-sectional view of an illustrative embodiment of a portion of a plasma-joining system for directing ionizing radiation into a plasma cavity consistent with this invention; 0019 FIG. 11 shows a cross-sectional view of yet another illustrative embodiment of a plasma-joining cavity and areas of objects to be joined that have been inserted into the cavity consistent with this invention; 0020 FIG. 12 shows a cross-sectional view of still another illustrative embodiment of a plasma-joining cavity, joining areas, and a brazing ring in the cavity consistent with this invention; and 0021 FIG. 13 shows a cross-sectional view of yet another illustrative embodiment of a plasma-joining cavity and joining areas where the joining areas can act as a partial seal consistent with this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS 0022 This invention may relate to plasma-assisted joining of at least two parts for a variety of applications, including, for example, brazing, welding, bonding, soldering, etc. In particular, various cavity shapes, plasma catalysts, and methods for joining are provided. 0023 The following commonly owned, concurrently filed U.S. patent applications are hereby incorporated by reference in their entireties: U.S. patent application Ser. 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. 1837.0016), No. 10/______ (Atty. Docket No. 1837.0017), No. 10/______ (Atty. Docket No. 1837.0018), No. 10/______ (Atty. Docket No. 1837.0020), No. 10/______ (Atty. Docket No. 1837.0021), No. 10/______ (Atty. Docket No. 1837.0023), No. 10/______ (Atty. Docket No. 1837.0024), No. 10/______ (Atty. Docket No. 1837.0025), 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). 0024 Illustrative Plasma-Assisted Joining System 0025 FIG. 1 shows illustrative plasma-assisted joining system 10 consistent with one aspect of this invention. In this embodiment, plasma-joining cavity 12 is formed in vessel 13 that is positioned inside radiation chamber (i.e., applicator) 14. In another embodiment (not shown), vessel 13 and radiation chamber 14 are the same, thereby eliminating the need for two separate components. 0026 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. It will be appreciated that 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 and 16 can be placed in cavity 12. As described more fully below, plasma-assisted joining consistent with this invention can involve generating a plasma in selected regions and preventing a plasma in others. 0027 As used herein, a plasma-joining cavity is any localized volume capable of igniting, modulating, and/or sustaining a plasma. Thus, 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 during joining. 0028 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, Pa. 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 joining processes, the material used to make the vessel may only need to withstand temperatures substantially below 3,000 degree Fahrenheit, such as 2,500 degrees or about 1,000 degree Fahrenheit, or even lower. 0029 In one successful experiment, a catalyzed joining 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 joining process being performed. Also, the cavity can be configured to prevent the plasma from rising/floating away from the primary processing region. 0030 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 radiation leakage (e.g., between about {fraction (1/16)} inch and about inch, such as about ). Also, if desired, a vacuum pump (not shown) can be connected to the chamber to remove any undesirable fumes that may be generated during joining. Although not shown in FIG. 1, cavity 12 and chamber 14 can have a separate gas port for removing gas. 0031 A radiation leak detector (not shown) was installed near source 26 and waveguide 30 and connected to a safety interlock system to automatically turn off the radiation (e.g., microwave) power supply if a leak above a predefined safety limit, such as one specified by the FCC and/or OSHA (e.g., 5 mW/cm2), was detected. 0032 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. 0033 Radiation energy can be supplied through optional 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 radiation power, for example, will be strongly absorbed by the plasma. 0034 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. 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. 0035 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 exhaust fumes. 0036 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. 0037 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 joining 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. 0038 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. 0039 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 Ser. No. 10/______ (Attorney Docket No. 1837.0033), which is hereby incorporated by reference in its entirety. 0040 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. 0041 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. 0042 Plasma Catalysts 0043 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. 0044 One method of forming a joining 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. 0045 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 substrate (e.g., at some predetermined orientation) while the plasma is ignited or sustained. 0046 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. 0047 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 (e.g., 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. 0048 In one experiment, a joining 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. 0049 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. 0050 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). 0051 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. 0052 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, S.C. Also, silicon-carbide fibers have been successfully used. 0053 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. 0054 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. 0055 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 Ser. 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. 0056 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 joining 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. 0057 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. 0058 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. 0059 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. 0060 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. 0061 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. 0062 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. 0063 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. 0064 FIG. 7, for example, shows radiation chamber 160 in which plasma-joining cavity 165 is placed. Plasma catalyst 170 is elongated and extends 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. 0065 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. 0066 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. 0067 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. 0068 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 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. 0069 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. 0070 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. 0071 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. 0072 Multi-mode Radiation Cavities 0073 A radiation waveguide, cavity, or chamber can be designed to support or facilitate propagation of at least one electromagnetic radiation mode. As used herein, the term mode refers to a particular pattern of any standing or propagating electromagnetic wave that satisfies Maxwell's equations and the applicable boundary conditions (e.g., of the cavity). In a waveguide or cavity, the mode can be any one of the various possible patterns of propagating or standing electromagnetic fields. Each mode is characterized by its frequency and polarization of the electric field and/or the magnetic field vectors. The electromagnetic field pattern of a mode depends on the frequency, refractive indices or dielectric constants, and waveguide or cavity geometry. 0074 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). 0075 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. 0076 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. 0077 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. 0078 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. 0079 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. 0080 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. 0081 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). 0082 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 Ser. 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. 0083 Mode-mixing 0084 For many joining 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 joining 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. 0085 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. 0086 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). 0087 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. 0088 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. 0089 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 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. 0090 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. 0091 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. 0092 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 (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. 0093 Multi-location Ignition 0094 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. 0095 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. 0096 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. 0097 Dual-Cavity Plasma Igniting/Sustaining 0098 A dual-cavity arrangement can be used to ignite and sustain a joining 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. 0099 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. 0100 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. 0101 Illustrative Plasma-assisted Joining 0102 As used herein, the term plasma-assisted joining, or simply joining, refers to any operation, or combination of operations, that involves the use of a plasma to put or bring together at least two parts and joining them so as to form a unit. Thus, plasma-assisted joining can include, for example, brazing, welding, bonding, soldering, fusing, etc. 0103 Plasma-assisted joining consistent with this invention may include placing at least a first joining area of a first part and a second joining area of a second part in proximity to one another in or near a joining cavity. The joining areas of the parts can be, for example, metal, non-metal, or a combination thereof. As described above, a gas may be directed into the cavity in an amount sufficient to at least form a plasma and then to either modulate or sustain the plasma. The plasma can be modulated or sustained in the cavity by continued absorption of radiation until the at least two joining areas are joined to one another or until a predetermined temperature indicative of a particular joining status is attained. 0104 As described more fully above, the distribution of plasma within a joining cavity can depend on the distribution of the applied radiation. For example, a strong plasma may only form at locations at which the electric field (e.g., the radiation density) is large. Because the strength of the electric field can be reduced, or even made to be zero near the inner surface of the cavity, plasma formation can be prevented at that surface. Therefore, the inner surface of a cavity can be contoured to selectively form a plasma only where parts should be heated to allow proper joining. Similarly, the electric field may not be sufficiently strong to form plasma at other locations, such as where the inner surface of the cavity (e.g., a metallic surface) is separated from the surface of the part by a distance less than about /4. In addition, controlling the plasma volume in the proximity of a particular surface area of the object can be used to control the thermal energy delivered to that surface area. 0105 FIG. 11, for example, shows a cross-sectional view of plasma processing cavity 360 formed in vessel 313 consistent with this invention. Joining areas 305 and 315 of parts 300 and 310, respectively, can be located in cavity 360. In this example, vessel 313 can be placed inside radiation chamber 303, which can be electrically conductive, and vessel 313, which can be substantially transparent to plasma-forming radiation. In this example, inner wall 365 of cavity 360 may be custom-configured to conform, in at least some areas, to the parts, especially where a small gap is desirable to suppress plasma heating. Alternatively, plasma formation can be prevented by shielding a portion of the object's surface with a substantially radiation opaque material, yet not necessarily by a contoured cavity surface. 0106 Inner surface 365 can be shaped to hold parts 300 and 310 such that joining areas 305 and 315 are spaced from inner surface 365 by distance 325 of at least about /4, wherein is the wavelength of the applied radiation. Other surface portions of parts 300 and 310 can be spaced from inner surface 365 by a distance of less than about /4, such as by distance 326 or 327. Also, distance 335, located between surface 330 and surface 340 of cavity 360 at aperture 320, can be less than about /4 to substantially confine the plasma within cavity 360 and substantially prevent any plasma from forming in the region where the gaps are less than about /4 there. 0107 An opening in the cavity wall 313 can be present and sufficient, however, to permit a gas to flow therethrough, if desired. Also, it will be appreciated that although one gas port is shown, aperture 320 can act as another port, or vessel 313 can have more than one gas port. It will also be appreciated that object 300 can act as a partial seal at aperture 320 and can be supported there. 0108 An electric bias may be applied to any of the joining areas during a plasma-assisted joining process consistent with the invention. Such a bias may facilitate heating of the joining areas by attracting the charged ions in the plasma to the joining areas, which may encourage uniform coverage of the plasma over the joining areas. The bias applied to the joining areas may be, for example, AC, DC, pulsed, continuous, or periodic. 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 ionized species. Further, the bias may be positive, negative, or alternate therebetween. It will be appreciated that the parts or joining areas may be placed on an electrically conductive plate and a potential bias may be applied to the plate during a plasma-assisted process consistent with the invention. 0109 FIG. 12 shows a side cross-sectional view of illustrative plasma joining cavity 460 formed in vessel 413 and parts 400 and 410 that are to be joined to each other. Brazing ring 420 can include any type of brazing material and may at least partially encircle part 410 and may abut an outside surface of block 400. This may be the case, for example, when part 410 is a metallic tube and part 400 is a metallic block, which may have recess 415. In this case, end 412 of tube 410 may be inserted into recess 415 in block 400. Alternatively, end 412 can just touch a portion of block 400 without any insertion. Brazing ring 420 can be formed, for example, from metal, metal powder, flux, and any combination thereof. As shown in FIG. 12, block 400, tube 410, and brazing ring 420 can be placed in cavity 460, with tube 410 extending through opening 450 in vessel 413. In this embodiment, joining can result from at least a partial melting and freezing of brazing ring 420. At least one plasma catalyst (as described above) may also be placed within or near cavity 460 to assist in the formation of the joining plasma, if desired. 0110 FIG. 13 shows a cross-sectional view of plasma joining cavity 560 formed in vessel 513, as well as parts 500 and 510 that are to be joined to each other. In this embodiment, non-joining areas 507 and 517, as well as joining areas 505 and 515, when placed close to vessel 513, can fully or partially seal aperture 520, although sealing is unnecessary. Joining areas 505 and 515 can be spaced from inner surface 565 of cavity 560 by a distance of at least about /4, wherein is the wavelength of the applied radiation used to form a plasma in the vicinity of joint areas 505 and 515, to permit plasma formation at those areas. It will be appreciated that any plasma catalyst can be used to ignite, modulate, and sustain a plasma in cavity 560 consistent with this invention. 0111 As described above, a temperature associated with the joining process may be monitored by at least one temperature sensor and used to control the plasma-assisted joining process. The temperature of any part, such as part 400 or part 410 of FIG. 12, or an inner surface of a cavity, such as surface 365 of FIG. 11, can therefore be monitored. If a color pyrometer is used, the color of radiation emitted from a surface can be monitored during the plasma-assisted joining process. When the brazing ring melts, it may produce some additional emission in the plasma that can generate a sharp spike in the output of the temperature sensor. When the temperature reaches a pre-determined level, for example, upon detecting a temperature spike, the spike can indicate that the brazing ring has melted and the process is complete. In this case, controller 44 can be programmed to turn off radiation source 26 immediately or after a preset delay time, if necessary, and close control valve 22. In another embodiment, control valve 22 may remain open for a period of time after the joining process for faster cooling of parts 11. Those skilled in the art will appreciate that the emission of additional radiation from the plasma upon melting of the brazing ring can be detected by any suitable optical sensor (e.g., a photodiode, a photomultiplier tube, a hollow cathode lamp etc.), with or without a suitable wavelength selective filter. 0112 In one embodiment consistent with this invention, a plasma can also be used to rapidly develop high-temperatures for the purpose of joining metals to non-metals, for example, metals to glass. Advantageously, the joining of two different materials can involve localized heating and melting near the joining areas only. Glass (or quartz) to metal seals, for example, can be used in making light bulbs (e.g., incandescent, quartz-halogen, sodium, mercury, and other types of light bulbs). 0113 For glass-metal joints capable of maintaining hermetic seals over a wide range of temperatures, the two materials can have nearly the same thermal expansion coefficients. Alternatively, we can have graded seals in which the joining material (e.g., brazing ring material) composition varies gradually at the joining area such that there is no abrupt transition in the thermal expansion characteristics at any point or region of the joint. In one embodiment, the joint can be formed while maintaining a gap between joining areas, with the powder metal/glass mixture placed in the gap with a suitable concentration variation. After the mixture is placed in the gap, the joint can be fused with plasma-assisted heating consistent with this invention. 0114 In another embodiment, the parts to be joined can be placed in individual cavities formed in one or more radiation-transmissive blocks, such as ceramic blocks. At least one plasma catalyst can be placed in each of the cavities to ignite and/or sustain the plasma there. In this example, radiation can be generated from one radiation source or several combined radiation sources. The radiation can be provided to the individual cavities 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. 0115 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.