WO2003096766A1

WO2003096766A1 - Plasma control using phase and/or frequency of multiple radiation sources

Info

Publication number: WO2003096766A1
Application number: PCT/US2003/014132
Authority: WO
Inventors: Devendra Kumar; Satyendra Kumar
Original assignee: Dana Corporation
Priority date: 2002-05-08
Filing date: 2003-05-07
Publication date: 2003-11-20
Also published as: AU2003232065A1

Abstract

Plasma control methods and apparatus are provided that use at least one phase or frequency controlled radiation source. A plasma apparatus, for example, can include a first radiation source (26), controlled by a first signal, configured to direct radiation into a radiation cavity (12) to form a plasma. The plasma apparatuscan also include a phase shifter (308) for shifting a phase of the signal to generate a phase-shifted signal. A second radiation source (27) can be controlled by the phase shifted signal, and configured to direct additional radiation into the cavity (12). The frequency of one or more of the radiation sources can also be varied or shifted. Various types of plasma catalysts are also provided.

Description

PLASMA CONTROL USING PHASE AND/OR FREQUENCY OF MULTIPLE RADIATION SOURCES

CROSS-REFERENCE OF RELATED APPLICATIONS

[001] Priority is claimed to U.S. Provisional Patent Application No. 60/378,693, filed May 8, 2002, No. 60/430,677, filed December 4, 2002, and No. 60/435,278, filed December 23, 2002, all of which are fully incorporated herein by reference.

FIELD OF THE INVENTION

[002] This invention relates to methods and apparatus for plasma- assisted processing, and in particular to using phase and frequency controlled radiation sources.

BACKGROUND OF THE INVENTION

[003] It is known that a single microwave radiation source can be used to generate a plasma by subjecting a gas to a sufficient amount of microwave radiation. A single microwave radiation source, however, may create a non- uniform distribution of energy. Moreover, the distribution of energy located by a single microwave radiation source may not easily be controlled, for example, to selectively ignite, modulate, and sustain a plasma at one or more desired locations.

[004] It is also known that a plasma can be ignited by subjecting a gas to a sufficient amount of microwave radiation at reduced pressures. However, vacuum equipment, which is required to lower the gas pressure, can be expensive, as well as slow and energy-consuming. Moreover, the use of such equipment can limit manufacturing flexibility.

BRIEF SUMMARY OF A FEW ASPECTS OF THE INVENTION

[005] Consistent with the present invention, apparatus and methods for controlling a plasma using multiple radiation sources are provided. In one embodiment, a plasma apparatus may include a radiation cavity in which a plasma can be formed from a gas. The radiation apparatus can also include a first radiation source, controlled by a signal, for directing radiation into the radiation cavity to facilitate formation of plasma in the radiation cavity. The plasma apparatus can also include a phase shifter for shifting a phase of the signal to generate a phase-shifted signal. The plasma apparatus can also include a second radiation source, controlled by the phase-shifted signal, for directing additional radiation into the cavity.

[006] In another embodiment consistent with this invention, a plasma apparatus is provided that can include a radiation cavity in which a plasma can be formed from a gas. The plasma apparatus can include a first radiation source for directing radiation into the radiation cavity to facilitate formation of plasma in the radiation cavity, where the first radiation source is controlled by a first signal that has a first frequency. The plasma apparatus can also include a second radiation source for directing radiation into the same cavity, where the second radiation source is controlled by a second signal that has a second frequency. The first and second frequencies can be the same or different.

[007] In yet another embodiment consistent with this invention, a plasma apparatus is provided that can include a radiation cavity in which a plasma can be formed from a gas. The plasma apparatus can include a phase shifter for receiving at least one of a first signal and a second signal and shifting one of the signals with respect to the other to create a phase shift. The plasma apparatus can also include first and second radiation sources, which are controlled by the first and second signals, respectively, for directing radiation into the radiation cavity to facilitate formation of plasma there.

[008] In still another embodiment consistent with this invention, a method is provided for controlling a multiple radiation source plasma apparatus. The method can include generating a control signal and splitting the control signal into at least a first signal and a second signal. The method can further include controlling a first radiation source for directing radiation into the radiation cavity to facilitate formation of a plasma there with the first signal. The method can further include controlling a second radiation source for directing its radiation output into the radiation cavity, with the second signal. [009] Additional plasma catalysts, and methods and apparatus for igniting, modulating, and sustaining a plasma consistent with this invention are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[010] Further aspects of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:

[011] FIG. 1 shows a schematic diagram of an illustrative apparatus that includes multiple radiation sources consistent with this invention;

[012] 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;

[013] FIG. 3 shows an illustrative plasma catalyst fiber with at least one component having a concentration gradient along its length consistent with this invention;

[014] FIG. 4 shows an illustrative plasma catalyst fiber with multiple components at a ratio that varies along its length consistent with this invention;

[015] FIG. 5A shows another illustrative plasma catalyst fiber that includes a core underlayer and a coating consistent with this invention;

[016] FIG. 5B shows a cross-sectional view of the plasma catalyst fiber of FIG. 5A, taken from line 5B-5B of FIG. 5A, consistent with this invention;

[017] FIG. 6 shows an illustrative embodiment of another portion of a plasma system including an elongated plasma catalyst that extends through ignition port consistent with this invention;

[018] FIG. 7 shows an illustrative embodiment of an elongated plasma catalyst that can be used in the system of FIG. 6 consistent with this invention;

[019] FIG. 8 shows another illustrative embodiment of an elongated plasma catalyst that can be used in the system of FIG. 6 consistent with this invention; [020] FIG. 9 shows an illustrative embodiment of a portion of a plasma system for directing ionizing radiation into a radiation chamber consistent with this invention;

[021] FIG. 10A shows an illustrative embodiment of an apparatus that includes two radiation sources and a phase shifter consistent with this invention;

[022] FIG. 10B shows an illustrative embodiment of an apparatus that includes two radiation sources and two signal sources consistent with this invention;

[023] FIG. 10C shows an illustrative embodiment of an apparatus that includes two radiation sources, a directional coupler, and a phase shifter consistent with this invention;

[024] FIG. 11 A shows a flow chart for an illustrative method consistent with this invention;

[025] FIG. 11 B shows a flow chart for another illustrative method consistent with this invention; and

[026] FIG. 11 C shows a flow chart for yet another illustrative method consistent with this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[027] Consistent with the present invention, plasma apparatus and methods that use multiple radiation sources are provided. As described more fully below, one or more of the radiation sources can be controlled with a signal having a variable phase. Each of the signals can be generated independently or by splitting and phase-shifting a signal. As also described more fully below, the frequency of one or radiation sources can be varied. In either case any desirable radiation interference pattern can be formed.

[028] Thus, consistent with this invention, it is possible to vary the locations of "hot spots" within a plasma processing cavity or region by varying either the phase or the frequency of at least one of the radiation sources. For example, by phase-shifting one control signal with respect to another, it is possible to vary the radiation interference pattern within the cavity, and thereby vary the radiation maxima and minima. When a plasma is formed, the maxima correspond to what are commonly referred to as "hot spots." Thus, it will be appreciated that by varying the phase or frequency of one radiation source with respect to another, it is possible to control the location and intensity of the hot spots. Such control can be useful when a user desires a substantially uniform time-averaged distribution of plasma or one or more localized plasma processing regions.

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

[030] Thus, this invention can be used for controllably generating heat and for plasma-assisted processing to lower energy costs and increase heat- treatment efficiency and plasma-assisted manufacturing flexibility. A plasma catalyst for initiating, modulating, and sustaining a plasma is also provided. The catalyst can be passive or active. A passive plasma catalyst can include any object capable of inducing a plasma by deforming a local electric field (e.g., an electromagnetic field) consistent with this invention without necessarily adding additional energy through the catalyst, such as by applying a voltage to create a spark. An active plasma catalyst, on the other hand, may be any particle or high energy wave packet capable of transferring a sufficient amount of energy to a gaseous atom or ion to remove at least one electron from the gaseous atom or molecule, in the presence of electromagnetic radiation.

[031] The following commonly owned, concurrently filed U.S. patent applications are hereby incorporated by reference in their entireties: U.S. Patent

Application No. 10/ , (Atty. Docket No. 1837.0008), No. 10/ , (Atty.

Docket No. 1837.0009), No. 10/ , (Atty. Docket No. 1837.0011),

No. 10/ , (Atty. Docket No. 1837.0012), No. 10/ , (Atty. Docket

No. 1837.0013), No. 10/ , (Atty. Docket No. 1837.0015), No. 10/ ,

(Atty. Docket No. 1837.0016), No. 10/ , (Atty. Docket No. 1837.0017),

No. 10/ , (Atty. Docket No. 1837.0018), No. 10/ , (Atty. Docket No. 1837.0020), No. 10/ , (Atty. Docket No. 1837.0021), No. 10/_

(Atty. Docket No. 1837.0023), No. 10/ , (Atty. Docket No. 1837.0024),

No. 10/ , (Atty. Docket No. 1837.0025), 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).

[032] Illustrative Plasma System

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

[034] In one embodiment, cavity 12 is formed in a vessel made of ceramic. Due to the extremely high temperatures that can be achieved with plasmas consistent with this invention, a ceramic capable of operating at about 3,000 degrees Fahrenheit can be used. The ceramic material can include, by weight, 29.8% silica, 68.2% alumina, 0.4% ferric oxide, 1% titania, 0.1% 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 types of plasma-processing, the material used to make the vessel may only need to withstand temperatures substantially below 3,000 degrees Fahrenheit, such as about 2,500 degrees, or about 1 ,000 degrees Fahrenheit, or even lower.

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

[036] Cavity 12 can be connected to one or more gas sources 24 (e.g., a source of argon, nitrogen, hydrogen, xenon, krypton) by line 20 and control valve 22, which may be powered by power supply 28 or any other supply. Line 20 may be tubing (e.g., between about 1/16 inch and about 14 inch, such as about 1/8"), but can be any channel or device capable of supplying a gas to cavity 12. Also, if desired, a vacuum pump (not shown) can be connected to chamber 14 to remove any undesirable fumes that may be generated during plasma processing. In one embodiment, gas can flow in and/or out of cavity 12 through one or more gaps in a multi-part vessel. Thus, gas ports consistent with this invention need not be distinct holes and can take on other forms as well, such as many small distributed holes.

[037] A radiation leak detector (not shown) can be installed near source 26 and waveguide 30 and connected to a safety interlock system to automatically turn 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.

[038] In one embodiment, the radiation apparatus may include radiation source 26 for directing radiation into the cavity. The radiation apparatus may further include radiation source 27 for directing additional radiation into the cavity. Although FIG. 1 depicts two radiation sources, it will be appreciated that a radiation apparatus consistent with the invention can operate with two or more sources. Radiation sources 26 and 27, which may be powered by electrical power supply 28, can direct radiation into chamber 14 through one or more waveguides 30 and 37. It will be appreciated by those of ordinary skill in the art that sources 26 and 27 can be connected directly to chamber 14 or cavity 12, thereby eliminating waveguides 30 and 37. The radiation energy entering cavity 12 is used to ignite a plasma within the cavity. This plasma can be substantially sustained and confined to the cavity by coupling additional radiation with the catalyst. [039] Each of radiation sources 26 and 27 may be a magnetron, a klystron, a gyrotron, a traveling-wave tube amplifier, or any other device capable of generating radiation. Radiation having any frequency less than about 333 GHz can be used consistent with this invention. For example, frequencies, such as power line frequencies (about 50 Hz to about 60 Hz), can be used, although the pressure of the gas from which the plasma is formed may be lowered to assist with plasma ignition. Also, any radio frequency or microwave frequency can be used consistent with this invention, including frequencies greater than about 100 kHz. In most cases, the gas pressure for such relatively high frequencies need not be lowered to ignite, modulate, or sustain a plasma, thereby enabling many plasma-processes to occur over a broad range of pressures, including atmospheric pressure and above.

[040] For example, the invention may be practiced by employing microwave sources at both 915 MHz and 2.45 GHz provided by Communications and Power Industries (CPI), although radiation having any frequency less than about 333 GHz can be used. A 3-stub tuner may allow impedance matching for maximum power transfer and a dual directional coupler (not shown) may be used to measure forward and reflected powers.

[041] Radiation energy can be supplied by radiation source 26 through circulator 32 and tuner 34 (e.g., 3-stub tuner). Tuner 34 can be used to minimize the reflected power as a function of changing ignition or processing conditions, especially before the plasma has formed because microwave power, for example, will be strongly absorbed by the plasma. Similarly, radiation energy from radiation source 27 may be supplied through circulator 31 and tuner 33, although the use of circulators and tuners are optional.

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

[043] Sensor 42 can develop output signals as a function of the temperature or any other monitorable condition associated with a work piece (not shown) within cavity 12 and provide the signals to controller 44. Dual temperature sensing and heating, as well as automated cooling 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 outputs connected to sources 26 and 27 as described above and another output connected to valve 22 to control gas flow into cavity 12.

[044] The equipment may be computer controlled using LabView 6i software, which may provide real-time temperature monitoring and microwave power control. Noise may be reduced by using shift registers to generate sliding averages of suitable number of data points. Also, the number of stored data points in the array may be limited to improve speed and computational efficiency. The pyrometer may measure the temperature of a sensitive area of about 1 cm², which may be used to calculate an average temperature. The pyrometer may sense radiant intensities at two wavelengths and fit those intensities using Planck's law to determine the temperature. It will be appreciated, however, that other devices and methods for monitoring and controlling temperature are also available and can be used consistent with this invention. Control software that can be used consistent with this invention is described, for example, in commonly owned, concurrently filed

U.S. Patent Application No. 10/ , (Attorney Docket No. 1837.0033), which is hereby incorporated by reference in its entirety.

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

[046] The exemplary radiation apparatus may also include a closed-loop deionized water cooling system (not shown) with an external heat exchanger cooled by tap water. During operation, the deionized water may first cool the magnetron, then the load-dump in the circulator (used to protect the magnetron), and finally the microwave chamber through water channels welded on the outer surface of the chamber. [047] Plasma Catalysts

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

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

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

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

[052] In one embodiment, a powder catalyst can be carried into the cavity and at least temporarily suspended with a carrier gas. The carrier gas can be the same or different from the gas that forms the plasma. Also, the powder can be added to the gas prior to being introduced to the cavity. For example, as shown in FIG. 2, radiation sources 52 and 54 can supply radiation to radiation cavity 55, in which plasma cavity 60 is placed. Powder source 65 can provide catalytic powder 70 into gas stream 75. In an alternative embodiment, powder 70 can be first added to cavity 60 in bulk (e.g., in a pile) and then distributed in the cavity in any number of ways, including flowing a gas through or over the bulk powder. In addition, the powder can be added to the gas for igniting, modulating, or sustaining a plasma by moving, conveying, drizzling, sprinkling, blowing, or otherwise, feeding the powder into or within the cavity.

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

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

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

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

[057] Thus, a passive plasma catalyst can include at least one portion of material that is relatively thin compared to its length. A bundle of catalysts (e.g., fibers) may also be used and can include, for example, a section of graphite tape. In one experiment, a section of tape having approximately thirty thousand strands of graphite fiber, each about 2-3 microns in diameter, was successfully used. The number of fibers in and the length of a bundle are not critical to igniting, modulating, or sustaining the plasma. For example, satisfactory results have been obtained using a section of graphite tape about one-quarter inch long. One type of carbon fiber that has been successfully used consistent with this invention is sold under the trademark Magnamite®, Model No. AS4C-GP3K, by the Hexcel Corporation, of Anderson, South Carolina. Also, silicon-carbide fibers have been successfully used.

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

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

[060] In addition to one or more electrically conductive materials, a passive plasma catalyst consistent with this invention can include one or more additives (which need not be electrically conductive). As used herein, the additive can include any material that a user wishes to add to the plasma. For example, in doping semiconductors and other materials, one or more dopants can be added to the plasma through the catalyst. See, e.g., commonly owned, concurrently filed

U.S. Patent Application No. 10/ , (Attorney Docket No. 1837.0026), which is hereby incorporated by reference in its entirety. The catalyst can include the dopant itself, or it can include a precursor material that, upon decomposition, can form the dopant. Thus, the plasma catalyst can include one or more additives and one or more electrically conductive materials in any desirable ratio, depending on the ultimate desired composition of the plasma and the process using the plasma.

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

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

[063] A passive plasma catalyst consistent with this invention can be homogeneous, inhomogeneous, or graded. Also, the plasma catalyst component ratio can vary continuously or discontinuously throughout the catalyst. For example, in FIG. 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.

[064] 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.

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

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

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

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

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

[070] In another embodiment, shown in FIG. 8, the catalyst can be formed from a plurality of electrically conductive segments 190 separated by and mechanically connected to a plurality of electrically non-conductive segments 195. In this embodiment, the catalyst can extend through the ignition port between a point inside the cavity and another point outside the cavity, but the electrically discontinuous profile significantly prevents sparking and energy leakage.

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

[072] An active plasma catalyst consistent with this invention can be any particle or high energy wave packet capable of transferring a sufficient amount of energy to a gaseous atom or molecule to remove at least one electron from the gaseous atom or molecule in the presence of electromagnetic radiation. Depending on the source, the ionizing particles can be directed into the cavity in the form of a focused or collimated beam, or they may be sprayed, spewed, sputtered, or otherwise introduced.

[073] For example, FIG. 9 shows radiation source 200 and radiation source 202 directing radiation into radiation chamber 205. Plasma cavity 210 can be positioned inside of chamber 205 and may permit a gas to flow therethrough via its gas ports. Source 220 directs ionizing particles 225 into cavity 210. Source 220 can be protected, for example, by a metallic screen which allows the ionizing particles to pass through but shields source 220 from radiation. If necessary, source 220 can be water-cooled.

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

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

[076] In addition to electrons, free energetic protons can also be used to catalyze a plasma. In one embodiment, a free proton can be generated by ionizing hydrogen and, optionally, accelerated with an electric field. [077] Multi-mode Radiation Cavities

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

[079] A transverse electric (TE) mode is one whose electric field vector is normal to the direction of propagation. Similarly, a transverse magnetic (TM) mode is one whose magnetic field vector is normal to the direction of propagation. A transverse electric and magnetic (TEM) mode is one whose electric and magnetic field vectors are both normal to the direction of propagation. A hollow metallic waveguide does not typically support a normal TEM mode of radiation propagation. Even though radiation appears to travel along the length of a waveguide, it may do so only by reflecting off the inner walls of the waveguide at some angle. Hence, depending upon the propagation mode, the radiation (e.g., microwave) may have either some electric field component or some magnetic field component along the axis of the waveguide (often referred to as the z-axis).

[080] The actual field distribution inside a cavity or waveguide is a superposition of the modes therein. Each of the modes can be identified with one or more subscripts (e.g., TEι₀ ("tee ee one zero"). The subscripts normally specify how many "half waves" at the guide wavelength are contained in the x and y directions. It will be appreciated by those skilled in the art that the guide wavelength can be different from the free space wavelength because radiation propagates inside the waveguide by reflecting at some angle from the inner walls of the waveguide. In some cases, a third subscript can be added to define the number of half waves in the standing wave pattern along the z-axis.

[081] For a given radiation frequency, the size of the waveguide can be selected to be small enough so that it can support a single propagation mode. In such a case, the system is called a single-mode system (i.e., a single-mode applicator). The TEι₀ mode is usually dominant in a rectangular single-mode waveguide.

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

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

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

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

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

[087] Whether or not a single or multi-mode cavity is used consistent with this invention, it will be appreciated by those of ordinary skill in the art that the cavity in which the plasma is formed can be completely closed or partially open. For example, in certain applications, such as in plasma-assisted furnaces, the cavity could be entirely closed. See, for example, 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.

[088] Mode-mixing

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

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

[091] Each mode of a multi-mode cavity may have at least one maximum electric field vector, but each of these vectors could occur periodically across the inner dimension of the cavity. Normally, these maxima are fixed, assuming that the frequency of the radiation does not change. However, by moving a mode-mixer such that it interacts with the radiation, it is possible to move the positions of the maxima. For example, mode-mixer 38 can be used to optimize the field distribution within cavity 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).

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

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

[094] Mode-mixing can also be achieved by launching radiation in the radiation chamber through a flexible waveguide. In one embodiment, the waveguide can be mounted inside the chamber. In another embodiment, the waveguide can extend into the chamber. The position of the end portion of the 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.

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

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

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

[098] Multi-location Ignition

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

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

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

[0102] Dual-Cavity Plasma Igniting/Sustaining

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

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

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

[0106] Illustrative Phase and Freguency Control of Radiation Sources

[0107] As shown in FIG. 10A, plasma apparatus 301 consistent with this invention may include radiation source 304 for directing radiation into a cavity (for example, cavity 12 of FIG. 1). First radiation source 304 may be connected to chamber 14 (via waveguide 310, for example), although the chamber is not shown in FIG. 10A. First radiation source 304 may be controlled by signal 303, such as a signal generated by signal source 302. A signal source may be any device capable of generating a waveform, such as a sinusoidal, square, pulsed, or other type of waveform. In one embodiment, signal 303 may be amplified by an amplifier (not shown), such as a traveling wave tube amplifier.

[0108] Plasma apparatus 301 may further include phase shifter 308 for shifting a phase of signal 303 to generate phase-shifted signal 305. Phase shifter 308 may be any device capable of delaying the phase of one or more signals in time. In one embodiment, phase shifter 308 is variable.

[0109] Plasma apparatus 301 may further include radiation source 306 for directing radiation into the same cavity as radiation source 304. Radiation source 306 may be controlled by phase-shifted signal 305 generated by phase shifter 308, for example. Phase-shifted signal 305 may also be amplified using an amplifier (not shown), such as a traveling wave tube amplifier. Radiation sources 304 and 306 may be, for example, a magnetron, a klystron, or a gyrotron.

[0110] In another embodiment, a phase-shifter (e.g., phase-shifter 308) may be configured to receive two or more control signals and shift one of the signals with respect to the others. For example, a phase-shifter could be used to delay one or more of the received signals by any desirable amount to create any desirable amount of phase-shift between them. In one embodiment, the phase shifter may be a variable phase shifter, such that the amount of phase-shift can be selected.

[0111] FIG. 10B shows another illustrative plasma apparatus 321 consistent with this invention. Apparatus 321 can include radiation source 324 for directing radiation into a cavity or any other region to facilitate plasma formation. Apparatus 321 can further include radiation source 328 for directing additional radiation into the same cavity or region. Radiation sources 324 and 328 may be coupled to the cavity or region by waveguides 330 and 332, respectively, or they may be coupled directly to the cavity, eliminating the need for waveguides entirely.

[0112] Apparatus 321 can include signal sources 322 and 326, for generating control signals 323 and 327, respectively. Control signals 323 and 327 can have the same or different frequencies, and, as described above, can be in any waveform, such as a sinusoidal, square, pulsed, or other type of waveform. As previously mentioned above, signals 323 and 327 may also be amplified by an amplifier (not shown).

[0113] Apparatus 321 can further include controller 325 for varying at least one of the radiation frequencies supplied by radiation sources 324 and 328. Controller 325 can control any of signal sources 322 and 326 and radiation sources 324 and 328. In this case, varying one of the frequencies with respect to another can vary the radiation interference pattern within a plasma cavity, and thereby vary the locations and intensities of the radiation maxima and minima (i.e., "hot spots" and "cold spots," respectively). Thus, it will be appreciated that by varying the frequency of at least one of the radiation sources, the locations and intensities of the hot spots can be controlled.

[0114] FIG. 10C shows another illustrative embodiment consistent with this invention in which a portion of the output of radiation source 344 (e.g., a magnetron) may be coupled via directional coupler 326 and used as a signal. The signal may then be supplied to phase-shifter 348 and, once phase-shifted, supplied and used to control another radiation source 350. As previously discussed, the outputs of radiation sources 344 and 350 may be directed to a common cavity or region (e.g., via waveguides 352 and 354) to create any desired radiation interference pattern.

[0115] FIG. 11 A shows a flow chart of an illustrative method for controlling multiple radiation sources consistent with this invention. The method may include, for example, generating a control signal (step 45). In one embodiment, the control signal may be generated by source 302 of FIG. 10A, for example. The control signal may be any waveform, such as a sinusoidal waveform, a square waveform, a saw-tooth waveform, a pulsed waveform, or any other type of waveform representing an amplitude variation in time or frequency domain. [0116] The method may further include splitting the control signal into at least a first signal and a second signal (step 47). In one embodiment, the control signal may be split into the first signal and the second signal using a signal splitter, with a phase difference between them.

[0117] The method may further include controlling an output of a first radiation source with the first signal (step 47) and controlling output of a second radiation source with the second signal (step 49). The phase difference between the first signal and the second signal may cause the radiations corresponding to these signals to interfere in a manner such that maxima and minima are formed. For example, if a waveform with a peak amplitude E1 (corresponding to first signal) and another waveform with a peak amplitude E2 (corresponding to second signal) were to interact in the chamber, maxima may be formed corresponding to hot spots. Such a hotspot may exhibit an intensity equal to the square of (E1+E2). The phase difference may be fixed or varied to achieve any desired electric field distribution in the chamber.

[0118] FIG. 11 B shows a flow chart for another method consistent with this invention using, for example, apparatus 321 of FIG. 10B. The first step may involve generating a first signal having a first frequency (step 401). The method may further include generating a second signal having a second frequency (step 403). Then, an output of first radiation source may be controlled with the first signal (step 405) and an output of the second radiation source may be controlled with the second signal (step 407). Controller 325 may be used to vary at least one frequency of the first and second signals to achieve a desired electric field distribution.

[0119] FIG. 11 C shows a flow chart of another method consistent with this invention using, for example, the apparatus shown in FIG. 10C. The method may include generating a first control signal (step 411) and controlling an output of a first radiation source with the first control signal (step 413). The method may further include using at least a part of the output of the first radiation source to form a second control signal for controlling an output of a second radiation source (step 415).

[0120] A plasma catalyst can be used with any of the plasma apparatus mentioned above to facilitate igniting, modulating, and sustaining a plasma using multiple radiation sources consistent with this invention. As previously explained, the use of a plasma catalyst can relax the conditions required to form a plasma, which can make the plasma more controllable, even at pressures at or above atmospheric pressure. Also, any type of plasma catalyst can be used consistent with this invention, including active and passive plasma catalysts. It will be appreciated that such catalysts may be particularly useful due to their substantially continuous catalyzing effect, as opposed to spark plugs, for example, which only spark periodically. Continuous catalysis can be especially useful during periodic processes that require multiple striking and restriking of a plasma.

Claims

We Claim:

1. A plasma apparatus comprising: a radiation cavity; at least a first radiation source for directing radiation into the radiation cavity to facilitate formation of plasma in the radiation cavity, wherein the first radiation source is controlled by a signal; a phase shifter for shifting a phase of the signal to generate a phase-shifted signal; and at least a second radiation source for directing radiation into the radiation cavity, wherein the second radiation source is controlled by the phase-shifted signal.

2. The plasma apparatus of claim 1 , further comprising a first amplifier to amplify the signal for said at least a first radiation source.

3. The plasma apparatus of claim 2, further comprising a second amplifier to amplify the phase-shifted signal for said at least a second radiation source.

4. The plasma apparatus of claim 1 , wherein at least one of the radiation sources comprises a magnetron, a klystron, a gyrotron, or a traveling wave tube amplifier.

5. The plasma apparatus of claim 1 , wherein the cavity contains a plasma catalyst.

6. The plasma apparatus of claim 5, wherein the plasma catalyst is at least one of an active catalyst and a passive catalyst.

7. The plasma apparatus of claim 6, wherein the catalyst comprises at least one of metal, inorganic material, carbon, carbon-based alloy, carbon-based composite, electrically conductive polymer, conductive silicone elastomer, polymer nanocomposite, and an organic-inorganic composite.

8. The plasma apparatus of claim 7, wherein the catalyst is in the form of at least one of a nano-particle, a nano-tube, a powder, a dust, a flake, a fiber, a sheet, a needle, a thread, a strand, a filament, a yarn, a twine, a shaving, a sliver, a chip, a woven fabric, a tape, and a whisker.

9. The plasma apparatus of claim 8, wherein the catalyst comprises carbon fiber.

10. The plasma apparatus of claim 6, wherein the catalyst is in the form of at least one of a nano-particle, a nano-tube, a powder, a dust, a flake, a fiber, a sheet, a needle, a thread, a strand, a filament, a yarn, a twine, a shaving, a sliver, a chip, a woven fabric, a tape, and a whisker.

11. The plasma apparatus of claim 3, wherein each of the first amplifier and the second amplifier is at least one of a klystron and a traveling wave tube amplifier.

12. A plasma apparatus comprising: a radiation cavity; at least a first radiation source for directing radiation into the radiation cavity to facilitate formation of plasma in the radiation cavity, wherein the first radiation source is controlled by a first signal having a first frequency; and at least a second radiation source for directing radiation into the radiation cavity, wherein the second radiation source is controlled by a second signal having a second frequency, wherein the first and second frequencies are different.

13. The plasma apparatus of claim 12, further comprising a first amplifier for amplifying at least one of the first signal and the second signal.

14. The plasma apparatus of claim 12, further comprising a controller for varying at least one of the first frequency and the second frequency.

15. The plasma apparatus of claim 12, wherein each of the first amplifier and the second amplifier is at least one of a klystron and a traveling wave tube amplifier.

16. The plasma apparatus of claim 12, wherein at least one of the first radiation source and the second radiation source is selected from a group consisting of a magnetron, a klystron, a gyrotron, a traveling wave tube amplifier, and any combination thereof.

17. The plasma apparatus of claim 12, wherein the chamber contains a plasma catalyst.

18. The plasma apparatus of claim 17, wherein the plasma catalyst is at least one of an active catalyst and a passive catalyst.

19. A plasma apparatus comprising: a radiation cavity; a phase shifter for receiving at least one of a first signal and a second signal, and shifting one of the signals with respect to the other to create a phase- shift; at least a first radiation source for directing radiation into the radiation cavity to facilitate formation of plasma in the radiation cavity, wherein the first radiation source is controlled by the first signal; and at least a second radiation source for directing radiation into the radiation cavity, wherein the second radiation source is controlled by the second signal.

20. The plasma apparatus of claim 19, wherein the phase-shifter is a variable phase-shifter, such that the phase shift can be varied to vary the plasma to plasma distribution.

21. The plasma apparatus of claim 19, wherein the first radiation source and the second radiation source is at least one of a magnetron, a klystron, a gyrotron, a traveling wave tube amplifier, and any combination thereof.

22. The plasma apparatus of claim 19, wherein the at least first radiation source is a magnetron and a part of its output is used as the second signal.

23. The plasma apparatus of claim 19, wherein the chamber contains a plasma catalyst.

24. The plasma apparatus of claim 23, wherein the plasma catalyst is at least one of an active catalyst and a passive catalyst.

25. A method for controlling a multiple radiation source plasma apparatus, the method comprising: generating a control signal; splitting the control signal into at least a first signal and a second signal; controlling an output of a first radiation source, for directing radiation into a plasma region, with the first signal; and controlling an output of a second radiation source for directing radiation into the plasma region with the second signal.

26. The method of claim 25, wherein there is a phase difference between the first signal and the second signal.

27. The method of claim 25, further comprising introducing a phase difference between the first signal and the second signal.

28. The method of claim 25, further comprising amplifying the first signal and the second signal.

29. The method of claim 25, wherein at least one of the radiation sources comprises a magnetron, a klystron, a gyrotron, or a traveling wave tube amplifier.

30. The method of claim 25, wherein the plasma region contains a plasma catalyst.

31. The method of claim 25, wherein the shifting can be varied to vary an energy distribution within the cavity.

32. The method of claim 25, further comprising igniting the plasma by subjecting a gas in the plasma region to electromagnetic radiation generated by the first radiation source having a frequency less than about 333 GHz in the presence of at least one passive plasma catalyst comprising a material that is at least electrically semi-conductive.

33. The method of claim 32, wherein the material comprises at least one of metal, inorganic material, carbon, carbon-based alloy, carbon-based composite, electrically conductive polymer, conductive silicone elastomer, polymer nanocomposite, organic-inorganic composite, and any combination thereof.

34. The method of claim 33, wherein the material is in the form of at least one of a nano-particle, a nano-tube, a powder, a dust, a flake, a fiber, a sheet, a needle, a thread, a strand, a filament, a yarn, a twine, a shaving, a sliver, a chip, a woven fabric, a tape, a whisker, and any combination thereof.

35. The method of claim 32, wherein the catalyst comprises carbon fiber.

36. The method of claim 32, wherein the at least one passive plasma catalyst comprises a plurality of elongated, electrically conductive items distributed in differing locations in the cavity.

37. The method of claim 25, further comprising 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 comprising at least one ionizing particle.

38. The method of claim 37, wherein the at least one ionizing particle comprises a beam of particles.

39. The method of claim 37, wherein the particle is at least one of an x- ray particle, a gamma ray particle, an alpha particle, a beta particle, a neutron, and a proton.

40. The method of claim 37, wherein the at least one ionizing particle is a charged particle.

41. The method of claim 37, wherein the ionizing particle comprises a radioactive fission product.

42. The method of claim 37, wherein the ionizing particle is a free electron, the method further comprising generating the electron by energizing an electron source.

43. The method of claim 42, wherein the energizing comprises heating the electron source.

44. The method of claim 37, wherein the particle comprises a free proton, the method further comprising generating the free proton by ionizing hydrogen.

45. The method of claim 37, wherein the cavity is at least partially open, permitting the gas to flow therethrough.