Plasma generator

Abstract

An apparatus for generating plasma by a transient hollow cathode discharge effect is disclosed. The apparatus includes a chamber including an inlet through which gas can enter the chamber and an outlet through which gas can exit the chamber, and a cathode electrode disposed in the chamber. The cathode electrode includes a plurality of hollow cathodes, an anode electrode spaced apart from the cathodes, a power supply, and a power supply controller configured to reduce the level of power below a level required to maintain plasma in the plurality of hollow cathodes after an electrical breakdown occurs. Each hollow cathode includes a through-hole through which gas can pass from one side of the cathode electrode to the other side of the cathode electrode. A modular apparatus is also disclosed that includes a plurality of plasma reactor modules arranged in series and / or parallel.

Description

[Technical Field]

[0001] The present invention relates to an apparatus for generating plasma. More particularly, embodiments of the present invention relate to an apparatus for generating plasma via a transient hollow cathode discharge effect. [Background technology]

[0002] Various forms of devices for generating plasma are known in the art and have a wide range of applications. Plasma is a state of matter distinct from solids, liquids, or gases, and contains a gaseous mixture of free electrons and positive ions. The unique chemical and physical properties of plasma can be utilized in many different applications, such as treating the surface of a material to achieve desired properties such as hydrophobicity, or removing contaminants from the surface of a physical object.

[0003] It is in this context that the present invention has been made. Summary of the Invention [Means for solving the problem]

[0004] According to a first aspect of the present invention, there is provided an apparatus for generating a plasma via a transient hollow cathode discharge effect, the apparatus comprising: a chamber having an inlet through which a gas can flow and an outlet through which the gas can flow; a cathode electrode disposed within the chamber, the cathode electrode including a plurality of hollow cathodes, each having a thickness-wise hole through which a gas can pass from one side of the cathode electrode to the other side of the cathode electrode; an anode electrode spaced from the cathode to define a gas flow path passing from the inlet through the thickness-wise holes of the plurality of hollow cathodes to the outlet; a power supply electrically connected to the anode electrode and the cathode electrode to supply power to generate a plasma at the plurality of hollow cathodes; and a power supply controller configured to reduce a power level of the power below a first power level required to maintain a plasma at the plurality of hollow cathodes after an electrical breakdown occurs.

[0005] According to some embodiments of the first aspect, the power supply controller is configured to reduce the power level by stopping the supply of power to the cathode and the anode.

[0006] According to some embodiments of the first aspect, the power supply controller is configured to reduce the power level after an electrical breakdown occurs by supplying power at a level equal to or greater than the first power level for a predetermined time, the predetermined time being sufficient for the electrical breakdown to occur.

[0007] According to some embodiments of the first aspect, the apparatus includes means for detecting a characteristic indicative of an occurrence of electrical breakdown, and the power supply controller is configured to reduce a power level of the power below the first power level in response to detecting the characteristic indicative of an occurrence of electrical breakdown. For example, in some embodiments, the characteristic includes a change in current and / or voltage.

[0008] According to some embodiments of the first aspect, the power supply controller is configured to control the power supply to supply power intermittently as a sequence of voltage pulses, and to set the frequency of the voltage pulses in response to a flow rate of gas passing through the chamber.

[0009] According to some embodiments of the first aspect, the inlet and the outlet are located on opposite sides of the cathode electrode from each other.

[0010] According to some embodiments of the first aspect, the chamber is configured such that gas can only flow from one side of the cathode electrode to the other side through the plurality of hollow cathodes.

[0011] According to some embodiments of the first aspect, the anode electrode is located on the opposite side of the inlet from the cathode electrode.

[0012] In some embodiments of the first aspect, the anode electrode is arranged to form a gas flow barrier that extends across all or part of a face of the cathode electrode facing the anode electrode, such that gas exiting the hollow cathode is deflected by the anode electrode to flow laterally across the face of the cathode electrode.

[0013] In some embodiments of the first aspect, the separation distance between the anode electrode and the cathode electrode is set such that the resistance to lateral gas flow in the gap between the anode electrode and the cathode electrode is less than the resistance to gas flow through the plurality of hollow cathodes, such that the gas flow rate through the chamber depends on the resistance to gas flow through the plurality of hollow cathodes.

[0014] In some embodiments of the first aspect, the separation distance between the anode and cathode electrodes is set such that the resistance to lateral gas flow in the gap between the anode and cathode electrodes is greater than the resistance to gas flow through the plurality of hollow cathodes.

[0015] According to some embodiments of the first aspect, the apparatus comprises an electrically insulating or semi-insulating layer disposed between the anode electrode and the cathode electrode, which limits the amount of energy delivered to the plasma after electrical breakdown occurs.

[0016] According to some embodiments of the first aspect, the apparatus is capable of generating a plasma while the gas in the chamber is at atmospheric pressure, in use.

[0017] According to some embodiments of the first aspect, the plurality of hollow cathodes are arranged in a plurality of groups of hollow cathodes, and the cathode electrode has a plurality of conductive paths, each of which connects the hollow cathodes in one group to each other to supply power to the group of hollow cathodes, and the hollow cathodes in adjacent groups are separated from each other by an electrically insulating region.

[0018] According to some embodiments of the first aspect, the plurality of hollow cathodes are arranged in a plurality of rows on the cathode electrode, with one group including a plurality of hollow cathodes on one row and an adjacent group including a plurality of hollow cathodes on an adjacent row.

[0019] According to some embodiments of the first aspect, the inner surface of each hollow cathode comprises a coating of a material that is toxic to biological samples, where the material comprises, for example, gold, silver, palladium and / or copper.

[0020] According to some embodiments of the first aspect, the power supply is a capacitive driving circuit that includes a voltage source having a high voltage terminal and a low voltage terminal, a first switch connected between the high voltage terminal and a first node, a second switch connected between the first node and a second node, a first inductor connected between the second node and a third node via a resistor, a second inductor connected between the third node and a high voltage output connected to the cathode electrode, a first capacitor connected between the first node and the low voltage terminal, and a second capacitor connected between the third node and a low voltage terminal further connected to the anode electrode.

[0021] According to some embodiments of the first aspect, the anode electrode has an integrated isolated capacitor, the anode electrode including a top electrode in the form of a first conductive layer, an intermediate electrode in the form of a second conductive layer, the first and second conductive layers being separated by an insulating dielectric and connected to each other via a conductive path passing through the insulating dielectric, and a bottom electrode in the form of a third conductive layer separated from the second conductive layer by an insulating dielectric, the second and third conductive layers together functioning as an integrated isolated capacitor.

[0022] According to a second aspect of the present invention, there is provided a modular apparatus comprising a plurality of plasma reactor modules, each of the plurality of plasma reactor modules comprising an apparatus according to the first aspect.

[0023] In some embodiments of the second aspect, two or more of the plasma reactor modules are connected in series, such that gas flowing out the outlet of one of the plasma reactor modules flows into the inlet of the next plasma reactor module in the series connection.

[0024] According to some embodiments of the second aspect, the number of series-connected plasma reactor modules is selected to achieve desired characteristics of the gas exiting the modular apparatus after passing through the multiple plasma reactor modules.

[0025] In some embodiments of the second aspect, two or more of the plasma reactor modules are connected in parallel to define multiple gas flow paths through the modular apparatus, such that gas entering the modular apparatus is divided into the multiple gas flow paths, and a portion of the gas flowing along each gas flow path need only pass through a corresponding one of the parallel-connected plasma reactor modules before exiting the modular apparatus.

[0026] According to some embodiments of the second aspect, the number of parallel-connected plasma reactor modules is selected to achieve a desired gas flow rate through the modular apparatus.

[0027] According to a third aspect of the present invention, there is provided a capacitive driving circuit comprising: a voltage source having a high voltage terminal and a low voltage terminal; a first switch connected between the high voltage terminal and a first node; a second switch connected between the first node and a second node; a first inductor connected between the second node and a third node via a resistor; a second inductor connected between the third node and a high voltage output connected to a cathode electrode; a first capacitor connected between the first node and the low voltage terminal; and a second capacitor connected between the third node and a low voltage terminal further connected to an anode electrode.

[0028] According to a fourth aspect of the present invention, there is provided a structure for use as an anode electrode in an electrical device, the structure having an integrated isolated capacitor, the anode electrode including a top electrode in the form of a first conductive layer, an intermediate electrode in the form of a second conductive layer, the first and second conductive layers being separated by an insulating dielectric and connected to each other via a conductive path passing through the insulating dielectric, and a bottom electrode in the form of a third conductive layer separated from the second conductive layer by an insulating dielectric, the second and third conductive layers together functioning as the integrated isolated capacitor.

[0029] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0030] [Figure 1] 1 is a cross-sectional view of an apparatus for generating plasma according to one embodiment of the present invention; [Figure 2] FIG. 2 is an illustration showing a portion of a cathode electrode including multiple hollow cathodes according to one embodiment of the present invention. [Figure 3] FIG. 2 is an illustration of a cathode electrode having multiple hollow cathodes arranged in groups and connected by respective conductive paths, according to one embodiment of the present invention. [Figure 4] FIG. 2 is an illustration of a first gas flow plate positioned on the inlet side of the cathode electrode according to one embodiment of the present invention. [Figure 5] FIG. 2 is an illustration showing a second gas flow plate positioned above a first gas flow plate according to one embodiment of the present invention. [Figure 6] FIG. 10 is an illustration showing an inlet cover disposed on a second gas flow plate according to one embodiment of the present invention. [Figure 7] FIG. 10 is an illustration of a third gas flow plate located on the outlet side of the cathode electrode according to one embodiment of the present invention. [Figure 8] FIG. 2 is an explanatory diagram showing an anode electrode disposed facing the outlet side of a cathode electrode according to one embodiment of the present invention. [Figure 9] FIG. 10 is an explanatory diagram showing an outlet cover disposed on a third gas flow plate and an anode electrode according to one embodiment of the present invention. [Figure 10] FIG. 1 is an illustration of a modular apparatus including multiple plasma reactor modules, according to one embodiment of the present invention. [Figure 11] FIG. 1 is an illustration of a modular apparatus comprising multiple plasma reactor modules arranged in parallel and in series, according to one embodiment of the present invention. [Figure 12] FIG. 1 is an explanatory diagram illustrating a power supply controller according to an embodiment of the present invention. [Figure 13]FIG. 1 is an illustration of an apparatus including an electrically insulating layer disposed between an anode electrode and a cathode electrode, according to one embodiment of the present invention. [Figure 14] FIG. 1 is a cross-sectional view of an apparatus for generating a plasma, in which gas enters the apparatus from the same side as the anode electrode, according to one embodiment of the present invention. [Figure 15] FIG. 1 is an illustration of a circuit for supplying power to an apparatus for generating plasma, according to one embodiment of the present invention. [Figure 16] FIG. 2 is an illustration of a hollow cathode having a conductive surface, including an inner conductive surface, that extends through a portion of the path of the hollow cathode on the inlet side of the cathode electrode, according to one embodiment of the present invention. [Figure 17] FIG. 1 is an illustration of a hollow cathode having a conductive surface that does not include a conductive surface inside the through-holes on the inlet side of the cathode electrode, according to one embodiment of the present invention. [Figure 18] 1 is a cross-sectional view of an apparatus for generating a plasma, with the inlet and outlet located on the same side as the cathode electrode, according to one embodiment of the present invention. [Figure 19] FIG. 1 is an illustration of a modular apparatus comprising multiple plasma reactor modules arranged in parallel, according to one embodiment of the present invention. [Figure 20] FIG. 2 is a circuit diagram illustrating connections between components comprising a capacitive driver circuit configured to provide power pulses to multiple plasma reactor modules, according to one embodiment of the present invention. [Figure 21] 21 is a perspective view of an anode structure that can be used in each plasma reactor module with the capacitive drive circuit of FIG. 20 according to one embodiment of the present invention. [Figure 22] FIG. 22 is a cross-sectional view of the anode structure of FIG. 21 according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0031] In the following detailed description, only certain exemplary embodiments of the present invention are shown and described by way of illustration only. Those skilled in the art will recognize that the described embodiments can be varied and modified in many ways within the spirit of the present invention. Accordingly, the drawings and descriptions are illustrative in nature and should not be regarded as restrictive. Like reference numerals refer to like elements throughout the specification.

[0032] Throughout the following description, the terms "gas" and "gas flow" are used to describe various aspects of the structure and operation of the apparatus in embodiments of the present invention. It should be understood that during operation, the fluid medium flowing through the apparatus actually comprises a mixture of gaseous species and plasma products. Thus, references to "gas" herein should be interpreted broadly to encompass plasma products.

[0033] Referring to FIG. 1 , an apparatus for generating plasma according to one embodiment of the present invention is shown in cross section. As will become apparent from the following description, the apparatus is configured to generate plasma via a transient hollow cathode discharge effect. The apparatus 100 includes a chamber 101 having one or more inlets 102 through which gas enters the chamber and one or more outlets 103 through which gas exits the chamber. The apparatus 100 further includes a cathode electrode 140 disposed within the chamber 101, an anode electrode 151 spaced apart from the cathode, a power supply 170, and a power supply controller 180. In this embodiment, the anode electrode 151 is disposed on the opposite side of the cathode electrode 140 from the one or more inlets 102; however, in other embodiments, the anode electrode 151 may be disposed on the same side as the cathode electrode 140 as the one or more inlets 102.

[0034] In this embodiment, device 100 is comprised of a stack of planar components 110, 120, 130, 140, 150, and 160, which may be referred to as "plates." Each plate has one or more holes or channels through which gas can flow, and the holes or channels of adjacent plates, when assembled in the order shown in FIG. 1 , are aligned to define a path through which gas can flow from one side of the device to the other. For convenience, the multiple holes and / or channels that, once connected, provide a continuous path for gas flow through the device will be referred to hereinafter as "chambers" 101. The term "chamber" is not limited to a particular size or shape and should be interpreted broadly as any space within the device through which gas can flow.

[0035] It should be understood that this particular structure is provided by way of example only and should not be considered limiting. In other embodiments, the device can be configured differently. For example, in other embodiments, the device includes a housing formed to define a chamber within the device. The housing can be formed of two or more pieces that, when assembled, define the chamber. The two or more pieces can be assembled such that the two or more pieces can be removed to provide access to the interior of the device, using mechanical fasteners such as bolts or clips, for maintenance or repair of components within the device, such as the cathode 140 or the anode 151. Each piece of the housing can be formed by any suitable manufacturing method, including, but not limited to, casting, injection molding, 3D printing, milling, etc.

[0036] The cathode electrode 140 includes a plurality of hollow cathodes 141, each having a through-thickness hole through which gas can pass from one side of the cathode electrode 140 to the other, as indicated by the dashed arrows in FIG. 1 , which represent the flow of gas through the device 100 during operation. FIG. 2 illustrates a portion of the cathode electrode 140 including six hollow cathodes 241, according to one embodiment of the present invention. In the embodiment illustrated in FIG. 2 , each hollow cathode 241 includes a ring-shaped electrode 241 a facing the anode 151 and a conductive path 241 b on the inner surface of the hollow cathode 241, connecting the anode-side ring electrode 241 a to the opposite side of the cathode electrode 140. In other embodiments, all or part of the anode-side ring electrode 241 a and / or the conductive path 241 b on the inner surface of the hollow cathode 241 can be omitted.

[0037] In embodiments in which the device 100 is used to remove biological contaminants, such as airborne virus particles or bacteria, from a gaseous medium, the inner surface of each hollow cathode 141, 241 may include a coating of a material toxic to biological samples, such as gold, silver, palladium, or copper, or alloys thereof, to more effectively purify the gaseous medium. In some embodiments, the material toxic to biological samples may be included in the form of nanoparticles to increase the surface of the material exposed to the gas flow.

[0038] In this embodiment, the chamber 101 is configured to allow gas to flow only from one side of the cathode electrodes 140 to the other through the multiple hollow cathodes 141. This ensures that gas exiting one or more outlets 103 must pass through at least one of the hollow cathodes 141. Such a configuration may be particularly advantageous in embodiments in which the apparatus 100 is used to process gaseous media passing through the chamber 101, as all gas exiting the apparatus 100 is exposed to the plasma environment within the hollow cathodes 141. However, in other embodiments, it may not be necessary to ensure that all gas passes through the hollow cathodes 141. For example, in embodiments in which the apparatus is used to generate a supply of ions that are then accelerated into an ion beam, it may be acceptable for the cathode electrodes 140 within the chamber to be imperfectly sealed, thereby allowing some gas to bypass the hollow cathodes 141.

[0039] Each hollow cathode electrode 141, 241 can have a suitable diameter capable of generating a transient hollow cathode discharge. For example, each hollow cathode 141, 241 can have a diameter on the order of 1 to 2 millimeters (mm), although it should be understood that this range is merely exemplary and that other diameters may be used in other embodiments. In some embodiments, the diameter of each hollow cathode 141, 241 may be less than 100 micrometers, while in other embodiments, the diameter may be greater than 1 centimeter. The diameter of the hollow cathode 141, 241 can be selected while considering other design parameters, such as the operating pressure of the gaseous medium separating the anode 151 and the cathode 140. The ratio of the diameter D of the hole in the cathode 140 to the thickness H, i.e., D / H, defines the electric field penetration into the hollow cathode 141 for a given applied voltage across the anode 151 and the cathode 140. The smaller the D / H value, the smaller the value of the electric field that penetrates into the backspace of the hollow cathode. Typical D / H ratios can be in the range of 1 to 0.5, although values ​​outside this range are still applicable in some embodiments.

[0040] The power supply 170 is electrically connected to the anode electrode 151 and the cathode electrode 140 to provide power for generating plasma at the plurality of hollow cathodes 141. The power supply 170 may include any suitable form of power supply suitable for providing power so that plasma can be generated via the transient hollow cathode discharge effect. Embodiments of the present invention utilize a transient hollow cathode discharge to create an ultra-high electric field region (e.g., 1×10) for efficient generation of high energy. 7 The principles of transient hollow cathode discharge are well established and will not be described in detail for the sake of brevity. However, without wishing to be bound by theory, a brief description of the transient hollow cathode electrode discharge phenomenon is provided below to facilitate understanding of the present invention.

[0041] A transient hollow cathode discharge phenomenon refers to the physical events that occur from the time a voltage is established across the hollow cathode structure and anode to the time a conductive plasma channel is formed between the anode and cathode. The magnitude of the applied voltage must be sufficient to ensure that the mean free path of electrons emitted from the hollow cathode and accelerated across the anode-cathode space (hereinafter referred to as the A-K gap) by the applied electric field is comparable to or exceeds the physical dimensions of the A-K gap. Thus, in any embodiment, the power supply 170 can be configured to apply a sufficiently large voltage across the cathode electrode 140 and the anode electrode 151, taking into account the distance between the cathode 140 and the anode 151.

[0042] Furthermore, the power supply 170 preferably allows the voltage across the cathode 140 and anode 151 to be established quickly enough (e.g., on the order of 100 nanoseconds) to allow the ionization growth process to proceed in a quasi-steady state. In this case, the amplitude of the applied voltage can be nearly constant within the timescale for the formation of a transient hollow cathode discharge. The rapid establishment of the voltage across the cathode 140 and anode 151 establishes a quasi-steady field configuration with a nearly uniform high electric field across most of the surface of the cathode 140, except near the hollow cathode 141. At each thickness hole, the electric field penetrates the hollow cathode 141 and seeps out to the back surface of the cathode electrode 140, albeit to a much smaller value than the A-K gap.

[0043] In some embodiments, the device can include one or more capacitors between the power supply 170 and one or more hollow cathodes 141. The capacitors can be located in close physical proximity to each of the one or more hollow cathodes they power, for example, by being placed on the cathode electrode 140 itself. This allows the power supply 170 to power the hollow cathodes 141 through the capacitors, which act as intermediate energy storage. The capacitors can be connected to each of the one or more hollow cathodes via a suitably low inductance, thereby enabling the capacitors to power the one or more hollow cathodes with a sufficiently fast rise time (typically on the order of 100 nanoseconds), allowing the ionization growth process described above to proceed in a quasi-steady state. Using capacitors in this manner can also be useful for achieving a sufficiently long decay time (typically tens of microseconds) to maintain a nearly constant voltage at one or more hollow cathodes during discharge formation during a transient hollow cathode discharge cycle.

[0044] The electric field within each hollow cathode 141 generates free electrons near the surface of the cathode electrode 140 in each hole 141 and in the hollow cathode backspace within the chamber 101. As the electrons exit the holes 141 on the anode side of the cathode electrode 140, they are rapidly accelerated toward the anode 151 and gain considerable energy due to the high electric field in the A-K gap. Because the mean free path of these free electrons is large compared to the distance of the A-K gap, there is little ion growth due to impact ionization within the A-K gap, and there is no ion growth due to electron multiplication.

[0045] At the same time, ions, which may be generated by electron collisions with the gaseous medium in the A-K gap or the surface of the anode 151, are accelerated back through the hollow cathode 141. However, due to their large mass difference, such ions move much slower than the electrons in the A-K gap. This creates a positive space charge that initially localizes near the surface of the anode 151 because the ions move slower than the electrons, and a tenuous plasma begins to form between the space charge and the anode 151. The presence of this conductive plasma effectively shifts the anode potential to the location of the space charge, thereby creating a virtual anode that reduces the effective length of the A-K gap and thus increases the magnitude of the electric field in the remaining non-conductive space in the A-K gap and within the hollow cathode 141. The increased electric field accelerates the electron growth process, causing the virtual anode to rapidly advance toward the cathode 140 and further increasing the electric field around the hollow cathode bore 141 until the virtual anode eventually penetrates into the hollow cathode 141. This ultimately leads to the creation of a positive space charge inside the hollow cathode 141, i.e., just inside the cathode backspace. When the virtual anode penetrates into the cathode backspace, there is a point-like plasma zone that is a massive source of electrons.

[0046] A conductive channel connecting the anode 151 and the cathode 140 is then formed, completing the electrical breakdown process and effectively terminating the THCD process. The power supply controller 180 is configured to reduce the power level of the power supplied by the power supply 170 after electrical breakdown occurs. Specifically, the power supply controller 180 is configured to reduce the power level below a level (which may be referred to as a first threshold power) necessary to sustain a plasma in the plurality of hollow cathodes 141. In this embodiment, the power supply controller 180 is configured to stop the supply of power to the cathodes 140 and the anodes 151, thereby effectively reducing the power level to zero. For example, the power supply controller 180 can open a switch connecting the power supply 170 to the cathodes 140 and / or the anodes 151, thereby breaking the electrical circuit and stopping the current flowing through the cathodes 140 and the anodes 151. In other embodiments, the power supply controller 180 can reduce the power level to a second power level below the first threshold power, which effectively turns off the plasma while still providing power at a lower, more limited level.

[0047] It should be understood that in practice, it may not be practical to stop or reduce the supply of power at the exact moment electrical breakdown occurs. In practice, power supply controller 180 may reduce the power level for a short, finite period of time after electrical breakdown occurs, where the length of time between electrical breakdown and power being reduced or turned off is short enough that any heating of the plasma during this time is negligible. In some embodiments, power supply controller 180 may actively monitor apparatus 100 during operation to detect characteristics indicative of electrical breakdown occurring and, in response to the detected characteristics, reduce the power level below the first threshold power. This allows power supply controller 180 to ensure that power continues to be supplied at a sufficiently high level until electrical breakdown occurs, and then respond quickly to the occurrence of electrical breakdown to avoid significant heating of the plasma.

[0048] In another embodiment, instead of detecting characteristics indicative of electrical breakdown, power supply controller 180 can be configured to reduce power below the first threshold power after electrical breakdown occurs by controlling power supply 170 to supply power at or above the first power level for a predetermined time, where the predetermined time is sufficient to cause electrical breakdown. In other words, power supply controller 180 can be pre-designed or pre-programmed to automatically reduce the power level after a certain time has elapsed since initiating supply of a high level of power equal to or greater than the first threshold power, without actively checking whether electrical breakdown has occurred. This can simplify the design of power supply controller 180 and apparatus 100 as a whole, by eliminating the need for such monitoring functionality.

[0049] By limiting the supply of electrical energy to electrodes 140, 151 when a conductive plasma channel forms after electrical breakdown, power supply controller 180 can avoid heating the channel into a high-temperature plasma. This allows apparatus 100 to generate relatively little heat during operation. Reducing the power level after electrical breakdown to avoid significant heating of the plasma also allows for a set of very large hollow cathodes to be integrated into a small volume without the need for active cooling that may be required to remove the thermal energy resulting from the formation of a high-temperature plasma.

[0050] A further advantage of reducing the power level after electrical breakdown occurs is that the apparatus 100 can be operated in a pulsed manner, in which the plasma is repeatedly struck and turned off for short time intervals without excessively heating the apparatus 100. In a pulsed mode of operation, the power supply controller 180 can reduce the power level to a level below the first threshold power, wait a period of time, and then increase the power level to a level equal to or greater than the first threshold power level to begin a new cycle of plasma generation. In this manner, the power supply controller 180 can be configured to control the power supply so that power is delivered intermittently as a series of voltage pulses.

[0051] In embodiments in which the apparatus 100 operates in a pulsed mode, the power supply controller can be configured to set the frequency of the voltage pulses in response to the gas flow rate passing through the chamber. In such embodiments, the delay between two pulses, i.e., the plasma generation cycle, is preferably longer than the recombination time of the various ionized species in the plasma, allowing the gaseous medium in the system to return from a conductive plasma state to a non-conductive state before the application of successive pulses. At the same time, in embodiments in which the apparatus 100 is used for continuous plasma processing of gaseous media, the delay between two pulses is preferably less than the residence time of the flowing medium passing through the reaction zone to achieve more effective processing. In other words, a higher pulse frequency can be set for a higher gas flow rate (i.e., a higher throughput system) to maintain effective processing of the gaseous medium passing through the apparatus 100.

[0052] In some embodiments, the apparatus can generate a plasma while the gas in the chamber is at atmospheric pressure. This can be achieved by appropriately selecting parameters such as the applied voltage, the A-K gap distance, the diameter of the hollow cathode, and the depth of the hollow cathode bore. In other embodiments, the apparatus can operate at pressures below atmospheric pressure.

[0053] Referring now to Figure 3, there is shown a cathode electrode including a plurality of hollow cathodes arranged in groups and connected by respective conductive paths, according to one embodiment of the present invention. The plurality of hollow cathodes 141 in the cathode electrode 140 shown in Figure 1 are arranged as shown in Figure 3. It should be understood that the cross section shown in Figure 1 is simplified to aid in understanding the invention and does not represent the actual physical cross section of the plurality of hollow cathodes 340 shown in Figure 3.

[0054] In this embodiment, the hollow cathodes are arranged in a regular array having orthogonal rows and columns. However, in other embodiments, the hollow cathodes may be arranged differently than the embodiment shown in Figure 3. For example, in one embodiment, the hollow cathodes may be closely spaced on a hexagonal lattice. In other embodiments, the hollow cathodes may be irregularly spaced across the surface of the cathode electrode.

[0055] Continuing to refer to FIG. 3 , in this embodiment, the hollow cathodes 340 are arranged in groups 341-1 and 341-2, and the cathode electrodes have a plurality of conductive paths 342. The cathode electrodes include two columns of hollow cathodes 340 arranged on either side of a central power line 343 that serves as a common voltage source, with the conductive paths 342 connected to the central power line 343. Each column includes 17 rows of five hollow cathodes 341, resulting in a total of (2 × 5 × 17) = 170 hollow cathodes. However, in other embodiments, the cathode electrodes include a different number of hollow cathodes. Each conductive path 342 connects the hollow cathodes 341 in one group to each other to supply power to the entire group. The hollow cathodes in adjacent groups 341-1 and 341-2 are separated from each other by electrically insulating regions 344. In this embodiment, where multiple hollow cathodes are arranged in rows on the cathode electrode, "adjacent" groups refer to groups 341-1, 341-2 of hollow cathodes on adjacent rows.

[0056] In this way, by arranging the hollow cathodes 341 in groups such that each group is separately connected to the common voltage source 343, it is possible to operate all of the hollow cathodes simultaneously, despite being powered by the common voltage source 343. As a result of this arrangement, each group of hollow cathodes 341-1, 341-2 is a similar distance from the central power line 343, which reduces the risk of any one hollow cathode being starved of electrical energy in the event of a breakdown.

[0057] The advantage of using a transient hollow cathode discharge to generate a plasma in multiple hollow cathodes is that it allows for self-synchronization of the initial ionization growth process among a collection of densely packed hollow cathodes without the need for ballast. This occurs because, in the transient hollow cathode configuration, the relatively long delay in the ionization growth process from the moment of voltage application results in more or less similar ionization growth rates among the group of hollow cathodes. The way in which the final electrical breakdown is controlled by ionization growth in the cathode backspace means that the initiation of electrical breakdown in one particular hollow cathode 341 in group 341-2 directly accelerates the breakdown of other adjacent hollow cathodes, i.e., those in groups 341, 341-2, and 341-3 shown in FIG. 3, through photoionization by UV radiation generated in the cathode backspace common to all hollow cathodes 341.

[0058] Referring now to FIG. 4 , a first gas flow plate 130 positioned over the inlet set of the cathode electrode is shown according to one embodiment of the present invention. The first gas flow plate 130 has rectangular holes 431 that are equal to or larger than the length and width of the array of hollow cathodes 340 on the cathode electrode 140, such that when the first gas flow plate 130 is positioned over the cathode electrode 140 as shown in FIG. 4 , each of the plurality of hollow cathodes 340 is in fluid communication with the space defined by the rectangular holes 431. Thus, gas present in the space within the rectangular holes 431 can pass through any one of the plurality of hollow cathodes and travel to the anode side of the cathode electrode 140. It will be understood that the shape of the holes 431 need not necessarily be rectangular, and that holes 431 of any other shape can be used in other embodiments. The thickness of the first gas flow plate 130 effectively raises the mixing zone, which is the region within the chamber where the gas streams entering through the multiple inlets 102 mix with each other before passing through the hollow cathode 141, away from the surface of the cathode electrode. This allows for better mixing of the inlet gases before passing through the hollow cathode 141. It will be appreciated that it may not be necessary for the gas flow plate 130 to be provided as a physically separate component to achieve this function. For example, a similar function may be achieved by the cathode electrode 140 having a raised surface around the area where the hollow cathode is located.

[0059] 5, a second gas flow plate is shown disposed on the first gas flow plate according to one embodiment of the present invention. Similar to the first gas flow plate 130, the second gas flow plate 120 has holes 521 that are equal to or larger than the length and width of the array of hollow cathodes 340 on the cathode electrode 140, such that when the first and second gas flow plates 130, 120 are disposed on the cathode electrode 140 as shown in FIG. 5, each of the plurality of hollow cathodes 340 is in fluid communication with the space formed by the holes 431, 521 in the first and second gas flow plates 130, 120.

[0060] In this embodiment, the holes 521 in the second gas flow plate 120 have an irregular octagonal shape and are larger than the holes 431 in the first gas flow plate 130. However, it will be understood that this is merely an example and that in other embodiments the holes 521 in the second gas flow plate 120 may have different shapes and / or different dimensions. The first and second gas flow plates 130, 120 together define a space in which gases entering the chamber 101 through the one or more inlets 102 can mix before passing through the hollow cathode 340.

[0061] 6, an inlet cover disposed on a second gas flow plate is shown according to one embodiment of the present invention. The inlet cover 110 has a plurality of inlets 102 in fluid communication with the spaces formed by the holes 431, 521 in the first and second gas flow plates 130, 120, such that the plurality of inlets 102 and holes 431, 521 define a chamber 101 portion on the inlet side of the device 100. The inlet cover 110 seals the chamber 101 so that the only way gas can enter the chamber 101 is through one of the inlets 102. While the first and second gas flow plates 130, 120 and the inlet cover 110 are described as separate components in this embodiment, in other embodiments, the chamber 101 and inlets 102 may be defined by appropriately shaped spaces formed within a single physical component.

[0062] Furthermore, in this embodiment, the multiple inlets 101 are positioned outside the periphery of the cathode electrode 140 region where the hollow cathodes 340 are formed (which may be referred to as the "hollow cathode region" of the cathode electrode 140). In this manner, gas entering the chamber 101 first strikes the surface of the first gas flow plate 130 surrounding the hollow cathode region and is deflected to flow laterally across the surfaces of the cathode electrode 140 and the hollow cathodes 340. This can ensure uniform distribution of the inlet gas throughout the multiple hollow cathodes 340, thereby achieving a similar gas flow rate through each hollow cathode, as opposed to concentrating the flow through a small number of hollow cathodes 340.

[0063] Referring now to FIG. 7 , a third gas flow plate disposed on the outlet side of the cathode electrode is illustrated according to one embodiment of the present invention. In this embodiment, the third gas flow plate 150 has a size and shape similar to the second gas flow plate 120 and includes holes 751 that are equal to or greater than the length and width of the array of hollow cathodes 340 on the cathode electrode 140. Thus, as shown in FIG. 7 , when the third gas flow plate 150 is disposed on the cathode electrode 140, each of the plurality of hollow cathodes 340 is in fluid communication with the space defined by the rectangular holes 751. Thus, gas flowing through the chamber 101 can flow through any one of the plurality of hollow cathodes 340, through the cathode electrode 140, and into the space defined by the rectangular holes 751. Thus, gas flowing through the chamber 101 can flow through any one of the plurality of hollow cathodes 340, through the cathode electrode 140, and into the space defined by the rectangular holes 751. It will be appreciated that the shape of holes 751 need not necessarily be rectangular, and that in other embodiments any other shape of holes 751 may be used. The thickness of third gas flow plate 150 affects the A-K gap distance and can be selected to achieve a desired A-K gap after apparatus 100 is assembled.

[0064] 8, an anode electrode is shown positioned facing the outlet side of the cathode electrode, according to one embodiment of the present invention. When assembled, the anode electrode 151 can extend across all or part of the surface of the cathode electrode 140 facing the anode electrode 151. In this embodiment, the anode electrode 151 is sized and shaped such that when assembled, it extends across at least the hollow cathode region of the cathode electrode 140, i.e., the region where the plurality of hollow cathodes 340 are formed. This ensures that when a voltage is applied between the cathode electrode 140 and the anode electrode 151, the electric field strength is uniform across all of the hollow cathodes 340.

[0065] Furthermore, in this embodiment, the anode electrode 151 is formed of an impermeable material, such as metal, so that the anode electrode 151 forms a barrier to gas flow. In this manner, gas exiting the hollow cathode 340 impinges on the surface of the anode electrode 151 and is deflected by the impermeable anode electrode 151 to flow laterally across the face of the cathode electrode 140. An advantage of this configuration is that gas exiting one hollow cathode is forced to flow laterally across the surface of the cathode electrode 140 and across the openings of the adjacent hollow cathode 340, thereby increasing the time the gas is exposed to the plasma environment within the apparatus 100 and thus increasing the treatment effectiveness. However, in other embodiments, a permeable anode may be used.

[0066] Furthermore, when an impermeable anode electrode 151 is used, the separation distance between the anode electrode 151 and the cathode electrode 140 can be set, for example, by appropriately selecting the thicknesses of the third gas flow plate 150 and the anode electrode 151, so that the resistance to lateral gas flow in the gap between the anode electrode 151 and the cathode electrode 140 is less than the resistance to gas flow of the plurality of hollow cathodes 141. This ensures that the gas flow rate through the chamber depends on the resistance to gas flow of the plurality of hollow cathodes. Arranging the anode-cathode gap so that the resistance to gas flow is less than that of the plurality of hollow cathodes 141 in this manner results in more rapid evacuation in the region behind the hollow cathodes, effectively allowing operation at a slightly higher pressure in the region behind the hollow cathodes (i.e., upstream of the hollow cathodes in the gas flow direction) than in the anode-cathode gap. This higher pressure enhances the ionization growth process in THCD formation.

[0067] Alternatively, in some embodiments, the separation distance between the anode and cathode electrodes is set so that the resistance to lateral gas flow in the gap between the anode electrode 151 and the cathode electrode 140 is greater than the resistance to gas flow through the plurality of hollow cathodes 141. This can increase the residence time of fluid flowing through the anode-cathode gap, which has the effect of improving treatment of the gas after passing through a hollow cathode 141 by repeatedly treating under other adjacent hollow cathodes 141 as the gas flows radially outward away from the first hollow cathode.

[0068] 9, an outlet side cover disposed over the third gas flow plate and anode electrode is shown according to one embodiment of the present invention. In this embodiment, the outlet side cover 160 is similar to the inlet side cover 110 and includes a plurality of outlets 103 in fluid communication with the space formed by the holes 751 in the third gas flow plate 150. The plurality of outlets 103 and the holes 751 thus define a chamber 101 portion on the outlet side of the device 100. The outlet side cover 160 seals the chamber 101 so that the only way for gas to exit the chamber 101 is through one of the outlets 103. While in this embodiment, the third gas flow plate 150 and the outer cover 160 are described as separate components, in other embodiments, the chamber 101 and the outlet 103 may be defined by a suitably shaped space formed within a single physical component.

[0069] 10, a modular apparatus including multiple plasma reactor modules arranged in series is shown, according to one embodiment of the present invention. Modular apparatus 1000 includes multiple plasma reactor modules 1001, 1002, 1003, 1004, 1005, and 1006, each of which includes a plasma generation device similar to that described above with reference to FIGS. 1-9. Plasma reactor modules 1001-1006 are connected in series so that gases exiting one or more outlets 103 of one plasma reactor module enter one or more inlets of the next plasma reactor module in the series connection.

[0070] While this embodiment shows a series arrangement of six plasma reactor modules 1001-1006, in other embodiments, any number of plasma reactor modules, i.e., two or more, can be connected in series. The number of series-connected plasma reactor modules can be selected to achieve desired characteristics for gases exiting the modular apparatus after passing through several plasma reactor modules. In embodiments in which the modular apparatus 1000 is used as a source of reactive species products, e.g., UV radiation produced by plasma processing of a gaseous medium composed primarily of air, typically at atmospheric pressure, with associated components of O, O, OH, and OH radicals, as well as water vapor, the number of series-connected plasma reactor modules can be selected to achieve a desired production rate for such reactive species products. Increasing the number of plasma reactor modules increases the residence time of gases within the apparatus exposed to the plasma environment, thereby increasing the production rate of such products.

[0071] The modular apparatus 1000 includes an inlet manifold 1010 configured to receive a gas flow through a common inlet and direct the gas to one or more inlets 102 of a first plasma reactor module in the series. The modular apparatus 1000 further includes an exhaust manifold 1020 configured to receive a gas flow through one or more outlets 103 of a last plasma reactor module in the series. The use of the inlet manifold 1010 may be particularly advantageous in embodiments where the first plasma reactor module 1001 has multiple inlets 102 because only a single connection needs to be provided from the inlet manifold 1010 to other equipment located upstream of the modular apparatus 1000. Similarly, the use of the outlet manifold 1020 may be particularly advantageous in embodiments where the last plasma reactor module 1006 has multiple outlets 103 because only a single connection needs to be provided from the outlet manifold 1020 to equipment located downstream of the modular apparatus 1000. However, in some embodiments, one or both of the inlet manifold 1010 and the outlet manifold 1020 can be omitted if desired. For example, in some embodiments, the outlet manifold 1020 can be omitted to effectively expose the outlet side of the device directly to the external environment, as described in more detail below with reference to FIG.

[0072] 11, a modular apparatus including multiple plasma reactor modules arranged in parallel and series is shown according to one embodiment of the present invention. The modular apparatus 1100 includes multiple plasma reactor modules connected in parallel to define multiple gas flow paths 1131, 1132, and 1133 through the modular apparatus 1100. In this embodiment, the modular apparatus 1100 includes multiple stages 1101, 1102, 1103, 1104, 1105, and 1106, each having three plasma reactor modules connected in parallel. The number of parallel-connected plasma reactor modules can be selected to achieve a desired overall gas flow rate through the modular apparatus 1100 by reducing flow resistance for a given plasma reaction rate requirement.

[0073] The multiple stages 1101, 1102, 1103, 1104, 1105, 1106 are themselves connected in series, similar to the apparatus described above with reference to Figure 10, so that gases exiting one stage enter the next stage. In some embodiments, the modular apparatus 1100 can include only a single plasma reactor module, and thus there are no series-connected plasma reactor modules.

[0074] Similar to the modular apparatus shown in FIG. 10 , the modular apparatus 1100 of this embodiment includes an inlet manifold 1110 and an outlet manifold 1120. In some embodiments, one or both of the inlet manifold 1110 and the outlet manifold 1120 can be omitted, if desired. Gas entering the modular apparatus 1100 is divided into multiple gas flow paths 1131, 1132, and 1133, as shown in FIG. 11 . Within each stage 1101-1106, a portion of the gas flowing along each of the gas flow paths 1131-1133 only needs to pass through a corresponding one of the plasma reactor modules within that stage before exiting the stage. If the modular apparatus includes other serially connected stages, as shown in FIG. 11 , the gas may pass through plasma reactor modules in other stages before exiting the modular apparatus, but it does not pass through other plasma reactor modules within the same stage.

[0075] As shown in FIG. 11 , the modular system can be connected in series and further include a spacer plate 1140 between adjacent plasma reactor modules. It will be understood that similar spacer plates 1140 can also be provided between the plasma reactor modules 1001-1006 in the embodiment shown in FIG. 10 . The spacer plate is disposed between the outlet cover 160 of one plasma reactor module and the inlet cover 110 of the next plasma reactor module in the series. The spacer plate 1140 is configured to electrically shield the cathode electrode 140 in one plasma reactor module from the anode 151 in the previous plasma reactor module in the series. In some embodiments, a similar shielding function can be provided by one or both of the outlet cover 160 and the inlet cover 110 of each of the two plasma reactor modules in the series, depending on the material properties from which the outlet cover 160 and the inlet cover 110 are formed; in this case, a separate spacer plate 1140 may be omitted.

[0076] 12, a power supply controller is shown in accordance with an embodiment of the present invention. In this embodiment, the power supply controller is configured to actively monitor the device 100 to detect a characteristic indicative of an electrical breakdown, as described above with reference to FIG. 1. As such, the device includes means 1282 for detecting the characteristic. In this embodiment, the power supply controller 1280 connects the anode electrode 151 and the intermediate energy storage capacitor C L A current-sensing resistor R is placed between S It is configured to monitor the voltage across the current-sensing resistor R S The voltage across is indicative of the current flowing through the device, which increases rapidly when electrical breakdown occurs and a conductive plasma is formed. Accordingly, in this embodiment, the means for detecting a characteristic 1282 includes a voltage detector 1282 that functions to detect the initiation of current flow in the plasma that is formed at the onset of electrical breakdown.

[0077] The control unit 1281 is connected to the current sensing resistor R S A signal from a voltage detector 1282 indicative of the voltage measured across the device may be monitored, and electrical breakdown may be determined to have occurred when the monitored signal from the voltage detector 1282 indicates an increase in current through the device above a threshold and / or when the rate of increase of the current over time exceeds a threshold rate of increase. In Figure 12, the means for detecting the characteristic 1282 is shown as part of the power supply controller 1280, although in other embodiments the means for detecting the characteristic 1282 may be physically separate from the power supply controller 1280.

[0078] In other embodiments, different types of current sensors, such as Hall effect sensors, can be used to detect changes in the current flowing from the power supply 1270 to the cathode 140 or anode 151, and different means for detecting the characteristic can be used as needed. As another example, in some embodiments, the characteristic can be a change in the intensity of electromagnetic radiation generated near the opening in the hollow cathode 141 or the anode 151, and the means for detecting the characteristic can be any suitable form of sensor, such as an optical sensor.

[0079] FIG. 12 also shows a capacitor C connected in parallel with the plasma reactor 1201 between the positive and negative terminals of the power supply 1270. L is shown. Capacitor C L (a) achieve a fast rise time of the applied voltage, (b) provide voltage gain due to the capacitor peaking effect, and (c) provide an energy limiting storage function that helps limit the energy available to the plasma once formed while reducing the power level delivered.

[0080] 13, an apparatus is shown that includes an electrically insulating layer disposed between an anode electrode and a cathode electrode, according to one embodiment of the present invention. The electrically insulating layer 1352, which in some embodiments may be a semi-insulating layer, is disposed between the anode electrode 1351 and the cathode electrode 1340 to limit the amount of energy delivered to the plasma after electrical breakdown occurs. For example, the electrically insulating layer 1352 may include an insulating or highly resistive semiconductor thin film.

[0081] The effect of the electrically insulating layer 1352 is to form a configuration similar to that of a dielectric barrier discharge (DBD), thereby limiting the energy delivered to the conductive plasma after it is formed. However, unlike conventional DBDs, in which the discharge takes the form of dense plasma spikes randomly distributed over a large surface, when used in embodiments of the present invention, the electrically insulating layer 1352 on the anode electrode 1351 forms a diffuse plasma channel at specific locations along the gas flow as a result of the transient hollow cathode discharge formation process. Compared to conventional DBD reactor systems, embodiments such as those shown in FIG. 13 allow for the use of smaller reactor volumes, thus enabling more compact devices for a given processing efficiency per unit of reactor volume.

[0082] Referring now to FIG. 14, a cross-sectional view of an apparatus for generating plasma in which gas enters the apparatus on the same side as the anode electrode is shown, according to one embodiment of the present invention. The illustrated apparatus is similar in some respects to the apparatus of FIG. 1 and includes an inlet cover 1410, a first gas flow plate 1420, a cathode electrode 1440, and a second gas flow plate 1450. The inlet cover 1410 may be configured similarly to the inlet cover 110 described above with reference to FIGS. 1 and 6, and the first gas flow plate 1420 may be configured similarly to the second gas flow plate 120 described above with reference to FIGS. 1 and 5. The second gas flow plate 1450 of this embodiment may be configured similarly to the second gas flow plate 130 described above with reference to FIGS. 1 and 4. However, it should be understood that the physical form and structure of the device shown in FIG. 14 is not limiting and that in other embodiments, the device may have an anode electrode 1451 on the same side as the cathode electrode 1440 having one or more gas inlets 1402, and may have a different form than that shown in FIG.

[0083] As represented by the dashed arrows in FIG. 14 , which indicate the direction of gas flow through the device during use, gas enters the device through one or more inlets 1402, then flows laterally through the gap between the anode electrode 1451 and the cathode electrode 1440 (which may be referred to as the anode-cathode gap or “A-K gap”), before passing through the cathode electrode 1440 via multiple hollow cathodes 1441. Because the anode electrode 1451 is located on the same side of the cathode electrode 1440 as the gas inlet, the outlet side of the device can be effectively open to the outside world. A device such as that shown in FIG. 14 can still operate at atmospheric pressure or a low vacuum.

[0084] With the illustrated configuration, the apparatus can be used to provide an ion-rich plasma cloud that can spread over a large surface area. Such a configuration can be advantageous in a variety of applications, including, but not limited to, surface treatment of substrates, such as plastic surfaces 1490 for printing. Plasma treatment of plastic surfaces can be used to improve their abrasiveness, thereby enhancing the effectiveness of the printing process by helping ink adhere more strongly to the plasma-treated surface. Plasma treatment using an apparatus such as that shown in FIG. 14 can also be used to impart desired properties to the substrate 1490, for example, to make the surface of the substrate 1490 hydrophobic or hydrophilic, by appropriate selection of materials, gases, and process conditions.

[0085] In some embodiments, a modular surface treatment apparatus can be constructed by connecting multiple plasma reactor modules similar to that shown in Figure 14 in parallel with respect to the gas flow path through the apparatus. The plasma reactor modules can be arranged over a large area to provide simultaneous plasma treatment of large surface areas of the substrate 1490, which can be particularly advantageous in high throughput industrial applications such as reel-to-reel processing of plastic sheets or other forms of substrate 1490.

[0086] Furthermore, in some embodiments, an apparatus similar to that shown in FIG. 14 can be used to provide a large-surface ion-rich plasma from which ions can be extracted to produce a large-surface ion beam. Used in this manner, the apparatus effectively behaves like a surface having an extremely low work function. In such embodiments, the gas supplied to the inlet(s) 1402 can be selected depending on the ion species desired at the outlet side of the apparatus. Yet another potential application of an apparatus such as that shown in FIG. 14 includes high-pressure etching or ashing of a substrate 1490, for example, to strip a photoresist layer from a photolithography substrate.

[0087] 15, there is shown a circuit for supplying power to an apparatus for generating a plasma, according to one embodiment of the present invention. The circuit shown includes a power supply 1570 for supplying power to the apparatus. A cathode electrode 1541 of the apparatus is connected to the negative terminal of the power supply, while an anode electrode 1551 of the apparatus is connected to the positive terminal of the power supply 1570. A capacitor C, which may be referred to as an intermediate energy storage capacitor, is connected to the power supply 1570. L is connected in parallel with the device, as described above with reference to Figure 12. A ballast 1590, including an inductor L in series with a resistor R, is connected in series between the cathode electrode 1541 and the power supply 1570. The ballast 1590 functions to limit the amount of energy delivered by the power supply 1570 to the conductive plasma once the plasma is formed. A capacitor C is also connected between the ballast 1590 and the cathode electrode 1541. L By connecting one terminal of the capacitor C, the amount of energy delivered to the plasma after electrical breakdown is L Thus, the ballast 1590 and the capacitor C L This combination functions to limit the power supply to the device after electrical breakdown occurs, thereby avoiding excessive energy consumption that would otherwise occur due to Joule heating of the plasma.

[0088] 15, in this embodiment, a power supply 1570 includes a first switch S1 and a second switch S2 connected in series between a high-voltage power supply and the positive terminal, and a main capacitor C connected in parallel with the high-voltage power supply and the output positive and negative terminals. One side of the main capacitor is connected to a node between the first switch S1 and the second switch S2. A power supply 1570 as described herein can be used in any of the above-described embodiments.

[0089] The operation of the power supply 1570 is as follows: At time t=t0, the power supply controller closes the first switch S1, charging the main capacitor C to the desired voltage. Once the main capacitor C is charged, the power supply controller opens the first switch S1 and closes the second switch S2, which transfers the energy in the main capacitor C to the intermediate energy storage capacitor C. L In this embodiment, the intermediate energy storage capacitor C L has a capacitance less than or equal to half of the main capacitor C, whereby energy is transferred to the cathode electrode 1541 and the anode electrode 1551, C and C L forms a ringing circuit, resulting in C L The rise time and waveform of this voltage can be adjusted by appropriately selecting the values ​​of inductance L and resistance R of the ballast 1590. The value of R can be chosen so that the LCR ringing circuit is slightly underdamped.

[0090] C L This voltage across is applied directly to the anode electrode 1551 and the cathode electrode 1541, initiating the THCD process. Then, at a later time t>t0, when electrical breakdown occurs and the initiation of a conductive plasma is established and detected, the power supply controller opens the second switch S2, thereby charging the intermediate energy storage capacitor C L, and the cathode and anode electrodes 1541, 1551 are insulated. At this point, only the energy stored in the intermediate energy storage capacitor CL is available to the plasma, which limits the degree to which the plasma heats and reduces energy consumption.

[0091] As mentioned above, in some embodiments, the power supply controller can reduce the power level to a lower level when an electrical breakdown occurs, rather than completely stopping the supply of power. This may be desirable in some situations, for example, to avoid the large back-EMF that would otherwise be generated when the second switch S2 is fully opened. In such an embodiment, the power supply 1570 can include a resistor that can be switchably connected across S2 before S2 is opened, so that a significantly lower power level is provided to the cathode and anode electrodes 1541, 1551 without the second switch S2 being opened. In this case, the second switch S2 can be fully opened after a predetermined time.

[0092] 16 and 17, alternative configurations of the hollow cathode shown in FIG. 2 are shown according to an embodiment of the present invention. In the embodiment of FIG. 16, the hollow cathode of cathode electrode 1640 has an inlet-side conductive surface 1641a of cathode electrode 1640, including an inner conductive surface 1641b that extends through a portion of the hollow cathode's path and terminates just before the anode-side end of the through-hole. In the embodiment shown in FIG. 17, the hollow cathode of cathode electrode 1740 has an inlet-side conductive surface 1741a of cathode electrode 1640 without including a conductive surface inside the through-hole. The hollow cathodes shown in FIGS. 2, 16, and 17 are all capable of supporting a transient hollow cathode discharge effect and can be used in any of the apparatuses for generating plasma via the transient hollow cathode discharge effect described above.

[0093] In each of the above-described embodiments, the inlet and outlet are located on opposite sides of the cathode electrode, such that gas enters the device on one side of the cathode electrode, passes through the hollow cathode, and then exits the device on the opposite side of the cathode electrode. For example, in the embodiment of FIG. 1, the device 100 is configured to define a gas flow path that passes from one or more inlets 102 through a plurality of hollow cathode thickness holes 141 to one or more outlets 103. In this manner, gas flowing through the device passes through the hollow cathode thickness holes 141 and is exposed to the plasma generated therein. This is achieved in the embodiment of FIG. 1 by locating the one or more inlets 102 on opposite sides of the cathode electrode 140 from the one or more outlets 103.

[0094] However, in other embodiments, the one or more inlets 102 and the one or more outlets 103 may be located on the same side as the cathode electrode 140. In the embodiment shown in Figure 18, the device 1800 includes a gas flow barrier 1801, e.g., a solid partition or baffle, located within the chamber on the opposite side of the cathode 1840 from the anode 1851, such that all or a portion of the gas entering the chamber via the one or more outlets 1802 flows through the hollow cathode 1841 on the inlet side of the gas flow barrier 1801 to the anode side of the cathode electrode 1840, and then flows back in the opposite direction through the hollow cathode 1841 on the outlet side of the gas flow barrier 1801 to reach the one or more outlets 103. This allows the one or more inlets 1802 and the outlets 1803 to be located on the same side of the device 1800, which may reduce the overall height of the device 1800.

[0095] Referring now to Figure 19, a modular apparatus including multiple plasma reactor modules arranged in parallel is shown according to one embodiment of the present invention. Modular apparatus 1900, similar to the apparatus described above in connection with Figure 11, includes six plasma reactor modules 1901, 1902, 1903, 1904, 1905, and 1906 connected in parallel to define multiple gas flow paths through apparatus 1900. Plasma reactor modules 1901-1906 are shown looking down from above at multiple hollow cathodes 1940 within each module 1901-1906.

[0096] Although only six plasma reactor modules 1901-1906 are shown in Figure 19, modular apparatus 1900 may further include one or more similar configurations of other plasma reactor modules stacked perpendicular to the plane of Figure 19, i.e., above or below the six plasma reactor modules 1901-1906 shown, to form a vertical stack of plasma reactor modules. In such an embodiment, each additional layer of plasma reactor modules above or below the six plasma reactor modules 1901-1906 may be connected in series with the layer on either side, thereby providing a modular apparatus with multiple plasma reactor modules connected in series and parallel, similar to the apparatus shown in Figure 11.

[0097] The exploded view in the lower left corner of FIG. 19 shows a trapezoidal arrangement of hollow cathodes 1940 in a first half of the plasma reactor module 1901. This trapezoidal arrangement of hollow cathodes 1940 can be referred to as a hollow cathode subassembly. It will be understood that the right side of the trapezoid shown in the exploded view includes a similar hollow cathode 1940 arrangement as shown on the left side, but only one half is shown for clarity. Similarly, it will be understood that the other five trapezoidal sections of the first plasma reactor module 1901 have a similar hollow cathode 1940 arrangement (rotated as necessary) as shown in the exploded view. The second through sixth plasma reactor modules 1902-1906 can all have a similar hollow cathode arrangement as the first plasma reactor module 1901, but again, only one structure of the modules is shown in FIG. 19 for clarity.

[0098] In comparison with the embodiment described above with reference to Figures 3-9, the actual cells of each plasma reactor module 1901-1906 in this embodiment are trapezoidal in shape, thereby enabling the construction of plasma reactor modules with hexagonal cross sections. The outer regions of the hexagonal plasma reactor modules 1901-1906 represent structures on which circuit boards or other suitable substrates housing hollow cathode electrodes are mounted. The anode units of each plasma reactor module 1901-1906 are similar in size to the hollow cathode units illustrated for the first plasma reactor module 1901, except that fluid communication holes are located around the periphery of the anode electrode units to provide a gas flow path so that gas can flow to the next layer in the stack.

[0099] The illustrated packing arrangement of the hollow cathodes 1940 offers advantages over the rectangular design shown in FIG. 3. In particular, the trapezoidal form factor leaves room for a central passageway 1901a to be formed when a group of six rotated trapezoids is arranged in a hexagon, as shown for the first plasma reactor module 1901. This central opening can accommodate a negative power line configured to supply negative high-voltage pulses to each of the six hollow cathode subassemblies 1940 within the plasma reactor module 1901. The six hollow cathode subassemblies 1940 can be connected to a common ground return located at the outer edge of the plasma reactor module 1901. This provides a coaxial configuration that helps minimize electromagnetic emissions resulting from the high-voltage drive pulse, particularly during the breakdown phase of the transient hollow cathode discharge.

[0100] A further advantage over the rectangular arrangement shown in FIG. 3 is that the parallel arrangement of six separate hollow cathode subassemblies in the hexagonal configuration shown ensures that failure of a particular subassembly will have minimal impact on the performance of each hexagonal layer, and thus the performance of the plasma reactor module 1901 as a whole.

[0101] A further advantage of the illustrated arrangement is that the close packing of the six hollow cathode assemblies with the central high voltage drive pulse supply allows for more efficient use of space in the overall modular apparatus, reducing the overall size of the apparatus for a given performance level. This is possible because there is no need to provide high voltage isolation between the different hexagonal plasma reactor modules 1901-1906 if the exterior surface of each plasma reactor module 1901-1906 is used as a common ground return.

[0102] 20-22, a capacitive driver circuit configured to provide power pulses to a plurality of plasma reactor modules according to one embodiment of the present invention is shown. FIG. 20 is a circuit diagram showing the connections between components that make up the capacitive driver circuit, FIG. 21 is a perspective view of an anode structure that can be used in each plasma reactor module, and FIG. 22 is a cross-sectional view of the anode structure in FIG. 21. A driver circuit such as that shown in FIG. 20 and / or an anode structure such as that shown in FIGS. 21 and 22 can be used in any of the plasma generator embodiments described above.

[0103] The capacitive driver circuit includes a voltage source (V), first and second switches S1 and S2, first and second inductors L2 and L3, and first and second capacitors C1 and C2. The output of the driver circuit is connected in parallel to the anode and cathode structures of each of a plurality of plasma reactor modules (APR) 2006 and 2007. In FIG. 20, each APR 2006 and 2007 includes a parallel-connected time-varying capacitor C A and time-varying resistor R A The illustrated drive circuitry can power any number of APRs, as indicated by the dashed lines at the bottom of FIG.

[0104] A first switch S1 is connected between the high-voltage side of the voltage source and a first node 2001. A second switch S2 is connected between the first node 2001 and a second node 2002. A first inductor L2 is connected between the second node 2002 and a third node 2003 via a resistor R2. A second inductor L3 is connected between the third node 2003 and a common high-voltage output 2004. The cathodes of the multiple APRs are connected to the common high-voltage output via their respective integral isolation capacitors C4 and one or more transmission lines, as shown in FIG. 20. A first capacitor C1 is connected between the first node 2001 and the low-voltage side of the voltage source, and a second capacitor C2 is connected between the third node 2003 and the low-voltage side of the voltage source. The low-voltage side of the voltage source is also connected to a common ground 2005, to which the anodes of the multiple APRs 2006 and 2007 are connected via respective transmission lines.

[0105] The anode structure shown in Figures 21 and 22 includes an integrated isolation capacitor C4, which has a relatively large capacitance compared to other capacitors in the drive circuit. The anode structure 2101 includes an upper anode electrode in the form of a first conductive layer 2101a and an intermediate anode electrode in the form of a second conductive layer 2101b. The first and second conductive layers 2101a, 2101b are separated by an insulating dielectric 2102 and connected through a conductive path, which in this embodiment is provided as a via 2104. Together, the upper and intermediate electrodes function as the anode of the plasma reactor module. The anode structure further includes a lower electrode 2103 in the form of a third conductive layer separated from the second conductive layer 2101b by the insulating dielectric 2102. Together, the second and third conductive layers 2101b, 2103 function as the integrated isolation capacitor C4.

[0106] The operation of the drive circuit over time can be divided into three distinct periods: (1) The initial application across the A‐K gap when ionized growth occurs under the hollow cathode configuration. (2) A transition to the hollow cathode discharge phase occurs when sufficient ionization is established across the A-K gap and a conductive plasma begins to form. (3) Plasma formation by current flow continues, and the locally accumulated electrostatic energy is converted into energy in the formed plasma and dissipated into plasma heating and radiation, thereby ending the high current drive phase.

[0107] The operation of the entire drive circuit, including the function of the isolation capacitor C4, will be described in more detail below. It will be appreciated that the operation of the various switches is controlled by a suitable controller arranged to generate the control signals required to open and close each switch at the appropriate times: 1. A current drive cycle begins with switch S2 opening and S1 closing, which charges capacitor C1 to a predetermined high voltage. 2. Once capacitor C1 is charged, switch S1 opens, isolating capacitor S1 from the voltage source, and S2 closes, transferring the charge on C1 to C2 via the resonant circuit defined by L2, with R2 providing the required damping. C2 has a smaller capacitance than C1, which allows for "ring-up" gain (i.e., an increase in the voltage initially stored on C1). 3. This process of charging C2 will also charge C4 and C A This also allows the isolating inductance L3 and the transmission line to be charged. For a duration that is long compared to the transit time through the transmission line, the transmission line will A ) combination acts as both a series inductance and a parallel capacitance. 4.C4 is C A Since it is much larger (say, about twice as large), most of the voltage established across C2 is A appears in between. 5. A high voltage across the A-K electrodes initiates an ionization growth process within the hollow cathode geometry, which, as described above, ultimately creates a virtual anode that moves from the physical anode surface towards the hollow cathode. 6. This reduction in distance between the virtual anode and cathode surfaces increases the effective capacitance across the A-K gap, hence C A This increases the electrostatic energy stored in the 7. As the virtual anode penetrates beyond the hollow cathode opening, the charge multiplication increases rapidly, a tenuous plasma forms, and substantial current begins to flow. The plasma across the A-K gap becomes resistive in nature. 8. Once continuous current flow is established, the plasma resistance decreases rapidly, supported by the electrostatic energy initially stored across the A‐K gap. 9. As the plasma resistance decreases, the current flow increases, since there is virtually no inductive component to limit the rate of current rise (simple RC discharge circuit). The increasing current flow leads to further heating of the plasma, thus decreasing the plasma resistance and further increasing the current flow. This positive feedback process (negative resistance) continues to produce a highly ionized plasma until the electrostatic energy initially stored across the A-K gap is completely consumed. The processes described in points 10.7–9 occur on a nanosecond (ns) time scale, whereas the processes described in points 2–3 occur on a time scale of several hundred ns. 11. As the local energy stored across the A-K gap is consumed, the drive current across the discharge decreases to a value determined by the charging current of C4 through L3 and the transmission line. This current is significantly lower than the high drive current during the hollow cathode discharge phase. The plasma begins to cool and the recombination process begins, returning the plasma to a high resistivity state. This high resistivity plasma reduces the charging current through C4, which further accelerates the cooling of the plasma. This point can be considered the end of the drive current. 12. By appropriate selection of circuit component values, particularly C2, C4, transmission line length, and the capacitance value of the A-K gap, the high current drive can be controlled to terminate at a desired point after the formation of the highly conductive plasma. 13. The introduction of C4 effectively provides an insulating element in the plasma discharge process. 14. In the event of a short circuit across the A-K gap, e.g., due to a mechanical and / or electrical fault, the equivalent circuit of this short circuit has a low resistance, and the current through the short circuit is determined by the charging current flowing through C4. By making C4 >> C2, this fault current can be limited to a low value, effectively isolating the shorted module from the other modules connected in parallel. This allows for a self-insulated and fault-tolerant design for systems with multiple plasma reactor modules (APR), such as the modular system described above.

[0108] Additionally, in some embodiments, a capacitive driver circuit such as that shown in Figure 20 may be used to power other types of devices other than APRs. Similarly, anodes with integrated isolation capacitors such as those shown in Figures 21 and 22 may be used in devices other than APRs, and are therefore not limited to use with anode electrodes in APRs.

[0109] Although particular embodiments of the invention have been described herein with reference to the drawings, it will be understood that many variations and modifications can be effected within the spirit and scope of the invention as defined in the appended claims.

Claims

1. 1. An apparatus for generating a plasma via a transient hollow cathode discharge effect, the apparatus comprising: a chamber having an inlet through which a gas can flow and an outlet through which the gas can flow; a cathode electrode disposed within the chamber, the cathode electrode including a plurality of hollow cathodes each having a thickness-wise hole through which the gas can pass from one side of the cathode electrode to the other side of the cathode electrode, the apparatus being configured to define a gas flow path passing from the inlet through the thickness-wise holes of the plurality of hollow cathodes to the outlet; an anode electrode spaced apart from the cathode; a power supply electrically connected to the anode electrode and the cathode electrode to supply power for generating plasma at the plurality of hollow cathodes; a power supply controller configured to reduce the power level of the electrical power to below a first power level required to sustain the plasma at the plurality of hollow cathodes after an electrical breakdown occurs; An apparatus comprising:

2. 10. The apparatus of claim 1, wherein the power supply controller is configured to reduce the power level by removing power from the cathode and the anode.

3. 3. The apparatus of claim 1 or 2, wherein the power supply controller is configured to reduce the power level after an electrical breakdown occurs by supplying the power at a level equal to or exceeding the first power level for a predetermined time, the predetermined time being a time sufficient for electrical breakdown to occur.

4. 3. The apparatus of claim 1, further comprising means for detecting a characteristic indicative of an occurrence of an electrical breakdown, and wherein the power supply controller is configured to reduce the power level of the power below the first power level in response to detecting the characteristic indicative of an occurrence of an electrical breakdown.

5. 5. The apparatus of claim 4, wherein the characteristic comprises a change in current and / or voltage.

6. 6. The apparatus of claim 1, wherein the power supply controller is configured to control the power supply to supply the power intermittently as a sequence of voltage pulses, and to set a frequency of the voltage pulses in response to a flow rate of gas passing through the chamber.

7. 7. The apparatus according to claim 1, wherein the inlet and the outlet are located on opposite sides of the cathode electrode.

8. 8. The apparatus of claim 7, wherein the chamber is configured to allow gas to flow only from one side of the cathode electrode through the plurality of hollow cathodes to the other side.

9. 9. The apparatus according to claim 1, wherein the anode electrode is located on the opposite side of the inlet from the cathode electrode.

10. 10. The apparatus of claim 9, wherein the anode electrode is positioned to form a gas flow barrier extending across all or part of a face of the cathode electrode opposite the anode electrode, such that gas exiting the hollow cathode is deflected by the anode electrode to flow laterally across the face of the cathode electrode.

11. 11. The apparatus of claim 10, wherein a separation distance between the anode electrode and the cathode electrode is set such that resistance to lateral gas flow in the gap between the anode electrode and the cathode electrode is less than resistance to gas flow through the plurality of hollow cathodes, whereby a gas flow rate through the chamber is dependent on resistance to gas flow through the plurality of hollow cathodes.

12. 11. The apparatus according to claim 1, wherein a separation distance between the anode electrode and the cathode electrode is set so that resistance to lateral gas flow in a gap between the anode electrode and the cathode electrode is greater than resistance to gas flow through the plurality of hollow cathodes.

13. 13. The apparatus of claim 1, further comprising an electrically insulating or semi-insulating layer disposed between the anode and cathode electrodes, which limits the amount of energy delivered to the plasma after electrical breakdown occurs.

14. Apparatus according to any one of the preceding claims, wherein, in use, the apparatus is capable of generating the plasma whilst the gas in the chamber is at atmospheric pressure.

15. 15. The apparatus according to claim 1, wherein the plurality of hollow cathodes are arranged in a plurality of groups of hollow cathodes, and the cathode electrode has a plurality of conductive paths, each of which connects the hollow cathodes in one group to each other to supply the power to the group of hollow cathodes, and the hollow cathodes in adjacent groups are separated from each other by an electrically insulating region.

16. 16. The apparatus of claim 15, wherein the plurality of hollow cathodes are arranged in a plurality of rows on the cathode electrode, with one group including a plurality of hollow cathodes on one row and an adjacent group including a plurality of hollow cathodes on an adjacent row.

17. 17. An apparatus according to any one of claims 1 to 16, wherein the inner surface of each hollow cathode comprises a coating of a material that is toxic to biological samples.

18. 18. The device of claim 17, wherein the material comprises gold, silver, palladium and / or copper.

19. 19. The device of claim 1, wherein the power supply is a capacitive driving circuit, the capacitive driving circuit comprising: a voltage source having a high voltage terminal and a low voltage terminal; a first switch connected between the high voltage terminal and a first node; a second switch connected between the first node and a second node; a first inductor connected between the second node and a third node via a resistor; a second inductor connected between the third node and a high voltage output connected to the cathode electrode; a first capacitor connected between the first node and the low voltage terminal; a second capacitor connected between the third node and the low voltage terminal further connected to the anode electrode; 1. An apparatus comprising:

20. 20. The device according to any one of claims 1 to 19, wherein the anode electrode has an integrated insulating capacitor, the anode electrode comprising: a top electrode in the form of a first conductive layer; an intermediate electrode in the form of a second conductive layer, the first and second conductive layers being separated by an insulating dielectric and connected to each other via a conductive path passing through the insulating dielectric; a bottom electrode in the form of a third conductive layer separated from the second conductive layer by the insulating dielectric; Including, the second and third conductive layers together function as the integrated isolation capacitor.

21. A modular apparatus comprising a plurality of plasma reactor modules, each of the plurality of plasma reactor modules comprising an apparatus according to any one of claims 1 to 20.

22. 22. The modular apparatus of claim 21, wherein two or more of the plasma reactor modules are connected in series, such that gases flowing out the outlet of one of the plasma reactor modules flow into the inlet of the next of the series-connected plasma reactor modules.

23. 23. The modular apparatus of claim 22, wherein the number of plasma reactor modules connected in series is selected to achieve desired characteristics of gases exiting the modular apparatus after passing through the plurality of plasma reactor modules.

24. 24. The modular apparatus of claim 21, 22, or 23, wherein two or more of the plasma reactor modules are connected in parallel to define a plurality of gas flow paths through the modular apparatus, whereby gas entering the modular apparatus is divided among the plurality of gas flow paths, and a portion of the gas flowing along each of the gas flow paths need only pass through a corresponding one of the parallel-connected plasma reactor modules before exiting the modular apparatus.

25. 25. The modular apparatus of claim 24, wherein the number of plasma reactor modules connected in parallel is selected to achieve a desired gas flow rate through the modular apparatus.

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