Discharge cell systems and methods

Described herein are systems and methods for ensuring plasma homogeneity in a discharge cell. The discharge cell may include a first hollow electrode and a second hollow electrode spaced away from the first electrode to define a discharge gap therebetween. A fluid inlet port may in fluid communication with an internal bore of the first electrode. A fluid outlet port may be in fluid communication with the discharge gap. A first pair of viewports may define a first optic pathway through the discharge gap. A second pair of viewports may define a second optic pathway through the discharge gap. A third pair of viewports may define a third optic pathway through the discharge gap, the third optic pathway defined through the hollow interior of the first and second electrodes.

TECHNICAL FIELD

The present invention relates generally to discharge cell systems and methods and, more particularly, to discharge cell configurations designed to provide a homogeneous, strongly excited, low-temperature, non-equilibrium plasma with controllable and repeatable parameters.

BACKGROUND

Discharge cells provide reference sources of gas/plasma excitation and can be utilized as diagnostic and/or calibration devices for many applications. Some designs, however, may not permit homogeneous plasma production with controllable and repeatable parameters. Thus, a need in the art exists for discharge cell systems and methods that provide a homogeneous, strongly excited, low-temperature, non-equilibrium plasma with controllable and repeatable parameters.

DETAILED DESCRIPTION

In various embodiments, systems and methods for ensuring plasma homogeneity in a discharge cell are provided. The discharge cell is designed to provide a homogeneous, strongly excited, low-temperature, non-equilibrium plasma with controllable and repeatable parameters. The discharge cell systems and methods described herein may provide a reference source of gas/plasma excitation that can be utilized as a calibration device for many existing and future diagnostic tools.

In various embodiments, a discharge cell includes a first hollow electrode and a second hollow electrode spaced away from the first electrode to define a discharge gap therebetween. A fluid inlet port may in fluid communication with an internal bore of the first electrode. A fluid outlet port may be in fluid communication with the discharge gap. A fluid pathway may be defined from the fluid inlet port, through the first internal bore and the first tip of the first electrode to the discharge gap, and to the fluid outlet port. A first pair of viewports may define a first optic pathway through the discharge gap. A second pair of viewports may define a second optic pathway through the discharge gap. A third pair of viewports may define a third optic pathway through the discharge gap, the third optic pathway defined through the hollow interior of the first and second electrodes.

In various embodiments, a method of ensuring plasma homogeneity in a discharge cell includes positioning a first hollow electrode within a housing, positioning a second hollow electrode within the housing, and defining a fluid pathway through the first electrode and the housing. The first electrode may have a first internal bore and a first tip. The second electrode may have a second internal bore and a second tip. The second electrode may be positioned to define a discharge gap between the first tip and the second tip. The housing may include a fluid inlet port and a fluid outlet port. The fluid pathway may extend from the fluid inlet port, through the first internal bore and the first tip, and to the fluid outlet port.

Formation of homogeneous plasma in molecular gases may be governed by various criteria. For example, formation of homogeneous plasma may be governed by the criteria established in U.S. Pat. No. 8,011,348, the disclosure of which is hereby incorporated by reference, for all purposes. Assuming the discharge has been generated by applying a high voltage electric pulse across a discharge gap, the criteria may be the following:

1) High-voltage pulse amplitude limited by the constraint, U [kV]>3·10−18×L×n, setting the value of reduced electric field E/n in the discharge gap after it is overlapped by the ionization wave at an E/n value greater than 300 Td;

2) High-voltage pulse amplitude limited by the constraint, U [kV]<3·10−17×L×n, setting the value of the reduced electric field E/n in the discharge gap after it is overlapped by the breakdown wave at the level of lower than 3000 Td;

3) High-voltage pulse leading edge rise time limited by the constraint, tf[ns]<3·10−18×L2×n/U;

4) High-voltage pulse leading edge rise time limited by the constraint, tf[ns]>RC;

5) High-voltage pulse duration limited by the constraint, tpul[ns]<3·1020×(L×R)/n;

6) High-voltage pulse duration limited by the constraint tpul[ns]>1017/n; and

7) Pulse interval limited by the constraint, 1026U/(n×L2)>fpul>V/L

R—impedance of the high-voltage PS with cable,

C—capacitance of the discharge cell,

n—molecular concentration in the unit of discharge section volume [cm−3],

fpul—discharge frequency [Hz], and

V—gas flow speed in the discharge section [cm/sec].

The above criteria or constraints may provide many benefits. For instance, the first criteria above may provide maximum discharge energy deposition into the electronic degrees of freedom and gas dissociation. The second criteria above may limit plasma electrons transfer into the run-away mode during the main stage of discharge and may minimize electron energy increase loss, electron beam formation, and X-ray emission. The third criteria above may allow increased voltage on the high-voltage electrode, thereby yielding an electric field intensity that is sufficient for electrons transfer into the run-away mode at the ionization wave front within a time span that is less than the time of overlapping of the discharge gap, thus ensuring the uniformity of filling the discharge gap with plasma. The fourth criteria above may allow the condition to match the high-voltage impulse generator with the discharge cell, which may ensure that the pulse energy transfer to plasma is highly effective. The fifth criteria above may limit the total energy input into gas-discharge plasma, thereby suppressing discharge instability development and providing a strong non-equilibrium pulse discharge plasma. The sixth criteria above may account for final time of electron multiplication in the discharge gap within the limits of fields limited by the first and second constraints. Such a condition may be required for gas ionization development in the gap after it is overlapped by the ionization wave, which then causes a reduction of the discharge gap resistance and a subsequent match of the discharge cell with the generator, thereby leading to an effective electric energy deposition into plasma. The seventh criteria above may provide a stable proceeding of chemical reactions in a continuous mode.

The above constraints may provide uniformity of gas excitation in a continuous mode (fpul>V/L) and high effectiveness of strong non-equilibrium regime of excitation by nanosecond discharge with high duty-cycle ratio (1026U/(n×L2)>fpul). Such ensures that when the time between pulses exceeds the pulse duration and provides time that is sufficient for plasma recombination and recovery of electric strength of the discharge gap, thereby assuring operation in the selected ranges of reduce electric fields.

FIG. 1illustrates a cross-sectional view of a discharge cell100, according to an example of an embodiment of the present disclosure. The discharge cell100may be arranged or otherwise designed to satisfy the criteria noted above. As shown inFIG. 1, the discharge cell100includes a first electrode102and a second electrode104spaced away from the first electrode102to define an interelectrode or discharge gap106between the first and second electrodes102,104. For example, the first electrode102may be positioned in a spaced relationship above the second electrode104, as shown inFIG. 1, or vice versa. ThoughFIG. 1illustrates the first and second electrodes102,104in vertical alignment, the first and second electrodes102,104may be spaced horizontally or diagonally, among others, from each other depending on the application. The size of the discharge gap106may be controlled to ensure a desired plasma production for a wide range of gas concentrations and gas types. For instance, the first electrode102may be moved towards or away from the second electrode104, or vice versa, to ensure the fulfillment of the seven criteria or constraints described above to ensure the creation/production of a homogeneous, strongly excited, low-temperature, non-equilibrium plasma in the discharge gap106.

The first and second electrodes102,104may be any electrical conductor used to create a circuit. Depending on the application, the first electrode102may be referred to as a low-voltage electrode, a negative electrode, a cathode, or an anode. The second electrode104may be referred to as a high-voltage electrode, a positive electrode, a cathode, or an anode.

The first and second electrodes102,104may include many configurations ensuring plasma homogeneity in the discharge gap106. For instance, the first electrode102may have a first internal bore110such that the first electrode102is hollow. In such embodiments, the first electrode102may be referred to as a first hollow electrode. In some embodiments, the first electrode102may be shaped as a hollow cone with a first base112and a first tip114. Such a hollow cone shape may 1) increase the effective area of emission from the electrode, 2) limit development of cathode or anode spots, and/or 3) increase the uniformity of the plasma in the discharge gap106. The first tip114of the first electrode102may be shaped with a sharp edge. The sharp edge may 1) increase the magnitude of the electric field at the edge of the first electrode102, 2) facilitate the start of the discharge, and/or 3) increase the uniformity of the plasma in the discharge gap106.

The second electrode104may be configured similarly to the first electrode102. For example, the second electrode104may have a second internal bore120such that the second electrode104is hollow and is referred to as a second hollow electrode. The second electrode104may be shaped as a hollow cone with a second base122and a second tip124. The hollow cone shape of the second electrode104may provide the same benefits outlined above, whether alone or in combination with the hollow cone shape of the first electrode102. The second tip124of the second electrode104may be shaped with a sharp edge, with the sharp edge of the second electrode104providing the same benefits outlined above, whether alone or in combination with the sharp tip edge of the first electrode102.

In various embodiments, the discharge cell100may be configured to facilitate replacement of fluid in the discharge gap106. For example, the configuration of the discharge cell100may ensure a complete change of fluid in the discharge gap106due to organization of fluid flow through at least one of the first and second electrodes102,104. Depending on the application, replacement of fluid within the discharge gap106may be provided from a low voltage electrode, such as the first electrode102, to ensure the absence of a parasitic discharge in the fluid supply and/or return lines. The fluid may be any one or combination of liquids, gases, and plasma. For example, the fluid may be an inert gas, though any other fluid is contemplated permitting plasma production/discharge in the discharge gap106.

As shown inFIG. 1, the discharge cell100may include a fluid inlet port130and a fluid outlet port132. In such embodiments, fluid may flow from the fluid inlet port130and to the fluid outlet port132to replace the fluid within the discharge gap106. The fluid inlet port130may be in fluid communication with the first internal bore110of the first electrode102. In such embodiments, a fluid pathway140may be defined from the fluid inlet port130through the first internal bore110and the first tip114of the first electrode102. The fluid outlet port132may be configured and/or positioned such that fluid exiting the first tip114of the first electrode102exits through the fluid outlet port132. In one or more embodiments, the fluid outlet port132may be adjacent to an external side surface142of the first electrode102. In this manner, fluid contaminated with plasma products may be pumped out through the fluid outlet port132along or near the external side surface142of an electrode. The fluid inlet port130and the fluid outlet port132may be defined by one or more elements connected to the discharge cell100. For instance, each of the fluid inlet port130and the fluid outlet port132may be defined by a valve, fitting, pipe, conduit, hose, or the like, or any combination thereof, connected to the discharge cell100. In some embodiments, the fluid inlet port130and the fluid outlet port132may be defined by one or more structures of the discharge cell100itself. For example, the fluid inlet port130and/or the fluid outlet port132may be defined at least partially by cutouts, apertures, or bores defined in or through portions of the discharge cell100.

In various embodiments, the discharge cell100may include one or more viewports150permitting the use of one or more optical and/or laser diagnostic methods of the plasma generated in the discharge gap106. Depending on the application, the viewports150may be arranged in opposing pairs to define one or more diagnostic pathways through the discharge gap106. For instance, the discharge cell100may include a first pair of viewports152defining a first optic pathway154through the discharge gap106. The first pair of viewports152may be positioned on a first set of opposing sides of the discharge cell100, such as on opposing left and right sides of the discharge cell100, to define the first optic pathway154along a first axis156. The discharge cell100may include a second pair of viewports160defining a second optic pathway162through the discharge gap106. The second pair of viewports160may be positioned on a second set of opposing sides of the discharge cell100, such as on opposing front and rear sides of the discharge cell100, to define the second optic pathway162along a second axis164. The discharge cell100may include a third pair of viewports170defining a third optic pathway172through the discharge gap106. The third pair of viewports170may be positioned on a third set of opposing sides of the discharge cell100, such as on opposing top and bottom sides of the discharge cell100, to define the third optic pathway172along a third axis174. In some embodiments, the first, second, and third optic pathways154,162,172may define mutually perpendicular axes. For example, the first, second, and third axes156,164,174may be mutually perpendicular to one another. In this manner, the first, second, and third optic pathways154,162,172may define a 6-way cross. The optic pathways may permit a laser line to be transmitted through the discharge gap106. The optic pathways may also permit other optic diagnostic methods of the discharge gap106.

The one or more viewports150may include many configurations. For instance, each viewport150may include a first end180, an opposing second end182, and an optical window184. The first end180may be positioned adjacent to the discharge gap106, with the second end182positioned away from the discharge gap106. The optical window184may be positioned adjacent to the second end182to space the optical window184away from the discharge gap106. Such a configuration may limit contamination of the optical window184with discharge products or material. Spacing the optical window184away from the discharge gap106may also create necessary space for beam focusing, such as required for laser diagnostic methods of the discharge gap106.

In various embodiments, the discharge cell100may include one or more housings securing the various components together and/or shielding the discharge cell100from contaminants. For instance, the discharge cell100may include an outer housing190and an inner housing192mounted within the outer housing190. The outer housing190may be conductive to limit the electromagnetic noise created by the discharge and to facilitate the plasma diagnostic methods of the discharge. For instance, the outer housing190may be constructed of aluminum or other conductive metal. As shown inFIG. 1, the fluid inlet port130and the fluid outlet port132may be defined through the outer housing190. In some embodiments, the fluid inlet port130and the fluid outlet port132may be defined through the outer housing190adjacent to the first electrode102to facilitate efficient replacement of fluid in the discharge gap106via the first electrode102. For instance, the fluid pathway140may be defined through the outer housing190via the fluid inlet port130, through the first internal bore110and first tip114of the first electrode102, along or adjacent to the external side surface142of the first electrode102, and through the outer housing190via the fluid outlet port132. The outer housing190may be mountable to an external base or structure, such as to a testing or holding apparatus. For instance, the outer housing190may be secured to an external structure or apparatus via one or more fasteners194or other attachment means.

The inner housing192may be a nonconductive casing mounted within the outer housing190. For example, the inner housing192may be a quartz tube or a glass cell, though other configurations are contemplated. At least a portion of each of the first and second electrodes102,104may be mounted within the inner housing192to define the discharge gap106between the first electrode102, the second electrode104, and the inner housing192. At least a portion of the inner housing192may define the fluid pathway140through which fluid is replaced in the discharge gap106. For example, as shown inFIG. 1, the inner housing192may include a cutout196to define a portion of the fluid pathway140from the discharge gap106to the fluid outlet port132. Depending on the application, the cutout196may be defined adjacent to the first base112of the first electrode102to facilitate fluid replacement along or near the external side surface142of the first electrode102.

As shown inFIG. 1, at least one of the viewports150, such as the first pair of viewports152and the second pair of viewports160, may extend through the outer housing190for positioning adjacent to the inner housing192. For instance, the first pair of viewports152and the second pair of viewports160may penetrate the outer housing190to position the first end180of the viewports adjacent to the inner housing192, such as in close proximity or in abutting engagement.

The discharge cell100may include various other components for convenience. For instance, the discharge cell100may include a direct current sensor measuring the electrical current passing through the low-voltage electrode. As shown inFIG. 1, the discharge cell100may include a power supply connection200. The power supply connection200may be adjacent to the second base122of the second electrode104to supply electrical power to the second electrode104.

FIG. 2illustrates a perspective view of the discharge cell100, according to an example of an embodiment of the present disclosure.FIG. 2illustrates the first electrode102, one viewport of the first pair of viewports152(e.g., a left viewport), the second pair of viewports160, and one viewport of the third pair of viewports170(e.g., a top viewport).FIG. 2also illustrates the outer housing190, a top portion of the inner housing192, the fluid inlet port130, the fluid outlet port132, and the power connection.FIG. 2further illustrates a fluid supply line210connected to the fluid inlet port130, and a fluid return line212connected to the fluid outlet port132. The first optic pathway154through the first pair of viewports152is also illustrated inFIG. 2.

FIG. 3is a flowchart detailing a method300of ensuring plasma homogeneity in a discharge cell, according to an example of an embodiment of the present disclosure. In block302, the method300includes positioning a first hollow electrode within a housing. In block304, the method300includes positioning a second hollow electrode within the housing. The first hollow electrode may be similar to the first electrode102ofFIG. 1, described above. For instance, the first hollow electrode may include a first internal bore and a first tip. The second hollow electrode may be similar to the second electrode104ofFIG. 1, described above. For instance, the second hollow electrode may include a second internal bore and a second tip. The second hollow electrode may be positioned within the housing to define a discharge gap between the first tip and the second tip of the first and second hollow electrodes, respectively. The discharge gap may be similar to the discharge gap106ofFIG. 1, described above.

Block302may include mounting at least a portion of the first hollow electrode within a nonconductive housing portion of the discharge cell, such as within the inner housing192ofFIG. 1, described above. Block304may include mounting at least a portion of second hollow electrode within the nonconductive housing portion of the discharge cell, such as within the inner housing192ofFIG. 1, described above. In some embodiments, the method300may include mounting the nonconductive housing portion within a conductive housing portion of the discharge cell, such as within the outer housing190ofFIG. 1, described above. The relative positions of the first and second hollow electrodes may be modified to control the size of the discharge gap, such as for different gas concentrations and/or gas types. For instance, the position of the first hollow electrode may be modified relative to the second hollow electrode to change one or more dimensions of the discharge gap to ensure homogeneity of plasma production within the discharge gap.

In block306, the method300includes defining a fluid pathway through the first hollow electrode and the housing. The fluid pathway may extend from a fluid inlet of the housing (e.g., the fluid inlet port130ofFIG. 1, described above), through the first hollow electrode, and to a fluid outlet of the housing (e.g., the fluid outlet port132ofFIG. 1, described above). In such embodiments, the method300may include moving fluid through the fluid pathway to replace the fluid in the discharge gap. In this manner, fluid within the discharge gap and contaminated with plasma products may be replaced with uncontaminated fluid moving through the fluid pathway. In some embodiments, fluid may be moved through the fluid pathway in a continuous manner during operation of the discharge cell.

In some embodiments, the method300may include monitoring the plasma production/discharge within the discharge gap. For instance, one or more pairs of viewports (e.g., the first, second, and third pairs of viewports ofFIG. 1, described above) may be mounted to the housing adjacent to the discharge gap to define one or more optic pathways through the discharge gap (e.g., the first, second, and third optic pathways154,162,172ofFIG. 1, described above). The optic pathways may permit one or more plasma diagnostic methods, such as one or more optical and/or laser diagnostic methods of the plasma generated in the discharge gap.