Patent Application
20250183007
Cold plasma is a critical technology in many application fields, including microelectronic fabrication, plasma medicine, flow control, lighting, propulsion, and sterilization. However, generating stable plasma is not a trivial task as energy-hungry machines are often required. Currently, igniting and sustaining plasma is usually performed by using either high-voltage pulses (e.g., 100s of V to kV) or high-power radio frequency (RF) sources (e.g., 10s of W). Therefore, even though low-power plasma with effective surface power density on the order of 0.1 to 1 W/cm2 is sufficient for many applications, including some medical ones, most current plasma sources are bulky and expensive units as they are inefficient in transferring energy to the plasma. Hence, efficient plasma with low power consumption would impact a wide range of applications such as plasma medicine, food and water decontamination, lighting, and reconfigurable RF electronics.
Although DC, pulse, and RF plasmas have been extensively explored, there is no comprehensive understanding of microwave plasma. This is despite the fact that microwave plasma occurs in the a-discharge regime with an extremely low sheath voltage drop, ensuring that the ignited plasma is stable with no electrode erosion as an important lifetime issue. Also, higher degrees of ionization and dissociation, higher densities of electrons and reactive species, lower heavy particle temperatures, and lower breakdown voltage are other advantages of microwave plasma compared to other types of electrically excited plasma. Non-resonant microwave plasma sources, however, are also realized by employing bulky and high-power supplies. In addition, they come at a prohibitively high cost except for high-end applications. Moreover, the resulting high voltages reduce power efficiency, require cumbersome safety protocols, and create a poor environment from an electromagnetic (EM) compatibility perspective.
Due to their ability to store and enhance EM energy, it is possible to employ microwave resonant structures to achieve high-efficiency plasma with low power consumption. The main principle is to utilize resonators that can concentrate the electromagnetic fields over a small gap. In this case, even with considerably low levels of input power, the magnitude of EM fields over those critical gaps can reach the breakdown threshold, resulting in gas breakdown and plasma formation. Since the effective size of the gap decreases after plasma formation, the required amount of power for sustaining plasma is usually even less. Before plasma ignition, the unloaded resonator produces strong fields necessary for gas breakdown. The higher the quality factor of the resonator, the higher the field enhancement. After ignition, however, the plasma impedance interacts with the resonator, quenching the resonator's quality factor. Thus, resonant structures also operate as so-called “ballasts” to avoid plasma instability, such as the glow-to-arc transition and streamer formation.
Different microwave resonant structures (e.g., quarter/half wavelength, ring, and dielectric) implemented using various technologies (microstrip, stripline, coplanar waveguide, coaxial, etc.) have been successfully examined for low-power plasma creation. However, (1) most of them do not operate in atmospheric pressure, which makes them difficult to be implemented in many practical scenarios, (2) the ignited plasma region is typically confined to a minimal volume and, hence, not optimal for many applications, and (3) it is impractical in most of the cases to scale up the resonant designs to larger effective areas.
In sum, plasma jets have many applications, such as in the biomedical field, or for plasma propulsion or plasma processing. Most plasma jets employ either RF or pulse excitation. However, these devices are typically not efficient, energy hungry, bulky, heavy, and expensive. Also, because of the high power/voltage involved, safety is a big concern. There are other microwave plasma jets, in both resonant and non-resonant modes, but still typically require high power consumption. Accordingly, there remains a need in the art for new and improved plasma jets.
Provided is a plasma jet assembly comprising a cavity resonator; a metallic material disposed in the cavity resonator; a radio frequency port configured to receive a radio frequency connector configured to couple electromagnetic energy into the cavity resonator; and a gas channel within the metallic material and configured to direct a flow of a gas (i) to a space adjacent the metallic material where an electric field concentrates upon the coupling of electromagnetic energy from the radio frequency connector, and (ii) in a direction out of the plasma jet assembly.
In certain embodiments, the cavity resonator is defined by an outer perimeter of via-holes formed in a printed circuit board. In particular embodiments, the metallic material is defined by an inner perimeter of via-holes formed in the printed circuit board within the outer perimeter of via-holes. In particular embodiments, the gas channel is defined by a central via-hole formed in the printed circuit board within the inner perimeter of via-holes. In particular embodiments, an input coupling line of the radio frequency port is disposed adjacent to the inner perimeter of via-holes without touching the inner perimeter of via-holes.
In certain embodiments, the cavity resonator is defined by a base surface and cavity walls of a main body.
In certain embodiments, the metallic material is a metallic post.
In certain embodiments, the radio frequency connector has a radio frequency pin disposed adjacent to the metallic post without touching the metallic post.
In certain embodiments, the plasma jet assembly further comprises a ceiling assembly having an inner surface and an outer surface, the inner surface disposed over the cavity resonator, and the ceiling assembly defining a plasma jet outlet. In particular embodiments, the space is formed between the metallic post and the ceiling assembly. In particular embodiments, the space is defined by a recess formed in the inner surface of the ceiling assembly.
Further provided is a plasma jet assembly comprising a printed circuit board comprising a cavity resonator defined by an outer perimeter of via-holes formed in the printed circuit board; an inner perimeter of via-holes formed in the printed circuit board within the outer perimeter of via-holes; a radio frequency port configured to receive a radio frequency connector configured to couple electromagnetic energy into the cavity resonator; a central via-hole formed in the printed circuit board and within the inner perimeter of via-holes, the central via-hole configured to direct a flow of a gas (i) to a space adjacent the inner perimeter of via-holes where an electric field concentrates upon the coupling of electromagnetic energy from the radio frequency connector, and (ii) in a direction through the central via-hole and out of the plasma jet assembly.
In certain embodiments, the printed circuit board comprises a first circuit layer and a second circuit layer with a substrate layer between the first circuit layer and the second circuit layer.
In certain embodiments, the substrate layer is composed of a sandwich of two substrate layers.
In certain embodiments, an input line of the radio frequency port is disposed adjacent to the inner perimeter of via-holes without touching the inner perimeter of via-holes.
In certain embodiments, the radio frequency port is disposed adjacent to the substrate layer.
Further provided is a plasma jet assembly comprising a cavity resonator defined by a base surface and cavity walls; a ceiling assembly having an inner surface and an outer surface, the inner surface disposed over the cavity resonator; a metallic post disposed in the cavity resonator; a radio frequency port configured to receive a radio frequency connector configured to couple electromagnetic energy into the cavity resonator; a space formed between the metallic post and the ceiling assembly; a plasma jet outlet defined by the ceiling assembly; and a gas channel within the metallic post and configured to direct a flow of a gas (i) to the space where an electric field concentrates upon the coupling of electromagnetic energy from the radio frequency connector, and (ii) in a direction through the plasma jet outlet and out of the plasma jet assembly.
In certain embodiments, the space is defined by a recess formed in the inner surface of the ceiling assembly.
In certain embodiments, the radio frequency connector has a radio frequency pin disposed adjacent to the metallic post without touching the metallic post.
In certain embodiments, the metallic post is axially aligned with the gas channel.
Further provided is a plasma jet assembly comprising a cavity resonator, a metallic post disposed in the cavity resonator, a radio frequency port configured to receive a radio frequency connector configured to couple electromagnetic energy into the cavity resonator, and a gas channel within the metallic post and configured to direct flow of a gas (i) to a space adjacent the metallic post where an electric field concentrates upon the coupling of electromagnetic energy from the radio frequency connector, and (ii) in a direction out of the plasma jet assembly.
In certain embodiments, the cavity resonator and the metallic post are housed within a main body, and the radio frequency connector is configured to couple electromagnetic energy from an RF source external to the main body.
In particular embodiments, the main body comprises a ceiling assembly defining a plasma jet outlet axially aligned with the metallic post. In particular embodiments, the ceiling assembly is removable. In particular embodiments, the ceiling assembly includes a recess configured to cover or partially cover the cavity resonator. In particular embodiments, the ceiling assembly defines a plasma jet outlet. In particular embodiments, the metallic post extends from a first end at a base surface within the cavity resonator to a second end adjacent to the ceiling assembly, wherein a gap is defined between the second end of the metallic post and the ceiling assembly, the gap being the space where the electric field concentrates upon the coupling of electromagnetic energy from the radio frequency connector.
In particular embodiments, the main body comprises a base surface surrounded by cavity walls. In particular embodiments, the metallic post is disposed centrally on the base surface.
In particular embodiments, the ceiling assembly includes an outer surface having the plasma jet outlet formed therein. In particular embodiments, the ceiling assembly includes an inner surface having a recess formed therein.
In particular embodiments, the gas channel has a first channel opening and a second channel opening, the first channel opening being at the second end of the metallic post, and the second channel opening being either at the first end of the metallic post or outside the main body.
In certain embodiments, the plasma jet assembly further comprises a gas transport tube disposed through the gas channel. In particular embodiments, the gas transport tube is a capillary tube.
In certain embodiments, the plasma jet assembly further comprises a radio frequency connector disposed through the radio frequency port. In particular embodiments, where the radio frequency connector has a radio frequency pin disposed adjacent to the metallic post without touching the metallic post. In particular embodiments, the radio frequency connector includes a coaxial cable.
In certain embodiments, the cavity resonator has a circular cross section.
In certain embodiments, the metallic post is axially aligned with the gas channel.
In certain embodiments, the plasma jet assembly comprises a printed circuit board having a first circuit layer and a second circuit layer with a substrate layer between the first circuit layer and the second circuit layer. In particular embodiments, the substrate layer is composed of two sandwiched substrate layers. In particular embodiments, the cavity resonator comprises an outer perimeter of via-holes in the printed circuit board. In particular embodiments, the metallic post comprises an inner perimeter of via-holes in the printed circuit board within the outer perimeter. In particular embodiments, the gas channel comprises a central via-hole within the inner perimeter. In particular embodiments, the plasma jet assembly further comprises a radio frequency connector disposed through the radio frequency port. In particular embodiments, the plasma jet assembly further comprises a radio frequency connector that transfers energy into the cavity resonator. In particular embodiments, the radio frequency connector includes a coaxial cable. In particular embodiments, the radio frequency port is disposed adjacent to the substrate layer.
Further provided is a method of using a plasma jet assembly described herein, the method comprising connecting a radio frequency connector to a signal generator and the radio frequency port; flowing a gas from a gas source into the gas channel; and activating the signal generator to couple electromagnetic energy into the cavity resonator and produce a jet of plasma out of the plasma jet assembly. In certain embodiments, the method further comprises disposing a gas transport tube in the gas channel. In certain embodiments, an electric field on the order of 105 V/m is generated in the space with an input power of milliwatts.
Further provided is a method of using a plasma jet assembly described herein, the method comprising connecting a radio frequency connector to a signal generator and the radio frequency port; flowing a gas from a gas source into the central via-hole; and activating the signal generator to couple electromagnetic energy into the cavity resonator and produce a jet of plasma out of the central via-hole. Further provided is a plasma jet assembly comprising a first substrate; a second substrate disposed over the first substrate; an outer perimeter of via-holes formed through the first substrate and the second substrate; a cavity resonator formed within the outer perimeter of via-holes; an inner perimeter of via-holes formed through the first substrate and within the outer perimeter of via-holes; a radio frequency port disposed adjacent to the first substrate and the second substrate, the radio frequency port configured to receive a radio frequency connector configured to couple electromagnetic energy into the cavity resonator; and a central via-hole formed through the first substrate and the second substrate and within the inner perimeter of via-holes, the central via-hole configured to direct a flow of a gas (i) to a space within the inner perimeter of via-holes where an electric field concentrates upon the coupling of electromagnetic energy from the radio frequency connector, and (ii) in a direction through the central via-hole and out of the plasma jet assembly.
In certain embodiments, the first substrate comprises a top side, a bottom side, and a first microwave laminate; and the second substrate comprises a top side, a bottom side, and a second microwave laminate.
In certain embodiments, the space includes a recess formed in the bottom side of the second substrate.
In certain embodiments, the first substrate comprises a plurality of coupling via-holes formed therethrough and extending from the RF port toward the cavity resonator.
In certain embodiments, a gas transport tube is disposed through the central via-hole.
Further provided is a plasma jet assembly comprising a first substrate; a second substrate disposed over the first substrate; an outer perimeter of via-holes formed through the first substrate and the second substrate; an inner perimeter of via-holes formed through the first substrate and within the outer perimeter of via-holes, the inner perimeter of via-holes including one or more gas channel via-holes configured to direct a flow of gas through and out of the plasma jet assembly; and a radio frequency port disposed adjacent to the first substrate and the second substrate.
In certain embodiments, the inner perimeter of via-holes includes at least two gas channel via-holes. In particular embodiment, the inner perimeter of via-holes includes at least four gas channel via-holes.
In certain embodiments, the plasma jet assembly further comprises a gas flow apparatus having a main body that defines a gas flow channel having a first end and a second end, wherein the gas flow channel is tapered from the first end to the second end. In particular embodiments, the gas flow apparatus includes a first member and a second member, the first member extending from the gas flow channel and being oriented perpendicular to the gas flow channel, and the second member extending from the gas flow channel opposite to the first member and being oriented perpendicular to the gas flow channel. In particular embodiments, a diameter of the gas flow channel is from about 1.33 mm to about 12 mm.
In certain embodiments, a bottom side of the second substrate includes an etched surface.
Advantageously, a plasma jet assembly as described herein can have higher efficiency, lower energy consumption, a compact form factor, and higher safety when compared to conventional plasma jets. A plasma jet assembly as described herein can be desirably applied to a variety of different fields and technologies, such as, but not limited to, plasma medicine, food/water/agricultural decontamination, material processing, propulsion, antimicrobial treatments, reconfigurable RF electronics, and flow controls.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
flow rates and input powers.
Throughout this disclosure, various publications, patents, and published patent specifications are referenced by an identifying citation. The disclosures of these publications, patents, and published patent specifications are hereby incorporated by reference into the present disclosure in their entirety to more fully describe the state of the art to which this invention pertains.
As used herein, the term “coupling” refers to the transfer of energy from one medium to another medium. Examples of coupling include, but are not limited to, direct coupling, resistive conduction, atmospheric plasma channel coupling, inductive coupling, capacitive coupling, evanescent wave coupling, radio waves, electromagnetic interference, and microwave power transmission.
As used herein, the terms “printed circuit board” and “PCB” refer to a medium to connect electronic components to one another in a controlled manner. The medium can include one or more layers, including, but not limited to, conductive layers, insulating layers, solder mask layers, and microwave laminates.
As used herein, the term “microwave laminate” can include substrates used for radio frequency (RF) and microwave communication systems and electronics. Generally, microwave laminates have a low dissipation factor, low levels of moisture absorption, and a low dielectric constant.
Provided herein is a plasma jet that utilizes microwave resonator technology. In some embodiments, the plasma jet can operate with different gas types (such as, but not limited to, helium or argon) and flow rates (for example, from 0.1 to a few slpm) with very high efficiency, which means extremely low power consumption. The resulting jet of plasma has a diameter that can be up to a couple of millimeters and a length up to ten millimeters, while both jet length and plasma properties can be controlled by input power and gas flow rate.
As described in the examples herein, non-limiting example plasma jets were designed, fabricated, and successfully measured at 2.45 GHz, which is a standard frequency for plasma applications. In a first example, the plasma jet was created with hundreds of milliwatts of input power and had a power efficiency of more than 80%. Compared to conventional plasma jets with pulse excitation, the plasma jet described herein has a much higher efficiency, and can operate in much lower power, and therefore is much safer, making the plasma jet useful and advantageous for many applications including, but not limited to, biomedical applications such as skin or dental treatments, disinfection (i.e., antimicrobial treatments), and cancer therapy. As also described in the examples herein, the average power consumption of the plasma jet can be even further decreased by using pulsed microwave excitation instead of continuous wave (CW). It is possible to utilize the same technique in lower frequencies (for example, 13.56 MHz or 26 MHz) to make high-efficiency plasma jets with higher plasma volume and thrust for other applications like plasma propulsion and material processing.
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In operation, the outer perimeter 214 of via-holes 212 in combination with the inner perimeter 215 of via-holes 212 facilitate an electric field concentration within the central via-hole 216. With enough input power to the cavity 210, via the RF cable, the high electric field concentration ionizes the gas funneling into the central via-hole 216 to produce the plasma jet (plasma plume). The plasma jet exits the PCB 202 via the first via opening of the central via-hole 216 due to the gas flowing in the direction of from the second circuit layer 208 to the first circuit layer 206 through the central via-hole 216. The gas thus acts not only as the source of ions to create plasma, but also as a carrier gas that directs the plasma is the direction of the gas flow which, in the embodiment depicted in
A method of using the plasma jet assembly 200 may include connecting the RF connector 224 to the signal generator and the RF port 222, disposing the gas transport tube in the central via-hole 216, connecting a gas transport tube to a gas source, and activating the gas source and the signal generator to produce a jet of plasma out of the central via-hole 216 on the first circuit layer 206 side.
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Examples of microwave laminates can include a variety of materials suitable for electromagnetic applications, including thermoset resins microwave laminates, ceramic microwave laminates, polytetrafluoroethylene (PTFE) microwave laminates, polymer microwave laminates, or combinations thereof, including composites. In addition, one or more of the top sides 318, 322 and the bottom sides 320, 324 of the substrates 302, 303 can include cladding, such as copper cladding. In certain embodiments, the first substrate 302 has a first top cladding 328 and a first bottom cladding 330, as shown in
In certain embodiments, the first microwave laminate comprises a ceramic thermoset polymer composite, such as, but not limited to, the TMM® 10i laminates sold by the Rogers Corporation. In certain embodiments, the dielectric constant of the first microwave laminate is about 9.80 and the dissipation factor of the first microwave laminate is about 0.0020. A height of the first microwave laminate can be about 0.508 mm, as a non-limiting example. In certain embodiments, the second microwave laminate comprises a ceramic thermoset polymer composite, such as, but not limited to, the TMM® 3 laminates sold by the Rogers Corporation. In certain embodiments, the dielectric constant of the second microwave laminate can be about 3.27 and the dissipation factor of the second microwave laminate can be about 0.0020. A height of the second microwave laminate can be about 0.508 mm, as a non-limiting example.
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The RF port 310 is disposed adjacent to the first substrate 302 and the second substrate 303. With reference to
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In certain embodiments, a gas transport tube 360 is disposed through the central via-hole 312, as shown in
In operation, the outer perimeter of via-holes 304 in combination with the inner perimeter of via-holes 308 facilitate an electrical field concentration in the space 358. With enough input power to the cavity resonator 306, via the RF connector, the high electric field concentration ionizes the gas funneling through the central via-hole 312 to produce a jet of plasma (a plasma plume), as shown in
A method of using the plasma jet assembly 300 can include connecting the RF connector to the signal generator and the RF port 310, disposing the gas transport tube 360 through the central via-hole 312, connecting the gas transport tube 360 to a gas source, and activating the gas source and the signal generator to produce a plasma jet out of the central via-hole 312 on the top side 322 of the second substrate 303.
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The first substrate 402 and the second substrate 403 can include microwave laminates, such as, but not limited to, TMM® 3 laminates sold by the Rogers Corporation. In certain examples, the microwave laminate of the second substrate 403 has a thickness of about 0.21 mm to about 1.905 mm, about 0.31 mm to about 1.27 mm, or about 0.635 mm. In certain examples, the microwave laminate of the first substrate 402 has a thickness of about 2.11 mm to about 19.05 mm, about 3.17 mm to about 12.7 mm, or about 6.35 mm. However, other dimensions are possible and encompassed within the scope of the present disclosure.
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The inner perimeter of via-holes 408 includes one or more gas channel via-holes 409 configured to direct a flow of gas through and out of the plasma jet assembly 400. In certain examples, the inner perimeter of via-holes 408 includes a plurality of the gas channel via-holes 409. As shown in
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A method of using the plasma jet assembly 400 can include connecting the RF connector to the signal generator and the RF port 410, connecting the gas flow apparatus 500 to the first substrate 402, connecting the gas flow apparatus 500 to a gas source, and activating the gas source and the signal generator to produce a plasma jet out of the gas channel via-holes 409 on the top side 422 of the second substrate 403.
These examples describe the development of a highly efficient microwave plasma jet based on evanescent-mode (EVA) cavity resonator technology. An EVA cavity resonator can be formed by loading a normal cavity with a post at the center. One important consequence of this loading is the electric field concentration in the gap between the resonator post and the top wall. This concentration of the electric field may be used, for instance, for the implementation of plasma-based high-power microwave limiters and switches. In the present disclosure, however, a gas flow mechanism is implemented to pass through the EVA critical region to realize a high-efficiency resonant microwave plasma jet. Theory of operation, design process, and modeling results are described below, along with the fabrication process, experimental setup, and measurement results. Plasma jet diagnostics are also described.
As noted above, an evanescent-mode cavity resonator can be formed by loading an enclosed cavity with a post in the center. Due to this loading, the electric field gets concentrated in the region between the post and the resonator ceiling, resulting in an effective capacitance. As a result, the size of an EVA cavity resonator is significantly reduced compared to an ordinary cavity resonator. The quality factor of EVA cavity resonators is typically high (≥500), which makes them good candidates for many applications, such as for tunable and selective filters.
Here, kgap is a coefficient used to adjust the Cgap due to the capillary tube in the gap in the structure introduced in this example. The resonant frequency of the structure can then be represented as
where Ctotal is the sum of the above three capacitors, as they are formed in a parallel configuration.
An EVA cavity resonator is advantageous for plasma jet creation because of the structural symmetry and the gas flow that can be conducted through the central post. In the structure disclosed herein, this has been accomplished by drilling a hole through the post and then utilizing a capillary tube within the post to pass the gas to the outlet through the critical gap area of the EVA resonator. In this case, the capacitive gap is partially filled by the dielectric of the tube. Because of the hole in the electrode and the presence of the capillary tube, the Cgap is affected, which is incorporated by the kgap coefficient in equation (3). With enough input power to the resonator and consequently high electric I field over the critical gap area of the EVA resonator, the gas going through this area ignites, and the plasma plume is forced out by the gas flow. In this example, a 2.45 GHz resonant frequency was chosen as it is one of the standard frequencies for microwave plasma generation, making it easier to benchmark the EVA cavity plasma jet. However, this technique can be extended to other frequencies.
To address the goal of achieving a highly efficient plasma jet, a very high |E|-field in the critical gap area, between the surface above the post and the ceiling, is important. This can be obtained by a high Q along with a good matching performance at the resonant frequency of 2.45. To drill a hole to pass a tube, such as a capillary tube with 1.12 mm of external diameter, a minimum post radius is important. For the cavity radius, an optimum b/a is around 3.59 for maximum Q. As gas breakdown takes place in the capacitive gap region, the gap size (gap, or g) has to be such that there can be enough volume of gas, while remaining practical for CNC fabrication. The 50-impedance matching is mainly adjusted by positioning the height and pin length of the coaxial connector. The connector pin length is adequately long to provide a strong coupling of the input energy to the resonator, having a high external coupling coefficient. The dimensions of the designed EVA plasma jet at 2.45 GHz are presented in Table 1. The microwave plasma jet operates at atmospheric pressure with a controlled gas flow rate system.
The designed EVA cavity-based plasma jet was simulated using ANSYS HFSS, a 3D electromagnetic (EM) simulation software. As seen in
The designed EVA cavity structure described in this example was fabricated by copper CNC machining, as shown in
As presented in
At the first step, the reflection coefficient of the assembled device was tested to check the resonance behavior. Although this was an OFF-mode test, it was conducted in the presence of the capillary tube because the capillary tube affects the Cgap and hence the resonant frequency. As seen in
The first step in generating a stable plasma is to make the gas breakdown. The breakdown voltage lies between the dark and glow discharge regimes. For discharge ignition, reaching the breakdown voltage (field) level in pre-breakdown is necessary. After the plasma is formed (post breakdown), it can typically be sustained at lower power than is needed for breakdown.
To investigate the effect of the gas flow rate on the plasma jet, the input power was set at a constant value of 2 W at the resonant frequency of 2.45 GHZ, and the gas flow rate was varied from 2 slpm to 7 slpm. As observed in the top row of
The designed EVA cavity plasma jet has an excellent OFF-mode reflection performance, as depicted in
Various optical diagnostics techniques were performed on atmospheric pressure microwave plasma jets (APPJs) based on the parameters to be measured. Electron density is one of the crucial parameters for different applications. Thomson scattering is an active spectroscopy method, allowing simultaneous electron density and temperature determination with high spectral resolution. While this method is widely used in high-density plasma characterization, it is expensive for low-density plasmas since a triple grating monochromator must be used to subtract the Rayleigh scattering component and stray light. Optical Emission Spectrometry (OES) is a passive spectroscopy method in which electron density is obtained by analyzing spectral line shapes and intensities. Two of the most commonly used broadening profiles to assess ne are spectral line emissions of the hydrogen atom, namely Balmer-alpha (H-α) at 656.279 nm and Balmer-beta (H-β) at 486.135 nm, because of their position in the visible spectral region and linear Stark effect.
In atmospheric pressure plasmas, a spectral profile is a convolution of Gaussian and Lorentzian profiles, known as the Voigt function. The Gaussian component of the obtained spectral profile depends on the mass of the hydrogen atoms, central wavelength, and gas temperature. The Lorentzian component of the Voigt profile, which dominates the spectral profile, consists of Doppler, Resonance, van der Waals, and Stark broadenings. The Resonance broadening occurs when perturbations of atomic levels are caused due to interaction between pairs of neutral atoms of the same kind, for example, (He+He). This broadening is negligible for hydrogen Balmer lines at atmospheric pressure and can be excluded. Thus, Doppler and van der Waals broadenings are considered for proper electron density estimation. When the emitting atoms have random motion, it introduces the Doppler effect, resulting in the broadening of atomic transitions, known as Doppler broadening. The full width at half maximum (FWHM) for Doppler broadening can be given as
where the gas temperature Tg is in Kelvin, Boltzmann's constant kb is in JK−1, and ma is the mass of the emitter. On the other hand, van der Waals broadening occurs when perturbations of atomic levels are caused due to interaction between different species, for example, (H+He). The FWHM of van der Waals broadening can be represented as
where the constant C depends on the gas type and is equal to 2.42 and 5.12 for helium and argon, respectively.
To estimate van der Waals and Doppler broadening, the knowledge of gas temperature is important. In nonthermal plasmas, the rotational temperature of diatomic molecules, mostly OH and N2+, give a close approximation of gas temperature, and this is a widely used method with great success. The spectral profiles are measured utilizing an OES technique using a Teledyne Princeton Instruments HRS-500-SS spectrometer with 0.05 nm optical resolution.
As seen in Table 2, the gas temperature increases from 296 (ambient) to 350 K for the change of input power from 500 mW to 10 W. All measured temperatures are with the accuracy of ±5 K. This temperature range proves this device as a safe option for many applications sensitive to high temperature. Upon measuring Tg, the Doppler and van der Waals broadenings are calculated using (6) and (7), respectively. Comparing the values in Table 2, it is evident that the van der Waals and the Doppler broadenings are minimal, and the Stark broadening is mostly dominant, making its FWHM suitable to measure ne. Hence, this disclosure presents measurements of the electron density in the introduced EVA plasma jet using the OES Stark broadening technique. In this study, the H-α profile was opted for calculating ne as it is more distinct than the H-β for the EVA jet device.
where ne is in cm−3.
To investigate the effect of input 2.45 GHz power on the EVA jet electron density, the gas flow rate was kept constant at 7 slpm, and ne was measured at input powers ranging from 0.5 to 10 W, as seen in
A resonant microwave plasma jet technology has been demonstrated in this example. The structure was formed by an EVA cavity resonator with a helium flow going through its critical gap region. Since this is a high-Q resonant structure with an electric field mainly concentrated over that critical gap area, gas breakdown occurs even with low values of input microwave power. The gas flow then pushes the plasma torch out of the resonator in a jet form, where its properties depend on both input power and gas flow rate. A prototype 2.45 GHz EVA plasma jet provided a maximum efficiency of >80%, achieved under low power and high flow rate, with electron density in the range of 1015 (cm−3) and a maximum jet temperature of 350 K at 10 W input power. The results demonstrate the realization of high-density plasma jets with only milliwatts of power by employing high-Q microwave resonant structures. If required, the plasma volume can be extended by either using an array of such jets or scaling the resonator to lower frequencies.
While atmospheric pressure plasmas (APP) present strong candidates for advancements in many applications, such as material processing, semiconductor fabrication, and plasma thrusters for spacecraft maneuvering, APP often require large-scale plasma generation. This example demonstrates a strategic approach to achieve a large volume plasma array through a substrate-integrated waveguide (SIW) implementation of EVA cavity technology. This can facilitate a plasma array generation of 3 mm wide.
This example leverages the EVA resonator's electric-field concentration properties to facilitate the transition from a microscale plasma jet into a microplasma jet array. Background gas flowing through the microgap via strategically designed gas channels experiences efficient ionization due to this field enhancement, resulting in power-efficient cold plasma jets. Four central post vias are employed as gas flow channels to facilitate plasma formation in the presence of a high electric field to achieve an array of plasma. A 3D-printed structure facilitates the flow through these channels, ultimately generating an array of four microplasma jets exiting the PCB technology-based SIW EVA resonator with as little as 800 mW of input microwave power. This example demonstrates the capability and usefulness of EVA resonators for large-volume plasma jets essential for various industrial and space applications.
The dimensions of the cavity and the central post determine specific values of these inherent electrical components and, in turn, the resonant frequency. Additionally, the post can be manipulated with a microgap on top, separating its top surface from the cavity ceiling. This microgap area acts like a capacitive element, resulting in a significant electric field concentration within the gap region. As the gas flows through the critical microgap region, this highly concentrated electric field facilitates gas breakdown at relatively low power thresholds.
The evanescent-mode cavity resonator described in this example was specifically designed to address the issue of a limited plasma volume of 1 mm in diameter and 6 mm in length. The structure leverages the advantages of evanescent-mode confinement while overcoming the volume concentration. 3 GHz was chosen as the operating frequency to elucidate the influence of this frequency on the generated plasma array characteristics and compare it with available commercial devices, mostly in microwave regions. Rogers TMM3 substrates were used on both top and bottom boards with thicknesses of 0.635 mm and 6.35 mm, respectively. The device's input power was routed using a coplanar waveguide line on the bottom board, which is good for 50-Ω input matching.
After fabrication of the two PCBs, a 250 μm circular etching process of 1 mm diameter was performed on the bottom surface of the top board, sitting right on top of the gas exiting vias using a PCB milling machine. This etching process promotes a more symmetrical and enhanced distribution of the |E|-field within the plasma generation region by mitigating potential inconsistency in asymmetries that could arise during the manual assembly later on. This etching was incorporated into the ANSYS HESS simulation model, which was validated by maintaining the desired resonant frequency at 3 GHz.
To generate a plasma array, the resonator design incorporates vias on both substrates to replicate the cavity boundaries. The central post within the cavity was realized using multiple stacked central vias, as illustrated in
To deliver a microwave signal to the evanescent-mode cavity resonator, a 50-Ω SMA connector is soldered onto the feed line on the bottom substrate. This SMA connector serves as the primary pathway for the external RF signal to excite the resonator. Following SMA connector attachment, the top and bottom substrates are secured together using screws. This creates the complete cavity structure and incorporates the designed gas flow channels within the bottom substrate. The inlet of the 3D-printed gas flow structure is then interfaced with a mass flow controller. This connection enables the precise regulation of the background gas flow into the gas channels realized within the central post vias of the bottom substrate.
Following this practical optimization, the modified microgap depth was incorporated into the simulation model to assess the agreement between the simulated and measured resonant frequencies. The simulation results indicated a resonant frequency of 2.998 GHz with a reflection coefficient, S11, of −21 dB. In comparison, the measured resonant frequency was 2.985 GHz with an S11 of −23.7 dB. The shift in resonant frequency is attributed to the manual placement of Teflon tapes at the bottom of the top board to prevent gas from leaking into the structure after the fabrication. This close agreement between the simulation and measurement data validates the effectiveness of the optimized microgap depth to achieve the desired resonant frequency, shown in
This example employed a signal generator to create the RF signal at the resonant frequency. This signal was amplified before being delivered to cavity resonator structure via a bidirectional coupler. One port of the coupler facilitated connection to a spectrum analyzer for real-time monitoring of the resonant frequency under the influence of the background gas. A mass flow controller regulated the background helium gas and directed it toward the gas channels within the cavity resonator structure.
Plasma ignition was successfully achieved at a flow rate of 1 standard liter per minute (slpm) for helium gas at an input power level ranging from 1 W to 2 W. As depicted in
This example demonstrates the feasibility of utilizing a PCB-based evanescent mode (EVA) cavity resonator structure for generating a wide-volume cold plasma jet array. The close agreement between the simulated and measured resonant frequencies validates the theory and the employed model. The device exhibits a high-quality factor, signifying efficient energy transfer within the resonator. The low input power required for plasma ignition (1-2 W) and sustained operation (as low as 700 mW) signifies the capability for large-volume and power-efficient plasma generation. This characteristic makes the plasma jet assembly useful for various applications, including plasma medicine, material processing, jet propulsion, etc. It should be appreciated that this example may be scaled for even larger plasma volumes.
Certain embodiments of the devices and methods disclosed herein are defined in the above examples. It should be understood that these examples, while indicating particular embodiments of the invention, are given by way of illustration only. From the above discussion and these examples, one skilled in the art can ascertain the essential characteristics of this disclosure, and without departing from the spirit and scope thereof, can make various changes and modifications to adapt the devices and methods described herein to various usages and conditions. Various changes may be made, and equivalents may be substituted for elements thereof without departing from the essential scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof.
This application claims priority to U.S. application Ser. No. 18/075,523, filed under 35 U.S.C. § 111 (a) on Dec. 6, 2022, now issued; which claims priority to U.S. Provisional Application No. 63/286,710, filed under 35 U.S.C. § 111 (b) on Dec. 7, 2021. The entire disclosure of each of the aforementioned applications is incorporated herein by reference for all purposes.
This invention was made with government support under grant number 2102100 awarded by the National Science Foundation. The government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 63286710 | Dec 2021 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 18075523 | Dec 2022 | US |
| Child | 19046956 | US |
