Microwave-driven Plasma Research Articles

A 213-nm microwave-driven far-UVC light source and its optimization based on backpropagation artificial neural network

Mercury lamps emitting 253.7 nm are well-known UV light devices but cause problems to environment. Here, a mercury-free light source is shown which is enabled by ZnI2 microwave-driven light-emitting plasma (ZnI2 MLEP), emitting 213-nm far-UVC. Its condition (pressure, ZnI2 mass and microwave power) was optimized using a backpropagation artificial neural network (ANN) to obtained the maximum 213-nm intensity (I213). The ANN was trained with experiment data and constructed the relationship between the condition and I213 with error less than 5 %. The I213 over all possible conditions allowed by the experimental accuracy was predicted. And the optimal condition was obtained and tested experimentally with the error of 5.6 %. After optimized by this method, the I213 of ZnI2 MLEP increases 53 % compared to conventional method. Based on the optimal condition, the ZnI2 MLEP was made into a standalone ZnI2 lamp with a recorded-high 213 nm intensity of 20.74 mW/cm2/nm. The comparison of spectra shows ZnI2 lamp have shorter wavelengths and higher UV radiant power density than Hg lamp. Coupled with its properties of environmentally friendly and electrodeless, the ZnI2 lamp have a potential to replace the Hg based UV lamps.

Plasma-Treated Water: A Comparison with Analog Mixtures of Traceable Ingredients.

Plasma-treated water (PTW) possess anti-microbial potential against Pseudomonas fluorescence, which is observable for both suspended cells and cells organized in biofilms. Against that background, the chemical composition of PTW tends to focus. Various analytical techniques have been applied for analyses, which reveal various traceable reactive oxygen and nitrogen compounds (RONS). Based on these findings, it is our aim to generate a PTW analog (anPTW), which has been compared in its anti-microbial efficiency with freshly generated PTW. Additionally, a solution of every traceable compound of PTW has been mixed according to their PTW concentration. As references, we treated suspended cells and mature biofilms of P. fluorescence with PTW that originates from a microwave-driven plasma source. The anti-microbial efficiency of all solutions has been tested based on a combination of a proliferation, an XTT, and a live-dead assay. The outcomes of the test proved an anti-microbial power of PTW that suggests more active ingredients than the traceable compounds HNO3, HNO2, and H2O2 or the combined mixture of the analog.

A Startup Control Method for Plasma Lamps by Using Fractional-N Phase-Locked Loop Microwave Driver

The microwave-driven plasma light source has the feature of the full spectrum, similar to sunlight, and has broad application prospects. Startups of plasma lamps are very difficult since the plasma density and the resonator impedance are changing constantly with the warm-up process and the RF driver can hardly couple the power to the plasma efficiently. This paper presents a startup control method for plasma lamps by using an integrated fractional-N phase-locked loop (PLL) as the RF signal source. An RF control module including a 21 dBm RF source with a band of 430–460 MHz and detection circuits for the RF driver is developed, and the startup process of the plasma lamp is further studied by analyzing the return loss of the resonator under the plasma load. According to the test result, the maximum frequency deviation of the RF control module is 3.3 kHz over the working temperature range of −40–80 °C. The designed RF control module is used to drive a high-power amplifier to start the plasma lamp automatically. It takes 79 s to achieve the stable arc operation from the gas breakdown. The return loss of the resonator is −26 dB, with an incident power of 171 W and reflected power of 418 mW, indicating that the RF driver and the plasma achieve good coupling. Compared with continuous wave, the luminous flux of the lamp powered by RF pulse improves by 18% under the same electric power. This startup control method has stable performance, small temperature drift, and an effective control function for plasma lamps.

Application of Machine Learning to Predict the Performance of an EMIPG Reactor Using Data from Numerical Simulations

Microwave-driven plasma gasification technology has the potential to produce clean energy from municipal and industrial solid wastes. It can generate temperatures above 2000 K (as high as 30,000 K) in a reactor, leading to complete combustion and reduction of toxic byproducts. Characterizing complex processes inside such a system is however challenging. In previous studies, simulations using computational fluid dynamics (CFD) produced reproducible results, but the simulations are tedious and involve assumptions. In this study, we propose machine-learning models that can be used in tandem with CFD, to accelerate high-fidelity fluid simulation, improve turbulence modeling, and enhance reduced-order models. A two-dimensional microwave-driven plasma gasification reactor was developed in ANSYS (Ansys, Canonsburg, PA, USA) Fluent (a CFD tool), to create 644 (geometry and temperature) datasets for training six machine-learning (ML) models. When fed with just geometry datasets, these ML models were able to predict the proportion of the reactor area with temperature above 2000 K. This temperature level is considered a benchmark to prevent formation of undesirable byproducts. The ML model that achieved highest prediction accuracy was the feed forward neural network; the mean absolute error was 0.011. This novel machine-learning model can enable future optimization of experimental microwave plasma gasification systems for application in waste-to-energy.

Plasma Enhanced Chemical Vapor Deposition of Organic Polymers

Chemical Vapor Deposition (CVD) with its plasma-enhanced variation (PECVD) is a mighty instrument in the toolbox of surface refinement to cover it with a layer with very even thickness. Remarkable the lateral and vertical conformity which is second to none. Originating from the evaporation of elements, this was soon applied to deposit compound layers by simultaneous evaporation of two or three elemental sources and today, CVD is rather applied for vaporous reactants, whereas the evaporation of solid sources has almost completely shifted to epitaxial processes with even lower deposition rates but growth which is adapted to the crystalline substrate. CVD means first breaking of chemical bonds which is followed by an atomic reorientation. As result, a new compound has been generated. Breaking of bonds requires energy, i.e., heat. Therefore, it was a giant step forward to use plasmas for this rate-limiting step. In most cases, the maximum temperature could be significantly reduced, and eventually, also organic compounds moved into the preparative focus. Even molecules with saturated bonds (CH4) were subjected to plasmas—and the result was diamond! In this article, some of these strategies are portrayed. One issue is the variety of reaction paths which can happen in a low-pressure plasma. It can act as a source for deposition and etching which turn out to be two sides of the same medal. Therefore, the view is directed to the reasons for this behavior. The advantages and disadvantages of three of the widest-spread types, namely microwave-driven plasmas and the two types of radio frequency-driven plasmas denoted Capacitively-Coupled Plasmas (CCPs) and Inductively-Coupled Plasmas (ICPs) are described. The view is also directed towards the surface analytics of the deposited layers—a very delicate issue because carbon is the most prominent atom to form multiple bonds and branched polymers which causes multifold reaction paths in almost all cases. Purification of a mixture of volatile compounds is not at all an easy task, but it is impossible for solids. Therefore, the characterization of the film properties is often more orientated towards typical surface properties, e.g., hydrophobicity, or dielectric strength instead of chemical parameters, e.g., certain spectra which characterize the purity (infrared or Raman). Besides diamond and Carbon Nano Tubes, CNTs, one of the polymers which exhibit an almost threadlike character is poly-pxylylene, commercially denoted parylene, which has turned out a film with outstanding properties when compared to other synthetics. Therefore, CVD deposition of parylene is making inroads in several technical fields. Even applications demanding tight requirements on coating quality, like gate dielectrics for semiconductor industry and semi-permeable layers for drug eluting implants in medical science, are coming within its purview. Plasma-enhancement of chemical vapor deposition has opened the window for coatings with remarkable surface qualities. In the case of diamond and CNTs, their purity can be proven by spectroscopic methods. In all the other cases, quantitative measurements of other parameters of bulk or surface parameters, resp., are more appropriate to describe and to evaluate the quality of the coatings.

Cold Atmospheric Plasma (CAP) for the Treatment of Actinic Keratosis and Skin Field Cancerization: Clinical and High-Frequency Ultrasound Evaluation.

IntroductionCold atmospheric plasma (CAP) has been clinically demonstrated to be an effective treatment for actinic keratosis (AK) in a number of case series. In this study, we evaluated the efficacy of CAP in the treatment of multiple AKs and assessed morphological changes induced on the skin field of cancerization both clinically and by high-frequency ultrasound (HFUS).MethodsPatients with multiple grade I–II AKs of the scalp and/or face who were resistant or intolerant to conventional field-directed treatments were enrolled. CAP treatments were performed using a microwave-driven argon plasma jet. At baseline and 3 months after the last CAP session, performance indexes were determined using three-dimensional digital pictures and HFUS investigations were performed on a representative Olsen grade II AK and a small spot of clinically unaffected skin within the test area. ResultsTwelve patients were enrolled in the study. All clinical variables showed a statistically significant reduction after CAP. HFUS evaluation revealed that the total, epidermal and dermal thicknesses of the target AKs had not changed with treatment. CAP therapy significantly increased dermal density in both the target AK and the surrounding photodamaged skin and signficantly decreased the thickness of the subepidermal low-echogenic band in the perilesional skin, which is an ultrasound sign of photodamage. ConclusionsCold atmospheric plasma was found to be an effective treatment for patients with multiple AKs. CAP was not followed by skin atrophy. HFUS examiniation showed the CAP improved features of chronic photodamage of the dermis of the skin underlying and surrounding the AK spots.

Identifying Double Bond Positions in Phospholipids Using Liquid Chromatography-Triple Quadrupole Tandem Mass Spectrometry Based on Oxygen Attachment Dissociation.

Lipids, a class of biomolecules, play a significant role in the physiological system. In this study, gas-phase hydroxyl radicals (OH·) and atomic oxygens (O) were introduced into the collision cell of a triple quadruple mass spectrometer (TQ-MS) to determine the positions of the double bond in unsaturated phospholipids. A microwave-driven compact plasma generator was used as the OH·/O source. The reaction between OH·/O and the precursor ions passing through the collision cell generates product ions that correspond to the double bond positions in the fatty acyl chain. This double bond position specific fragmentation process initiated by the attachment of OH·/O to the double bond of a fatty acyl chain is a characteristic of oxygen attachment dissociation (OAD). A TQ-MS incorporating OAD, in combination with liquid chromatography, permitted a high throughput analysis of the double bond positions in complex biomolecules. It is important to know the precise position of double bonds in lipids, since these molecules can have widely different functionalities based on the position of the double bonds. The assignment of double bond positions in a mixture of eight standard samples of phosphatidylcholines (phospholipids with choline head groups) with multiple saturated fatty acyl chains attached was successfully demonstrated.

Growth of Plasma-Treated Corn Seeds under Realistic Conditions

In this study, the effect of the plasma treatment on corn seeds is investigated. Corn seeds were treated uniformly without burning or blackening by three kinds of plasma apparatus: RF plasma in vacuum, microwave-driven atmospheric-pressure plasma, DBD atmospheric-pressure plasma, and two other treatments: vacuum exposure only, and using plasma-activated water in the seed coating process, to investigate growth rate changes under realistic conditions. Each treatment was performed on a total of 1512 corn seeds. Seeds from each experimental condition were treated with the recommended rate of Poncho/VOTiVO with Acceleron, a commercial biological seed treatment that helps to protect the seeds from fungus, insects, and nematodes after planting. The 1512 seeds were divided evenly into three replications with 84 seeds planted for each replication at six unique locations across central Illinois. The results for germination, growth, and product yield over the 2017 growing season is presented. Overall no statistically significant difference in the yield of corn harvested was found between the control and any of the five treatments. This is likely due to the already near-100% germination rate of the corn hybrid used in the study and the use of the Poncho/VOTiVO protective coating on every sample.

A Microwave Driven PE-ALD for Ultrathin Al2O3/ZnO Synthesis over Perovskite Layer

Abstract To enhance perovskite solar cell performance, separating the ZnO electron transport layer and perovskite layer by a dense, ultrathin alumina insulating layer is needed. Also the insulating layer can effectively prevent the degradation rate of perovskite. The present work design and self-building of a microwave-driven plasma enhanced atomic layer deposition system. The plasma enhanced atomic layer deposition process with O2 gas and H2O vapor, and the characteristics of the ultrathin alumina films on silicon wafer and FTO/TiO2 layer have been performed. The deposition was carried out using trimethyl-aluminum as the precursor, and argon gas for purging. During the designated deposition, substrate temperature remained constantly at 100 and 200 ◦C, and the chamber pressure was ∼2 x 10-2 torr. Two deposition cycles were performed, 100 and 200 cycles. Various thin film characterization techniques including contact angle, and atomic force microscope, and X-ray diffraction were applied for investigating the alumina film properties. The contact angle result shows as-deposited alumina film was well synthesized over the TiO2 layer.

Microwave-Driven In-liquid Plasma in Chemical and Environmental Applications. III. Examination of Optimum Microwave Pulse Conditions for Prolongation of Electrode Lifetime, and Application to Dye-Contaminated Wastewater

Generating in-liquid plasma using continuous microwave radiation has proven problematic as the surface of the electrode undergoes significant deterioration because of the generated plasma. This article describes a method by which this problem can be resolved by the utilization of pulsed microwave radiation from a magnetron microwave generator and presents results in the search for optimal pulsed microwave irradiation conditions; these would avoid damage to the electrode and would afford reduced power consumption. Results show that continuous generation of in-liquid plasma that avoids electrode (antenna) damage requires strict and very limited pulsed oscillation conditions. Evaluation of this device was investigated by the discoloration of a rhodamine-B (RhB) dye-contaminated wastewater, for which it was shown that higher treatment efficiency can be obtained compared to more traditional methods such as the UV photolysis (UV), the UV-assisted photocatalytic TiO2 method (UV/TiO2), and the NaClO methodology (NaClO). The energy consumed during the 3 min needed to discolor 50 mL of a 0.10 mM aqueous RhB dye solution was 6.3 × 10−3 kWh per mg of RhB; complete mineralization of the dye solution by the in-liquid plasma occurred within 15 min (loss of TOC).

Temporal and spatial evolution of nanosecond microwave-driven plasma

In this paper, a method for simultaneously acquiring the temporal and spatial evolution of characteristic plasma spectra in a single microwave pulse is proposed and studied. By using multi-sub-beam fiber bundles coupled with a spectrometer and EMICCD (Electron-multiplying intensified charge-coupled device), the spatial distribution and time evolution of characteristic spectra of desorbed gases at the dielectric/vacuum interface during nanosecond microwave-driven plasma discharge are observed. Arrays of small align tubes punctured with metal walls of feed horn are filled with separate fibers of matched sizes and equal lengths. The output ends of fibers arranged in a single longitudinal column are connected to the entrance slit of a spectrometer, where the optical spectrum inputs to a high-speed EMICCD, to detect the rapid-varying time and space spectra of nanosecond giga-watt microwave discharges. The evolution of spectral clusters of N2 (C-B), N2+ (B-X), and the hydrogen atoms is discovered and monitored. The whole duration of light emission is much longer than the microwave pulse, and the intensities of ion N2+ (B-X) spectra increase after microwave pulses with rise times of 25–50 ns. The brightness distribution of plasma spectra in different space is observed and approximately consistent with the simulated E-field distribution.

Human health risk evaluation of a microwave-driven atmospheric plasma jet as medical device

PurposeThe aim of this study was the characterisation of a microwave-driven atmospheric plasma jet (APJ) dedicated for medical applications. The scientific focus includes harmless sterilization of surfaces and therapeutic treatments in dentistry. MethodesThe plasma was investigated with respect to potential health risks for human beings, which could occur especially by the gas temperature, heat flow, patient leakage current, UV emission and ozone emission from the plasma jet, according to DIN SPEC 91315:2014-06 (General requirements for plasma sources in medicine) [1]. ResultsThe results of the experiments indicate a high potential of the plasma jet to be used as a medical device exhibiting low gas temperatures up to 34 °C. The calculated leakage currents are mostly below the 10 μA threshold. The limiting UV exposure duration for the APJ with a calculated maximum effective irradiance of 2.6 μW/cm2 is around 19 min, based on the exposure limits of the international commission on non-ionizing radiation protection guidelines (ICNIRP) [2]. A significant ozone concentration was observed mainly in the axial effluent gas flow. Ozone concentration strongly decreases with increasing distance from the plasma source exit nozzle. ConclusionThe investigated APJ exhibits physical properties that might not constitute health risks to humans, e.g. during treatment in dentistry. Thus, the APJ shows a high potential for application as a device in dental therapy.

Diagnostic of ultrafast temporal plasma evolution in high-power microwave discharge

For the most advanced multi-frame camera in the world, the shortest response time between two frames is no shorter than 1.5 ns. Thus, there is no effective way to diagnose the ultrafast sub-nanosecond dynamic of a microwave-driven plasma discharge in a single pulse. Different-length multi-sub-beam optical fibers, together with a spectrometer and an EMICCD camera, are proposed and designed to detect the nanosecond discharge spectra in a single pulse, like a real-time multi-frame spectral camera. This novel method could realize a time interval between two consecutive frames shorter than 0.1 ns by a length difference of 2 cm for sub-fibers, achieving the measurement of ultrafast plasma dynamics. Temporal evolution of electron density as well as energy of electrons and ions during nanosecond microwave discharge is further studied by de-convolving the Stark broadening and thermal Doppler broadening and by calculating the ratio of emission coefficients.

Spatially resolved modeling and measurements of metastable argon atoms in argon-helium microplasmas

Microwave-driven plasmas operating near atmospheric pressure have been shown to be a promising technique for producing the high density of argon metastable atoms required for optically pumped rare gas laser systems. Stable microwave-driven plasmas can be generated at high pressures using microstrip-based resonator circuits. We present results from computational modeling and laser absorption measurements of argon metastable densities in such plasmas operating in argon-helium gas mixtures at pressures up to 300 Torr. The model and measurements resolve the plasma characteristics both perpendicular to the substrate surface and along the resonator length. The measurements qualitatively and in many aspects quantitatively confirm the accuracy of the model. The plasmas exhibit distinct behaviors depending on whether the operating gas is mostly argon or mostly helium. In high-argon plasmas, the metastable density has a large peak value but is confined very closely to the electrode surfaces as well as being reduced near the discharge gap itself. In contrast, metastable densities in high helium-fraction mixtures extend through most of the plasma. In all systems, increasing the power extends the region of metastable along the resonator length, while the extent away from the substrate surface remains approximately constant.

Construction and testing of a closed microwave plasma oven system for the treatment of radioactive waste

ABSTRACTThe production of nuclear energy as well as the application of other nuclear technologies produces large volumes of low- and intermediate-level radioactive wastes. In many countries, these wastes are sorted and treated separately in well-developed treatment schemes. Plasma treatment, in particular, with microwave-driven plasma torches promises many advantages. To explore the feasibility of that technology, plasma treatment with a plasma torch controlled by a 2.45-GHz microwave signal of up to 200 W was investigated in previous work. It could be shown that all materials can be successfully treated, that is, combusted or melted in an open laboratory setup. Here, design and construction of a plasma oven with controlled material and gas flow with the surrounding are reported. In exploratory experiments, it was found that the efficiency can be increased by utilizing a rotating crucible which is hit by the plasma zone at an angle of 40°. In addition, it was shown that shredding the material to 2–3 mm was advantageous. Further milling did not improve the results significantly. These results were transferred to the oven design. A further increase in treatment efficiency of up to ∼45% has been found when performing the plasma treatment in the oven. The formation of NOx, a toxic component of the flue gas, was systematically investigated. The flue gas concentrations are high above legal limits during combustion so flue gas scrubbing is required. The basic design goals of the plasma oven were met and further investigations can be undertaken to investigate more details of plasma processing of radioactive waste.

Investigation of some process parameters using microwave plasma technology for the treatment of radioactive waste

ABSTRACTThe production of nuclear energy and the application of other nuclear technologies produce large volumes of low- and intermediate-level radioactive wastes. To investigate a novel means of treating such wastes, plasma is investigated for its efficacy. Plasma treatment promises to simultaneously treat all waste types without any previous sorting or pre-treatment. Microwave-driven plasma torches have the advantage of high-energy efficiency and low-electrode wear. In small-scale experiments, several design variations of an open plasma oven were assembled in order to investigate constraints caused by the materials and oven geometry. The experimental set-up was modified several times in order to test the design characteristics and the variation of plasma-specific proprieties related to the radioactive waste treatment and in order to find a suitable solution with the minimum complexity that allows a representative reproducibility of the results obtained. A plasma torch controlled by a 2.45 GHz microwave signal of up to 200 W was used, employing air as the primary plasma gas with a flow rate of ∼2 L/min. Different organic and inorganic materials in different shapes and sizes were treated besides a standardized mixture resembling mixed wastes from nuclear plants. The results prove that the chosen microwave plasma torch is suitable for a combined combustion and melting of organic and in-organic materials. Investigation of the specimen size to be treated is influential in this process: the power is still too low to melt larger samples, but the temperature is sufficient to treat all kinds of material. When glass particles are added, materials melt together to form an amorphous substance, proving the possibility to vitrify material with this plasma torch. By optimization of the oven configuration, the time needed to combust 25 g of standard sample was reduced by ∼50%. Typical energy efficiencies were found in the range of 8–20% for melting of metal chipping, and ∼90% for melting of zinc powder.

Extreme ultraviolet radiation emitted by helium microwave driven plasmas

The extreme ultraviolet radiation emitted by helium microwave-driven (2.45 GHz) plasmas operating at low-pressure conditions was investigated. Novel data regarding emitted spectral lines of excited helium atoms and ions in the 20–33 nm wavelength range and their intensity behavior with variation of discharge operational conditions are presented. The intensity of all the spectral emissions was found to strongly increase with the microwave power delivered to the plasma. Furthermore, the intensity of the ionic spectral emissions decreases by nearly one order of magnitude as the pressure was raised from 0.2 to 0.5 mbar.

Generation of surface-wave microwave microplasmas in hollow-core photonic crystal fiber based on a split-ring resonator.

We report on a new and highly compact scheme for the generation and sustainment of microwave-driven plasmas inside the core of an inhibited coupling Kagome hollow-core photonic crystal fiber. The microwave plasma generator consists of a split-ring resonator that efficiently couples the microwave field into the gas-filled fiber. This coupling induces the concomitant generation of a microwave surface wave at the fiber core surround and a stable plasma column confined in the fiber core. The scheme allowed the generation of several centimeters long argon microplasma columns with a very low excitation power threshold. This result represents an important step toward highly compact plasma lasers or plasma-based photonic components.

Apparatus for generating quasi-free-space microwave-driven plasmas.

An apparatus for generating quasi-free-space microwave-driven plasmas has been designed, constructed, and tested. The plasma is driven by a multi-kW, ∼5 GHz microwave beam focused at the center of a vacuum chamber using a Koch-type metal plate lens. Sustained plasma discharges have been generated in argon at pressures ranging from 150 to 200 mTorr, at beam power levels ranging from 5 to 10 kW, and at gas flow rates of approximately 200 SCCM.

Two-dimensional profile measurement of plasma parameters in radio frequency-driven argon atmospheric pressure plasma jet

The two-dimensional profiles of the electron density, electron temperature, neutral translational temperature, and molecular rotational temperature are investigated in an argon atmospheric pressure plasma jet, which is driven by the radio frequency of 13.56 MHz by means of the laser scattering methods of Thomson, Rayleigh, and Raman. All measured parameters have maximum values at the center of the discharge and decrease toward the plasma edge. The results for the electron temperature profile are contrary to the results for the microwave-driven plasma. From our experimental results, the profiles of the plasma parameters arise from the radial contraction of plasmas and the time averaged profile of the electric field, which is obtained by a microwave simulation performed under identical conditions to the plasma jet. In the case of the neutral temperature, a higher translational temperature than the rotational temperature is measured, and its discrepancy is tentatively explained in terms of the low ion-neutral charge exchange rate and the additional degrees of freedom of the molecules. The description of our experimental results and the underlying physics are addressed in detail.