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Atmospheric pressure plasmas generated from a helium gas with admixtures of water vapor have numerous applications in biomedicine. It is important that the chemistry of such plasmas can be tightly controlled so that they may be tailored for their intended use. In this study, computational modeling is used to vary the pulse repetition frequency of a nanosecond-pulsed, pin-to-pin He + 0.25% H 2 O discharge in the range of 1–100 kHz to determine the influence of the pulse repetition frequency on the resulting densities of reactive oxygen species and the rates of dominant reaction pathways involving them. The plasma is simulated using the 0D plasma-chemical kinetics model GlobalKin. The pulse shape is kept constant. The afterglow duration is, therefore, dependent on the repetition frequency. Analysis of the bulk plasma chemistry after the plasma has reached equilibrium shows that the peak electron density is only weakly dependent on the pulse repetition frequency. Increasing the pulse repetition frequency is shown to increase the density of H, O, and OH radicals, while the relationship between the repetition frequency and the densities of species with longer lifetimes, namely, H 2 O 2 and O 3 , is found to be more complex. These are formed throughout the afterglow, and their density depends on the availability of reactant species, the afterglow duration, and the background gas temperature. This work concludes that the pulse repetition frequency is not a simple control parameter, especially for species that are predominantly produced in the afterglow. Detailed modeling is required for accurate control of species densities using the pulse repetition frequency. [ABSTRACT FROM AUTHOR]

Interactions at the interface between atmospheric pressure plasmas and liquids are being investigated to address applications ranging from nanoparticle synthesis to decontamination and fertilizer production. Many of these applications involve activation of droplets wherein the droplet is fully immersed in the plasma and synergistically interacts with the plasma. To better understand these interactions, two-dimensional modeling of radio frequency (RF) glow discharges at atmospheric pressure operated in He with an embedded lossy dielectric droplet (tens of microns in size) was performed. The properties of the sheath that forms around the droplet were investigated over the RF cycle. The electric field in the bulk plasma polarizes the dielectric droplet while the electron drift in the external electric field is shadowed by the droplet. The interaction between the bulk and sheath electric fields produces a maximum in E/N (electric field/gas number density) at the equator on one side of the droplet where the bulk and sheath fields are aligned in the same direction and a minimum along the opposite equator. Due to resistive heating, the electron temperature Te is maximum 45° above and below the equator of the droplet where power deposition per electron is the highest. Although the droplet is, on the average, negatively charged, the charge density on the droplet is positive on the poles and negative on the equator, as the electron motion is primarily due to diffusion at the poles but due to drift at the equator. [ABSTRACT FROM AUTHOR]

Cold plasma as a new anticancer agent is described by Freund I et al. i [25] The anticancer effect of oxidized saline generated using a spark discharge is investigated using CT26 colorectal cancer cells I in vitro i and I in vivo i and compared against oxidized saline generated by the well-investigated medical plasma jet I kINPen i . In summary, atmospheric plasmas play an important role in many current and emerging fields, including plasma medicine, agriculture, plasma processing, catalysis, and aerospace engineering. Plasma generated in contact with liquid is charactered by Ng I et al i .[35] It is shown that the plasma can be tailored to tune the composition of plasma-functionalized liquids. The significant effect of radiation trapped photon emission on ionization wave dynamics demonstrates the need to consider these effects in plasma reactors where self-absorbed radiation is typically ignored. [Extracted from the article]

Indium nitride nanostructures have been formed by atmospheric and sub-atmospheric pressure plasma nitridation of molten indium. By nitriding the molten indium metal layer with Ar-10%N2 atmospheric pressure plasma at 435 K, micrometer-sized particles (0.7–4 μm) made up by agglomeration of nanoparticles (5–50 nm) and a variety of nanostructures (nanowires, microrods, and hollow microtubes) with various diameters (60–700 nm) have been formed, as observed by scanning electron microscopy. Transmission electron microscope observations show that these structures are composed of wurtzite type indium nitride nanocrystals (5–20 nm). By nitriding the molten indium metal with N2 sub-atmospheric pressure plasma at 673 K, hexagonal crystallites having facets (40–200 nm) have been grown to pile up. It is found that high quality indium nitride crystallites of wurtzite type single crystals can be grown at a relatively low growth temperature of 673 K, as revealed by transmission electron microscopy and Raman spectroscopy. Based on the experimental results, the main active nitrogen species for indium nitride formation are assumed to be nitrogen atoms and formation mechanisms of indium nitride nanoparticles and nanowires have been discussed. [ABSTRACT FROM AUTHOR]

A high-pressure direct current magnetron discharge is analyzed using a self-consistent two-dimensional fluid model. This discharge has potential interest in plasma switching applications due to its ability to generate rather dense (plasma density of ∼1019 m−3) stable nonequilibrium plasmas at pressures of ∼13.3 Pa (100 mTorr). This discharge has several advantages over nanosecond pulsed high pressure discharges used in conventional plasma switchers since it operates at much lower voltages and lower gas pressures, with the advantage of much longer cathode lifetime. In the present paper, we analyze the operating modes of this magnetron discharge for conditions close to that of Sommerer et al. [J. Phys. D: Appl. Phys. 52, 435202 (2019)]. We observe the subnormal and normal modes of the magnetron operation whose physics differs from that typical for unmagnetized plasmas. [ABSTRACT FROM AUTHOR]

The propagation of atmospheric pressure plasmas (APPs) on and through porous dielectric materials is being investigated for plasma-catalysis and functionalizing biomedical materials for tissue scaffolding and bone regeneration. Such plasma functionalization improves wettability and cell attachment, and so uniformity of the treatment of the pore surfaces is important. The method of propagation of APPs through porous media is not well characterized. In this paper, we discuss results from a computational investigation of humid air APPs propagating through short fully interconnected pore-chains in a dielectric substrate. The properties of the dielectric and pores (diameter 150 μm) were chosen to resemble bone scaffolding. We found that photoionization is an important feature in plasma propagation through pore-chains to seed electrons in the following pore in the chain. This seeding of electrons in regions of high electric field allows for the formation of micro-streamers and surface ionization waves. This is particularly important when the openings between pores are small. The orientation of the pore-chain with respect to the applied electric field has a significant impact on plasma generation, mode of propagation, and fluences of short-lived, reactive species to the surfaces of the pores. The uniformity of fluences of charged and short-lived neutral species to the pore surfaces decreases as the angle of the pore chain deviates from being aligned with the applied electric field. Diffusion within pores improves the uniformity of fluences to pore surfaces for long-lived species on longer time scales compared to their post-discharge uniformity. [ABSTRACT FROM AUTHOR]

It has been shown that plasma generated in contact with liquid can be tailored to tune the composition of plasma functionalized liquids. For biomedical applications, it is necessary to understand the generation of the plasma treated liquids to modulate the composition and thus the biological response. In this work, two distinct discharge compositions were realized by modifying the location of the ground electrode in a pin-to-liquid plasma system. Through this simple modification to the configurations, the spatiotemporal characteristics of the discharge were significantly affected which, in turn, affected the composition of the generated plasma activated water (PAW). Colorimetric testing of the PAW generated from each system revealed that only one configuration was able to generate PAW with a high concentration of H2O2. Using time-, space-, and wavelength-resolved imaging of excited plasma species [OH, N2 (SPS), N2+ (FNS), and atomic O], the differences in PAW composition were linked to the differences observed in the discharge dynamics between the two configurations. [ABSTRACT FROM AUTHOR]

Self-organized structures that are spread perpendicular to the radio frequency (RF) current direction have been observed in low temperature RF capacitively coupled plasmas. A fluid plasma model that includes thermoelectric electron energy transport is used to understand how these structures form. The electron thermoelectric transport coefficient is calculated using Bolsig+ for different chemistries and is found to be large for Ar plasma. Thermoelectric electron energy transport, which is driven by particle diffusion, opposes electron thermal conduction and can localize the plasma, leading to periodic structures. To examine these structures in radio frequency (RF) capacitive plasmas, two-dimensional Ar plasma at 13.5 MHz is first simulated without and then with thermoelectric electron energy transport. The charged species densities are perturbed in the simulations, and the growth or decay of different modes with time is observed. The periodicity of the structure is found to be determined by the relative strength of thermoelectric electron energy transport compared to energy conduction and losses. The effect of operating variables such as chemistry and pressure and design variables such as inter-electrode gap and steps in the electrode have been studied. For Ar plasma as pressure is decreased, the plasma peaks become stronger since thermoelectric electron energy transport is enhanced. Within limits, steps in the electrodes can be used to control the location of the periodic structures. For N2 plasma, the periodic structure does not appear as thermoelectric electron energy transport is weak. The spacing between plasma peaks is found to be dependent on pressure, chemistry, and inter-electrode gap. [ABSTRACT FROM AUTHOR]

Atmospheric pressure plasmas in argon are of particular interest due to the production of highly excited and reactive species enabling numerous plasma-aided applications. In this contribution, we report on absolute optical emission and absorption spectroscopy of a radio frequency (RF) driven capacitively coupled argon glow discharge operated in a parallel-plate configuration. This enabled the study of all key parameters including electron density and temperature, gas temperature, and absolute densities of atoms in highly electronically excited states. Space and time-averaged electron density and temperature were determined from the measurement of the absolute intensity of the electron-atom bremsstrahlung in the visible range. Considering the non-Maxwellian electron energy distribution function, an electron temperature (T e ) of 2.1 eV and an electron density (n e ) of 1.1 × 10 19 m − 3 were obtained. The time-averaged and spatially resolved absolute densities of atoms in the metastable (1 s 5 and 1 s 3) and resonant (1 s 4 and 1 s 2) states of argon in the pure Ar and Ar/He mixture were obtained by broadband absorption spectroscopy. The 1 s 5 metastable atoms had the largest density near the sheath region with a maximum value of 8 × 10 17 m − 3 , while all other 1s states had densities of at most 2 × 10 17 m − 3 . The dominant production and loss mechanisms of these atoms were discussed, in particular, the role of radiation trapping. We conclude with comparison of the plasma properties of the argon RF glow discharges with the more common He equivalent and highlight their differences. [ABSTRACT FROM AUTHOR]

A simple procedure has been proposed to estimate atmospheric-pressure (AP) plasma parameters based on the electric field simulation of a very-high-frequency plasma system including the impedance matching unit. The capacitively coupled plasma is generated between metal electrodes with a narrow gap, where the standard Langmuir probe method cannot be applied. The amplitudes of current density (J0) and voltage (V0) between the electrodes are determined by the three-dimensional computer simulation of the system in the impedance matched state using two experimental parameters (i.e., the separations between two parallel-plate capacitors) in the matching unit. Approximate analytical solutions relating the central electron density (n0) and the sheath thickness to J0 and V0 are derived based on a simplified inhomogeneous plasma model with collisional sheaths. The average electron temperature Te is estimated using a power balance relation between the total powers absorbed and lost in the ionizing plasma. The results on power dependences of n0 and Te show reasonable agreement with the predictions by the particle and power balance relations. In the case of AP plasma of Ar or He mixed with impurity or process gas molecules, it is impossible to determine Te. However, n0 can be obtained and the collisional energy loss per electron–ion pair creation (ɛc) can be estimated, which brings certain information on the change of plasma chemistry. Since the proposed procedure is simple and non-intrusive, it might become a useful tool for discussing AP plasma properties in process developments. [ABSTRACT FROM AUTHOR]

An atmospheric pressure plasma (APP) system offers advanced, cost-effective processing routes for surface cleaning without a vacuum chamber. The appeal of APP systems in surface cleaning, however, is reduced by lack of a predictive link among the processing parameters, surface-plasma reactions, and plasma chemistry responsible for efficient removal. Here, we present a comprehensive multiphysics model of surface cleaning using a microwave assisted atmospheric plasma system as an alternative to chemical cleaning of surfaces. This model can quantitatively predict the processing time for the removal of the contaminant species from the surface. The presented model investigates the dependencies of removal rates and the nature of the contaminant species on the processing parameters. We demonstrate these dependencies by using polypropylene as a model hydrocarbon impurity. The complex and cooperative effects of microwave power, gas flow rate, torch-substrate distance, and tilt angle have been explored for understanding factors behind efficient cleaning. Our results show that the rate of hydrocarbon degradation is highly dependent on gas temperatures over the surface, flow pattern, and torch-substrate distance and depends less on the angle of attack. This study helps to optimize the values for operational parameters of atmospheric plasma processing that speeds up the experiments toward achieving a higher surface cleaning rate. [ABSTRACT FROM AUTHOR]

The electron energy distribution function (EEDF) in low pressure plasmas is typically evaluated by using the second derivative d 2 I / d V 2 of a Langmuir probe I – V characteristic (Druyvesteyn formula). Since measured probe characteristics are inherently noisy, two-time numerical differentiation requires data smoothing techniques. This leads to a dependence on the employed filtering technique, and information particularly in the region near the plasma potential can easily get lost. As an alternative to numerical differentiation of noisy probe data, a well-known AC probe technique is adopted to measure d 2 I / d V 2 directly. This is done by superimposing a sinusoidal AC voltage of 13 kHz on the probe DC bias and performing a Fourier analysis of the current response. Parameters such as the modulation amplitude (up to 1.5 V) and the number of applied sine oscillations per voltage step of the DC ramp are carefully chosen by systematic parameter variations. The AC system is successfully benchmarked in argon and applied to hydrogen plasmas at a laboratory inductively coupled plasma experiment (4– 10 Pa gas pressure, 300– 1000 W RF power). It is shown that the EEDF is reliably accessible with high accuracy and stability in the low energy range. Hence, a trustworthy determination of basic plasma parameters by integration of the EEDF can be provided. [ABSTRACT FROM AUTHOR]

Nowadays, multi-element cathodes are frequently employed to grow multi-element thin films and coatings using cathodic arc deposition processes. During cathode erosion, the cathode spot sequentially ignites on the cathode surface and imposes melting–solidification cycles that lead to material intermixing and the formation of a modified layer on the cathode surface. To allow us to study these surface modifications, a 10 μm thick Mo/Al multilayer coating was sputter-deposited onto a standard Ti arc cathode. This cathode was eroded by a dc steered arc discharge for a short duration enabling the observation of single craters formed by type 1 and 2 cathode spots. Furthermore, separated clusters of overlapping craters and a fully eroded surface caused by different stages of erosion were differentiated when scanning the erosion track in the lateral direction. Cross sections of single craters were prepared by focused ion beam techniques while metallographic methods were applied to obtain cross sections of overlapping craters and the modified layer. The layers of the multilayer coating acted as trace markers providing new insights into the material intermixing within craters, the material displacements during crater formation, the plasma pressure acting on the craters, and the temperature gradient (heat-affected zone) below the craters. The observations are discussed within the framework of established arc crater formation models. [ABSTRACT FROM AUTHOR]

This paper investigates pressure influence on polycrystalline diamond formation using sawtooth-waveform modulated induction thermal plasma. Modulated induction thermal plasma was used to promote nucleation of diamond particles in the first stage. The operating pressure was set to 32, 60, and 90 Torr. The deposited diamond films were analyzed by field emission scanning electron microscope and Raman spectroscopic observation. Experimental results indicated that a lower pressure condition provided a higher deposition rate and a qualitatively better diamond film. Spectroscopic and high-speed video camera observation showed that a lower pressure condition expanded the thermal plasma flow axially onto the substrate. Numerical simulation was also made for Ar/CH 4 /H 2 induction thermal plasma to study the influence of pressure on the thermal plasma flow. Calculation results showed that lower pressure involves a higher particle flux of neutral hydrocarbon species on to the substrate surface because of higher convective transport of these species before their ionization. These results imply that neutral hydrocarbon would play a more important role for diamond deposition than hydrocarbon ions in the present thermal plasma diamond film deposition. [ABSTRACT FROM AUTHOR]

This article is focused on understanding of the effect of a low-pressure argon plasma on split ring resonator (SRR) metamaterial resonance. After an experimental characterization of SRR arrays that denies reaching critical density at 2.45 GHz, we present an alternative SRR, called sparse SRR, tunable in resonance frequency, that admits higher plasma density in its structure. This innovative structure is characterized with plasma to search for left-handed material signature. Several plasma effects such as gradients and sheaths are quantified. It is shown that plasma shifts the SRR resonance to higher frequencies. The amplitude of the resonance shift can not be estimated without taking into account the sheath thickness. Moreover, the plasma gradient causes the appearance of several distinguishable dips of resonances. These different effects are studied with simulations, highlighting the difficulty to create a left-handed material based on plasma-metamaterial. [ABSTRACT FROM AUTHOR]

A nonthermal atmospheric pressure plasma source based on a dc-driven intermittent spark discharge in a coaxial electrode configuration with volumetrically distributed discharge filaments is presented. Spreading the intermittent spark discharge over the volume of the cylindrical discharge chamber is achieved owing to the mutual action of acoustic and magnetic fields on the discharge. The magnetic field scans discharge filaments over the cross section of the cylindrical electrode, while the acoustic field spreads the discharge along the electrode. Electrical parameters and characteristics of the developed nonthermal atmospheric pressure plasma source with volumetrically distributed plasma microchannels are presented. [ABSTRACT FROM AUTHOR]

We developed an atmospheric pressure plasma (APP) process for forming metal oxides and self-assembled monolayers (SAMs) without a vacuum process. SAMs are frequently utilized for surface modification. In particular, a combination of aluminum oxide (AlOx) and alkylphosphonic acid is used as the gate dielectric in flexible electronics, with a low operating voltage of 2 V because of its simple fabrication process at room temperature in air. However, when using a combination of SAMs and AlOx as the gate dielectric, it is necessary to form a metal oxide with sufficient thickness using a vacuum plasma process to reduce the leakage current, which is not suitable for use in flexible large-area electronics. In this study, we resolved this problem by utilizing an APP process and substrate heating instead of the vacuum plasma process. Heating a substrate above 100 °C forms a sufficient AlOx layer. The leakage current of plasma-processed AlOx with SAMs was reduced by three orders of magnitude compared to that of natural AlOx with SAMs. X-ray photoelectron spectroscopy revealed a time dependence of the thickness of AlOx. An exposure time of 100 s was sufficient to form an AlOx layer with a thickness of 4 nm. An organic transistor with this APP treatment of AlOx and SAMs showed a mobility of 1.4 cm2/V s and an on/off ratio of 104 within 2 V. We also fabricated an inverter and a ring oscillator circuit with an inverter gain of more than 300 and an oscillation frequency of 40 Hz. [ABSTRACT FROM AUTHOR]

We propose a method for diagnostics of the density of weakly magnetized plasma using a resonance microwave probe, which allows decreasing the minimum measurable value of the density by three orders of magnitude, thus expanding the dynamic range of the measuring system significantly. Within the framework of this method, measurements are performed at a fixed frequency corresponding to the resonance frequency of a microwave probe in the absence of plasma, while minor variations in the resonance frequency of the system within the limits of the width of the inherent resonance are recorded by the method of amplitude-phase measurements. The paper presents a theoretical model of the measurement method and its experimental verification. [ABSTRACT FROM AUTHOR]

Density function theory calculations have been performed to study the stability of small helium-vacancy (He-V) complexes near tungsten (W) surfaces of different orientations. The results show that the stability of vacancies and He-V complexes near W surfaces depends on surface orientation. However, as the depth below the surface increased beyond about 0.65–0.8 nm, the stability of He-V complexes is similar to the bulk. The formation energies of single vacancies and di-vacancies at depths less than 0.2 nm below the W(110) surface are higher than for W(100) or W(111) surfaces, but have lower energies at depths between 0.2 and 0.65 nm. The formation energies of He-V complexes below W surfaces are sensitive to the geometric orientation of the He and vacancy, especially below the W(111) surface. Within about 0.2 nm of the top layer of the three W surfaces, neither a vacancy nor a di-vacancy can trap He. Because of the lower formation energy of He-V complexes and higher He binding energy to vacancies below the W(110) surface, the He desorption from the W(110) surface is less likely to occur than from the W(100) and W(111) surfaces. Our results provide fundamental insight into the differences in surface morphology changes observed in single W crystals with different surface orientations under He plasma exposure. [ABSTRACT FROM AUTHOR]

A model of cathode spots in high-current vacuum arcs is developed with account of all the potentially relevant mechanisms: the bombardment of the cathode surface by ions coming from a preexisting plasma cloud; vaporization of the cathode material in the spot, its ionization, and the interaction of the produced plasma with the cathode; the Joule heat generation in the cathode body; melting of the cathode material and motion of the melt under the effect of the plasma pressure and the Lorentz force and related phenomena. After the spot has been ignited by the action of the cloud (which takes a few nanoseconds), the metal in the spot is melted and accelerated toward the periphery of the spot, with the main driving force being the pressure due to incident ions. Electron emission cooling and convective heat transfer are dominant mechanisms of cooling in the spot, limiting the maximum temperature of the cathode to approximately 4700–4800 K. A crater is formed on the cathode surface in this way. After the plasma cloud has been extinguished, a liquid-metal jet is formed and a droplet is ejected. No explosions have been observed. The modeling results conform to estimates of different mechanisms of cathode erosion derived from the experimental data on the net and ion erosion of copper cathodes. [ABSTRACT FROM AUTHOR]