RF Plasma Jet Research Articles

Electric field measurements in a He:N2 atmospheric pressure RF plasma jet by E-FISH

Abstract The electric field in the bulk of a self-sustained radio frequency atmospheric plasma jet (RF-APPJ) in a helium:nitrogen mixture is measured for the first time, to the best of the authors' knowledge, by electric field induced second harmonic generation (E-FISH). It is shown that the electric field in the bulk of the RF-APPJ is unexpectedly high, having an amplitude of about 1.6 kV/cm, and that it exhibits a phase shift of approximately -0.2π relative to the voltage waveform. Both findings are in contrast to low pressure capacitively coupled plasmas (CCPs), where the bulk electric field is negligible and the phase shift between the current and the voltage equals approximately to -π/2. The electron density in the bulk is estimated from the measured phase shift between the electric field, i.e. the current in the bulk, and the applied voltage by using an equivalent RC-circuit model for the discharge. The measured reduced electric field combined with the estimated electron density in the middle of the jet is used to calibrate the values of excitation rates of the N2 (C3Πu) state obtained by Phase Resolved Optical Emission Spectroscopy (PROES) in arbitrary units. The results of the electric field measurements are proposed as an input for numerical models or as a benchmark for calculations. Last but not least, special attention was paid to the calibration of the E-FISH measurement in a discharge powered by AC voltage, which includes artifacts supposedly caused by the windows of the setup.

Influence of pre-treatment with non-thermal atmospheric pressure plasma on bond strength of TP340 titanium-PEEK direct bonding

Direct bonding of a TP340 titanium to PEEK by hot pressing via pre-treatment of non-thermal atmospheric pressure plasma jet has been demonstrated. The plasma irradiation effect on the bonding surface on the bond strength after hot pressing was investigated. The tensile shear strength of TP340-PEEK joined by hot pressing after plasma pre-treatment was measured by comparing specimens bonded using conventional hot pressing and those bonded using adhesives. The plasma treatment to the TP340 side resulted in the formation of TiO2, which is chemically fed to oxide formation due to the irradiation of oxygen radicals generated by the plasma, resulting in a bond strength of less than 1 MPa, similar to the bond strength of the untreated specimens. The plasma irradiation effect on the PEEK side on the bond strength of TP340-PEEK bonded samples was also investigated. The bonding strength was increased by plasma irradiation to PEEK. As the plasma irradiation time was increased, the bonding strength gradually increased to 9.2 MPa, which is about 19 times higher than the bonding strength without plasma irradiation. These results suggest that oxygen radicals in the atmospheric pressure RF plasma jet produced oxygen-containing surface functional groups on the PEEK surface, which increased the strength of the TP340-PEEK direct joining.

NO formation by N2/O2 plasma catalysis: The impact of surface reactions, gas-phase reactions, and mass transport

Pathways and timescales relevant to facilitate plasma-assisted N2-O2 reactions are assessed by measuring the consumption of plasma-derived N and the formation of NO in the gas phase and over Ag catalytic surfaces. These measurements are enabled by a setup that enables N2 activation in an atmospheric pressure RF plasma jet, enables O2 addition in the plasma afterglow, facilitates reactions over an Ag wire catalyst, and allows species density quantification by molecular beam mass spectrometry. Gas-phase reactions consume N but do not form NO with high selectivity. The presence of the non-porous Ag wire catalyst increases the rate of N conversion to NO, though mass transfer processes, not surface reactions, dictate the rate of N consumption. When O2 concentrations and the ratio of the surface area of the catalyst to the void volume of the reactor are high (3–5 mol% O2, 10900 m−1), N conversion to NO reaches 100 % selectivity. When both N2 and O2 are fed through the plasma jet, gas-phase NO production increases 10×, although plasma and gas-phase processes do not exclusively produce NO. Above a threshold NO density, N cannot diffuse to the catalyst surface faster than it is consumed in the gas phase by reactions with NO. Thus, the use of heterogeneous catalysts to enhance plasma-driven NxOy formation and control NxOy product selectivity is limited to cases where diffusive transport of N from the gas phase to the catalyst surface is faster than consumption of N from gas-phase reactions with NO.

Impact of catalysis on n-butane oxidation in an RF atmospheric pressure plasma

The plasma catalytic oxidation of n-butane diluted in a helium oxygen RF plasma jet is used to study volatile organic compound removal to unravel plasma catalytic synergisms. The plasma conversion is tested for a stoichiometric n-butane oxygen mixture for varying plasma power and using a manganese oxide catalyst. It is shown that the interplay between plasma and catalyst is very complex. The catalyst enhances the power coupling, but also serves as a sink for oxygen atoms due to surface recombination. The surface processes are dominated by reactions of radicals and excited species from the plasma. The oxidation of n-butane at the catalyst surface is slightly enhanced. In total, however, n-butane oxidation without the catalyst is more efficient than with the catalyst, which constitutes an anti-synergism.

A 2D steady state analytical model for atmospheric pressure RF plasma jet

This paper presents a 2-dimensional analytical model for plasma inside the discharge tube of an RF atmospheric pressure plasma jet (APPJ) in coaxial geometry. Since the model is a steady state one, it requires that the frequency of the applied RF be high enough so that the formation of streamers and plasma bullets is suppressed and that continuous plasma is present throughout the RF cycle. It is also assumed that power absorption is due to electron-atom collisions in the bulk plasma. The model takes into account gas flow, yielding a flow-diffusion axial flux that aids the exit of the plasma into the atmosphere. The inputs to the model are the reactor dimensions, average power fed (P av ) and the flow velocity of the gas (ug) using which the model yields the plasma density along with its radial and axial profiles, electron temperature, axial and radial particle fluxes, gas heating, flux, and energy of the plasma particles escaping from the discharge tube into the ambient air. Experiments were undertaken on an RF-powered jet at 13.56 MHz. Substituting the average RF power, measured using voltage and current probes, into the model gives the plasma density n ≈ 3.9 × 1017(≅2.8 × 1019)m−3 at for He (Ar). The corresponding electron temperature given by the model is for He (Ar). It is hoped that the developed model would be able to provide a comprehensive methodology for analyzing APPJs.

Coupling the COST reference plasma jet to a microfluidic device: a new diagnostic tool for plasma-liquid interactions

Plasma-liquid interaction processes are central to plasma applications in medicine, environment, and material processing. However, a standardized platform that allows the study of the production and transport of plasma-generated reactive species from the plasma to the liquid is lacking. We hypothesize that use of microfluidic devices would unlock many possibilities to investigate the transport of reactive species in plasma-treated liquids and, ultimately, to measure the effects of these species on biological systems, as microfluidics has already provided multiple solutions in medical treatment investigations. Our approach combines a capacitively coupled RF plasma jet known as the COST reference plasma jet with simple 3D printed microfluidic devices. This novel pairing is achieved by carefully controlling capillary effects within the microfluidic device at the plasma-liquid interaction zone. The generation and transport of reactive species from the plasma to the liquid inside the microfluidic device are analyzed using a colorimetric hydrogen peroxide concentration assay. A capillary flow model is provided to explain the two main regimes of operations observed in the device and their merits are discussed. Overall, the proposed plasma-microfluidic prototype shows great potential for the fundamental study of plasma-liquid interactions and opens the way to the use of standard microfluidic devices with plasma sources developing a plasma column or a plasma plume.

Germination Energy, Germination Capacity and Microflora of Allium cepa L. Seeds after RF Plasma Conditioning

This paper presents the results of an experiment on the effect of the cold plasma (He+O2 or He+Air) pre-sowing stimulation of seeds of the Wolska cultivar of onion on the process of their germination. Four groups of seeds characterized by different exposure times (60, 120, 240 and 480 s) were used. Untreated seeds were used as a control. The distance between the electrode and the tested material was 50 mm. Pre-sowing plasma stimulation improved germination parameters such as germination capacity and germination energy for all the tested groups relative to the control. The highest fractions of germinated seeds were observed for an exposure time of 120 s. Analysis of the data showed a statistically significant impact of RF plasma on the seed germination parameters of the onion. SEM analysis showed that the interaction with plasma produced tension in the cells, leading to a change in their shape. No visible damage to the onion seed cells was observed, apart from the effect of depletion of the upper wax layer. The best influence on pathogenic fungi was when the group of seeds underwent 240 and 480 s of exposure to plasma fumigation, especially using the He+Air RF plasma jet.

Influence of pulse modulation frequency on helium RF atmospheric pressure plasma jet characteristics

AbstractThis work investigates the influence of pulse modulation frequency ranging from 50 Hz–10 kHz on the helium RF atmospheric pressure plasma jet's fundamental characteristics. The impact of modulation frequency on plasma jet discharge behaviour, geometrical variation, reactive species emission, and plasma parameters (gas temperature Tg, electron excitation temperature Texc, and electron density (ne)) are studied using various diagnostics such as optical imaging, emission spectra, and thermal diagnostic. From the experiments, it is observed that operating the plasma jet at low pulse modulation frequencies (around 50 Hz) provides enhanced plasma dimensions, higher electron densities and greater optical emission from reactive species (viz., He I, O, OH, N2+, etc.) as compared to the higher modulation frequencies. Besides the low power consumption, three times less gas temperature of the modulated plasma jet than the continuous wave mode makes it more advantageous for the applications. Moreover, the influence of duty cycle (D) and applied RF power (P) on the plasma jet characteristics are also discussed. It is found that 10–40% duty cycle operation provides the most favourable attributes. More importantly, the concern of shorter plasma length in RF plasma jets is overcome by operating at 10–20% duty cycle with increased applied power. This work thoroughly characterizes helium atmospheric pressure RF plasma jet with a wide range of pulse mode operating parameters, which could help to select appropriate operating conditions for various industrial and biomedical applications.

N2 vibrational excitation in atmospheric pressure ns pulse and RF plasma jets

Time-resolved N2 vibrational temperature and translational–rotational temperature in quasi-two-dimensional atmospheric pressure plasma jets sustained by ns pulse and RF discharges in nitrogen/noble gas mixtures are measured by the broadband vibrational Coherent Anti-Stokes Raman Scattering (CARS) . The results indicate a much stronger vibrational excitation in the RF plasma jet, due to the lower reduced electric field and higher discharge power. In a ns pulse discharge in N2/He, N2 vibrational temperature is significantly lower compared to that in N2/Ar, due to the more rapid vibration–translation (V–T) relaxation of nitrogen by helium atoms. In the RF plasma jets in N2/Ne and N2/Ar, the vibrational excitation increases considerably as the nitrogen fraction in the mixture is reduced. The experimental data in the RF plasma jet in N2/Ar jet are compared with the kinetic modeling predictions. The results indicate that nitrogen vibrational excitation in N2/Ar plasma jets with a small N2 fraction in the mixture (several percent) is controlled primarily by electron impact, anharmonic vibration–vibration (V–V) pumping, and V–T relaxation by N atoms. In comparison, V–V energy transfer from the vibrationally excited molecules in the first excited electronic state, N2(A3Σu +, v), which are generated primarily by the energy transfer from the metastable Ar atoms, has a minor effect on the vibrational populations of the ground electronic state, N2(X1Σg +, v). Although the discharge energy fraction going to electronic excitation is significant, the predicted quasi-steady-state N2(A3Σu +) number density, controlled by the energy pooling and quenching by N atoms, remains relatively low. Because of this, the net rate of N2(X1Σg +) vibrational excitation by the V–V energy transfer from N2(A3Σu +) is much lower compared to that by the direct electron impact. The results show that atmospheric pressure RF plasma jets can be used as sources of highly vibrationally excited N2 molecules and N atoms.

Tailored Power of an RF Plasma Jet With Admixture of Nitrogen or Oxygen and Its Effects on Human Immune Cells

Parameter studies of plasma treatment are informative about the optimal use of this technology in biomedical applications such as the argon radio-frequency plasma jet kINPen. However, the interdependence of the plasma-dissipated power in relation to input current and feed gas modulation on the resulting biological consequences has not been studied so far. To this end, a parameter study is presented, and the effect on human immune cell viability was investigated across different input current power and argon with oxygen/nitrogen feed gas admixture settings. It was found that with both nitrogen and oxygen admixtures, a concentration-dependent change in plasma-dissipated power emerged, which converged at 27.5 and 26.5 mA, respectively. The extent of cytotoxicity in immune cells confirmed the relevance of these findings, which were in congruency with the plasma-dissipated powers identified. These findings underline the critical role and input parameter-dependent action of plasma sources for biomedical application.

Energy efficiency of voltage waveform tailoring for the generation of excited species in RF plasma jets operated in He/N2 mixtures

Based on tunable diode laser absorption spectroscopy (TDLAS) measurements of the spatially averaged and peak helium metastable atom densities in a capacitively coupled micro atmospheric pressure plasma jet operated in He/N2 mixtures, the energy efficiency of metastable species (He-I 23S1) generation is compared for three different scenarios: single frequency operation at (i) 13.56 MHz and (ii) 54.12 MHz, and voltage waveform tailoring (VWT) at (iii) ‘valleys’-waveforms synthesized from four consecutive harmonics of 13.56 MHz. For each case, the dissipated power is measured based on a careful calibration procedure of voltage and current measurements. It is shown that the range of powers, at which the jet can be stably operated, is noticeably expanded by VWT. The results are compared to particle-in-cell/Monte Carlo collisions simulation results and very good agreement is found. The computational results show that the choice of the surface coefficients in the simulation is important to reproduce the experimental data correctly. Due to the enhanced control of the spatio-temporal electron power absorption dynamics and, thus, of the electron energy distribution function by VWT, this approach does not only provide better control of the generation of excited and reactive species compared to single frequency excitation, but in case of helium metastables the energy efficiency is also shown to be significantly higher in case of VWT.

(Invited) Electrodeless Organic Electrosynthesis By Plasma-Liquid Interface

The plasma-liquid interface can promote selective redox reactions involving organic molecules without the need for solid electrodes. More specifically, our group’s work on model organic compounds has revealed that selective reduction occurs near the surface of water in contact with an RF plasma jet generated using helium or argon as working gas, for positions located near the centerline of the impinging free jet. The complementary oxidation half reaction occurs further away from the centerline, and the products of reduction and oxidation can be isolated by placing a semipermeable membrane between the reduction and oxidation zones, which are termed electrodeless cathode and anode respectively. The reduction potential in the liquid near the surface at the plasma jet centerline can be tuned by changing the plasma parameters. Correlations between reduction potential and plasma parameters, specifically electron density and temperature as characterized by Thomson scattering, will be presented and a simple model proposed to theoretically connect the two. Finally, envisioned applications for this unique type of electrodeless organic electrosynthesis will be discussed in the fields of water quality, renewable carbon utilization, and pharmaceuticals. The work described in this presentation is collaborative between research groups at Washington University in Saint Louis, Princeton Plasma Physics Laboratory, and the University of Michigan; and was supported by the Department of Energy under award DE-SC0020352, the Princeton Collaborative Low Temperature Plasma Research Facility (PCRF), and the National Science Foundation through grant CBET-2033714.

Atomic oxygen generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and O2

Absolute atomic oxygen densities measured space resolved in the active plasma volume of a COST microplasma reference jet operated in He/O2 and driven by tailored voltage waveforms are presented. The measurements are performed for different shapes of the driving voltage waveform, oxygen admixture concentrations, and peak-to-peak voltages. Peaks- and valleys-waveforms constructed based on different numbers of consecutive harmonics, N, of the fundamental frequency f 0 = 13.56 MHz, different relative phases and amplitudes are used. The results show that the density of atomic oxygen can be controlled and optimized by voltage waveform tailoring (VWT). It is significantly enhanced by increasing the number of consecutive driving harmonics at fixed peak-to-peak voltage. The shape of the measured density profiles in the direction perpendicular to the electrodes can be controlled by VWT as well. For and peaks-/valleys-waveforms, it exhibits a strong spatial asymmetry with a maximum at one of the electrodes due to the spatially asymmetric electron power absorption dynamics. Thus, the atomic oxygen flux can be directed primarily towards one of the electrodes. The generation of atomic oxygen can be further optimized by changing the reactive gas admixture and by tuning the peak-to-peak voltage amplitude. The obtained results are understood based on a detailed analysis of the spatio-temporal dynamics of energetic electrons revealed by phase resolved optical emission spectroscopy.

Influence of a Helium–Nitrogen RF Plasma Jet on Onion Seed Germination

This paper presents an experiment using a radio frequency atmospheric pressure plasma jet to generate cold plasma for pre-sowing stimulation of Wolska onion seeds. Impact of the He + N2 afterglow plasma on germination was investigated. Eight groups of seeds characterized by different exposure times (2, 5, 10, 15, 60, 120, 240, and 480 s) and distance from the electrodes (20 mm and 50 mm) were used. Pre-sowing plasma stimulation of the seeds improved the germination capacity and germination energy for all tested groups, relative to control. The impact of radio frequency plasma on the onion seed germination parameters was statistically significant. The highest germination parameters were obtained for seeds stimulated for 240 s at a distance of 50 mm. No significant differences in physical and morphological properties of onion seeds were found.

The effects of grounded electrode geometry on RF-driven cold atmospheric pressure plasma micro-jet

With the argument that two-electrode DBD-like systems are much more operational than single-electrode systems in biomedical applications, targets sensitive to temperature and electric shock, the effects of parameters associated with the geometry of the grounded electrode such as its shape, size, and position it at the output of the atmospheric pressure RF plasma jet in two-electrode systems is investigated. By varying the position of the typical narrow ring grounded electrode on the dielectric tube toward the powered electrode, the ratio of the axial to radial electric field components depend on the externally applied potential to the plasma has been investigated and shown that the axial component of the electric field is maximized at certain position(s) of the grounded electrode. The analysis of the data indicates that there is an inverse relationship between the magnitude of the axial electric field in the plasma channel and the discharge ignition voltage, and a direct relationship with the plasma jet length. It is known that by increasing the width of the ground electrode until the full covering of dielectric, the jet length increases from the dielectric output to the neighborhood near the needle electrode, and reduces the discharge ignition threshold and consequently power consumption of the jet, but increasing its width to greater than the above values does not have a significant effect on jet output. It has also been shown that by tapering the dielectric end and fully covering it with its conical-shaped electrode, the output jet length increases and decreases its width.

(Invited) Electrochemistry in Plasmas: From Flames to RF Plasma Jets

As electrochemists, we are interested in electron attachment and detachment processes. Traditionally, we control the availability of electrons via an electrically conducting solid and measure electron transfer across the solid/liquid interface. Of course, there are exceptions to this picture, e.g. liquid/liquid interfaces, but often liquids are involved to provide an electrolyte medium to support the chemical species. Gaseous electrolytes have typically been ignored due to their feeble electrical conductivity. However, recently with the advent of new accessible approaches to form stable plasmas, these electrically conducting gases are attracting some significant interest and are now being investigated as exotic electrochemical environments.The defining property of plasmas is presence of free electrons; because of this they may be considered as both electrodes or electrolytes1, 2. We describe results supporting both modes.As electrodes, we show that metal oxides on surfaces may be reduced to zero valent metals using a helium atmospheric plasma jet.3 We show that free electrons do indeed reduce a copper oxide film, which may be carefully controlled by surface bias.4 A gaseous flame doped with electroactive species may be considered as electrolytes. Using a three-electrode system5, we may measure unique voltammograms for a series of small organic molecules and amino acids.6 Except for leucine and isoleucine, all were distinguishable. The reduction signatures originate from specific electron attachment reactions of radicals formed via incomplete combustion and fragmentation of the parent molecules. In this case without a solvent we have an extended potential window and our voltammograms extend between 0 and -10 V, which gives unprecedented access to chemistry not previously accessible in liquids. Moreover, mass transport properties are far better than in liquids, as such the fluxes of electroactive species to the electrode a much greater.

Resonant microwaves probing acoustic waves from an RF plasma jet

Microwave cavity resonance spectroscopy is introduced and demonstrated as a new approach to investigate the generation of acoustic waves by a pulsed radio-frequency driven atmospheric-pressure plasma jet. Thanks to recent advancements in the diagnostic method, the lower detection limit for pressure changes in air is ∼0.3 Pa. Good agreement with conventional pressure transducer measurements with respect to the temporal evolution, the pressure amplitude and the spectral response is found. Fourier analysis revealed that the acoustic waves induced by the plasma can most likely be attributed to standing waves in the discharge geometry. Additionally, the plasma-induced acoustic waves of a few (tens of) Pa are proposed as an active mechanism in plasma medicine.

Helium metastable species generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and N2

Spatially resolved tunable diode-laser absorption measurements of the absolute densities of He-I (23S1) metastables in a micro atmospheric pressure plasma jet operated in He/N2 and driven by ‘peaks’- and ‘valleys’-type tailored voltage waveforms are presented. The measurements are performed at different nitrogen admixture concentrations and peak-to-peak voltages with waveforms that consist of up to four consecutive harmonics of the fundamental frequency of 13.56 MHz. Comparisons of the measured metastable densities with those obtained from particle-in-cell/Monte Carlo collision simulations show a good quantitative agreement. The density of helium metastables is found to be significantly enhanced by increasing the number of consecutive driving harmonics. Their generation can be further optimized by tuning the peak-to-peak voltage amplitude and the concentration of the reactive gas admixture. These findings are understood based on detailed fundamental insights into the spatio-temporal electron dynamics gained from the simulations, which show that voltage waveform tailoring allows to control the electron energy distribution function to optimize the metastable generation. A high degree of correlation between the metastable creation rate and the electron impact excitation rate from the helium ground state into the He-I ((3s)3S1) level is observed for some conditions which may facilitate an estimation of the metastable densities based on phase resolved optical emission spectroscopy measurements of the 706.5 nm He-I line originating from the above level and metastable density values at proper reference conditions.

Comparison of electron heating and energy loss mechanisms in an RF plasma jet operated in argon and helium

The μ-APPJ is a well-investigated atmospheric pressure RF plasma jet. Up to now, it has mainly been operated using helium as feed gas due to stability restrictions. However, the COST-Jet design including precise electrical probes now offers the stability and reproducibility to create equi-operational plasmas in helium as well as in argon. In this publication, we compare fundamental plasma parameters and physical processes inside the COST reference microplasma jet, a capacitively coupled RF atmospheric pressure plasma jet, under operation in argon and in helium. Differences already observable by the naked eye are reflected in differences in the power-voltage characteristic for both gases. Using an electrical model and a power balance, we calculated the electron density and temperature at 0.6 W to be , 1.2 eV and , 1.7 eV for argon and helium, respectively. In case of helium, a considerable part of the discharge power is dissipated in elastic electron-atom collisions, while for argon most of the input power is used for ionization. Phase-resolved optical emission spectroscopy reveals differently pronounced heating mechanisms. Whereas bulk heating is more prominent in argon compared to helium, the opposite trend is observed for sheath heating. This also explains the different behavior observed in the power-voltage characteristics.

Resonant microwaves probing the spatial afterglow of an RF plasma jet

The electron density and effective electron collision frequency in the spatial afterglow of a pulsed radio frequency driven atmospheric-pressure plasma jet are obtained by using microwave cavity resonance spectroscopy in a temporal manner with an ∼1 μs resolution. During the “plasma on” phase, values of 1.7 ± 0.3×1018 m−3 for the electron density and 0.12 ± 0.01 THz for the electron collision frequency were found. These values and standard deviations represent the collective measurement set with repetition rates ranging from 125 to 8000 Hz. The spread in the plasma parameters during this phase within one repetition frequency is smaller than 3%. It is observed that remnant species, e.g., metastables, of previous discharges influence the decay of the plasma. The work reported is enabled by recent developments in the applied diagnostic with respect to the resolution in the plasmas' permittivity. Moreover, a multiplying probe is used for the electrical characterization of the plasma and the presence of the cavity did not influence the plasma impedance. This strongly suggests that the cavity did not affect the discharge.