Microscale Atmospheric Pressure Plasma Jet Research Articles

Gas Flow-Dependent Modification of Plasma Chemistry in μAPP Jet-Generated Cold Atmospheric Plasma and Its Impact on Human Skin Fibroblasts.

The micro-scaled Atmospheric Pressure Plasma Jet (µAPPJ) is operated with low carrier gas flows (0.25-1.4 slm), preventing excessive dehydration and osmotic effects in the exposed area. A higher yield of reactive oxygen or nitrogen species (ROS or RNS) in the µAAPJ-generated plasmas (CAP) was achieved, due to atmospheric impurities in the working gas. With CAPs generated at different gas flows, we characterized their impact on physical/chemical changes of buffers and on biological parameters of human skin fibroblasts (hsFB). CAP treatments of buffer at 0.25 slm led to increased concentrations of nitrate (~352 µM), hydrogen peroxide (H2O2; ~124 µM) and nitrite (~161 µM). With 1.40 slm, significantly lower concentrations of nitrate (~10 µM) and nitrite (~44 µM) but a strongly increased H2O2 concentration (~1265 µM) was achieved. CAP-induced toxicity of hsFB cultures correlated with the accumulated H2O2 concentrations (20% at 0.25 slm vs. ~49% at 1.40 slm). Adverse biological consequences of CAP exposure could be reversed by exogenously applied catalase. Due to the possibility of being able to influence the plasma chemistry solely by modulating the gas flow, the therapeutic use of the µAPPJ represents an interesting option for clinical use.

Following O and OH in He/O2 and He/H2O gas mixtures—from the gas phase through the liquid phase to modifications on a biological sample

Applied cold atmospheric plasma allows for the controlled delivery of reactive oxygen and nitrogen species tailored for specific applications. Through the manipulation of the plasma parameters, feed gases, and careful consideration of the environment surrounding the treatment target, selective chemistries that preferentially influence the target can be produced and delivered. To demonstrate this, the COST reference microscale atmospheric pressure plasma jet is used to study the generation and transport of O and OH from the gas phase through the liquid to the biological model target cysteine. Relative and absolute species densities of OH and O are measured in the gas phase through laser induced fluorescence (LIF) and two-photon absorption LIF respectively. The transport of these species is followed into the liquid phase by hydrogen peroxide quantification and visualized by a fluorescence assay. Modifications to the model biological sample cysteine exposed to OH and H2O2 dominated chemistry (He/H2O (0.25%)) and O dominated chemistry (He/O2 (0.6%)) is measured by FTIR spectroscopy. The origin of these species that modify cysteine is considered through the use of heavy water (HO) and mass spectrometry. It is found that the reaction pathways differ significantly for He/O2 and He/H2O. Hydrogen peroxide is formed mainly in the liquid phase in the presence of a substrate for He/O2 whereas for He/H2O it forms in the gas phase. The liquid chemistry resulting from the He/O2 admixture mainly targets the sulfur moiety of cysteine for oxidation up to irreversible oxidation states, while He/H2O treatment leads preferentially to reversible oxidation products. The more O or OH/H2O2 dominated chemistry produced by the two gas admixtures studied offers the possibility to select species for target modification.

Measuring plasma-generated ·OH and O atoms in liquid using EPR spectroscopy and the non-selectivity of the HTA assay

Plasma-generated hydroxyl radicals (·OH) and oxygen atoms (O) produced by the COST reference plasma jet, a micro-scaled atmospheric pressure plasma jet, were investigated using a variety of experimental techniques. Several gas admixtures were studied to distinguish the contributions of the two reactive oxygen species. Large discrepancies between inferred aqueous ·OH densities were noted when using a 2-hydroxyterephthalic acid (HTA) fluorescence assay and electron paramagnetic resonance (EPR) measurements with the spin trap 5,5-dimethyl-1-pyrroline N-oxide—especially when oxygen was present in the feed gas. A series of follow-up experiments including optical emission spectroscopy, H2O2 quantification, and EPR measurements of atomic oxygen using the spin trap 2,2,6,6-tetramethylpiperidine, revealed that the inconsistencies between the measured aqueous ·OH were likely due to the propensity of atomic oxygen to hydroxylate TA in a manner indistinguishable from ·OH. This renders the HTA assay non-selective when both ·OH radicals and atomic oxygen are present, which we report for all three gas admixtures in our experiments. Additionally, considerable degradation of both HTA and the spin adducts measured using EPR spectroscopy was apparent, meaning actual radical densities in the plasma-treated liquid may be considerably higher than implied. Degradation rates compared favorably to previously measured gas phase densities of atomic oxygen in the predecessor of the COST jet and reported degradation of other chemical probes. These results show the prolific role of atomic oxygen in plasma-induced liquid chemistry and caution against diagnostic techniques that are unable to account for it.

Microscale Atmospheric Pressure Plasma Jet as a Source for Plasma‐Driven Biocatalysis

AbstractThe use of a microscale atmospheric pressure plasma jet (μAPPJ) was investigated for its potential to supply hydrogen peroxide in biocatalysis. Compared to a previously employed dielectric barrier discharge (DBD), the μAPPJ offered significantly higher H2O2 production rates and better handling of larger reaction volumes. The performance of the μAPPJ was evaluated with recombinant unspecific peroxygenase from Agrocybe aegerita (rAaeUPO). Using plasma‐treated buffer, no side reactions with other plasma‐generated species were detected. For long‐term treatment, rAaeUPO was immobilized, transferred to a rotating bed reactor, and reactions performed using the μAPPJ. The enzyme had a turnover of 36,415 mol mol−1 and retained almost full activity even after prolonged plasma treatment. Overall, the μAPPJ presents a promising plasma source for plasma‐driven biocatalysis.

Plasma-sensitive Escherichia coli mutants reveal plasma resistance mechanisms.

Non-thermal atmospheric pressure plasmas are investigated as augmenting therapy to combat bacterial infections. The strong antibacterial effects of plasmas are attributed to the complex mixture of reactive species, (V)UV radiation and electric fields. The experience with antibiotics is that upon their introduction as medicines, resistance occurs in pathogens and spreads. To assess the possibility of bacterial resistance developing against plasma, we investigated intrinsic protective mechanisms that allow Escherichia coli to survive plasma stress. We performed a genome-wide screening of single-gene knockout mutants of E. coli and identified 87 mutants that are hypersensitive to the effluent of a microscale atmospheric pressure plasma jet. For selected genes ( cysB, mntH, rep and iscS) we showed in complementation studies that plasma resistance can be restored and increased above wild-type levels upon over-expression. To identify plasma-derived components that the 87 genes confer resistance against, mutants were tested for hypersensitivity against individual stressors (hydrogen peroxide, superoxide, hydroxyl radicals, ozone, HOCl, peroxynitrite, NO•, nitrite, nitrate, HNO3, acid stress, diamide, heat stress and detergents). k-means++ clustering revealed that most genes protect from hydrogen peroxide, superoxide and/or nitric oxide. In conclusion, individual bacterial genes confer resistance against plasma providing insights into the antibacterial mechanisms of plasma.

The transport and surface reactivity of O atoms during the atmospheric plasma etching of hydrogenated amorphous carbon films

A remote microscale atmospheric pressure plasma jet with a He/O2 gas mixture is used to etch a hydrogenated amorphous carbon layer. The etched profiles are measured by means of imaging spectroscopic reflectometry, a powerful technique providing a 2D map of the film thickness (etched profile) and also film properties. Additionally, the 2D axially symmetric fluid model of the gas flow and species transport combined with the basic kinetic model of the reaction of O atoms with O2 molecules has been solved to study the transport and surface reactivity of O atoms. The model provides a spatially resolved and surface-integrated O atom loss rate at the surface. The situation with convection-dominated species transport and fast recombination reactions of O atoms in the volume leads to a strong dependence of the etched profile on the O2 admixture and O atom surface loss probability β. By comparing etched profiles with the simulation results, the O atom surface reaction probability of β = 0.2%–0.6% could be estimated. The modeled O atom loss rate at the surface was always higher and with the same trend as the etching rate, corroborating that O atoms are the main etching species. The presented data and simulation results show that the fastest surface-integrated etching rate is achieved not under conditions with the highest O density on the jet axis, but at lower O2 admixtures due to reduced recombination losses in the gas phase.

Atmospheric plasma generates oxygen atoms as oxidizing species in aqueous solutions

A remote microscale atmospheric pressure plasma jet (µAPPJ) with He, He/H2O, He/O2, and He/O2/H2O gas mixtures was used to study the transport of reactive species from the gas phase into the liquid and the following aqueous phase chemistry. The effects induced by the µAPPJ in water were quantitatively studied using phenol as a chemical probe and by measuring H2O2 concentration and pH values. These results were combined with the analysis of the absolute densities of the reactive species and the modeling of convective/diffusion transport and recombination reactions in the effluent of the plasma jet. Additionally, modified plasma jets were used to show that the role of emitted photons in aqueous chemistry is negligible for these plasma sources. The fastest phenol degradation was measured for the He/O2 plasma, followed by He/H2O, He/O2/H2O, and He plasmas. The modeled quantitative flux of O atoms into the liquid in the He/O2 plasma case was highly comparable with the phenol degradation rate and showed a very high transfer efficiency of reactive species from the plasma into the liquid, where more than half of the O atoms leaving the jet nozzle entered the liquid. The results indicate that the high oxidative effect of He/O2 plasma was primarily due to solvated O atoms, whereas OH radicals dominated the oxidative effects induced in water by plasmas with other gas mixtures. These findings help to understand, in a quantitative way, the complex interaction of cold atmospheric plasmas with aqueous solutions and will allow a better understanding of the interaction of these plasmas with water or buffered solutions containing biological macromolecules, microorganisms, or even eukaryotic cells. Additionally, the µAPPJ He/O2 plasma source seems to be an ideal tool for the generation of O atoms in aqueous solutions for any future studies of their reactivity.

Absolute OH and O radical densities in effluent of a He/H2O micro-scaled atmospheric pressure plasma jet

The effluent of a micro-scaled atmospheric pressure plasma jet (μ-APPJ) operated in helium with admixtures of water vapor ( ppm) has been analyzed by means of cavity ring-down laser absorption spectroscopy and molecular beam mass spectrometry to measure hydroxyl (OH) radical densities, and by two-photon absorption laser-induced fluorescence spectroscopy to measure atomic oxygen (O) densities. Additionally, the performance of the bubbler as a source of water vapor in the helium feed gas has been carefully characterized and calibrated. The largest OH and O densities in the effluent of and , respectively, have been measured at around 6000 ppm. The highest selectivity is reached around 1500 ppm, where the OH density is at ∼63% of its maximum value and is 14 times larger than the O density. The measured density profiles and distance variations are compared to the results of a 2D axially symmetric fluid model of species transport and reaction kinetics in the plasma effluent. It is shown that the main loss of OH radicals in the effluent is their mutual reaction. In the case of O, reactions with other species than OH also have to be considered to explain the density decay in the effluent. The results presented here provide additional information for understanding the plasma-chemical processes in non-equilibrium atmospheric pressure plasmas. They also open the way to applying μ-APPJ with He/H2O as a selective source of OH radicals.

Concepts and characteristics of the ‘COST Reference Microplasma Jet’

Biomedical applications of non-equilibrium atmospheric pressure plasmas have attracted intense interest in the past few years. Many plasma sources of diverse design have been proposed for these applications, but the relationship between source characteristics and application performance is not well-understood, and indeed many sources are poorly characterized. This circumstance is an impediment to progress in application development. A reference source with well-understood and highly reproducible characteristics may be an important tool in this context. Researchers around the world should be able to compare the characteristics of their own sources and also their results with this device. In this paper, we describe such a reference source, developed from the simple and robust micro-scaled atmospheric pressure plasma jet (μ-APPJ) concept. This development occurred under the auspices of COST Action MP1101 ‘Biomedical Applications of Atmospheric Pressure Plasmas’. Gas contamination and power measurement are shown to be major causes of irreproducible results in earlier source designs. These problems are resolved in the reference source by refinement of the mechanical and electrical design and by specifying an operating protocol. These measures are shown to be absolutely necessary for reproducible operation. They include the integration of current and voltage probes into the jet. The usual combination of matching unit and power supply is replaced by an integrated LC power coupling circuit and a 5 W single frequency generator. The design specification and operating protocol for the reference source are being made freely available.

Stability and excitation dynamics of an argon micro-scaled atmospheric pressure plasma jet

A megahertz-driven plasma jet at atmospheric pressure—the so-called micro-scaled atmospheric pressure plasma jet (μAPPJ)—operating in pure argon has been investigated experimentally and by numerical modelling. To ignite the discharge in argon within the jet geometry, a self-made plasma tuning unit was designed, which additionally enables measurements of the dissipated power in the plasma itself. Discharges in the α-mode up to their transition to the γ-mode were studied experimentally for varying frequencies. It was found that the voltage at the α–γ transition behaves inversely proportional to the applied frequency f and that the corresponding power scales with an f 3/2law. Both these findings agree well with the results of time-dependent, spatially one-dimensional fluid modelling of the discharge behaviour, where the f 3/2 scaling of the α–γ transition power is additionally verified by the established concept of a critical plasma density for sheath breakdown. Furthermore, phase resolved spectroscopy of the optical emission at 750.39 nm as well as at 810.37 nm and 811.53 nm was applied to analyse the excitation dynamics of the discharge at 27 MHz for different applied powers. The increase of the power leads to an additional maximum in the excitation structure of the 750.39 nm line emission at the α–γ transition point, whereas the emission structure around 811 nm does not change qualitatively. According to the fluid modelling results, this differing behaviour originates from the different population mechanisms of the corresponding energy levels of argon.

Summarizing results on the performance of a selective set of atmospheric plasma jets for separation of photons and reactive particles

A microscale atmospheric-pressure plasma jet is a remote plasma jet, where plasma-generated reactive particles and photons are involved in substrate treatment. Here, we summarize our efforts to develop and characterize a particle- or photon-selective set of otherwise identical jets. In that way, the reactive species or photons can be used separately or in combination to study their isolated or combined effects to test whether the effects are additive or synergistic. The final version of the set of three jets—particle-jet, photon-jet and combined jet—is introduced. This final set realizes the highest reproducibility of the photon and particle fluxes, avoids turbulent gas flow, and the fluxes of the selected plasma-emitted components are almost identical in the case of all jets, while the other component is effectively blocked, which was verified by optical emission spectroscopy and mass spectrometry. Schlieren-imaging and a fluid dynamics simulation show the stability of the gas flow. The performance of these selective jets is demonstrated with the example of the treatment of E. coli bacteria with the different components emitted by a He-only, a He/N2 and a He/O2 plasma. Additionally, measurements of the vacuum UV photon spectra down to the wavelength of 50 nm can be made with the photon-jet and the relative comparison of spectral intensities among different gas mixtures is reported here. The results will show that the vacuum UV photons can lead to the inactivation of the E.coli bacteria.

Resonance broadening of argon lines in a micro-scaled atmospheric pressure plasma jet (argon μAPPJ)

Optical emission from atmospheric pressure micro-jet operating with pure argon (argon μAPPJ) flow has been detected with a moderate resolution spectrometer. Large broadening of the several argon (Ar) lines has been observed in the near infrared spectral region. This effect was attributed to resonance broadening of the s2 (Paschen notation) level in 3p54s configuration. In the present work, corresponding line profiles are suggested for plasma diagnostics. For this, a general case of resonance broadening coefficient of noble gases is discussed. As broadening reflects the Ar density, and the static gas pressure of the jet is in equilibrium with the ambient, the local gas temperature can be inferred. An estimation of gas temperature from the width of the 750 nm Ar line is in agreement with rotational temperature of OH radicals determined from the A2Σ+ → X2Π (0, 0) band. At low temperatures (300–600 K) and at partial Ar pressure near atmospheric, the resonance width of the suggested lines is very sensitive to small temperature variations. High temperature sensitivity and large width make the resonance broadened lines very attractive for diagnostics of low temperature discharges at elevated pressure, e.g., as they are used in plasma-medicine.

Characteristics of a propagating, self-pulsing, constricted ‘γ-mode-like’ discharge

Investigations on the self-pulsing operation regime of a modified micro-scaled atmospheric pressure plasma jet (μ-APPJ) are presented. Using a wedge-shaped electrode configuration, a self-pulsing behavior of the device is achieved, which is characterized by the repetitive ignition of a constricted ‘γ-mode-like’ discharge at the gas inlet, which propagates with the gas flow towards the nozzle, where it extinguishes. The ‘γ-mode-like’ feature coexists with the homogeneous alpha-glow. Synchronized voltage/current and optical emission measurements are presented in order to correlate the evolution of electrical quantities such as voltage, current, dissipated power and phase with changes in the discharge structure. First insights are gained into the underlying discharge dynamics responsible for a stable self-sustainment, propagation and extinction of the constricted discharge. The results indicate that processes induced by helium metastables play a major role. Maximal electron densities on the order of ne = 3.2 · 1012 cm−3 and dissipated power of 18.9 W are achieved in this novel operation regime.

Atomic nitrogen: a parameter study of a micro-scale atmospheric pressure plasma jet by means of molecular beam mass spectrometry

Absolute atomic nitrogen densities (N) in the effluent of a micro-scale atmospheric pressure plasma jet (µ-APPJ) operated in He with small admixtures of molecular nitrogen (N2) are measured by means of molecular beam mass spectrometry. Focusing on changes of the external plasma parameters, the dependency of the atomic nitrogen density on the admixture of molecular nitrogen to the plasma, the variation of applied electrode voltage and the variation of distance between the jet nozzle and the sampling orifice of the mass spectrometer are analysed. When varying the N2 admixture, a maximum density of atomic nitrogen of approximately 1.5 × 1014 cm−3 (~6 ppm) is reached at about 0.25% N2 admixture. Moreover, the N density increases approximately linearly with the applied voltage. Both results are comparable to atomic oxygen (O) behaviour of the µ-APPJ operated at equal plasma conditions except for admixing molecular O2 instead of nitrogen (Ellerweg et al 2010 New J. Phys. 12 013021). The N density decreases continuously with increasing distance, but the decrease is slower than in the case of O atoms in He/O2 plasma. N atoms with a density of 2.0 × 1013 cm−3 (~0.8 ppm) are still detected at 40 mm distance from the jet nozzle in controlled He/N2 atmosphere. The simple fluid simulation of N diffusion does not reproduce the measured densities of N. Nevertheless, a simulation taking into account atomic nitrogen reactions with gas impurities are able to reproduce the measured data, indicating that these reactions are an important loss mechanism of N atoms. The presented results are relevant for the future investigation of interactions of reactive nitrogen species with biological substrates.

Chemical kinetics and reactive species in atmospheric pressure helium–oxygen plasmas with humid-air impurities

In most applications helium-based plasma jets operate in an open-air environment. The presence of humid air in the plasma jet will influence the plasma chemistry and can lead to the production of a broader range of reactive species. We explore the influence of humid air on the reactive species in radio frequency (rf)-driven atmospheric-pressure helium–oxygen mixture plasmas (He–O2, helium with 5000 ppm admixture of oxygen) for wide air impurity levels of 0–500 ppm with relative humidities of from 0% to 100% using a zero-dimensional, time-dependent global model. Comparisons are made with experimental measurements in an rf-driven micro-scale atmospheric pressure plasma jet and with one-dimensional semi-kinetic simulations of the same plasma jet. These suggest that the plausible air impurity level is not more than hundreds of ppm in such systems. The evolution of species concentration is described for reactive oxygen species, metastable species, radical species and positively and negatively charged ions (and their clusters). Effects of the air impurity containing water humidity on electronegativity and overall plasma reactivity are clarified with particular emphasis on reactive oxygen species.

Detection of atomic oxygen and nitrogen created in a radio-frequency-driven micro-scale atmospheric pressure plasma jet using mass spectrometry

In this paper we show mass spectrometry results for a radio-frequency-driven micro-atmospheric pressure plasma jet (μ-APPJ) discharge obtained using a mass analyzer with triple differential pumping allowing us to sample directly in ambient atmospheric pressure environment (Hiden HPR-60). The flow of the buffer gas (mixture of helium and 1% oxygen) was 2 slm and 3 slm and the excitation frequency was 13.56 MHz. We monitored production of atomic oxygen and nitrogen in the plasma for different flows and powers given by the RF power supply. These measurements were made for energies of electrons emitted from the ionization filament below the threshold for dissociation of O2 and N2. In addition to oxygen and nitrogen atoms, yields for O2, N2, NO and O3 are recorded for different powers and gas flows. It is shown that the μ-APPJ is symmetrical and operates in α-mode. The power transmitted to the discharge was below 5 W in all measurements.

Influence of target surfaces on the atomic oxygen distribution in the effluent of a micro-scaled atmospheric pressure plasma jet

Micro-scaled plasma jets with reactive process gases, e.g. oxygen, are applied for localized surface treatment. Here, investigations of the development and the spatial distribution of atomic oxygen are reported in the post-discharge effluent of a micro-scaled atmospheric pressure plasma jet. These measurements are supplemented by installation of planar targets of various materials in the effluent. The reactive species are detected by means of two-photon laser-induced fluorescence spectroscopy from the discharge, through the free effluent, up to distances of about 200 µm in front of a planar surface. Ozone density profiles are measured by UV absorption spectroscopy. The effect of the effluent on gold and plastic substrates and vice versa is investigated. In the free effluent, the atomic oxygen density falls off exponentially to about 2 × 1015 cm−3 at a distance of 6 mm from the jet's nozzle. The implementation of a plastic target does not disturb the O distribution, resulting in a strictly localized etching of the target. In contrast, mounting of a gold target increases the oxygen density and spreads its distribution close to the target. For correlation, surface modifications by plasma treatment of plastic and gold substrates are analysed by UV laser microscopy and x-ray photoelectron spectroscopy.

The Role of VUV Radiation in the Inactivation of Bacteria with an Atmospheric Pressure Plasma Jet

AbstractA modified version of a microscale atmospheric pressure plasma jet (µ‐APPJ) source, so‐called X‐Jet, is used to study the role of plasma‐generated VUV photons in the interaction of bacteria. The new jet geometry allows effective separation of reactive particles (O atoms or O3 molecules) from the plasma‐generated photons. The measurements of the size of inhibition zones formed in the layer of vegetative Escherichia coli bacteria, VUV emission intensity, and the mass spectrometry measurements show that photochemistry in the gas phase followed by photochemistry products impacting on bacteria is more effective in bacterial inactivation than direct VUV radiation damage. Protonated water cluster ions are detected indicating that water impurity has to be considered carefully. The combination of the presence of water cluster ions and O2 molecules at the surface leads to the strongest effect. Additionally, the interaction of plasma‐generated VUV photons with the effluent of the He/O2 plasma, but not with ground state O2, leads to faster inactivation of bacteria, most probably due to enhanced formation of O and O3.magnified image

Separation of VUV/UV photons and reactive particles in the effluent of a He/O2 atmospheric pressure plasma jet

Cold atmospheric pressure plasmas can be used for treatment of living tissues or for inactivation of bacteria or biological macromolecules. The treatment is usually characterized by a combined effect of UV and VUV radiation, reactive species and ions. This combination is usually beneficial for the effectiveness of the treatment but it makes the study of fundamental interaction mechanisms very difficult. Here we report on an effective separation of VUV/UV photons and heavy reactive species in the effluent of a microscale atmospheric pressure plasma jet (μ-APPJ). The separation is realized by an additional flow of helium gas under well-defined flow conditions, which deflects heavy particles in the effluent without affecting the VUV and UV photons. Both components of the effluent, the photons and the reactive species, can be used separately or in combination for sample treatment. The results of treatment of a model plasma polymer film and vegetative Bacillus subtilis and Escherichia coli cells are shown and discussed. A simple model of the He gas flow and reaction kinetics of oxygen atoms in the gas phase and at the surface is used to provide a better understanding of the processes in the plasma effluent. The new jet modification, called X-Jet for its appearance, will simplify the investigation of interaction mechanisms of atmospheric pressure plasmas with biological samples.

Phase resolved optical emission spectroscopy of coaxial microplasma jet operated with He and Ar

Atmospheric pressure plasma jet sources are currently in the focus of many researchers for their promising applications in medical industry (e.g. treatment of living tissues), surface modification or material etching or synthesis. Here we report on the study of excitation mechanisms of a coaxial microplasma jet with a hollow capillary as an inner electrode and a ceramic tube with metal ring as outer electrode. This microplasma jet is operated in He and Ar gas and it is investigated by means of electrical measurements, optical emission spectroscopy and space and phase resolved wavelength integrated optical spectroscopy. Measurements of a microscale atmospheric pressure plasma jet with parallel metal electrodes operated in He are shown for comparison as well. Four different modes are distinguished with He as plasma forming gas. The α discharge in annular space between the electrodes, observed at low applied voltages, is very similar to the discharge in the jet with parallel electrodes. As the voltage increases a γ discharge appears, first localized at the tip of the capillary. As the voltage increases further the γ discharge appears in the annular space as well. A hollow cathode plasma is observed at any voltage used on the symmetry axis of the jet. Only one mode of plasma operation is observed in argon gas with distinctively different behavior. We hypothesize that it is comparable to a single microdischarge of a filamentary dielectric barrier discharge.