Pressure Argon Plasma Research Articles

Basic Research in Plasma Medicine - A Throughput Approach from Liquids to Cells

In plasma medicine, ionized gases with temperatures close to that of vertebrate systems are applied to cells and tissues. Cold plasmas generate reactive species known to redox regulate biological processes in health and disease. Pre-clinical and clinical evidence points to beneficial effects of plasma treatment in the healing of chronic ulcer of the skin. Other emerging topics, such as plasma cancer treatment, are receiving increasing attention. Plasma medical research requires interdisciplinary expertise in physics, chemistry, and biomedicine. One goal of plasma research is to characterize plasma-treated cells in a variety of specific applications. This includes, for example, cell count and viability, cellular oxidation, mitochondrial activity, cytotoxicity and mode of cell death, cell cycle analysis, cell surface marker expression, and cytokine release. This study describes the essential equipment and workflows required for such research in plasma biomedicine. It describes the proper operation of an atmospheric pressure argon plasma jet, specifically monitoring its basic emission spectra and feed gas settings to modulate reactive species output. Using a high-precision xyz-table and computer software, the jet is hovered in millisecond-precision over the cavities of 96-well plates in micrometer-precision for maximal reproducibility. Downstream assays for liquid analysis of redox-active molecules are shown, and target cells are plasma-treated. Specifically, melanoma cells are analyzed in an efficient sequence of different consecutive assays but using the same cells: measurement of metabolic activity, total cell area, and surface marker expression of calreticulin, a molecule important for the immunogenic cell death of cancer cells. These assays retrieve content-rich biological information about plasma effects from a single plate. Altogether, this study describes the essential steps and protocols for plasma medical research.

123 - Induction of Immunogenic Cell Death upon Combinatorial Therapy with Cold Physical Plasma and Chemotherapeutic Agents in Melanoma

Introduction Chemotherapeutic agents remain as the essential regimen against malignant melanoma. Variety of combination therapies have been employed to sensitize tumor cells, circumvent drug resistance, and reduce systemic toxicity. However, there is still need for innovative combinational approaches to resolve these difficulties. In the current study, we investigated the potential synergism of cold physical plasma in conjunction with chemotherapeutic agents and identified mechanisms of synergistic immunogenic cell death (ICD) in tumor cells in vitro Methods In this study, we pretreated murine B16F10 and human SK-Mel 28 melanoma cells with increasing concentrations of Doxorubicin, Epirubicin, Oxaliplatin, Vorinostat and SAHA for 24h. This was followed by 30 secs exposure cold physical plasma effluent of an atmospheric pressure argon plasma jet (kINPen). After 6-hour incubation, IC50 was determined using alamar blue assay and compared with single treatment modalities. Cell culture supernatants were used to quantify markers of immunogenic cell death (CART, ATP, HMGB1, CXCL10) by ELISA, immunofluorescence and flow cytometry. Total cell counts, intracellular superoxide levels (DHE), mitochondria membrane potential (TMRE), cell membrane integrity (sytox green) and cellular morphology were quantitatively assessed using live cell and high throughput confocal imaging (Operatta; Perkin Elmer). Co culture and transwell migration assays was performed on THP1 monocytes and RAW 264.7 macrophages. For SK-Mel 28 tumor cells, 3D spheroid models were also investigated. Results Combination therapy of chemotherapeutic agents and cold physical plasma significantly decreased the IC50 values in melanoma cell lines in comparison to single treatment modalities in 2D and 3D culture conditions. The tumor cell killing was mediated by elevation of superoxide and loss of mitochondrial membrane potential. Co culturing SK-Mel 28 cells with THP1 monocytes or RAW macrophages with B16F10 following combination therapy led to pronounced tumor cell killing. ICD markers such as ATP, HMGB1, CART, and CXCL10 were induced in both single treatment conditions however were significantly elevated upon combination therapy demonstrating a synergistic effect. Transwell migration assay revealed migration of THP1 monocytes and RAW 264.7 macrophages following incubation with conditioned medium. In this study, we have demonstrated that cold physical plasma is a valuable tool to induce ICD in tumors pretreated with conventional chemotherapeutic agents.

Low temperature plasma promoting fibroblast proliferation by activating the NF-\u03baB pathway and increasing cyclinD1 expression

The potential applications of low temperature plasma (LTP) in wound healing have aroused the concern of many researchers. In this study, an argon atmospheric pressure plasma jet was applied to generate LTP for treatment of murine fibroblast cell (L929) cultured in vitro to investigate the effect of NF-κB pathway on fibroblast proliferation. The results showed that, compared with the control, L929 cells treated with plasma for less than 20 s had significant increases of proliferation; the productions of intracellular ROS, O2− and NO increased with prolongation of LTP treatment time; NF-κB pathway was activated by LTP in a proper dose range, and the expression of cyclinD1 in LTP-treated cells increased with the same trend as cell proliferation. After RNA interference to block p65 expression, with the same treatment time, RNAi-treated cells proliferated more slowly and expressed less cyclinD1 than normal cells. Furthermore, pretreatment with N-acetyl-L-cysteine (NAC) markedly prevented the plasma-induced changes in cells. In conclusion, the proliferation of L929 cells induced by LTP was closely related to NF-κB signaling pathway, which might be activated by appropriate level of intracellular ROS. These novel findings can provide some theoretical reference of LTP inducing cell proliferation and promoting wound healing.

A Comparison of Floating-Electrode DBD and kINPen Jet: Plasma Parameters to Achieve Similar Growth Reduction in Colon Cancer Cells Under Standardized Conditions

A comparative study of two plasma sources (floating-electrode dielectric barrier discharge, DBD, Drexel University; atmospheric pressure argon plasma jet, kINPen, INP Greifswald) on cancer cell toxicity was performed. Cell culture protocols, cytotoxicity assays, and procedures for assessment of hydrogen peroxide (H2O2) were standardized between both labs. The inhibitory concentration 50 (IC50) and its corresponding H2O2 deposition was determined for both devices. For the DBD, IC50 and H2O2 generation were largely dependent on the total energy input but not pulsing frequency, treatment time, or total number of cells. DBD cytotoxicity could not be replicated by addition of H2O2 alone and was inhibited by larger amounts of liquid present during the treatment. Jet plasma toxicity depended on peroxide generation as well as total cell number and amount of liquid. Thus, the amount of liquid present during plasma treatment in vitro is key in attenuating short-lived species or other physical effects from plasmas. These in vitro results suggest a role of liquids in or on tissues during plasma treatment in a clinical setting. Additionally, we provide a platform for correlation between different plasma sources for a predefined cellular response.

Application of atmospheric-pressure argon plasma jet for bread mold decontamination

Atmospheric-pressure argon plasma (APAP) is a promising non-thermal technology for microbial control and prevention minimally affecting quality of foods. Effect of APAP jet on the growth of bread molds, including two Aspergillus sp., Rhizopus stolonifer, and Penicillium roqueforti, isolated from white bread were investigated. The molds were isolated, verified, cultured to fully grown on potato dextrose agar (PDA), and subsequently treated with APAP jet using plasma generating power at 24 W for 5, 10, and 20 min, respectively. The inhibition of mold growth was investigated by comparing fungal dry weights and the effect on fungal cell structure was observed using compound light microscope. The results indicated that the 20-min treatment time is most effective in retarding the growth of the three bread molds. However, this level of generating power did not lead to destruction of the cellular structures for all the four fungi. Plasma generating power and treatment time are significant parameters determining the success of bread mold decontamination and further investigation on real bread matrix is needed.

Characterization of a microwave-excited atmospheric-pressure argon plasma jet using two-parallel-wires transmission line resonator

This paper presents a method to produce a microwave-excited atmospheric-pressure plasma jet (ME-APPJ) with argon. The plasma was generated by a microwave-driven micro-plasma source that uses a two-parallel-wire transmission line resonator (TPWR) operating at around 900 MHz. The TPWR has a simple structure and is easier to fabricate than coaxial transmission line resonator (CTLR) devices. In particular, the TPWR can sustain more stable ME-APPJ than the CTLR can because the gap between the electrodes is narrower than that in the CTLR. In experiments performed with an Ar flow rate from 0.5 to 8.0 L·min−1 and an input power from 1 to 6 W, the rotational temperature was determined by comparing the measured and simulated spectra of rotational lines of the OH band and the electron excitation temperature determined by the Boltzmann plot method. The rotational temperature obtained from OH(A-X) spectra was 700 K to 800 K, whereas the apparent gas temperature of the plasma jet remains lower than ∼325 K, which is compatible with biomedical applications. The electron number density was determined using the method based on the Stark broadening of the hydrogen Hβ line, and the measured electron density ranged from 6.5 × 1014 to 7.6 × 1014 cm−3. TPWR ME-APPJ can be operated at low flows of the working gas and at low power and is very stable and effective for interactions of the plasma with cells.

Influence of atmospheric pressure argon plasma treatment on the quality of peanut oil

The effects of exposure to atmospheric pressure argon plasma on the quality of edible oil were studied. Peanut oil was exposed to the plasma that was generated in voltages ranging from 150–170 V and the variations of peroxide value, iodine value, acid value, and brightness of the resultant products were monitored. Besides, the storage stability of the radiated peanut oil was investigated as well. Plasma treatment did not change the acid value, but reduced the iodine value and increased the peroxide value and brightness of peanut oil in a voltage‐dependent manner. Compared with control, exposure to the plasma generated in 170 V significantly decreased the contents of oleic acid and linoleic acid by 2.14 and 2.99% respectively, but exhibited no effect on that of palmitic acid. Storage results revealed that the acid value of the treated peanut oils kept increasing in the 30‐day storage period, but the peroxide and iodine values peaked on the 14th and 4th day respectively and then declined gradually. Besides, the voltage‐dependent variation pattern was observed for all the three parameters monitored. Since the peroxide, iodine, and acid values of the treated peanut oil are still in the ranges allowed by the Codex Alimentarius standards, it is concluded that the atmospheric pressure argon plasma treatment does not deteriorate the quality of peanut oil and this technology has potential application in the edible oil industry.A total of 5 mL peanut oil is transferred to a weighing bottle (diameter 40 mm) and exposed to atmospheric pressure argon plasma at a flow rate of 0.5 L/h for 100 s. The voltage of the power supply is set to 150, 155, 160, 165, or 170 V to explore the effects of plasma density on peanut oil quality. Treatment by the plasma generated in voltages 0∼170 V shows no effect on the acid value, but significantly reduces the iodine value, increases the peroxide value, and brightness of peanut oil in a voltage‐dependent manner. Exposure to the plasma generated in 170 V significantly reduces the contents of oleic acid and linoleic acid by 2.14 and 2.99%, respectively, but exhibits no effect on that of palmitic acid.Practical applications: Peanut oil is rich in oleic acid, linoleic acid and other polyunsaturated fatty acids, as well as vitamin E and other nutrients, it also has high nutritional value and pleasant, peculiar flavor. However, aflatoxin B1 (AFB1) which comes from contaminated peanuts is often detected in peanut oil. An alternative that has been presented for the prevention and control of AFs in food is plasma. The plasma is a quasineutral gas of charged and neutral particles which exhibits collective behavior. The domestic and international research indicates that low temperature plasma has a good restraint of tumor cells, the promotion of gene transfection, coagulation and many other aspects. To our knowledge, the report about atmospheric pressure plasma treatment of aflatoxin has not been found in literature. Atmospheric pressure plasma for degradating AFs has the advantages of good safety, high efficiency, environmental friendliness, and is the developing direction of AFs control. In recent years, the development of atmospheric pressure plasma technology have outpaced the traditional low pressure plasma technology because it does not require the use of vacuum equipment, and it shows its advantage of handling large areas of objects more suited to the practical production line. Accordingly, in this paper, acid value, iodine value, peroxide value, chromatic value, fatty acid content and composition, and storage stability are chosen as detection indexes, to study the effects of atmospheric pressure plasma with different voltage on peanut oil quality, in order to provide theoretical basis for the application of atmospheric pressure plasma technology in vegetable oil industry.

Investigation on the energy spectrums of electrons in atmospheric pressure argon plasma jets and their dependences on the applied voltage

This work presents a systematical investigation on the spatiotemporal evolution of the energy spectrum of electrons in atmospheric pressure argon plasma jets and its dependence on the applied voltage. The investigations are carried out by means of the numerical simulation based on a particle-in-cell Monte-Carlo collision model. The characteristics of the spatiotemporal evolution of the energy spectrum of electrons (ESE) in the discharge space have been presented, and especially the mechanisms of inducing these characteristics have also been revealed. The present work shows the following conclusions. In the evolution of ESE, there is a characteristic time under each applied voltage. Before the characteristic time, the peak value of ESE decreases, the peak position shifts toward high energy, and the distribution of ESE becomes wider and wider, but the reverse is true after the characteristic time. The formation of these characteristics can be mainly attributed to the transport of electrons toward a low electric field as well as a balance between the energy gained from the electric field including the effect of space charges and the energy loss due to inelastic collisions in the process of electron transport. The characteristic time decreases with the applied voltage. In addition, the average energy of electrons at the characteristic time can be increased by enhancing the applied voltage. The results presented in this work are of importance for regulating and controlling the energy of electrons in the plasma jets applied to plasma medicine.

Cold plasma interactions with plants: Morphing and movements of Venus flytrap and Mimosa pudica induced by argon plasma jet

Low temperature (cold) plasma finds an increasing number of applications in biology, medicine and agriculture. In this paper, we report a new effect of plasma induced morphing and movements of Venus flytrap and Mimosa pudica. We have experimentally observed plasma activation of sensitive plant movements and morphing structures in these plants similar to stimulation of their mechanosensors in vivo. Application of an atmospheric pressure argon plasma jet to the inside or outside of a lobe, midrib, or cilia in Dionaea muscipula Ellis induces trap closing. Treatment of Mimosa pudica by plasma induces movements of pinnules and petioles similar to the effects of mechanical stimulation. We have conducted control experiments and simulations to illustrate that gas flow and UV radiation associated with plasma are not the primary reasons for the observed effects. Reactive oxygen and nitrogen species (RONS) produced by cold plasma in atmospheric air appear to be the primary reason of plasma-induced activation of phytoactuators in plants. Some of these RONS are known to be signaling molecules, which control plants' developmental processes. Understanding these mechanisms could promote plasma-based technology for plant developmental control and future use for plant protection from pathogens. Our work offers new insight into mechanisms which trigger plant morphing and movement.

Numerical modelling of high-pressure arc discharges: matching the LTE arc core with the electrodes

A widely used approach to simulation of high-pressure arc discharges is based on the system of magneto-hydrodynamic equations written in the approximation of local thermodynamic equilibrium (LTE). In this work, boundary conditions on the surface of the electrodes are formulated with the use of equations of balance of energy in the non-equilibrium near-electrode layers that separate the LTE bulk plasma and the electrodes. As an example, numerical simulations of a free-burning arc in atmospheric-pressure argon plasma in the current range from 20 to 200 A are reported. Simulation results are in reasonably good agreement with those given by more sophisticated models and with the experiment. Simulations performed for cathodes of slightly different geometries have predicted a strong effect produced by details of the cathode geometry over the distribution of the current density along the cathode surface and therefore over the plasma temperature, an interesting and potentially important result that is worth further numerical investigation and experimental verification.

Confirmation of OH as good thermometric species for gas temperature determination in an atmospheric pressure argon plasma jet

We have proved that the temperatures determined from OH rotational spectra accurately describe the actual gas temperatures in an atmospheric pressure argon plasma jet, expanding in a hot ambient atmosphere. For this, we have operated the radiofrequency jet discharge inside a heated oven at various decreasing forwarded powers and increasing gas flow rates. We compared the rotational temperature values with the temperature of the oven. When extrapolated to zero power or infinite gas flow, the data describing the dependence of the rotational temperatures upon power and mass flow rate indicate the correct oven temperature. We demonstrate that the population of the rotational levels of the OH molecule is in equilibrium with the translational motion of the argon atoms in the plasma jet, from room temperature (RT) to 500 °C.

Role of Ambient Gas Composition on Cold Physical Plasma-Elicited Cell Signaling in Keratinocytes

A particularly promising medical application of cold physical plasma is the support of wound healing. This is presumably achieved by modulating inflammation as well as skin cell signaling and migration. Plasma-derived reactive oxygen and nitrogen species (ROS/RNS) are assumed the central biologically active plasma components. We hypothesized that modulating the environmental plasma conditions from pure nitrogen (N2) to pure oxygen (O2) in an atmospheric pressure argon plasma jet (kINPen) will change type and concentration of ROS/RNS and effectively tune the behavior of human skin cells. To investigate this, HaCaT keratinocytes were studied in vitro with regard to cell metabolism, viability, growth, gene expression signature, and cytokine secretion. Flow cytometry demonstrated only slight effects on cytotoxicity. O2 shielding provided stronger apoptotic effects trough caspase-3 activation compared to N2 shielding. Gene array technology revealed induction of signaling and communication proteins such as immunomodulatory interleukin 6 as well as antioxidative and proproliferative molecules (HMOX1, VEGFA, HBEGF, CSF2, and MAPK) in response to different plasma shielding gas compositions. Cell response was correlated to reactive species: oxygen-shielding plasma induces a cell response more efficiently despite an apparent decrease of hydrogen peroxide (H2O2), which was previously shown to be a major player in plasma-cell regulation, emphasizing the role of non-H2O2 ROS like singlet oxygen. Our results suggest differential effects of ROS- and RNS-rich plasma, and may have a role in optimizing clinical plasma applications in chronic wounds.

Electron temperature and density of non-thermal atmospheric pressure argon plasma jet by convective wave packet model

The propagation velocities (u g ) of argon plasma jet are obtained by intensified charge coupled device (ICCD) camera images at fixed gate width time of 5 ns. The propagation velocities in upstream and downstream regions are in the order of 104 - 105 m/s. The plasma ambipolar diffusion velocities are measured to be in the order of 10 - 102 m/s. Plasma jet discharges are generated by sinusoidal power supply in varying voltages from 2 to 4 kV at repetition frequency of about 40 kHz. By employing convective wave packet model, the electron temperature (T e ) inside plasma bullet for argon non-thermal atmospheric pressure plasma jet is estimated to be about 1.18 eV. Also, the electron density (n e ) is found to be 8.0 × 1014 - 2.5 × 1015 cm−3.

Osteoblast growth, after cleaning of biofilm-covered titanium discs with air-polishing and cold plasma.

To investigate the effects of a combined biofilm removal with an optimized air polishing and a cold plasma device on cells in vitro. A 7-day-old biofilm was removed from rough titanium discs with an air-polishing device with erythritol powder (AP) or with a cold atmospheric pressure argon plasma (CAP) device or in combination of both (AP+CAP). The removal efficacy was evaluated by subsequent cell seeding of osteoblast-like cells (MG-63). The cell spreading was analysed after 5 days of incubation by scanning electron microscopy. Separately, the surface hydrophilicity was analysed by measuring the water contact angle (WCA) of the disc for each treatment method. The mechanical plaque removal with AP rendered specimen conducive for cell growth, 85% of the surface was covered with cells. An advantage of the combination of AP+CAP was not detectable compared to AP (cell coverage ranged from 57% up to 75%). After sole CAP treatment, microorganisms re-grew and destroyed all cells. The WCA was reduced by all treatment methods. An AP treatment has the potential to remove biofilm from rough implant surfaces completely. In contrast to our hypothesis, the combination of plasma and AP treatment did not enhance osteoblast spreading.

The influence of grounded electrode positions on the evolution and characteristics of an atmospheric pressure argon plasma jet

An atmospheric pressure plasma jet (APPJ) in Ar with various grounded electrode arrangements is employed to investigate the effects of electrode arrangement on the characteristics of the APPJ. Electrical and optical methods are used to characterize the plasma properties. The discharge modes of the APPJ with respect to applied voltage are studied for grounded electrode positions of 10 mm, 40 mm and 80 mm, respectively, and the main discharge and plasma parameters are investigated. It is shown that an increase in the distance between the grounded electrode and high-voltage electrode results in a change in the discharge modes and discharge parameters. The discharges transit from having two discharge modes, dielectric barrier discharge (DBD) and jet, to having three, corona, DBD and jet, with increase in the distance from the grounded to the high-voltage electrodes. The maximum length of the APPJ reaches 3.8 cm at an applied voltage of 8 kV. The discharge power and transferred charges and spectral line intensities for species in the APPJ are influenced by the positions of the grounded electrode, while there is no obvious difference in the values of the electron excited temperature (EET) for the three grounded electrode positions.

Modeling a Langmuir probe in atmospheric pressure plasma at different EEDFs

In this study, we present a computational model of a cylindrical electric probe in atmospheric pressure argon plasma. The plasma properties are varied in terms of density and electron temperature. Furthermore, results for plasmas with Maxwellian and non-Maxwellian electron energy distribution functions are also obtained and compared. The model is based on the fluid description of plasma within the COMSOL software package. The results for the ion saturation current are compared and show good agreement with existing analytical Langmuir probe theories. A strong dependence between the ion saturation current and electron transport properties was observed, and attributed to the effects of ambipolar diffusion.

In situ impedance measurement of microwave atmospheric pressure plasma

The impedance of atmospheric pressure argon plasma jets driven by microwave frequency is determined in situ by a novel ‘two frequency method’. In the conventional method of reflection coefficient measurement, the frequency of the driving microwave power is scanned, which inevitably affects the plasma characters and leads to uncertainty in the estimated plasma impedance. In our proposed method, the frequency-scanning signal additional to the driving power is used to measure over a wide frequency range, which enables accurate determination of the plasma impedance based on an equivalent circuit model. The measured resistance and reactance of the plasma increase with the driving power in agreement with the transmission line theory. Based on this in situ measurement of the plasma impedance, the net power coupled to the plasma has been determined. The overall power efficiency remains approximately unchanged around 45% for different input power levels owing to the competing effects between the impedance mismatch and the volume change of the plasma.

Characteristic effects onto C13H12N2O3 molecule dissolved in solvents of argon plasma at atmospheric pressure

We could easily argue that the decomposition of the chemical chain molecules is a compelling application when it comes to the atmospheric pressure plasma. In this paper, we have investigated the effect of the atmospheric pressure argon plasma on 4-((2-methoxyphenyl)diazenyl)benzene-1,3,-diol molecule (abbreviated as 4MBD) at room temperature. 4MBD molecule is one of the industrial dye molecules used widely. When considering the ecological life, this molecule is very harmful and dangerous. As such, we suggest a new decomposing method for such molecules. Atmospheric pressure plasma jet is principally treated for the breakdown of the molecule in question. Fourier transform infrared spectrometry and UV–Vis spectrophotometry tools are used to characterization of the molecules subsequent to the plasma applications to 4MBD molecule in ethanol and methanol solvents. The atmospheric-pressure plasma jet of argon (Ar) as non-equilibrium has been formed by ac-power generator with frequency—24 kHz and voltage—12 kV. Characterizations for solutions prepared with ethanol and methanol solvents of 4MBD molecule have been examined after applying (duration 3 min) the atmospheric pressure plasma jet. The molecule is broken at 6C–7N–8N=9C stretching peak in consequence of the plasma treatment. The new plasma photo-products for ethanol and methanol solutions are produced as 6C–7N–8N=9C (strong, varying) and 12C=17O (strong, wide) stretching peaks. Also, the bathochromic drifts are discerned.

U-Shaped Ultrahigh Frequency Atmospheric Pressure Plasma Jet With Magnetic Loop Antenna

An atmospheric pressure argon plasma was generated using a magnetic loop antenna with a pulse modulated ultrahigh frequency signal of 850 MHz. Finite-element simulations were performed to study the patterns of gas flow and electromagnetic waves. Fast photography of the plasma jet was used to study temporal behavior of the plasma jet. Emission and mass spectrometry were used to identify active species in the plasma jet. The detected dominant species included excited argon atoms and singly charged ions of argon, nitrogen, oxygen, nitrogen oxide, and water. To study the effect of the plasma jet on biological samples, a thin onion epidermal membrane was treated with the plasma jet. The hydrophobic layer displayed hydrophilic characteristics after treatment for 1 min.

Characterization of an Atmospheric-Pressure Argon Plasma Generated by 915 MHz Microwaves Using Optical Emission Spectroscopy

The paper presents the investigations of an atmospheric-pressure argon plasma generated at 915 MHz microwaves using the optical emission spectroscopy (OES). The 915 MHz microwave plasma was inducted and sustained in a waveguide-supplied coaxial-line-based nozzleless microwave plasma source. The aim of presented investigations was to estimate parameters of the generated plasma, that is, excitation temperature of electrons Texc, temperature of plasma gas Tg, and concentration of electrons ne. Assuming that excited levels of argon atoms are in local thermodynamic equilibrium, Boltzmann method allowed in determining the Texc temperature in the range of 8100–11000 K. The temperature of plasma gas Tg was estimated by comparing the simulated spectra of the OH radical to the measured one in LIFBASE program. The obtained Tg temperature ranged in 1200–2800 K. Using a method based on Stark broadening of the Hβ line, the concentration of electrons ne was determined in the range from 1.4 × 1015 to 1.7 × 1015 cm−3, depending on the power absorbed by the microwave plasma.