Atmospheric Plasma Physics and Technology Technical Articles

• Shakir, S., Mynampati, S., Pashaie, B., and Dhali, S. K., “RF-generated ambient-afterglow plasma,” J. Appl. Phys. 99, 073303 (2006).  Abstract: Atmospheric pressure plasmas have gained importance due to their potential application in polymer surface treatment, surface cleaning of metals, thin film deposition, and destruction of biological hazards. In this paper a radio-frequency driven atmospheric pressure afterglow plasma source in argon and helium is discussed. The light intensity measurement shows that the radio-frequency discharge is continuous in time unlike the intermittent nature of a low frequency dielectric-barrier discharge. The discharge, under ambient conditions, can be generated in argon, helium, and nitrogen. Spectroscopic measurements show that metastables are capable of producing oxygen atoms and other excited species. The argon afterglow, in particular, is capable of dissociating oxygen molecules in the ambient gas. An afterglow model has been developed to study the interaction of the plasma with the ambient gas. Results from applications of the plasma to surface treatment of metals and polymers, and bacterial decontamination are briefly discussed.
• Li, S. Z., Kang, J. G., and Uhm, H. S., “Electrical breakdown characteristics of an atmospheric pressure RF capacitive plasma source,” Phys. Plasmas 12, 093504 (2005). Abstract: The electrical breakdown characteristics of the rf capacitive plasma source are investigated theoretically and experimentally. The plasma source is the electrode type consisting of the concentric cylinders for generating nonequilibrium plasma at atmospheric pressure. The theoretical model based on the diffusion-controlled breakdown mechanism is proposed to analyze the electrical breakdown phenomenon in this rf capacitive plasma source of the coaxial cylinders. The electron temperature at the electrical breakdown is calculated from the theoretical model, thereby evaluating the electrical breakdown voltages. The experimental data of the electrical breakdown voltage are measured with respect to the variation of the geometric parameters of plasma source, the gas temperature, and the concentration of the foreign reactive gases (oxygen and nitrogen) mixed in the helium gas. The theoretical results of the electrical breakdown voltage agree remarkably well with experimental data. This indicates that not only the electron temperature is important in determining the electrical breakdown voltage, but also the geometric variables, the gas temperature, and the scattering cross sections of molecules play significant roles.
• Shi, J. J., and Kong, M. G., “Expansion of the plasma stability range in radio-frequency atmospheric-pressure glow discharges,” J. Appl. Phys. 87, 201501 (2005). Abstract: Reliable applications of atmospheric-pressure glow discharges (APGDs) depend critically on their plasma stability. A common technique of ensuring APGD stability is to keep their operation well within their stability range by decreasing their discharge current. However, this reduces the achievable densities of the reactive plasma species and, thereby, compromises the application efficiency. In this letter, the use of high excitation frequencies in radio-frequency APGD is shown to substantially expand their stability range. It is also demonstrated that high-frequency operation introduces an added benefit of higher electron energy and greater electron density, thus enabling more abundant reactive plasma species and improved application efficiency.
• Kang, J. G., Kim, H. S., Ahn, S. W., and Uhm, H. S.,“Development of the RF plasma source at atmospheric pressure,” Surf. Coat. Technol. 171, 144 (2003). Abstract: A radio frequency (RF) plasma source operates by feeding helium or argon gas through two coaxial electrodes driven by a 13.56 MHz RF source. In order to prevent an arc discharge, a dielectric material is loaded outside the center electrode. A stable, arc-free discharge is produced at a flow rate of 1.5 l/min of helium gas. The temperature of the gas flame varies from 100 to 150 °C depending on the RF power. The breakdown voltage also changes when the flow rate varies. The plasma generation in a hot chamber is much more efficient than that in a cold chamber. The plasma characteristics are diagnosed by using optical emission spectroscopy. One of the applications of the RF plasma source is the printed circuit board (PCB) cleaning process, needed for environmental protection. The PCB cleaning device forms an asymmetric biaxial reactor.
• Kong, M. G., and Deng, X. T., “Electrically efficient production of a diffuse nonthermal atmospheric plasma,” IEEE Trans. Plasma Sci. 31, 7 (2003). Abstract: Summary form only given. Nonthermal atmospheric pressure gas discharges operated in the diffuse mode have commanded increasing interest largely because their low gas temperature, spatial uniformity, and temporal stability are highly desirable for many materials processing applications. Most such plasmas are generated capacitively with a sinusoidal excitation voltage, and their electrical signature has a distinct characteristic of having one discharge current pulse every half cycle. Of particular interest is that the width of the discharge current pulse is a small fraction of half period of the applied excitation voltage and that the current pulse usually precedes the peak of the excitation voltage in any given half cycle. This suggests that for a very significant part of its application the excitation voltage may not contribute to the production of electrons and that of reactive species, both of which are important for surface modifications required for many materials processing applications. Therefore it is of interest to explore whether this potentially inefficient use of the input electric power can be remedied by, for example, appropriate shaping of the waveform of the excitation voltage. To study this, we have developed a simple numerical model and its associated computer code for one-dimensional simulation of atmospheric pressure helium discharges. To compare to the usual route of using sinusoidal excitation, we consider three pulsed excitation voltage waveforms, namely (1) peak-leveled sinusoidal, (2) square wave, and (3) Gaussian. For each case, plasma voltage and current are computed to identify the parametric range within which the generated atmospheric plasmas are diffuse and nonthermal discharges.
• Yuan, X. H., and Raja, L. L., “Computational study of capacitively coupled high-pressure glow discharges in helium,” IEEE Trans. Plasma Sci.31, 495 (2003).  Abstract:  The structure of a capacitively coupled high-pressure glow (HPG) discharge in high-purity helium is investigated using a detailed one-dimensional modeling approach. Impurity effects are modeled using trace amounts of nitrogen gas in helium. Average electron temperatures and densities for the HPG discharge are similar to their low-pressure counterpart. Helium-dimer ions dominate the discharge structure for sufficiently high-current densities, but model impurity nitrogen ions are found to be dominant for low-discharge currents. Helium dimer metastable atoms are found to be the dominant metastable species in the discharge. The high collisionality of the HPG plasma results in significant discharge potential drop across the bulk plasma region, electron Joule heating in the bulk plasma, and electron elastic collisional losses. High collisionality also results in very low ion-impact energies of order 1 eV at the electrode surfaces.
• Shi,J.J., Deng, X. T., Hall, R., Punnett, J. D., and Kong, M. G., “Three modes in a radio frequency atmospheric pressure glow discharge,” J. Appl. Phys. 94, 6303 (2003). Abstract: Fundamentally not requiring a vacuum chamber, atmospheric pressure glow discharges (APGDs) offer an exciting prospect for a wide range of material processing applications. To characterize their operation and establish their operation range, a radio frequency (rf) APGD is studied experimentally with measurement of discharge voltage, current, dissipated plasma power and plasma impedance. Different from the current understanding that rf APGD are operative only in the abnormal glow mode, we show the presence of two additional modes namely the normal glow mode and the recovery mode. It is shown that all three modes are spatially uniform and possess key characteristics of a glow discharge. So rf APGD have a much wider operation range than previously believed. To provide further insights, we investigate the transition from the abnormal glow mode to the recovery mode. It is established that the cause responsible for the mode transition is sheath breakdown, a phenomenon that is known in low- and moderate-pressure glow discharges but has not been reported before for atmospheric-pressure glow discharges. Finally we demonstrate that plasma dynamics, hence plasma stability, in these three modes are influenced crucially by the impedance matching between the plasma rig and the power source.
• Park, J., Henins, I., Herrmann, H. W., and Selwyn, G. S., “Gas breakdown in an atmospheric pressure radio-frequency capacitive plasma source,” J. Appl. Phys. 89, 15 (2001). Abstract: Gas breakdown is studied in an atmospheric pressure rf capacitive plasma source developed for materials applications. At a rf frequency of 13.56 MHz, breakdown voltage is largely a function of the product of the pressure and the discharge gap spacing, approximating the Paschen curve. However, breakdown voltage varies substantially with rf frequency due to a change in the electron loss mechanism. A large increase in breakdown voltage is observed when argon, oxygen, or nitrogen is added to helium despite their lower ionization potential. Discussion is given for optimal breakdown conditions at atmospheric pressure
• Guerra-Mutis, M. H., Pelaez, C .V., and Cabanzo, R., “Glow plasma jet – experimental study of a transferred atmospheric pressure glow discharge,” Plasma Sources Sci. Technol. 12, 165 (2003). Abstract: In this paper we present the experimental study of a glow plasma jet (GPJ) obtained from a transferred atmospheric pressure glow discharge (APGD) operating at 60 Hz. The characterization of the emission spectra for both electrical discharges is presented and the electrical circuit features for APGD generation are discussed. The potentiality of GPJ as a source of active species for depletion of contaminants in liquid hydrocarbon fractions is also established.
• Cada, M., Hubicka, Z., Sicha, M., Churpita, A., Jastrabik, L., Soukup, L., and Tichy, M., “Probe diagnostics of the RF barrier-torch discharge at atmospheric pressure,” Surf. Coat. Technol. 174, 530 (2003). Abstract: Accurate control of plasma microparameters at the position of the substrate is crucial factor in applications of the barrier-torch plasma source for technological purposes. We present measurements of the electron temperature Te in the RF barrier-torch discharge by means of the planar RF-compensated Langmuir probe. The probe was mounted at the substrate position. The error caused by collisions of charged particles with neutrals in the space-charge sheath around the probe (collision probe working regime) at atmospheric pressure is discussed. In order to minimize this error the single probe technique was used to acquire the probe data, which were then recalculated to get the double probe characteristic. From this the electron temperature Te has been obtained in usual manner. The Te was measured at the position of the substrate in the single- and multi-torch barrier atmospheric plasma-jet systems. Using He as a working gas Te was found to be in the interval Te=2.7~6 eV depending on the applied RF power and system configuration. The neutral gas temperature has been measured by optical diagnostics and found to be 400-800 K. The plasma of the RF barrier-torch discharge is therefore strongly non-isothermal even at such high operation pressure.
• Zhu, W. C., Wang, B. R., Yao, Z. X., and Pu, Y. K., “Discharge characteristics of an atmospheric pressure radio-frequency plasma jet,” J. Phys. D: Appl. Phys. 38, 1396 (2005). Abstract: An atmospheric pressure plasma jet using radio-frequency (rf) (13.56 MHz) power is developed to produce a homogeneous glow discharge at low temperatures (between 50°C and 150°C on the inner electrode). Discharge parameters (power, voltage, current and the phase angle) are measured and the influence of the operating parameters on the discharge characteristics is investigated for a He/O₂ gas mixture. By varying the input power, a ‘phase saturation’ region and the ‘arc failure’ mode are identified. The optimized flow rate ratio between oxygen and helium is found to be 0.1/40 slpm in our experiment. At this ratio, a low power consumption and wide operational range for rf power (200 W) are obtained
• Niemi, K., Schulz-von, der. Gathen. V., and Dobele, H. F., “Absolute atomic oxygen density measurements by two-photon absorption laser-induced fluorescence spectroscopy in an RF-excited atmospheric pressure plasma jet,“ Plasma Sources Sci. Technol. 14, 375 (2005). Abstract: The atmospheric pressure plasma jet is a capacitively coupled radio frequency discharge (13.56 MHz) running with a high helium flux (2 m3 h−1) between concentric electrodes. Small amounts (0.5%) of admixed molecular oxygen do not disturb the homogeneous plasma discharge. The jet effluent leaving the discharge through the ring-shaped nozzle contains high concentrations of radicals at a low gas temperature—the key property for a variety of applications aiming at treatment of thermally sensitive surfaces. We report on absolute atomic oxygen density measurements by two-photon absorption laser-induced fluorescence (TALIF) spectroscopy in the jet effluent. Calibration is performed with the aid of a comparative TALIF measurement with xenon. An excitation scheme (different from the one earlier published) providing spectral matching of both the two-photon resonances and the fluorescence transitions is applied.
• Laimer, J., Haslinger, S., Meissl, W., Hell, J., and Stori, H., “Investigation of an atmospheric pressure radio-frequency capacitive plasma jet,” Vacuum79, 209 (2005). Abstract: We have constructed a large area atmospheric pressure plasma jet (LAAPPJ) composed of two planar electrodes with an area 38 cm2 at a distance of 2.5 mm.The LAAPPJ was operated with helium at a gas flow rate of 1.5 mmol/s. The electrical properties of the discharge were studied by measuring the voltage and the discharge current simultaneously using a high voltage probe, a current probe and a digital oscilloscope. Pictures of the discharge were taken with a digital camera. Two discharge modes can be sustained within certain limits. At RF input powers of 50–420 W an α-discharge can be sustained, however, below approximately 300 W the electrodes are only partially covered with the glow. At higher powers a breakdown occurs and a transition to a γ-discharge takes place, whereby the α-discharge dies out. The γ-discharge only covers a part of the electrodes and can be sustained down to powers of 50 W. Below this power the discharge extinguishes.
• Wang, H. B., Sun, W. T., Li, H. P., Bao, C. Y., and Zhang, X. Z., “Characteristics of radio-frequency, atmospheric-pressure glow discharges with air using bare metal electrodes,” Appl. Phys. Lett.89, 161502 (2006). Abstract: In this letter, an induced gas discharge approach is proposed and described in detail for obtaining a uniform atmospheric-pressure glow discharge with air in a γ mode using water-cooled, bare metal electrodes driven by radio-frequency (13.56 MHz) power supply. A preliminary study on the discharge characteristics of the air glow discharge is also presented in this study. With this induced gas discharge approach, radio-frequency, atmospheric-pressure glow discharges using bare metal electrodes with other gases which cannot be ignited directly as the plasma working gas, such as nitrogen, oxygen, etc., can also be obtained.
• Li, S. Z., Lim, J. P., and Uhm, H. S., “Discharge characteristics of an atmospheric-pressure capacitively coupled radio-frequency argon plasmas,“ Phys. Lett. A 360, 304 (2006). Abstract: In this Letter, the electrical discharge characteristics of plasmas generated in coaxial cylindrical electrodes capacitively powered by radio-frequency power supply at atmospheric pressure are investigated with respect to argon gas. The electrical discharge parameters, the current and voltage characteristics (I–V) and the current and power characteristics (I–P), are measured for argon plasmas, and the electron temperatures and electron densities are estimated based on the equivalent circuit model and by making use of the power balance equation. Furthermore, the influence of the additive gas, oxygen gas, on the electrical discharge characteristics is also investigated in the argon plasmas, which is closely related to the electron temperature of plasmas.
• Li, S. Z., Lim, J. P., Kang, J. G., and Uhm, H. S., “Comparison of atmospheric-pressure helium and argon plasmas generated by capacitively coupled radio-frequency discharge,” Phys. Plasmas 13, 093503 (2006). Abstract: In this paper, the electrical discharge characteristics of plasmas generated in coaxial cylindrical electrodes capacitively powered by a radio-frequency power supply at atmospheric pressure are investigated with respect to helium and argon gases. The electrical discharge parameters, voltage V, current I, and power P, are measured for both helium and argon plasmas, and the electron temperatures and electron densities for them are evaluated by means of the equivalent circuit model and the power balance equation. By comparison of the discharge characteristics of the helium and argon plasmas, it is found that the discrepant macroscopic characteristics of helium and argon plasma, viz., current and voltage characteristics and current and power characteristics, are owed to their own intrinsic microscopic parameters of the helium and argon atoms, such as the first excited energy, the ionization energy, the total cross section, and the atom mass. Furthermore, the influences of the additive gas, oxygen gas, on the electrical discharge characteristics are also investigated in the helium and argon plasmas, which are closely related to the electron temperature of plasmas.
• Li, G., Li, H. P., Wang, L. Y., Wang, S., Zhao, H. X., Sun, W. T., Xing, X. H., and Bao, C. Y., “Genetic effects of radio-frequency, atmospheric-pressure glow discharges with helium,” J. Appl. Phys. Lett.89, 073902 (2006). Abstract: Due to low gas temperatures and high densities of active species, atmospheric-pressure glow discharges
  APGDs would have potential applications in the fields of plasma-based sterilization, gene mutation, etc. In this letter, the genetic effects of helium radio-frequency APGD plasmas with the plasmid DNA and oligonucleotide as the treated biomaterials are presented. The experimental results show that it is the chemically active species, instead of heat, ultraviolet radiation, intense electric field, and/or charged particles, that break the double chains of the plasmid DNA. The genetic effects depend on the plasma operating parameters, e.g., power input, helium flow rate, processing distance, time, etc.
• Laimer, J., and Störi, H., “Glow Discharges Observed in Capacitive Radio-Frequency Atmospheric-Pressure Plasma Jets,” Plasma Process. Polym. 3, 573 (2006). Abstract: The APPJ is the subject of intensive recent research. Basically, the APPJ consists of an atmospheric-pressure RF discharge between two bare metallic electrodes and of a superimposed gas flow consisting predominantly of a rare gas. In its usual operating regime, a uniform glow discharge, which was identified as the α mode of a RF discharge, can be sustained over a wide range of gap spacings and RF powers. Maximum electron densities of 0.22 × 1012 cm−3 were assessed for helium and of 2.75 × 1012 cm−3 for argon by evaluation of electric measurements using a simple equivalent circuit model. At low RF powers, only partial coverage of the electrodes was observed. At high RF powers, a breakdown of the α sheath occurred, and a transition to either a pure γ mode or a coexistingα and γ mode took place. The γ mode covers only a small fraction of the electrode surface. In helium, it was easy to ignite directly a pure α discharge, but not in argon. Due to the high over-voltage needed, the ignition in argon mostly led to a coexisting α and γ mode. However, through RF power reduction a transition to a pure α discharge can be initiated.
• Li, S. Z., Wang, D. Z., Zhu, W. C., and Pu, Y. K., “Evaluations of electron density and temperature in atmospheric-pressure radio-frequency helium plasma jet,” Jpn. J. Appl. Phys. 45, 9108 (2006). Abstract: In this study, the electrical discharge characteristics of an atmospheric-pressure helium plasma jet using radio-frequency power supply are investigated by measuring the electrical parameters (voltage, current, power, and phase angle). The plasma parameters (electron temperature and electron density) are estimated on the basis of the electrical experiment data using the equivalent circuit model and the power balance equation. By varying input power, a “phase saturation” region is identified. The optimized flow rate ratio between oxygen and helium is found to be 0.1/40 slpm in the experiment.
• Sun, W. T., Liang, T. R., Wang, H. B., Li, H. P., and Bao, C. Y., “The back-diffusion effect of air on the discharge characteristics of atmospheric-pressure radio-frequency glow discharges using bare metal electrodes,” Plasma Sources Sci. Technol. 16, 290 (2007). Abstract: Radio-frequency (RF), atmospheric-pressure glow discharge (APGD) plasmas using bare metal electrodes have promising prospects in the fields of plasma-aided etching, deposition, surface treatment, disinfection, sterilization, etc. In this paper, the discharge characteristics, including the breakdown voltage and the discharge voltage for sustaining a stable and uniform α mode discharge of the RF APGD plasmas are presented. The experiments are conducted by placing the home-made planar-type plasma generator in ambient and in a vacuum chamber, respectively, with helium as the primary plasma-forming gas. When the discharge processes occur in ambient, particularly for the lower plasma-working gas flow rates, the experimental measurements show that it is the back-diffusion effect of air in atmosphere, instead of the flow rate of the gas, that results in the obvious decrease in the breakdown voltage with increasing plasma-working gas flow rate. Further studies on the discharge characteristics, e.g. the luminous structures, the concentrations and distributions of chemically active species in plasmas, with different plasma-working gases or gas mixtures need to be conducted in future work
• Sun, W. T., Li, G., Li, H. P., Bao, C. Y., Wang, H. B., Zeng, S., Gao, X., and Luo, H. Y., “Characteristics of atmospheric-pressure, radio-frequency glow discharges operated with argon added ethanol,” J. Appl. Phys. 101, 123002 (2007). Abstract: Rf, atmospheric-pressure glow discharge (APGD) plasmas with bare metal electrodes have promising prospects in the fields of plasma-aided etching, thin film deposition, disinfection and sterilization, etc. In this paper, the discharge characteristics are presented for the rf APGD plasmas generated with pure argon or argon-ethanol mixture as the plasma-forming gas and using water-cooled, bare copper electrodes. The experimental results show that the breakdown voltage can be reduced significantly when a small amount of ethanol is added into argon, probably due to the fact that the Penning ionization process is involved, and a pure α-mode discharge can be produced more easily with the help of ethanol. The uniformity of the rf APGDs of pure argon or argon-ethanol mixtures using bare metallic electrodes is identified with the aid of the intensified charge coupled device images.
• Staack, D., Farouk, B., Gutsol, A. F., and Fridman, A., “Spatially resolved temperature measurements of atmospheric-pressure normal glow microplasmas in air,” IEEE Trans. Plasma Sci. 35, 1448 (2007). Abstract: The rotational and vibrational temperatures of DC normal glow air discharges were measured by comparing modeled optical emission spectra of the N2 second positive system with spectroscopic measurements from the discharges. By using an imaging spectrometer and optical assembly, the temperature measurements were spatially resolved to about 6 mum. Results are presented for a 3.8-mA discharge at an electrode spacing of 400 mum. Rotational temperatures are highest in the near-cathode region around 1500 K and decrease toward the anode to about 1100 K. Throughout the discharge, higher rotational temperatures correspond with lower vibrational temperatures. The maximum vibrational temperature measured is around 5000 K. Emission from the N2+ first negative system was also measured and is only intense in the negative glow (NG) region. The temperature near the anode is sensitive to the anode material. Gold, stainless steel, and tungsten electrodes were studied. Oxidizing anode materials can create a bright and hot anode spot several hundred kelvin warmer than for nonreacting anode materials. In addition, comparisons between spatially resolved and previously studied emission-averaged temperatures indicate that the emission-averaged temperatures correspond to those of the NG regions.
• Sanchez-Gonzalez, R., Kim, Y., Rosocha, L. A., and Abbate, S., “Methane and ethane decomposition in an atmospheric-pressure plasma jet,” IEEE Trans. Plasma Sci. 35, 1669 (2007). Abstract: This paper reports on studies obtained from RF-driven atmospheric-pressure plasma-jet excitation of methane and ethane. Differentiation with other works is achieved in that others have considered hydrocarbon decomposition at either low pressure or high temperature. In our experiments, we can clarify the effect of pure-plasma treatment of hydrocarbons, as opposed to the thermal effect of gas heating that results in pyrolysis. Gas-chromatography analysis was used to detect and quantify the main decomposition products. Kinetic modeling of the pertinent chemistry was performed by dividing the reactive system in two main parts: a plasma region where the electron impact processes leading to decomposition are considered and a postplasma region where recombination of nonstable species occurs. A reasonable qualitative agreement between the experimentally measured by-product concentrations and the calculations was achieved. It is observed that our proposed recombination mechanism correctly predicts ethane and ethylene formation from a discharge and methane, ethylene, propane, and acetylene formation from the discharge. By means of calculations, the main role of radicals in the pertinent hydrocarbon chemistry under nonthermal plasma conditions is confirmed.
• Li, H. P., Sun, W. T., Wang, H. B., Li, G., and Bao, C. Y., “Electrical features of radio-frequency, atmospheric-pressure, bare-metallic-electrode glow discharges,” Plasma Chem. Plasma Process. 27, 529 (2007). Abstract: Radio-frequency (RF), atmospheric-pressure glow discharge (APGD) plasmas with bare metallic electrodes have promising prospects in the fields of plasma-aided etching, deposition, disinfection and sterilization, etc. In this paper, an induced gas discharge approach is proposed for obtaining the RF, atmospheric-pressure, γ-mode, glow discharges with pure nitrogen or air as the primary plasma-working gas using bare metallic electrodes. The discharge characteristics, including the discharge mode, the breakdown voltage and discharge voltage for sustaining α mode and/or γ mode discharges, of the RF APGD plasmas of helium, argon, nitrogen, air or their mixtures using a planar-type plasma generator are presented in this study. The uniformity (no filaments) of the discharges is confirmed by the images taken by an iCCD with a short exposure time (10 ns). The effects of different gap spacings and electrode materials on the discharge characteristics, the variations of the sheath thickness and the electron number density are also studied in this paper.
• Haslinger, S., Laimer, J., and Stoei, H., “Stability conditions of argon and helium gas mixtures in an atmospheric pressure plasma jet,” Vacuum 82, 142 (2007). Abstract: Non-equilibrium plasmas can be generated by atmospheric pressure glow discharges, amongst others by atmospheric pressure plasma jets (APPJ), which feature a capacitive radio-frequency discharge between bare metallic electrodes. We investigated the stability conditions for discharges in an APPJ operated with helium-argon mixtures. Uniform [alpha]-discharges can be sustained in mixtures ranging from pure helium to pure argon. The ignition voltage increases drastically with argon content. There is also an upper limit for the existence of the [alpha]-mode, where [alpha]-sheath breakdown occurs. Critical plasma parameters for the [alpha]-mode were determined by equivalent circuit models and discussed in respect to dependences on the different models. A critical electron density of 2.4×1011 cm-3 was obtained for pure helium. It increases steadily with argon content and reaches a value of 1.2×1012 cm-3 for pure argon. Sheath thicknesses for [alpha]-sheath breakdown were calculated in the range of 0.17-0.29 mm for helium-argon mixtures.
• Foest, R., Kindel, E., Lange, H., Ohl, A., Stieber, M., and Weltmann, K. D., “RF capillary jet – a tool for localized surface treatment,” Contrib. Plasma Phys. 47, 119 (2007). Abstract: The UV/VUV spectrum of a non-thermal capillary plasma jet operating with Ar at ambient atmosphere and the temperature load of a substrate exposed to the jet have been measured. The VUV radiation is assigned to N, H, and O atomic lines along with an Ar*2 excimer continuum. The absolute radiance (115-200 nm) of the source has been determined. Maximum values of 880 μW/mm2sr are obtained. Substrate temperatures range between 35 °C for low powers and high gas flow conditions and 95 °C for high powers and reduced gas flow. The plasma source (13.56, 27.12 or 40.78 MHz) can be operated in Ar and in N2. The further addition of a low percentage of silicon containing reactive admixtures has been demonstrated for thin film deposition. Several further applications related to surface modification have been successfully applied.
• Balcon, N., Aanesland, A., and Boswell, R., “Pulsed RF discharges, glow and filamentary mode at atmospheric pressure in argon,” Plasma Source Sci. Technol.16, 217 (2007). Abstract: The properties of a pulsed radio frequency capacitive discharge are investigated at atmospheric pressure in argon. The discharge can operate in two different modes: a homogeneous glow discharge or turn into filaments. By pulsing the 13.56 MHz generator both the filamentary and the glow modes can be selected depending on the pulse width and period. For a 5 µs pulse width (~70 RF cycles in the pulse), short pulse periods (less than 100 µs) result in a filamentary discharge while long pulse periods (greater than 1 ms) result in a glow discharge.
• Anghel, S. D., and Simon, A., “Measurement of electrical characteristics of atmospheric pressure non-thermal He plasma,” Meas. Sci. Technol. 18, 2642 (2007). Abstract: This work develops a method for determining the electrical characteristics of an atmospheric pressure non-thermal He plasma. It is applied on an original low power plasma source that has some specific properties, as follows: (1) it uses high sinusoidal voltages generated in an LRC series resonant circuit; (2) the frequency of the sinusoidal waveform that maintains the discharge (<1 MHz) is lower than those used till now (>13.56 MHz); (3) the circuit is powered with dc voltages lower than 5 V and dc currents up to 1 A; and (4) the plasma torch is made of the usual materials and has a single electrode. As a function of the power and the gas (helium) flow rate, the generated plasma has three developing stages: pointed, extended (oval ball) and jet. The developed calculation algorithm of the electrical characteristics of the plasma uses only the parameters that can be measured without disturbing the plasma. The algorithm was validated by direct measurement of the plasma current. The electrical measurements show that the plasma has a purely resistive behaviour. As a function of the absorbed power and the gas flow rate, the plasma resistance varies in the range of 104 − 105 Ω.
• Ye, R. B., and Zheng, W., “Temporal-spatial-resolved spectroscopic study on the formation of an atmospheric pressure microplasma jet,” Appl. Phys. Lett. 93, 071502 (2008). Abstract: Temporal-spatial-resolved optical emission spectroscopy was employed to shed light on the dynamic behavior and the propagation mechanism of a plasma, originating from a dielectric barrier discharge in helium inside a quartz tube for microplasma jet formation. The plasma propagated, regardless of the gas flow direction, in an accelerating manner at a high velocity up to 17 km/s, suggesting that the propagation was sustained by photoionization. A theoretical analysis demonstrated that the enhancement of the local electric field ahead of the ionization front was mainly responsible for the acceleration of the plasma near the electrode.
• Ye, R., and Zheng, W., “Unipolar discharge phenomena in atmospheric pressure helium plasma generated in a quartz tube,” J. Phys. D: Appl. Phys. 41, 125202 (2008). Abstract: Discharge in a fine tube is a simple way of generating a dielectric barrier discharge microplasma jet under atmospheric pressure (AP). Since its electrodes are directly connected by a dielectric, discharge behavior in this kind of tubular-electrode plasma could be significantly different from that of a parallel-plate plasma source having planar electrodes. In this paper, experimental results for unipolar discharges, novel discharge phenomena arising from a tubular-electrode plasma, are presented. The AP helium plasma was generated using a 5.7 mm i.d. quartz tube. In unipolar discharges, breakdown in the gas only occurred at the positive or the negative half of a sinusoidal applied high voltage, and the discharge tube worked like a gas diode that outputs a unidirectional current signal. By means of voltage–current and voltage–charge (Lissajous figure) measurements, it was suggested that the surface charges on the dielectric were mainly responsible for the formation of unipolar discharge. In addition, the residual current at the instant of voltage polarity change was found to be a critical factor determining the transition from bipolar to unipolar discharge.
• Walsh, J. L., and Kong, M. G., “Contrasting characteristics of linear-field and cross-field atmospheric plasma jets,” Appl. Phys. Lett. 93, 111501 (2008). Abstract: This letter reports an experimental study of two types of atmospheric pressure plasma jets in terms of their fundamental properties and their efficiency in etching polymeric materials. The first plasma jet has a cross-field configuration with its electric field perpendicular to its gas flow field, whereas the second is a linear-field device having parallel electric and flow fields. The linear-field jet is shown to drive electron transportation to the downstream application region, thus facilitating more active plasma chemistry there. This is responsible for its etching rate of polyamide films being 13-fold that of its cross-field counterpart.
• Pipa, A. V., Bindemann, T., Foest, R., Kindel, E., Ropcke, J., and Weltmann, K. D., “Absolute production rate measurements of nitric oxide by an atmospheric pressure plasma jet (APPJ),” J. Phys. D: Appl. Phys. 41, 194011 (2008). Abstract: Tunable diode laser absorption spectroscopy (TDLAS) has been applied to measure the absolute production rate of NO molecules in the gas phase of an atmospheric pressure plasma jet (APPJ) operating at rf (13.56 MHz) in argon with small (up to 1%) admixtures of air. The resulting NO production rates were found to be in the range (0.1–80) × 10−3 sccm or (0.05–35) ×1018 molecules s−1 depending on the experimental conditions. Maximum rates were obtained at 0.2% air. For TDLAS measurements the APPJ was arranged inside an astigmatic multi-pass cell of Herriott type with 100 m absorption length. The insertion into a closed volume differs slightly from the normal, open operation with the jet propagating freely into air. Therefore, the measuring results are compared with optical emission of the open jet to verify equivalent experimental conditions. The dependence of the optical emission of NO (237 nm) on power and gas mixture has been measured. The similar shape of the dependence of absorption and emission signals gives evidence that the comparability of experimental conditions is sufficiently satisfied. It is concluded that the NO production rate of the APPJ in ambient air can be characterized using TDLAS and provides reliable results in spite of differing experimental conditions due to the set-up.
• Li, X., Tang, C. J., Dai, X. Y., and Yin, Y. X., “Study of atmospheric pressure abnormal glow discharge,” Plasma Sources Sci. Technol.10, 185 (2008). Abstract: Atmospheric pressure abnormal glow discharge (APAGD) was carried out simply with a transformer of 1: 500 driven by a alternating current with a frequency of 50 Hz. Typical stable discharge parameters, namely voltage of 400 V to 850 V and current of 60 mA to 110 mA were measured by oscillograph. Simulation of the discharge process suggested that the stable discharge was supported by the impedance from the secondary coil of the transformer, which offered a negative feedback to prevent the discharge from turning into an arc. An interpretation was given for the oscillogram of the sinuous discharge current and square voltage. Furthermore, the electron temperature and electron density averaged in the discharge channel of APAGD were estimated.
• Li, H. P., Li, G., Wang, S., Le, P. S., and Bao, C. Y., “Radio-frequency glow discharges of different gases using bare metallic electrodes at atmospheric pressure,” IEEE Trans. Plasma Sci.36, 1418 (2008). Abstract: The images of the atmospheric-pressure glow discharges driven by the radio-frequency power supply using He, Ar, N2, O2, air, or their mixture as the primary plasma-working gas are presented in this paper. The plasma jet, produced using the planar-type plasma generator behaving like a “brush”, is useful for the effective treatment of different materials exposed to the plasma jet.
• Schulz-von, der. Gathen. V., Schaper, L., Knake, N., Reuter, S., Niemi, K., Gans, T., and Winter, J., “Spatially resolved diagnostics on a microscale atmospheric pressure plasma jet,” J. Phys. D: Appl. Phys. 41, 194004 (2008). Abstract: Despite enormous potential for technological applications, fundamentals of stable non-equilibrium micro-plasmas at ambient pressure are still only partly understood. Micro-plasma jets are one sub-group of these plasma sources. For an understanding it is particularly important to analyse transport phenomena of energy and particles within and between the core and effluent of the discharge. The complexity of the problem requires the combination and correlation of various highly sophisticated diagnostics yielding different information with an extremely high temporal and spatial resolution. A specially designed rf microscale atmospheric pressure plasma jet (μ-APPJ) provides excellent access for optical diagnostics to the discharge volume and the effluent region. This allows detailed investigations of the discharge dynamics and energy transport mechanisms from the discharge to the effluent. Here we present examples for diagnostics applicable to different regions and combine the results. The diagnostics applied are optical emission spectroscopy (OES) in the visible and ultraviolet and two-photon absorption laser-induced fluorescence spectroscopy. By the latter spatially resolved absolutely calibrated density maps of atomic oxygen have been determined for the effluent. OES yields an insight into energy transport mechanisms from the core into the effluent. The first results of spatially and phase-resolved OES measurements of the discharge dynamics of the core are presented.
• Knake, N., Reuter, S., Niemi, K., Schulz-von, der. Gathen. V., and Winter, J., “Absolute atomic oxygen density distributions in the effluent of a microscale atmospheric pressure plasma jet,“ J. Phys. D: Appl. Phys. 41, 194006 (2008). Abstract: The coplanar microscale atmospheric pressure plasma jet (μ-APPJ) is a capacitively coupled radio frequency discharge (13.56 MHz, ~15 W rf power) designed for optimized optical diagnostic access. It is operated in a homogeneous glow mode with a noble gas flow (1.4 slm He) containing a small admixture of molecular oxygen (~0.5%). Ground state atomic oxygen densities in the effluent up to 2 × 1014 cm−3 are measured by two-photon absorption laser-induced fluorescence spectroscopy (TALIF) providing space resolved density maps. The quantitative calibration of the TALIF setup is performed by comparative measurements with xenon. A maximum of the atomic oxygen density is observed for 0.6% molecular oxygen admixture. Furthermore, an increase in the rf power up to about 15 W (depending on gas flow and mixture) leads to an increase in the effluent’s atomic oxygen density, then reaching a constant level for higher powers.
• Farouk, T., Farouk, B., Gutsol, A., and Fridman, A., “Atmospheric pressure radio frequency glow discharges in argon: effects of external matching circuit parameters,” Plasma Sources Sci. Technol. 17, 035015 (2008). Abstract: Numerical simulations of radio frequency atmospheric pressure argon glow discharges were performed using a one-dimensional hybrid model. The discharge simulations were carried out for a parallel plate electrode configuration with an inter-electrode gap of 1.0 mm together with an external matching circuit. The external matching circuit parameters were found to have significant effect on the discharge characteristics. The results indicate that the discharge can operate at either the α or γ mode depending on the matching circuit parameters. The two modes of operation were found to be distinctly different. The predicted Ar* density was considered to provide qualitatively the visual appearance of the α or γ mode discharge. The α mode was found to have a luminous region in the center of the discharge. On the other hand, the γ mode had luminous regions very close to the electrodes which were followed by alternating dark and bright regions. The appearance of the simulated γ mode was found to resemble that of an atmospheric pressure direct current glow discharge. The predicted gas temperature indicated the γ mode to have higher gas temperature compared with the α mode.
• Dinescu, G., and Ionita, E. R., “Radio frequency expanding plasmas at low, intermediate, and atmospheric pressure and their applications,” 33 rd EPS. 301, 1919 (2008). Abstract: We report on the operation and characteristics of radio frequency (RF) plasma beam sources based on the expansion of the discharge outside of limited spaces with small interelectrode gaps. The appropriate electrode configuration, combined with high mass flow values and appropriate power levels, leads to small- or large-size plasma jets, working stably at low, intermediate, and atmospheric pressures. The sources are promising tools for a wide range of applications in thin film deposition, surface modification, and cleaning, including the case of temperature-sensitive substrates.
• Anghel, S. D., Simon, A., and Frentiu, T., “Spectroscopic investigations on a low power atmospheric pressure capacitively coupled helium plasma,” Plasma Source Sci. Technol. 18, 2642 (2008). Abstract“ An atmospheric pressure helium plasma is characterized based on its optical emission. The plasma is generated at 13.56 MHz (continuous wave) at power levels ranging from 4 to 35 W. It is in contact with a single electrode and is part of a resonant electric circuit. The excitation, vibrational and rotational temperatures are in the range 1880–5660 K, 1960–3150 K and 760–1398 K, respectively. The temperatures are correlated with the plasma power, plasma volume and gas dynamic flow. Atomization and excitation capabilities of the plasma were tested for three easily excitable elements (Zn, Na and Li) introduced in the plasma by pneumatic nebulization from liquid solution.

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