For experimental tests of fluctuation theory in ideal plasmas and plasmas seeded with dust, the ideal environment would be that of stable quiescent plasma. In most laboratory plasmas the homogeneous state of the positive column is often unstable, rare exceptions are the so-called brush cathode discharges, proposed in the 60s, where a specially manufactured cathode allows stable operation in the abnormal glow regime and the only fluctuations present are those due the thermal motion of the particles. Such a device, the BAri Brush Electrode (BABE), has recently been built in a novel configuration that combines the advantages of the inverse design with those of the reflex geometry. The region between the two anodes is essentially field-free and extremely stable in wide range of plasma densities and collisionalities. Unprecedented low fluctuation levels of delta n/n <= 10(-5) in He and delta n/n <= 5 x 10(-6) in Ar discharges have been achieved.

In this paper we report on a spectroscopic study on CH observables including laser inducedfluorescence (LIF) on CH and emission spectroscopy on the CH Ger¨o band. Directobservation by LIF has been found to be very difficult, both because of technical difficultiesand an overall low CH density in the discharge. Analysis of CH(A) emission shows that it isdue to CH4 dissociative excitation processes, by He(23S) in He-based mixtures, and byelectron impact in N2-CH4. The analysis of spectra evidences the need for more preciseknowledge on the collision quenching of electronically excited states and on the electronenergy distribution function.

Atmospheric pressure air dielectric barrier discharges were applied to phosphate-buffered saline and complete Dulbecco's modified Eagle medium (DMEM) supplemented with 10% fetal bovine serum to investigate their oxidative chemical modifications induced in liquids that are relevant for cell culture experiments in the field of Plasma Medicine. The study mainly identified long-living reactive species. Hydrogen peroxide, superoxide anion radicals, and nitrite and nitrate ions were detected in DMEM. The density of the reactive species was correlated with the energy dose delivered to the liquids by the discharges.

A possible way to store both renewable energy and CO2 in chemical energy is to produce value-added chemicals and fuels starting from CO2 and green electricity. This can be done by exploiting the non-equilibrium properties of gaseous electrical discharges. Discharges, in addition, can be switched on and off quickly, thus being suitable to be coupled with an intermittent energy source. In this study, we have used a nanosecond pulsed discharge to dissociate CO2 and CH4 in a 1:1 mixture at atmospheric pressure, and compared our results with literature data obtained by other discharges. The main products are CO, H-2, C2H2, water and solid carbon. We estimate an energy efficiency of 40% for syngas (CO and H-2) production, higher if other products are also considered. Such values are among the highest compared to other discharges, and, although not very high on an absolute scale, are likely improvable via possible routes discussed in the paper and by coupling to the discharge a heterogeneous catalysis stage.

The atmospheric pressure plasma deposition of hydroxyl functionalized hydrocarbon films is reported in this work, with a reactor fed with water aerosol and ethylene. The effects of power and feed flow rates onto film chemistry have been investigated. Coatings produced with this approach can find application in the biomedical field, among others, as platforms for cell adhesion and proliferation. Results show that operating at 4 kHz provides a much higher amount of hydroxyl group in the coating compared with samples obtained at 11 kHz. After water immersion, the stability of the films and their amount of hydroxyl groups remain high. A simplified deposition mechanism is proposed.

Bio-composite coatings, consisting of an organic matrix embedding a bioactive molecule, have been deposited by means of atomizer-assisted atmospheric pressure plasma. Ethylene was chosen as the precursor of the matrix, while the atomizer was fed with a water solution of lysozyme. Coatings chemical composition was investigated by XPS, FTIR and MALDI-TOF spectroscopies, and it has been proved that the one-step inclusion of protein domains in the composite coatings is successful and lysozyme chemical structure is only slightly altered. The amount of embedded lysozyme is as high as 14?g/cm² as evaluated from water release test. Finally, the activity of the plasma-embedded protein is close to that of pure lysozyme as verified against Micrococcus lysodeikticus ATCC 4698 through an agar plate diffusion test.

The goal of this work was to achieve proper chemical modification of poly-e-caprolactone porous scaffolds by means of He/O-2 Atmospheric Pressure Plasma-Jet treatments with the aim of improving their affinity with cells. The surface chemical composition and wettability of plasma processed scaffolds was characterized, their cytocompatibility was tested with human Saos-2 osteoblasts cells. Plasma-grafted polar groups were found to improve the hydrophilicity of the scaffolds, which resulted in better clusterization and cytoskeleton morphology of cells cultured on them.

Low environmental impact is a main issue in the design of novel combustion systems, as aircraft engines. In this context, the present work investigates the possibility to increase the combustion efficiency of a lean flame through the use of sinusoidally driven dielectric barrier discharge (DBD) plasma actuator. The effect of the plasma discharge on a lean non premixed methane/air flame in a Bunsen-type burner has been studied for two different configurations: the normal diffusive flame (NDF) and the inverse diffusive flame (IDF). The flame behavior was investigated by chemiluminescence imaging through an intensified CCD camera. Optical filters were installed in front of the camera, aiming to selectively record signal from the chemiluminescent species OH*, CH*, or CO2*. This allowed evaluating the changes occurring in presence of plasma actuation in term of flame emissions. It was shown that the plasma effects are significantly influenced by the burner and DBD configuration. A plasma power of approximately 25 W permitted to increase the air mass flow rate at which lean blowout appears; it rose up to 30% for low methane flow rate and up to 10% at high fuel flow rate.

In this paper we describe in detail the application of laser induced fluorescence (LIF) to the OH density measurement in a dielectric barrier discharge (DBD) at atmospheric pressure in Ar�H2 O, He�H2 O mixtures, and with small N2 additions. Measurements are reported in which OH density is measured in a pulsed DBD, together with its decay in the post-discharge. The variation of macroscopic discharge parameters, such as the applied voltage, the water vapour content, the gas mixture composition and the discharge duration, has a large effect on the OH loss rate and a smaller one on OH density. These effects are described and briefly discussed as a valuable help for the understanding of the complex microscopic kinetics of water containing discharges.

In this paper we give a general description of the LIF technique by way of OH absolute measurements in two atmospheric pressure plasma jet systems, namely a plasma gun and a RF plasma jet. We address in detail: the modelling of the LIF measurement, taking into account collisional processes and the spatial non-uniformity of the laser beam; absolute calibration; rotational temperature measurements.

This paper offers an outline of laser induced fluorescence (LIF) diagnostics and practical recommendations for its use in atmospheric pressure discharges. LIF principles, technical requirements and rationalization of experimental outcomes by modelling are addressed. Important issues that are particularly relevant to small scale, spatially inhomogeneous discharges, like plasma-jets, are emphasized. For the first time, all collision processes and the spatial non-homogeneity of the laser beam are together accounted for in the LIF model. Saturation characteristics are discussed and used for the assessment of model parameters. A calibration procedure is discussed and implemented. Gas temperature measurements by LIF are also addressed. The whole description of the technique is given, without loss of generality, through the example of its application to the OH radical. Notes on other diatomic radicals, CH, NO and CN, are given along the paper. Some results in a RF plasma-jet are presented as an example of application in a discharge system where all the concepts developed in the paper are applied.

A CO2 nanosecond repetitively pulsed discharge (NRP) is a harsh environment for laser induced fluorescence (LIF) diagnostics. The difficulties arise from it being a strongly collisional system in which the gas composition, pressure and temperature, have quick and strong variations. The relevant diagnostic problems are described and illustrated through the application of LIF to the measurement of the OH radical in three different discharge configurations, with gas mixtures containing CO2 + H2O. These range from a dielectric barrier NRP with He buffer gas, a less hostile case in which absolute OH density measurement is possible, to an NRP in CO2+H2O, where the full set of drawbacks is at work. In the last case, the OH density measurement is not possible with laser pulses and detector time resolution in the ns time scale. Nevertheless, it is shown that with a proper knowledge of the collisional rate constants involved in the LIF process, a collisional energy transfer-LIF methodology is still applicable to deduce the gas composition from the analysis of LIF spectra.

The aim of this paper is the use of optical sensors to recognize lean blowout in a non-premixed methane/air burner, Bunsen-type, and the use of plasma actuators for flame control and stabilization. The burner is optically accessible to permit the imaging acquisitions of the flame region. The plasma actuation regards alternatively the air flow and the fuel flow. The electric field is generated using a fixed configuration of plasma actuator and the dielectric barrier discharge (DBD) but using two different power supplies: a nanosecond repetitively pulsed high voltage (HV) and a sinusoidal DBD HV. The comparison between the two types of actuation is the core of this paper, together with the analysis of the results obtained when actuation acted on the air or on the fuel. For the analysis, the lean blowout (LBO) limits recorded in the presence and absence of plasma actuation to investigate the plasma actuation success. The flame behavior is acquired using a compact digital camera, an intensified charge-coupled device (CCD) in order to capture the differences between the baseline conditions and the actuated cases. It is shown that the plasma significantly allows stabilizing the flame under lean conditions where it would not exist without plasma.

In this paper we present laser induced fluorescence (LIF) measurements of OH relative density in a He-H2Oatmospheric pressure dielectric barrier discharge, with an estimation of the absolute density based on thequantitative analysis of the LIF signal and on the decay of OH density in the post-discharge. The possibleinterference of H2O2 photo-dissociation is discussed and finally excluded. Densities of the order of1013 cm3 have been estimated in mixtures with water vapour content ranging from 2.3 to 23 Torr partialpressure. LIF diagnostic characteristics and sensitivity in theOH case at atmospheric pressure are discussedin comparison with absorption techniques.

Laser induced fluorescence by one (LIF) and two photon (TALIF) excitation has been employed to characterize NO, O species in the expanded stream of N(2)-O(2) air like low pressure plasma jet. The gas, excited in a coaxial RF capacitive discharge at pressure P(1), expands through a de Laval nozzle into a vessel at P(2)=0.25 Ton at expansion ratio P(1)/P(2) of about 35. The multiple expansion compression waves of the jet are traced by laser induced fluorescence of NO and O dissociation products expanding through the nozzle. The quantitative O and NO densities, obtained by in-situ calibration of TALIF and LIF signals are discussed.

Optical, electrical and discharge product measurements were performed in order to reveal thebehaviour of N2(A 3
+u ) metastables in a surface dielectric barrier discharge (DBD) driven inN2 with small NO admixtures in a modulated ac regime. Metastable species were detectedboth in a thin surface plasma layer and in the space afterglow through plasma induced emissionand optical-optical double resonance laser induced fluorescence, respectively. Basically, theobserved N2(A 3
+u ) species follow the discharge current oscillations in the plasma layer,while they evolve on a millisecond time scale due to diffusion through the space afterglow.Emission spectrometry provides a rough estimation of average metastable concentration in athin surface plasma layer ~ 9 × 1013 cm-3. Fluorescence measurements then allow an upperestimation of metastable concentration in the space afterglow
1 × 1012 cm-3.

Experiments on the interaction of metal targets with a Nd:YAG laser beam ((Formula presented.) = 1,064 nm, intensity (Formula presented.)-(Formula presented.)) are carried out in a finite Nitrogen pressure environment. The observed (Formula presented.) spectra are unambiguous evidence of the existence of an ionization and excitation source, arriving at the observation volume prior to the plume. Such a source can be either prompt electrons or VUV radiation. The analysis reveals that the prompt electron interpretation requires energies in excess of 1 keV, incompatible with any acceleration mechanisms relevant for such laser intensities. On the other hand, VUV radiation is sufficiently strong to explain the observed spectra. © 2014 Springer-Verlag Berlin Heidelberg.

The wide range of applications of cold plasmas originates from their specialcharacteristic of being a physical system out of thermodynamic equilibrium. This propertyenhances its reactivity at low gas temperature and allows to obtain macroscopic effectswith a moderate energy consumption. In this review, the basic concepts of non-equilibriumin ionized gases are treated by showing why and how non-equilibrium functions of thedegrees of freedom are formed in a variety of natural and man-made plasmas with particularemphasis on the progress made in the last decade. The modern point of view of a molecularbasis of non-equilibrium and of a state-to-state kinetic approach is adopted.Computational and diagnostic techniques used to investigate the non-equilibrium conditionsare also surveyed. Graphical abstract: [Figure not available: see fulltext.]

We have measured OH densities by time-resolved broad-band absorption spectroscopy in a He-H2O-O2 dielectric barrier discharge, with the aim to investigate the effect of water and oxygen addition on OH production. The results are examined in the light of literature data, in particular model calculations of reactive oxygen species (ROS) concentrations in a rf discharge with similar gas mixtures. Although relevant to different systems, the comparison is nevertheless interesting. At low water content and no oxygen a fair agreement is found with the model. The observed dependence on water and oxygen, at almost constant discharge power, is instead found to be smoother than the calculated one, probably because in our discharge the power invariance does not imply a constant production of OH

We report results of a novel time-resolved broad-band absorption spectroscopy experiment for OH density measurement applied to a pulsed dielectric barrier discharge in Ar/H2O mixtures. The measurement is aimed at the calibration of our previous OH LIF measurements in the same discharge. The apparatus is simple and cheap, being based on a UV LED light source and a non-intensified, non-cooled, gateable linear CCD array as a detector. The set-up is capable of ruling out both medium/long-term drifts of the UV source and of the discharge, and discharge emission from the measurement. Performances of the set-up are discussed, together with possible improvements for its use as a standalone technique.

We have measured OH densities by time-resolved broad-band absorption spectroscopy in a He-H2O-O-2 dielectric barrier discharge, with the aim to investigate the effect of water and oxygen addition on OH production. The results are examined in the light of literature data, in particular model calculations of reactive oxygen species (ROS) concentrations in a rf discharge with similar gas mixtures. Although relevant to different systems, the comparison is nevertheless interesting. At low water content and no oxygen a fair agreement is found with the model. The observed dependence on water and oxygen, at almost constant discharge power, is instead found to be smoother than the calculated one, probably because in our discharge the power invariance does not imply a constant production of OH.

Laser induced fluorescence measurements of the rate coefficients of N+2 (B 2
+u , v = 0)collision quenching by N2 and O2 are presented. The values of (8.84 ± 0.37) × 10-10 cm3 s-1and (10.45 ± 0.45) × 10-10 cm3 s-1 have been found for N2 and O2, respectively. The presentresults agree well with the literature data obtained by selective methods for ion B stateexcitation. The data are discussed in the framework of the spectroscopic evaluation of thereduced electric field in electrical discharges at high pressure, which makes use of the secondpositive system and first negative system emissions of nitrogen.

Optical - mainly spectroscopic - diagnostics applied to atmospheric pressure discharge arediscussed in this contribution, with emphasis on the aspects that are peculiar of the environment inwhich they are carried out, namely the time and space resolution needed and the collisionprocesses in electronic states. A survey of the state of the arts is given , with a detailed descriptionof our contribution to the field.

Our recent work on optical emission spectroscopy (OES) and laser-induced fluorescence (LW) applied to dielectric barrier discharge (DBD) at atmospheric pressure (ATP) is reported. The interplay of discharge kinetics and diagnostic techniques features is evidenced, with emphasis on collision processes that, with their variety of state-to-state characteristics and rate constants, determine the observation of a peculiar collision-dominated non-equilibrium system.

In the plasma dry reforming, CH4 and CO2 are dissociated by electron collisions producing fragments that in turn react to form new molecules. While the production of CO, H2, and small hydrocarbons has been investigated extensively, much less is known about the plasma synthesis of oxygenated molecules, which is the target of the present work. In addition to experimental results, gas phase reactions for the production of acetic, formic and propanoic acid were investigated by DFT calculations.

In recent years, the application of plasma actuators in different engineering fields was considered particularly interesting. It was successfully applied for the cold flow control in aero engines and turbo-devices. One important application concerns the use of non-equilibrium plasma for plasma-assisted ignition and combustion control. The reduction of nitric oxides (NOx) in aircraft engines, gas turbines, or internal combustion engines has become a major issue in the development of combustion systems. A way to reduce the NOx emissions is to burn under homogenous lean conditions. However, in these regimes the flame becomes unstable and it leads to incomplete combustion or even extinction. Thus, the major issue becomes to stabilize the flame under lean conditions. In this context the present work aims to demonstrate the possibility to increase the combustion efficiency of a lean flame through the use of nanosecond repetitively pulsed plasma (NRPP). A NRPP produced by electric pulses with amplitude up to 40 kV, pulse rise time lower than 4 ns and repetition rate up to 3.5 kHz has been used to stabilize and improve the efficiency of a lean non premixed methane/air flame in a non-premixed Bunsen-type burner. The burner is optically accessible permitting the imaging acquisitions of the flame region. The flame behavior was acquired using a high rate CCD camera in order to capture the differences between the baseline conditions and the actuated cases. Moreover a post-processing technique showing the jagging of the flame in different conditions was applied to evaluate the changes occurring in presence of plasma actuation in term of flame area weighted respect to the luminosity intensity. It was shown that the plasma significantly allows stabilizing the flame under lean conditions where it would not exist without plasma.

Laser induced fluorescence is intensively used for the detection of OH in many atmospheric pressure discharge devices. At this pressure, a quantitative knowledge of the collision phenomena in the upper excited state is critical. Here we report the measurement at T = 300 K of a set of rate constants of electronic quenching and vibrational relaxation of the OH(A(2) Sigma(+), upsilon' = 0, 1) electronic state, by collision with N-2, O-2, H2O, CO2, CO, H-2, D-2, CH4, C2H2, C2H4, C2H6. These are the main gases in applications like plasma medicine, hydrocarbons reforming and CO2 conversion. Available literature data are revisited, and new data are added, mostly relevant to upsilon' = 1 quenching and vibrational relaxation.

We have investigated the treatment of a mixture of fatty acid methyl esters by an RF-plasma jet with He-H2O and He-O2 gas feed. We have measured the concentration of the hydroxyl radical in the jet by laser induced fluorescence, looking for correlation with the lipid reactivity. We have detected four product families, whose yields increase with the OH exposure: volatile products, polymerization products, reduced chains, and oxidized chains. Theoretical calculations give insights on the radical attack to the lipid chain and show that none of the products can be attributed exclusively to reactions with OH. Therefore, the observed reactant conversion as function of the OH exposure must be interpreted as a qualitative relationship with the total amount of radical species present in the plasma jet.

The interplay between plasma processes and the biological environment is a long and intriguing story that spans different applications, from surface modification of biomaterials to the direct interaction of plasma with cells. This makes plasma processes very powerful tools in such biomedical fields as tissue engineering and sterilization, which are much different than the typical field in which plasmas are used. In vitro cell culture experiments represent the best way to fully understand the more subtle and fundamental interactions between the chemical species produced by glow discharge and cells. Among the different kind of cells that can be used, cell lines allow high reproducibility and control of results. This article reviews 3 main items, ranging from low-pressure plasma modifications of 2-and 3-dimensional materials to dielectric barrier discharges directly on cells, with respect to the authors' scientific work.

Cells positioned at the bottom of a Petri dish were exposed, in a home-made plasma source, to pulsed Dielectric Barrier Discharges operated in air in order to investigate the effect of the plasma species on their viability and growth. Processes with different number of pulse, respectively 1,3,9 and 27 pulse, were performed to study the influence on viability and cell growth of two different cell lines, Saos 2 and NHDF. Atmospheric air discharges applied on the two selected cell lines have shown an effect strongly dependent on cell type. At certain doses we have measured increased activity of the NHDF fibroblasts cell line. On the other side, an inhibition of cell adhesion and growth on the Saos 2 osteoblastoma cell line, directly dependent on the plasma doses, was clear. This study shows that by properly tuning the dose of exposure of cells to air plasma it is possible to induce both positive and negative effects on cell growth, that would be useful in several branches of Medicine. © 2012 Materials Research Society.

In this study, the charge trapping effect in alumina dielectric surfaces has been deeplyinvestigated by means of a dedicated dielectric barrier discharge apparatus in differentdischarge regimes and gas mixtures. This work further validates our previous findings in thecase of air discharges in a filamentary regime. Long lasting charge trapping has beenevidenced by ex situ thermoluminescence characterizations of alumina dielectric barrier platesexposed to a plasma. The density of trapped surface charges was found to be higher in theglow discharge with respect to pseudo-glow and filamentary regimes, and for all regimes theminimum trap activation temperature was 390K and the trap energy was less than or around1 eV. This implies that in the case of glow discharges a higher reservoir of electrons is present.Also, the effect was found to persist for several days after running the discharge.