G. Dilecce

Electron energy distribution functions under N2 discharge and post-discharge conditions: A self-consistent approach

Abstract Electron energy distribution functions (EDF) under N 2 discharge and post-discharge conditions have been calculated by self-consistently solving the Boltzmann equation, the vibrational master equation (including dissociation) and the kinetics of the most important electronic states of N 2 . The results show that the relaxation of EDF in the post-discharge is strongly linked to the residence time of N 2 in the discharge, which determines the initial conditions. In particular at low residence times the role of metastable states in affecting EDF through superelastic electronic collisions (SEC) overcomes the corresponding one from superelastic vibrational collisions (SVC). On the contrary, SVC dominate at long residence times, when a well-developed vibrational distribution has been built up by the discharge. At intermediate residence times both SVC and SEC affect the relaxation of EDF, which in general presents a non-monotonic behaviour.

LIF and fast imaging plasma jet characterization relevant for NTP biomedical applications

In the field of biomedical application, many publications report on non-thermal plasma jet potentialities for cell behaviour modifications in cancer treatment, wound healing or sterilization. However most previous plasma jet characterizations were performed when jets expend freely in air. Only recently has the influence of the targeted surface been properly considered. In this work, modifications induced by various types of targets, mimicking the biological samples, in the plasma propagation and production of hydroxyl radicals are evidenced through time-resolved intensified charge-coupled device imaging and laser-induced fluorescence (LIF) measurements. A LIF model, also specifically dedicated to estimate air and water penetration inside the jet, is used and proves to be well adapted to characterize the plasma jet under biomedical application conditions. It is shown that the plasma produced by the plasma gun counter-propagates after impinging the surface which, for the same operating parameters, leads to an increase of almost one order of magnitude in the maximum OH density (from ~2 × 1013 cm−3 for open-air propagation to ~1 × 1014 cm−3 for a grounded metal target). The nature of the target, especially its electrical conductivity, as well as gas flow rate and voltage amplitude are playing a key role in the production of hydroxyl radicals. The strong interplay between gas flow dynamics and plasma propagation is here confirmed by air and water distribution measurements. The need for a multi-diagnostic approach, as well as great care in setting up the in situ characterization of plasma jets, is here emphasized. Special attention must not only be paid to voltage amplitude and gas flow rate but also to the nature, humidity and conductivity of the target.

Excitation of ) and ) states in a pulsed positive corona discharge in , - and -NO mixtures

Time resolved multichannel emission spectroscopy has been applied to study emission in the 200-500 nm spectral range produced by a pulsed positive corona discharge. The discharge was driven in coaxial geometry by an HV power supply (100 kV/1 kA) at atmospheric pressure in nitrogen, and in and mixtures. Emission of NO- (-) and 2.PG (-) bands has been studied in order to trace the development of and electronic states during both discharge and post-discharge periods. Analysis of spectroscopic data indicates an evolution of the electron mean energy during the discharge pulse in the range 20-1 eV and post-discharge kinetics of the NO and electronic states which is predominantly controlled by , v = 0,1) metastable species.

Laser diagnostics of high-pressure discharges: laser induced fluorescence detection of OH in He/Ar?H2O dielectric barrier discharges

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–H2O, He–H2O 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.

Vibrational relaxation of N2(C, v) state in N2 pulsed rf discharge: electron impact and pooling reactions

Abstract The decay of Δv = −2 band sequence of N2 Second Positive emission system has been investigated in a N2 rf pulsed 500 Hz post discharge in the pressure range 0.1–3 Torr. The intensity of each band head of the sequence has been used to infer the N2(C, v′ = 0–4) relative vibrational distribution. The shape of the distribution varies significantly with the post discharge time and the pressure. A population inversion in the levels 0–1 and 2–3 characterizes the distributions measured at post discharge times higher than 100 μs. The distributions at the various post discharge times have been fitted by a model which takes into account three processes for the (C, v′) excitation: the electron impact from (X, v) states by fast electrons (a), from (A, w) state by slow electrons (b), and the pooling reaction N2(A, w) + N2(A, w′) (c). In the discharge the excitation is predominantly given by process (a). At 1 ms in the post discharge the electronic process (b) still survives, and, together with process (c), provides excitation to C state.

Time and space resolved analysis of N 2 (C 3 Π u ) vibrational distributions in pulsed positive corona discharge

Time- and space-resolved emission spectroscopy was applied to study the N2(C 3 � u → B 3 � g) and NO(A 2 � + → X 2 � r) emission induced by pulsed positive primary streamers in coaxial geometry generated in high-purity

{\rm N}_{2}(A\,^{3}\Sigma _{\rm u}^{+}) density measurement in a dielectric barrier discharge in N2 and N2 with small O2 admixtures

This paper deals with the measurement of metastable state density in a dielectric barrier discharge in nitrogen and nitrogen with small admixtures of oxygen, operating in a Townsend-like discharge regime. The measurement is made by optical?optical double resonance-LIF, calibrated by a method based on the measurement of the ratio of nitrogen second positive system and NO-? emissions, and of NO density by LIF. A metastable density of the order of 1013?cm?3 was found in a nitrogen diffuse discharge. Addition of small oxygen concentrations to the discharge drives a transition to the filamentary regime that appears to be caused not by a marked decrease of the metastable density in the discharge but rather by a considerable increase of its quenching rate. Such an increase, due to collision quenching by O2 and O, strongly reduces the survival of the metastable between two discharge pulses. These observations are consistent with the idea that the diffuse regime can be due to a space charge memory effect due to the nitrogen triplet metastable, which is cancelled by the introduction of oxygen in the gas feed.

Electron-energy distribution function measurements in capacitively coupled rf discharges

Measurements by Langmuir probes have been performed in a capacitively coupled, parallel‐ plate rf discharge of He and Ar with pressures ranging from 0.1 to 2 Torr and discharge power from 10 to 50 W at 27 MHz. The first part of this paper is devoted to a discussion of the preparation of the electrostatic probe technique, for which a rf discharge is well known to be a very hostile environment, and of its intrinsic limitations, in order to assess the reliability of the measurements. Results showing a marked non‐Maxwellian character of the electron‐energy distribution function are then presented and discussed in the light of the most recent theories about the sustainment of this kind of gas discharge.

Rate constants for deactivation of N2(A)v=2–7 by O, O2, and NO

Rate constants for N2(A,v) quenching by O, for levels v=2–7, by O2 for levels v=3–7, and by NO for levels v=2–4, have been measured in this work. This is the first data set for the quenching by O of vibrational levels v>3. The results of this work are based on the measurement by laser induced fluorescence (LIF) of N2(A,v) decay in a rf pulsed postdischarge, supported by LIF measurements of NO density. O atom density is deduced by N2(A,v=0,1) decay using the known rate constants of N2(A,v=0,1) quenching by O, O2, and NO. Finally, from appropriate scaling of LIF results for the various v levels, N2(A,v) vibrational distributions are deduced, showing a quite low vibrational excitation of the triplet metastable, characterized by an average Boltzmann vibrational temperature of the order of 2000–2500 K with some superimposed structures.

OH density measurement by time-resolved broad band absorption spectroscopy in an Ar–H2O dielectric barrier discharge

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.