Vincent Puech

On atmospheric-pressure non-equilibrium plasma jets and plasma bullets

Atmospheric-pressure non-equilibrium plasma jets (APNP-Js), which generate plasma in open space rather than in a confined discharge gap, have recently been a topic of great interest. In this paper, the development of APNP-Js will be reviewed. Firstly, the APNP-Js are grouped based on the type of gas used to ignite them and their characteristics are discussed in detail. Secondly, one of the most interesting phenomena of APNP-Js, the ?plasma bullet?, is discussed and its behavior described. Thirdly, the very recent developments on the behavior of plasma jets when launched in a controlled environment and pressure are also introduced. This is followed by a discussion on the interaction between plasma jets. Finally, perspectives on APNP-J research are presented.

Collision cross sections and transport parameters in neon and xenon

A self-consistent set of electron impact cross sections is reported for 24 neon and 13 xenon levels. Semi-empirical expressions are used which allow cross section calculations for impact energies from excitation thresholds up to the relativistic energy range. Therefore, they are effective for modelling electric discharges as well as electron-beam produced plasmas in any mixtures containing neon and xenon atoms, such as excimer lasers. The calculated cross sections are in general agreement with the experimental data obtained in electron impact energy loss spectra experiments. In addition, the Boltzmann equation has been solved, in the hydrodynamic approximation, for reduced electric fields in the range 10-3-103 Td. The predicted transport parameters are in agreement with the recent experimental data except for the 1s5 neon level. For this state, the measured excitation coefficient conflicts with that calculated.

Analysis of the self-pulsing operating mode of a microdischarge

The self?pulsing regime of a microhollow cathode discharge in argon is reported. The plasma is generated inside the hole drilled in an anode?dielectric?cathode device. The hole dimension ranges from 200 to 400??m and the gas pressure ranges from 40 to 200?Torr. It is shown by optical spectroscopy and fast CCD imaging that the current pulse is related to a fast expansion of the plasma outside the microhole on the cathode backside. The pulse current duration ranges from 0.4 to 2??s depending on the gas pressure. The self-pulsing regime occurs at medium current range (0.1?1?mA). At lower current the discharge is steady and the plasma is confined inside the hole (abnormal regime); at higher current, the plasma is steady and the plasma expands outside the hole on the cathode backside. The self-pulsing frequency is a linear function of the averaged discharge current and decreases with the device capacitance. The dependence of the self-pulsing characteristics (frequency, light emission, power deposition, etc) on the gas pressure follows a Paschen-like law; this is interpreted in considering that the fast expansion of the plasma outside the hole is similar to a gas breakdown. A simple electrical model, using a bistable voltage-controlled variable resistor to simulate the evolution of the plasma impedance, provides qualitative results in good agreement with the experiments.

Dynamics of colliding microplasma jets

Because of their capabilities to generate plasmas that are not confined between electrodes, low-temperature plasma jets offer unique opportunities for applications such as material processing and biomedicine. The need to generate multiple jets in order to cover larger treatment areas has recently become desirable. However, the interaction between neighbouring jets is unavoidable. It is therefore crucial to elucidate the physical processes that occur between jets. In this paper we present the case of two counter-propagating jets generated by two DBD-based devices. We show that the plasma bullets emitted by the two jets interact with each other as soon as they leave their respective DBD device, resulting in a decrease in their velocities. The bullets do not actually meet but rather approach each other at a minimum approach distance. The location of the region of minimum approach is not midway between the nozzles of the jet devices but rather depends on the operating conditions. In addition, we discovered the emergence of a ?secondary? discharge exactly in the region of minimum approach. This discharge exhibits a pink glow, reminiscent of the pink afterglow occurring in some nitrogen discharges. Time-resolved spectroscopic measurements and current measurement analysis showed that the pink glow is a transient negative glow discharge that cannot be attributed to kinetic processes associated with re-excitation of nitrogen molecules. It is rather ignited by electrons accelerated from both jets towards the region of minimum approach. This process is found to be exactly timed with the measured current reversal.

Synergistic Effect of H2O2 and NO2 in Cell Death Induced by Cold Atmospheric He Plasma.

Cold atmospheric pressure plasmas (CAPPs) have emerged over the last decade as a new promising therapy to fight cancer. CAPPs’ antitumor activity is primarily due to the delivery of reactive oxygen and nitrogen species (RONS), but the precise determination of the constituents linked to this anticancer process remains to be done. In the present study, using a micro-plasma jet produced in helium (He), we demonstrate that the concentration of H2O2, NO2− and NO3− can fully account for the majority of RONS produced in plasma-activated buffer. The role of these species on the viability of normal and tumour cell lines was investigated. Although the degree of sensitivity to H2O2 is cell-type dependent, we show that H2O2 alone cannot account for the toxicity of He plasma. Indeed, NO2−, but not NO3−, acts in synergy with H2O2 to enhance cell death in normal and tumour cell lines to a level similar to that observed after plasma treatment. Our findings suggest that the efficiency of plasma treatment strongly depends on the combination of H2O2 and NO2− in determined concentrations. We also show that the interaction of the He plasma jet with the ambient air is required to generate NO2− and NO3− in solution.

Kinetic of the NO removal by nonthermal plasma in N2/NO/C2H4 mixtures

NO removal is studied in N2/NO and in N2/NO/C2H4 mixtures through time-resolved laser-induced fluorescence in the afterglow of a pulsed homogeneous discharge. NO density measurements are compared with predictions of a 0D model on a large range of parameter values, such as the specific deposited energy and the ethene initial concentration. It is shown that dissociation of NO through collision with the N2(a′1Σu−) state play the main part in the NO removal kinetic. Moreover, quenching of N2(a′1 Σu−) by C2H4 leads to a drastic decrease of the NO removal efficiency when ethene is added to N2/NO. The determined rate coefficient value for the quenching mechanism is (4±2)×10−10 cm3 s−1.

Relativistic electron-beam-produced plasmas. I. Collision cross sections and loss function in argon

The generalised oscillator strength formalism is used to calculate a complete and self-consistent set of collision cross sections for incident electron energies ranging from thresholds up to 10 MeV. Results are reported for about 40 states with an emphasis given on the ionisation processes. The study of the loss function points out the importance of the inner-shell ionisation, which consumes nearly as much energy as the excitation and the ionisation of the outer shell. Moreover, the authors show that the mean energy of the secondaries ejected from a given shell is about twice the ionisation potential of this shell. As a result, many energetic secondary electrons coming from inner-shell ionisation are present in the plasmas produced by relativistic electron beams.

TRIGGERED BREAKDOWN IN LOW-PRESSURE HOLLOW CATHODE (PSEUDOSPARK) DISCHARGES

Triggered breakdown in hollow cathode discharges in geometries similar to those used for pseudospark switches and pseudospark pulsed electron beams has been investigated experimentally and with a two‐dimensional model previously developed. A systematic study of the influence of the discharge conditions (applied voltage and pressure), geometry, and trigger conditions (trigger intensity and position) on the time to breakdown in helium is presented, and some data are also shown for argon. Excellent qualitative agreement is found between the model predictions and the experimental results. The relation between the time to breakdown and the geometrical distribution of injected charge is discussed, and the understanding gained from these model results is used to suggest guidelines for trigger optimization. Conditions wherein significant oscillations in the current—a ‘‘current quenching’’ effect—are observed in the prebreakdown current wave form are discussed.

DNA oxidation by singlet delta oxygen produced by atmospheric pressure microdischarges

Arrays of microcathode sustained discharges were developed for the production of singlet delta oxygen (SDO) at atmospheric pressure. SDO densities higher than 3.5×1016 cm−3 have been efficiently produced and transported over distances longer than 50 cm. These arrays appear to be an ideal tool for examining the reactivity of SDO with biological components. Experiments were performed indicating that SDO is able to oxidize 2′-deoxyguanosine, a DNA constituent. It is shown that the 4-OH-8-oxodGuo formation is proportional to the number of SDO molecules while other reactive species could also be involved in the production of the nucleosides dSp and dZ.

Influence of water on NO removal by pulsed discharge in N2/H2O/NO mixtures

Kinetic mechanisms of NO removal are studied in N2/NO and N2/H2O/NO gas mixtures. A very short duration (60?ns) photo-triggered discharge is used to create a homogeneous plasma at a total pressure between 230 and 460?mbar. Measurements of the NO density are performed in the afterglow by time-resolved laser-induced fluorescence, for a time scale between 2 and 200??s after the current pulse excitation. Plasma homogeneity allows effective comparison between experimental results and predictions of a fully self-consistent discharge and kinetic modelling. It is shown that the NO removal efficiency is mainly determined through loss mechanisms balance for nitrogen metastable singlet states. In the absence of H2O, NO is in great part dissociated owing to collisions with singlet states. When water vapour is added, these states are destroyed through collisions with H2O with a rate constant k = (3.0?1.5)?10-10?cm3?s-1, and it leads to the decrease of the NO removal efficiency. This reaction is invoked for the first time.