Emmanuel Odic

Production of active species in N2–O2 flowing post-discharges at atmospheric pressure for sterilization

A flowing afterglow of very pure molecular nitrogen at atmospheric pressure with admixture of controlled amounts (from some tens to some thousands ppm) of molecular oxygen is studied. For flows of 40 slm, spectroscopic measurements down the discharge allow us to estimate concentrations in atomic nitrogen and in singlet-S metastable oxygen atoms. With UV emission due to nitrogen oxide, all three reactive agents exhibit sporicidal effects, and their relative role is estimated.

R&D around a photoneutralizer-based NBI system (Siphore) in view of a DEMO Tokamak steady state fusion reactor

ince the signature of the ITER treaty in 2006, a new research programme targeting the emergence of a new generation of neutral beam (NB) system for the future fusion reactor (DEMO Tokamak) has been underway between several laboratories in Europe. The specifications required to operate a NB system on DEMO are very demanding: the system has to provide plasma heating, current drive and plasma control at a very high level of power (up to 150 MW) and energy (1 or 2 MeV), including high performances in term of wall-plug efficiency (η  >  60%), high availability and reliability. To this aim, a novel NB concept based on the photodetachment of the energetic negative ion beam is under study. The keystone of this new concept is the achievement of a photoneutralizer where a high power photon flux (~3 MW) generated within a Fabry–Perot cavity will overlap, cross and partially photodetach the intense negative ion beam accelerated at high energy (1 or 2 MeV). The aspect ratio of the beam-line (source, accelerator, etc) is specifically designed to maximize the overlap of the photon beam with the ion beam. It is shown that such a photoneutralized based NB system would have the capability to provide several tens of MW of D0 per beam line with a wall-plug efficiency higher than 60%. A feasibility study of the concept has been launched between different laboratories to address the different physics aspects, i.e. negative ion source, plasma modelling, ion accelerator simulation, photoneutralization and high voltage holding under vacuum. The paper describes the present status of the project and the main achievements of the developments in laboratories.

Treatment of Organic Pollutants by Corona Discharge Plasma

It is well known since several decades that an electrical discharge produces a plasma which may induce chemical reactions that otherwise would not occur. However, only few decades ago, scientists started to investigate the possibility to use the plasma generated by the electrical discharges for treating gaseous pollutants. The first pollutants to be considered were the nitrogen and sulfur oxides produced by the combustion of fossil fuels in the power stations. Later on, scientists investigated the applications of plasma generated by electrical discharges to the treatment of volatile organic compounds like organic solvents. Presently there is some interest in investigating the application of such plasmas to the treatment of pollutant in liquid phase. The catalyzing performances of the plasma depend on its characteristics, which depend on the type of discharge. The discharge itself depends on the shape of the electrodes, on the nature of the inter-electrode region, on the voltage and current waveforms used for producing the plasma. The paper will briefly report on the important steps of the researches in this field; it will propose some features, which should characterize the different plasmas and illustrate the different discharges investigated. Finally taking into account the most recent results, a tentative economic evaluation of the processes based on this technology will be discussed.

Corona discharges with water electrospray for Escherichia coli biofilm eradication on a surface

Low-temperature plasma (cold), a new method for the decontamination of surfaces, can be an advantageous alternative to the traditional chemical methods, autoclave or dry heat. Positive and negative corona discharges in air were tested for the eradication of 48-h Escherichia coli biofilms grown on glass slides. The biofilms were treated by cold corona discharge plasma for various exposure times. Water electrospray from the high voltage electrode was applied in some experiments. Thermostatic cultivation of the biofilm, and confocal laser scanning microscopy (CLSM) of the biofilm stained with fluorescent dyes were used for biocidal efficiency quantification. Up to 5 log10 reduction of bacterial concentration in the biofilm was measured by thermostatic cultivation after exposure to both corona discharges for 15min. This decontamination efficiency was significantly enhanced by simultaneous water electrospray through the plasma. CLSM showed that the live/dead ratio after treatment remained almost constant inside the biofilm; only cells on the top layers of the biofilm were affected. DAPI fluorescence showed that biofilm thickness was reduced by about 1/3 upon exposure to the corona discharges with electrospray for 15min. The biofilm biomass loss by about 2/3 was confirmed by crystal violet assay.

Atmospheric pressure argon surface discharges propagated in long tubes: physical characterization and application to bio-decontamination

Pulsed corona discharges propagated in argon (or in argon with added water vapor) at atmospheric pressure on the interior surface of a 49 cm long quartz tube were investigated for the application of surface bio-decontamination. H2O molecule dissociation in the argon plasma generated reactive species (i.e. OH in ground and excited states) and UV emission, which both directly affected bacterial cells. In order to facilitate the evaluation of the contribution of UV radiation, a DNA damage repair defective bacterial strain, Escherichia coli DH-1, was used. Discharge characteristics, including propagation velocity and plasma temperature, were measured. Up to ~5.5 and ~5 log10 reductions were observed for E. coli DH-1 bacteria (from 106 initial load) exposed 2 cm and 44 cm away from the charged electrode, respectively, for a 20 min plasma treatment. The factors contributing to the observed bactericidal effect include desiccation, reactive oxygen species (OH) plus H2O2 accumulation in the liquid phase, and UV-B (and possibly VUV) emission in dry argon. The steady state temperature measured on the quartz tube wall did not exceeded 29 °C; the contribution of heating, along with that of H2O2 accumulation, was estimated to be low. The effect of UV-B emission alone or in combination with the other stress factors of the plasma process was examined for different operating conditions.

Temporal evolution of temperature and OH density produced by nanosecond repetitively pulsed discharges in water vapour at atmospheric pressure

We report on an experimental study of the temporal evolution of OH density and gas temperature in spark discharges created by nanosecond repetitively pulsed discharges in pure water vapour at 475 K and atmospheric pressure. The plasma was generated by 20 kV, 20 ns pulses, at a repetition frequency of 10 kHz. The temperature was measured during the discharge by optical emission spectroscopy of the second positive system of N2, and between two discharges by two-colour OH-planar laser induced fluorescence (OH-PLIF) using two pairs of rotational transitions. Between two successive discharges, the relative density of OH was measured by OH-PLIF and was found to decay very slowly, with a 1/e decay time of about 50 µs. With the use of a chemical kinetics model, the OH density was placed on an absolute scale.

Progress in the development of the neutron flux monitoring system of the French GEN-IV SFR: Simulations and experimental validations

The neutron flux monitoring system of the French GEN-IV sodium-cooled fast reactor will rely on high-temperature fission chambers installed in the reactor vessel and capable of operating over a wide-range neutron flux. The definition of such a system is presented and the technological solutions are justified with the use of simulation and experimental results.

Cathode Surface Morphology Effects on Field Emission: Vacuum Breakdown Creation of Field Emitters

The effect of stainless steel cathode surface morphology on field electronic emission in high vacuum (~10-5 Pa) is studied. The surface rugosity is shown to affect the emission intensity; high-aspect ratio surface features lead to locally enhanced electric field, thereby increasing the field electronic emission. A breakdown in the vacuum, caused either by impact of charged dust particles or other impurities, or by overheating and vaporization of field emitters, can lead to cratering on the cathode surface. This cratering drastically changes the local surface rugosity, enhancing the local field and leads to greater emission intensity. Treatment of the cathode surface by glow discharge at higher pressure is known to reduce field emission by ionic bombardment and sputtering of emission sites; an image of such a glow-type discharge in the same apparatus used to study field emission is presented.

Investigation of water dissociation by Nanosecond Repetitively Pulsed Discharges in superheated steam at atmospheric pressure

Nanosecond Repetitively Pulsed (NRP) discharges in atmospheric pressure water vapor at 450 K are studied with time-resolved optical emission spectroscopy (OES). A 20-ns highvoltage pulse is applied across two pin-shaped electrodes at a frequency of 10 kHz, with an energy of 2 mJ per pulse. Emission of OH(A-X) as well as atomic states of O and H are observed. The emission of these species increases during the 20-ns pulse, then decreases. Then, after about 150 ns, we observe again a strong increase of emission of these species. To determine the gas temperature, we add a small amount (1%) of molecular nitrogen to the ow of water vapor. The rotational temperature measured from N 2(C 3 u - B 2 g) second positive system of N2 is measured and compared with the rotational temperature measure with the OH(A-X) transition. The electron density is obtained by the Stark broadening of the H emission line at 486 nm. The electron number density increases to about 6 10 15 cm 3 during the pulse, then decays to 10 14 cm 3 after 150 ns. But then, a surprising behavior occurs: the Full-Width at Half-Maximum (FWHM) of the H emission line increases again sharply, with no electric eld applied, up to 5 nm, and then decays slowly to 1 nm over the next microsecond.

Non-Thermal Plasma Treatment of Contaminated Surfaces: Remote Exposure to Atmospheric Pressure Dielectric Barrier Discharge Effluent

Iatrogenic infections due to contaminated medical devices are significant problem in the field of medicine, and have motivated the search for alternative surface disinfection/sterilization methods and technologies. During the last decade, a strong effort has been made in the field of non-thermal plasmas, including fundamental work from a physical, but also biological point of view. Non-thermal plasmas are used in industry for the modification of surface properties such as to improve wettability and adherence, and also for the deposition of thin films. The present work considers the treatment of surfaces contaminated by either bacteria or proteins with the effluent gas exiting from an atmospheric pressure dielectric barrier discharge. The discharge reactor consisted of a coaxial cylindrical geometry DBD reactor energized by a 30 kHz applied voltage. The effluent gas was used to treat surfaces contaminated with Escherichia coli (strain DH10B) or RNAse A (124 amino acids, 13.7kDa, known to be thermal-resistant). Results show that the decontamination of surfaces by the effluent gas from a humid argon DBD is effective, and that the effectiveness is greater the closer the biological samples are placed to the DBD source. The results also indicate that the mechanism of bacterial inactivation is based on a combination of stable oxidative species such as ozone and hydrogen peroxide as well as shorter lived species such as hydroxyl radical.