Thibault Darny

Rare gas flow structuration in plasma jet experiments

Modifications of rare gas flow by plasma generated with a plasma gun (PG) are evidenced through simultaneous time-resolved ICCD imaging and schlieren visualization. The geometrical features of the capillary inside which plasma propagates before in-air expansion, the pulse repetition rate and the presence of a metallic target are playing a key role on the rare gas flow at the outlet of the capillary when the plasma is switched on. In addition to the previously reported upstream offset of the laminar to turbulent transition, we document the reverse action leading to the generation of long plumes at moderate gas flow rates together with the channeling of helium flow under various discharge conditions. For higher gas flow rates, in the l min−1 range, time-resolved diagnostics performed during the first tens of ms after the PG is turned on, evidence that the plasma plume does not start expanding in a laminar neutral gas flow. Instead, plasma ignition leads to a gradual laminar-like flow build-up inside which the plasma plume is generated. The impact of such phenomena for gas delivery on targets mimicking biological samples is emphasized, as well as their consequences on the production and diagnostics of reactive species.

New insights on the propagation of pulsed atmospheric plasma streams: From single jet to multi jet arrays

Atmospheric pressure plasma propagation inside long dielectric tubes is analyzed for the first time through nonintrusive and nonperturbative time resolved bi-directional electric field (EF) measurements. This study unveils that plasma propagation occurs in a region where longitudinal EF exists ahead the ionization front position usually revealed from plasma emission with ICCD measurement. The ionization front propagation induces the sudden rise of a radial EF component. Both of these EF components have an amplitude of several kV/cm for helium or neon plasmas and are preserved almost constant along a few tens of cm inside a capillary. All these experimental measurements are in excellent agreement with previous model calculations. The key roles of the voltage pulse polarity and of the target nature on the helium flow patterns when plasma jet is emerging in ambient air are documented from Schlieren visualization. The second part of this work is then dedicated to the development of multi jet systems, using tw...

Numerical and experimental study of the dynamics of a s helium plasma gun discharge with various amounts of N 2 admixture

This paper presents a combined 2D numerical and experimental study of the influence of N 2 admixture on the dynamics of a He–N 2 discharge in the 10 cm long dielectric tube of a plasma gun setup. First, the comparison between experiments and simulations is carried out on the ionization front propagation velocity in the tube. The importance of taking into account a detailed kinetic scheme for the He–N 2 mixture in the simulations to obtain a good agreement with the experiments is put forward. For the μs driven plasma gun, the two-and three-body Penning reactions occurring in the plasma column behind the ionization front, are shown to play a key role on the discharge dynamics. In the experiments and simulations, the significant influence of the amplitude of the applied voltage on the ionization front propagation velocity is observed. As the amount of N 2 varies, simulation results show that the ionization front velocity, depends on a complex coupling between the kinetics of the discharge, the photoionization and the 2D structure of the discharge in the tube. Finally, the time evolution of axial and radial components of the electric field measured by an electro-optic probe set outside the tube are compared with simulation results. A good agreement is obtained on both components of the electric field. In the tube, simulations show that the magnitude of the axial electric field on the discharge axis depends weakly on the amount of N 2 conversely to the magnitude of the off-axis peak electric field. Both, simulations and first measurements in the tube or within the plasma plume show peak electric fields of the order of 45 kV·cm −1 .

Plasma jet-induced tissue oxygenation: potentialities for new therapeutic strategies

The lack of oxygen is a major reason for the resistance of tumor cells to treatments such as radiotherapies. A large number of recent publications on non-thermal plasma applications in medicine report cell behavior modifications and modulation of soluble factors. This in vivo study tested whether such modifications can lead to vascular changes in response to plasma application. Two in situ optical-based methods were used simultaneously, in real time, to assess the effect of non-thermal plasma on tissue vasculature. Tissue oxygen partial pressure (pO2) was measured using a time-resolved luminescence-based optical probe, and the microvascular erythrocyte flow was determined by laser Doppler flowmetry. When plasma treatment was applied on mouse skin, a rapid pO2 increase (up to 4 times) was subcutaneously measured and correlated with blood flow improvement. Such short duration, i.e. 5 min, plasma-induced effects were shown to be locally restricted to the treated area and lasted over 120 min. Further investigations should elucidate the molecular mechanisms of these processes. However, improvement of oxygenation and perfusion open new opportunities for tumor treatments in combination with radiotherapy, and for tumor blood vessel normalization based strategies.

Helical Plasma Propagation of Microsecond Plasma Gun Discharges

Atmospheric pressure helium plasma inside a dielectric capillary and in the plume may exhibit stable and fascinating helix patterns. Such plasma propagation mode is achieved over an extended range of pulse repetition and gas flow rates. A key role of electric field is inferred from voltage, and electromagnetic configuration perturbation impacts on the plasma pattern. Helical plasma propagation is suggested to represent a specific case of previously reported wall-hugging plasma propagation.

Electric field measurements during plasma jet operation on/in biological samples and tissues

Many recent experiments or treatments related to biological applications of cold plasmas have been recently performed using plasma jets. It concerns either applications in vitro or in vivo. The observed effect are most of the time attributed to the very rich chemistry generated by the interaction of the rare gas plasma plume with the surrounding environment constituted either from the ambient air or this last one in complex interaction with liquids at the interface with the target (nutritious medium for in vitro experiments, organ liquids for in vivo applications). Recent experiments performed on tissue oxygenation [1] or breast cancer treatments on immunocompetent mice [2] lead to the conclusion that probably the involved chemistry could not, alone, completely allow to describe the observed phenomena. This, especially under very soft treatment conditions, was suggesting possible triggering of some immune system chain processes, but also possible modifications in the microenvironment of tissue and tumors. In this context, there is still an unknown role of the electric field associated with the ionization front or generated in the environment of the plasma plume tip. In this work, we measured the electric field in real conditions of the GREMI plasma jet applied to various biological samples (tissues and liquids), in addition to free jet conditions, using a new device (bi-component EOP Kapteos probe) that allows time (ns) and space (1 mm3) resolved measurements in two radial and longitudinal directions. We will present data obtained along the discharge capillary together with measurements along and in the plasma plume and at different level in biological samples. Both in the capillary and the plume, high longitudinal induced electric fields have been measured just ahead of the apparition of a strong radial electric field in good agreement with Kushner model results. More interesting, is the measurement of a significant electric field presence rather deep in tissue of euthanatized animals (down to two millimeters) at levels that may affect cell permeability or behavior.

Unexpected Plasma Plume Shapes Produced by a Microsecond Plasma Gun Discharge

The plasma plume shapes produced at the outlet of a 1 mm inner diameter plasma gun capillary are investigated over an extended range of both neon flow rates and pulse repetition rate (prr). Various complex, but stable, plume shapes are observed. It appears first that for a given flow rate, the plume shape can be modulated with prr sweeping, revealing fascinating multidart shape when discharge is triggered at a fundamental frequency and up to the sixth harmonic. Second, reproducible similar plume patterns, either multidarts or conical shaped, can be obtained for given appropriate flow rate/prr parameter couples. These phenomena tend to show the effect of the force exchange between the neutral gas flow and the discharge species resulting in plume patterning.

Analysis of NO x production in an RF kINPen and a µs driven plasma gun

Summary form only given. NOx and associated acids are investigated with regards to their role for biomedical application. NOx are produced by two different plasma sources, a RF kINPen and a μS Plasma gun, both are cold atmospheric pressure plasma jets. The first one is powered by a radio frequency voltage [1, 2] of 1 MHz, the second one is a dielectric barrier discharge powered by a μs pulse voltage [3, 4]. For these two sources, NOx are produced by many complex chemicals reactions occurring in the interaction of the active plume with surrounding air. With the mid infrared quantum cascade laser absorption spectroscopy [1], we are able to measure the absolute density of NO2 and O3 produced by both plasma sources. Correlating the measurements in situ with optical emission spectroscopy, great differences in the excitation processes of fundamental species such as O, N2, OH, N2 and N2+ can be observed. With a parametrical study, varying the voltage and the pulse repetition frequency, we are able to modulate the NO2 densities production from a few tens to a few hundreds of ppb. Such parametric analysis also shows that large variations in the population of the different molecular bands of the NO gamma system, are associated with the plasma gun operation regime. These physical parameters play a major role in the NOx production and yield vital clues to discriminate major kinetic reactions involved in the NO production and in relation with future numerical simulations.