Claire Douat

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.

Interactions Between Two Counterpropagating Plasma Bullets

Cold plasma jets produced by pulsed discharges have recently attracted a lot of attention because of their unusual physical properties enabling the development of new applications, particularly for plasma medicine. ICCD pictures of these jets revealed that they were not continuous plasmas but composed of plasma bullets propagating at very high velocity. In this paper, pictures of the interaction of two counterpropagating bullets have been taken to give insights into the physics of the bullets and to clarify which kind of models could be relevant to describe the bullet properties.

Space-time resolved density of helium metastable atoms in a nanosecond pulsed plasma jet: influence of high voltage and pulse frequency

Using tunable diode laser absorption spectroscopy, the spatio-temporal distributions of the helium He(23 S1) metastable atoms’ density were measured in a plasma jet propagating in ambient air. The plasma jet was produced by applying short duration high voltage pulses on the electrodes of a DBD-like structure, at a repetition rate in the range 1–30 kHz. In addition to the metastable density, the spatial distribution of helium 587 nm emission intensity was also investigated to give insight into the excitation mechanisms of the He(33 D) excited state inside the dielectric tube, in which no laser measurement can be performed. It is demonstrated that the shape of the radial distribution of helium He(23 S1) metastable atoms strongly depends on the polarity of the applied voltage and on the repetition frequency. For positive applied voltages, a dramatic constriction of the excited species production is observed whenever the pulse repetition frequency is higher than 6 kHz, and the voltage higher than 5 kV. This shrinking of the jet structure induces an increase by one order of magnitude of the metastable atoms’ density in the jet centre which reaches values as high as 1014 cm−3. Beyond a critical distance, associated to a transition between a positive streamer and a negative one, the distribution of the excited atoms gets back to an annular structure. For the negative polarity, no shrinking effect correlated to the pulse repetition frequency was observed. The on-axis constriction of the excited species for the high repetition rate and positive polarity is attributed to a memory effect induced by the negative ions, having a lifetime of hundreds of microseconds, left between successive pulses at the periphery of the helium gas flow.

Production of nitric/nitrous oxide by an atmospheric pressure plasma jet

Absolute densities of nitrous species were studied in an atmospheric pressure RF plasma jet. The measurement of NO and N2O densities has been performed mainly by means of ex situ quantum-cascade laser absorption spectroscopy via a multi-pass cell in Herriot configuration. The dependence of the species production on individual parameters such as power, flow and oxygen, nitrogen and water admixture is shown. NO and N2O densities are found to increase with absorbed power, while an increase in the gas flow induces a decrease of these densities due to a reduction in residence time of the gas in the plasma. Actually, a change of these two parameters, absorbed power and gas flow, induces a variation of energy density. The higher energy density, the higher NO and N2O densities. The NO and N2O densities are strongly gas mixture dependent. A change of that parameter allows to choose between a NO-rich or a N2O-rich regime. NO and N2O densities increase as a function of the N2 admixture, while increasing oxygen, above a minimum value, reduces the densities of both NO and N2O. When adding water instead of oxygen to the gas mixture the reduction in the NO density is much less. For maximal NO and N2O formation a ratio of about He/N2/O2 = 99.5/0.36/0.07 is found to be the most efficient in the μ-APPJ. However, it was found that the absorbed power in the plasma always reduces with increasing admixtures. The validation of the results obtained with quantum-cascade absorption spectroscopy with mass spectrometry shows how the two measurement techniques can complement each other. Finally a comparison of our results and others works is presented.

Spatio-Temporally Resolved Mapping of Helium Metastable Density in an Atmospheric Pressure Plasma Jet

Tunable diode laser absorption spectroscopy was used to measure the spatio-temporal evolution of helium metastable atoms He (23S) in an atmospheric plasma jet. The production of these atoms is clearly correlated to the propagation of an ionization wave, the so-called plasma bullet. Whatever the applied voltage, the spatial structure of the density rapidly evolves from a donut shape to a conical form as the plasma propagates. For applied voltages higher than 4.5 kV, a complex transient structure appears at the tip of the jet.

Cavity-hollow cathode-sputtering source for titanium films

A cavity-hollow cathode was investigated as low-cost sputtering source for titanium. An argon discharge is produced inside a hollow cathode consisting of two specifically formed disks of titanium. An additional cavity further enhances the pendulum effect of the electrons. Measurements with small Langmuir probes yielded evidence for the formation of a space charge double layer above the cathode. The sputtered atoms form negatively charged clusters. After further acceleration by the double layer the clusters impinge on the substrates. Titanium thin films were produced on highly oriented pyrolytic graphite. The films were investigated by a scanning tunnel microscope and X-ray photoelectron spectroscopy.

Measurement of helium metastables in micro plasma jet

Summary form only given. Recently, interest in plasma micro-jet at atmospheric pressure has increased due to their unique capabilities and novel applications, such as biomedicine1, thin film disposition2 and chemical decontamination3. In this work we report on the measurement, by laser absorption spectroscopy, of the distribution of the helium metastable state densities inside both the main discharge and the plasma jet propagating in free atmosphere.