Fabien Tholin

Simulation of the discharge propagation in a capillary tube in air at atmospheric pressure

This paper presents simulations of an air plasma discharge at atmospheric pressure initiated by a needle anode set inside a dielectric capillary tube. We have studied the influence of the tube inner radius and its relative permittivity ?r on the discharge structure and dynamics. As a reference, we have used a relative permittivity ?r = 1 to study only the influence of the cylindrical constraint of the tube on the discharge. For a tube radius of 100??m and ?r = 1, we have shown that the discharge fills the tube during its propagation and is rather homogeneous behind the discharge front. When the radius of the tube is in the range 300?600??m, the discharge structure is tubular with peak values of electric field and electron density close to the dielectric surface. When the radius of the tube is larger than 700??m, the tube has no influence on the discharge which propagates axially. For a tube radius of 100??m, when ?r increases from 1 to 10, the discharge structure becomes tubular. We have noted that the velocity of propagation of the discharge in the tube increases when the front is more homogeneous and then, the discharge velocity increases with the decrease in the tube radius and ?r. Then, we have compared the relative influence of the value of the tube radius and ?r on the discharge characteristics. Our simulations indicate that the geometrical constraint of the cylindrical tube has more influence than the value of ?r on the discharge structure and dynamics. Finally, we have studied the influence of photoemission processes on the discharge structure by varying the photoemission coefficient. As expected, we have shown that photoemission, as it increases the number of secondary electrons close to the dielectric surface, promotes the tubular structure of the discharge.

Experimental and numerical study of the propagation of a discharge in a capillary tube in air at atmospheric pressure

This paper presents an experimental and numerical study of a pulsed air plasma discharge at atmospheric pressure propagating in a capillary glass tube. In this work, we have compared the discharge structures and the axial propagation velocities of discharges. First, we have studied a needle-to-plane configuration without tube. For applied voltages in the range 7–18 kV, we have observed in experiments and in simulations that a plasma ball starts to develop around the needle tip. Then, for applied voltages less than 14 kV, in experiments, the discharge rapidly splits into several streamer channels with a main axial streamer. In simulations, we have computed only the main axial discharge. For applied voltages higher than 14 kV, in experiments and in simulations, we have observed that the discharge propagates with a cone shape in the gap. For all studied voltages, a good experiment/modelling agreement is obtained on the axial propagation velocity of the discharge, which increases with the applied voltage. Then, we have studied the propagation of discharges inside capillary tubes with radii in the range 37.5–300 µm. In experiments and simulations, we have observed that for small tube radius, the discharge front is quite homogeneous inside the tube and becomes tubular when the tube radius increases. Experimentally, we have observed that the velocity of the discharge reaches a maximum for a tube radius slightly less than 100 µm. We have noted that for a tube radius of 100 µm, the discharge velocity is three to four times higher than the velocity obtained without tube. This clearly shows the influence of the confinement by a capillary tube on the discharge dynamics. In this work, we have only simulated discharges for tube radii in the range 100–300 µm. We have noted that both in experiments and in simulations, the velocity of the discharge in tubes increases linearly with the applied voltage. As the radius of the tube decreases, the discharge velocity derived from the simulations slightly increases but is less than the experimental one. We have noted that the discrepancy on the discharge velocity between experiments and simulations increases as the voltage increases.

Simulation of the hydrodynamic expansion following a nanosecond pulsed spark discharge in air at atmospheric pressure

This paper presents 2D simulations of the dynamics of formation of a nanosecond spark discharge between two point electrodes in air at atmospheric pressure at 300 and 1000 K, the induced air heating and the following hydrodynamic expansion. As a first step, we have considered that 30% of the discharge energy instantaneously heats the ambient air. At the end of the voltage pulse, it is shown that the energy density and the air temperature distributions are non-uniform in the interelectrode gap. Rapidly after the nanosecond voltage pulse, a cylindrical shock wave is formed and propagates with a velocity very close to the speed of sound of the surrounding ambient air. Furthermore, the rapid dilatation of the hot channel formed on the discharge path is observed for t 1 µs, as in experiments. Then we have carried out a parametric study on the influence of the value of the fraction of discharge energy going to fast heating on the hydrodynamic expansion at 1000 K, assuming an instantaneously fast gas heating. For all values in the range of 15% to 60% studied in this work, we have observed a very similar gas dynamics. Then, we have considered that the nanosecond spark discharge heats the ambient air at 1000 K with a longer relaxation time of 1 µs, and in this case we have observed the propagation of a weak pressure wave and no dilatation of the hot channel on the discharge path. Finally, the comparison with experiments seems to validate the hypothesis that the 10 ns spark discharges studied in this work at 300 and 1000 K, significantly heat the ambient air on very short time-scales. (Some figures may appear in colour only in the online journal)

Influence of temperature on the glow regime of a discharge in air at atmospheric pressure between two point electrodes

This paper presents simulations of the dynamics of air discharges between two point electrodes at atmospheric pressure for two different gas temperatures 300 and 1000 K. Simulation results show that in the early stages of the glow regime, two streamer discharges propagate in the gap and form after their connection a conducting channel between electrodes. In a recent experimental work on nanosecond repetitively pulsed (NRP) discharges at 1000 K between two point electrodes with an interelectrode gap distance of 5 mm, it was found that a glow regime is obtained if the average electric field in the gap is at least equal to the breakdown field. Simulation results show that for the conditions studied in the experiments, the time of connection of both discharges is close to the 10 ns duration of the voltage pulse if the average electric field in the gap in the conduction phase is equal to the breakdown field. Under these conditions, a glow regime is obtained as a conducting channel has just the time to form between electrodes during the voltage pulse and no significant gas heating may occur. At 300 K, we found that a minimal value of the Laplacian electric field of 8–9 kV cm−1 at atmospheric pressure is necessary to have a stable propagation of the positive streamer without branching in the point-to-point geometry. Then, based on simulation results, we discuss the conditions of existence of the glow regime in NRP discharges at atmospheric pressure and 300 K in a 1 cm interelectrode gap.

Images of a Nanosecond Repetitively Pulsed Glow Discharge Between Two Point Electrodes in Air at 300 K and at Atmospheric Pressure

For many applications of atmospheric pressure plasmas, a crucial issue is to obtain glow discharges at 300 K. We have generated such plasmas with a nanosecond repetitively pulsed method. We present experimental and simulated optical emission images of the dynamics of the formation of the glow regime at the early stages of its development.

Simulation of the stable ‘quasi-periodic’ glow regime of a nanosecond repetitively pulsed discharge in air at atmospheric pressure

This paper presents simulations of the dynamics of nanosecond repetitively pulsed discharges between two point electrodes in atmospheric pressure air at 300 and 1000?K. At 300?K, the preionization left by successive discharges at the end of interpulses mainly consists of positive and negative ions with a density of about 109?cm?3 for a repetition frequency of 10?kHz. When photoionization is taken into account with a level of seed charges of about 109?cm?3, the dynamics and the characteristics of the discharge during a voltage pulse are shown to depend only weakly on the nature of negative seed charges (electrons or ions). At 1000?K, the preionization left by successive discharges at the end of interpulses consists of positive and negative ions and electrons with a density of about 1010?cm?3 for a repetition frequency of 10?kHz. Simulation results show that the dynamics and the characteristics of the discharge during a voltage pulse remain rather close whatever the preionization level considered in the range 109?1011?cm?3, corresponding to nanosecond repetitively pulsed discharges in the frequency range 1?100?kHz. The simulation of several consecutive nanosecond voltage pulses at a frequency of 10?kHz shows that, at 1000?K, the discharge can reach in a few voltage pulses a stable ?quasi-periodic? glow regime in agreement with experiments. Finally, the influence of an external air flow aligned with the electrode axis on the conditions to obtain a stable ?quasi-periodic? glow regime is discussed.

Propagation of an Air Discharge at Atmospheric Pressure in a Capillary Glass Tube: Influence of the Tube Radius on the Discharge Structure

The experimental and simulated optical emissions of an air discharge at atmospheric pressure propagating in a capillary glass tube are compared for two tube radii: 100 and 300 μm. For 300 μm, both experimental and calculated optical emissions show a tubular structure of the discharge. For 100 μm, the emission of the discharge appears to be more homogeneous radially.

Numerical modeling of a glow discharge through a supersonic bow shock in air

The interaction between a glow discharge and the bow shock of a Mach 3 air flow around a truncated conical model with a central spike is modeled, and comparison is made with prior experimental results. The KRONOS workflow for plasma modeling in flow fields, which has recently been developed at ONERA, was used for the modeling. Based on the quasi-neutral approximation, it couples hypersonic and reactive flow fields with electron chemistry, including the effect of non-Maxwellian electron energy distribution function. The model used for the discharge involves 12 species and 82 reactions, including ionization, electronic and vibrational excitation, and attachment. The simulations reproduce the main features of the discharge observed experimentally well, in particular, the very recognizable topology of the discharge. It was found from the simulations that behind the bow shock, in the afterglow, the negative ion flow ensures the electrical conduction and the establishment of the glow discharge. The influence of...