Christophe O. Laux

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.

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.

Simulation of shock tube radiation measurements with a Collisional-Radiative model

The prediction of the nonequilibrium radiative heat flux experienced by a spacecraft during Earth re-entry relies on an approach based on a combination of both numerical simulation and ground test experiments. This paper presents an analysis of the equilibrium and nonequilibrium radiation measured in the NASA Ames Electric Arc Shock Tube (EAST) facility for typical Moon or Mars return conditions. The selected conditions ranged from 10.54 to 11.17 km/s with a free-stream pressure of 0.1 Torr. The observed emitting spectral features, atomic lines of nitrogen and oxygen, present in the Vacuum Ultraviolet (VUV) and Near-Infrared (NIR) were analyzed. The electron concentration was inferred from Stark-broadened atomic lines of N and Hα. The post-shock intensity profile was compared to the prediction of the collisional-radiative model. It was shown that the observed nonequilibrium peak was governed by heavy particle impact excitation processes.

Application of linear sliding discharges for flow control: study of the energy coupling mechanisms

The pulsed sliding discharge can deposit energy along a 10-cm straight line in air in a very short duration. This fast heating process causes the formation of shock waves in the surrounding air and makes the pulsed sliding discharge a possible design for plasma actuation. In this paper we intend to link the mechanical impulse created by the discharge to the energy deposited within the plasma channel. For this purpose, detailed impulse measurements and shock wave visualization are performed over a range of test cases, and a snowplow model is implemented to relate the shock wave evolution with the energy deposited in the plasma. We demonstrate a linear dependency between the mechanical impulse imparted by the sliding discharge and the energy deposited within it, and we show the existence of two discharge regimes depending on the circuit inductance. The snowplow model describes accurately the shock wave structure in both regimes when 30% of the energy dissipated in the discharge is used to heat the gas. Additionally, this model is valid over the transition from strong shock to weak shock. It will be used in the future to compute the impulse generated by the discharge, and these calculations will be compared to the results presented in this paper.

Temporal and Spatial Evolution of OH Concentration in a Lean Premixed Propane-Air Flame Assisted by Nanosecond Repetitively Pulsed Discharges

Planar laser-induced fluorescence (LIF) has been used to study the evolution of the OH concentration in a weakly turbulent lean premixed flame both spatially and temporally, when applying Nanosecond Repetitively Pulsed (NRP) discharges. A 2-kW turbulent leanpremixed propane-air flame is stabilized by a cylindrical bluff-body. The discharge is created by voltage pulses of amplitude 7 kV, duration 10 ns, applied at a frequency of 30 kHz between two pin electrodes placed in the recirculation zone downstream of the bluffbody. The average electric power deposited by the plasma is up to 20 W, typically less than 1% of the thermal power of the flame. LIF images of the recirculation zone are recorded starting from 15 microseconds after discharge initiation. CH* chemiluminescence images of the flame are also recorded to trace the location of the flame. The results show that OH is produced in the plasma region and is then convected towards the shear layer. The key mechanism of the reduction of flame lift-off height by NRP discharges is the continuous ignition of the fresh combustible mixture in the shear layer, where it comes in contact with the active species produced by the NRP discharges.

Investigation of the Hydrodynamic Expansion Following a Nanosecond Repetitively Pulsed Discharge in Air

We report on an experimental study of the hydrodynamic expansion following a Nanosecond Repetitively Pulsed (NRP) discharge in atmospheric pressure air at 300 and 1000 K. The discharge is created by voltage pulses of amplitude 10 kV, duration 10 ns, applied at a frequency of 1-10 kHz between two pin electrodes. The electrical energy of each pulse is of the order of 1 mJ. We recorded single-shot schlieren images starting from 50 nanosecond after the discharge. The time-resolved images show the shock-wave propagation and the expansion of the heated gas channel. The temporal evolution of the gas temperature behind the shock front is estimated from the measured shock-wave velocity by using the Rankine-Hugoniot relations. The results show that the gas heats up by almost 1100 K within 50 ns after the pulse. This fast gas heating is consistent with a two-step mechanism involving electron-impact excitation of N 2 followed by the dissociative quenching of the excited electronic states of N 2 by O 2 .