L Magne

Transient phenomena in closed electron drift plasma thrusters : insights obtained in a french cooperative program

This paper presents some aspects of the research developed in the frame of a coordinated program launched in France in 1996 and devoted to plasma thrusters for space technologies. Relevant results of physical studies have been selected from the literature with the addition of recent original results. The thrusters within the scope of this research are diagnostic equipped versions of industrial realizations, in a thrust level range of 0.1 N and electrical power 1.5 kW. The optical and electrical diagnostics concern studies of the thruster plasma and of the thruster plume. Transient phenomena in these two regions, related to discharge current fluctuations or oscillations on a typical time scale of 40 µs, have been space-time characterized. This has been achieved by developing a large panel of diagnostics including RFEA, Langmuir probes, OES, fast camera imaging and electron drift Hall current probe. They lead to a coherent representation of these phenomena , in rather good qualitative agreement with 1D modelling. But they emphasize also the importance of 2D effects. Insights obtained through combined LIF (on Xe+ ions) and OES diagnostics are also presented. They concern the ionization-acceleration region in the thruster plasma, where intrusive diagnostics are disturbing in nature, and open a new step for a significant improvement of the detailed understanding of these thrusters. Such improvements are required when looking at the final goal of a predicable modelling simulation able to help the design of optimized structures at various thrust levels, in spite of the important work devoted to these devices in the former USSR and by Russian teams in Moscow at the MIREA, MAI-RIAME and KOURCHATOV Institutes.

Atomic oxygen recombination on fused silica: modelling and comparison to low-temperature experiments (300 K)*

This work is devoted to the study of atomic oxygen recombination on a glass surface, mainly in connection with atomic sources development. In this paper we present a non-stationary model for atomic oxygen recombination on a fused silica surface. Kinetics equations for oxygen atoms, taking into account heterogeneous reactions between gaseous atoms and the surface (Eley-Rideal mechanisms), as well as homogeneous processes involving surface migration of adsorbed species (Langmuir-Hinshelwood mechanisms), are solved. Surface reaction coefficients are calculated, and the choice of numerical values for surface parameters is discussed. The solution to the equations is compared to our previous experiments concerning the influence of the surface state on atomic recombination. An estimation is made of surface reaction coefficient values.

Production and reactivity of the hydroxyl radical in homogeneous high pressure plasmas of atmospheric gases containing traces of light olefins

A photo-triggered discharge has been used to study the production kinetic mechanisms and the reactivity of the hydroxyl radical in a N2/O2 mixture (5% oxygen) containing ethane or ethene for hydrocarbon concentration values in the range 1000?5000?ppm, at 460?mbar total pressure. The discharge (current pulse duration of 60?ns) has allowed the generation of a transient homogeneous non-equilibrium plasma, and the time evolution of the OH density has been measured (relative value) in the afterglow (up to 200??s) by laser induced fluorescence (LIF). Experimental results have been explained using predictions of a self-consistent 0D discharge and plasma reactivity modelling, and reduced kinetic schemes for OH have been validated. It has been shown that recombination of H- and O-atoms, as well as reaction of O with the hydroperoxy radical HO2, plays a very important role in the production of OH radicals in the mixture with ethane. H is a key species for production of OH and HO2 radicals. As for ethane, O, H and HO2 are key species for the production of OH in the case of ethene, but carbonated radicals, following the partial oxidation of the hydrocarbon molecule by O, also play a non-negligible role. The rate constant for O- and H-atom recombination has been estimated to be 3 ? 10?30?cm6?s?1 at near ambient temperature, consistent with LIF measurements on OH for both mixtures with ethane and ethene.

Kinetic of the NO removal by nonthermal plasma in N2/NO/C2H4 mixtures

NO removal is studied in N2/NO and in N2/NO/C2H4 mixtures through time-resolved laser-induced fluorescence in the afterglow of a pulsed homogeneous discharge. NO density measurements are compared with predictions of a 0D model on a large range of parameter values, such as the specific deposited energy and the ethene initial concentration. It is shown that dissociation of NO through collision with the N2(a′1Σu−) state play the main part in the NO removal kinetic. Moreover, quenching of N2(a′1 Σu−) by C2H4 leads to a drastic decrease of the NO removal efficiency when ethene is added to N2/NO. The determined rate coefficient value for the quenching mechanism is (4±2)×10−10 cm3 s−1.

Influence of water on NO removal by pulsed discharge in N2/H2O/NO mixtures

Kinetic mechanisms of NO removal are studied in N2/NO and N2/H2O/NO gas mixtures. A very short duration (60?ns) photo-triggered discharge is used to create a homogeneous plasma at a total pressure between 230 and 460?mbar. Measurements of the NO density are performed in the afterglow by time-resolved laser-induced fluorescence, for a time scale between 2 and 200??s after the current pulse excitation. Plasma homogeneity allows effective comparison between experimental results and predictions of a fully self-consistent discharge and kinetic modelling. It is shown that the NO removal efficiency is mainly determined through loss mechanisms balance for nitrogen metastable singlet states. In the absence of H2O, NO is in great part dissociated owing to collisions with singlet states. When water vapour is added, these states are destroyed through collisions with H2O with a rate constant k = (3.0?1.5)?10-10?cm3?s-1, and it leads to the decrease of the NO removal efficiency. This reaction is invoked for the first time.

Atomic oxygen recombination on fused silica : experimental evidence of the surface state influence

The time post discharge of a low-pressure pulsed dc discharge in pure oxygen is used to investigate the atomic oxygen recombination on fused silica surface. With the intention of studying this recombination for different surface states, we perform before each pulsed experiment a wall treatment by means of dc discharges under different experimental conditions. Then, we monitor the decrease of the atomic oxygen in time post discharge by time resolved VUV resonant absorption spectroscopy. We have shown that it is possible to obtain for a given wall treatment, a pulse after pulse variation of this decrease. We have attributed this variation to a filling of the chemisorption sites. Finally, we have determined the surface reaction probability of atomic oxygen on fused silica surface and we have compared it to published values.

Singlet oxygen production in a microcathode sustained discharge

The authors report experimental results showing that high yields of singlet oxygen O2(aΔg1) can be generated in a three-electrode microcathode sustained discharge (MCSD) configuration. This configuration consists of a microhollow cathode discharge (MHCD) acting as a plasma cathode to sustain a stable glow discharge between the MHCD and a third, planar electrode placed at a distance of 8mm. Experiments were performed in pure oxygen and in mixtures of oxygen with rare gases (He or Ar) at pressures up to 130Torr. O2(aΔg1) relative yields of 7.6% were measured 23cm downstream in the afterglow of the MCSD discharge.

Experimental study and modelling of a low-pressure N2-O2 time afterglow

The time afterglow of a pulsed discharge is used to investigate the neutral-particle kinetics in N2-O2 low-pressure mixtures. The pressure is in the range 0.5-2 Torr at 300 K and the mixture composition in the range 0-20% of oxygen. Time-resolved emission spectroscopy on N2(B), N2(C), NO(A) and NO(B) is employed to monitor energy transfers involving the metastable state N2(A). The influence of N(4S) and O(3P) atoms on the kinetics is accurately treated using absolute concentration measurements by time-resolved absorption spectroscopy in the VUV range. It is shown that by an appropriate choice of the discharge repetition rate, the vibrational excitation of N2(X) can be neglected. A chemical model, containing few unknown parameters, is developed in order to fit the experimental fluorescences. The NO(X) kinetics are investigated and its absolute concentration is deduced. Furthermore, it is shown that the N2(A) density is probably higher for pulsed discharges than for stationary low-pressure DC discharges.

OH kinetic in high-pressure plasmas of atmospheric gases containing C2H6 studied by absolute measurement of the radical density in a pulsed homogeneous discharge

The absolute value of the hydroxyl radical was measured in the afterglow of an homogeneous photo-triggered discharge generated in N2/O2/H2O/C2H6 mixtures, using a UV absorption diagnostic synchronized with the discharge current pulse. Measurements show that OH is efficiently produced even in the absence of water vapour in the mixture, and that the radical production is closely linked to the degradation kinetic of the hydrocarbon. Experimental results for dry mixtures, both for OH and for the removal of ethane in the discharge volume, are compared with predictions of a self-consistent 0D discharge and the kinetic model. It appears that the oxidation reaction of the ethane molecule by O(3P) atoms plays a minor role. Dissociation of the hydrocarbon through quenching collisions of the nitrogen metastable states are of great importance for a low oxygen concentration value. Also, the oxidation of ethane by O(1D) cannot be neglected at high oxygen concentration. The most probable exit channel for N2 states quenching collisions by ethane is the production of ethene and hydrogen molecules. Afterwards C2H4 should be dissociated to produce H and H2. As previously suggested from the study of the OH density time evolution in relative value, the recombination of H and O atoms appears as a main process for the production of OH in transient low temperature plasmas generated in atmospheric gases at high pressure. Another important reaction is the reduction of the HO2 radical by O, this radical coming from the addition of H on the oxygen molecule. H atoms come from numerous kinetic processes, amongst which is the dissociation of ethene.

Sub-nanosecond time resolved light emission study for diffuse discharges in air under steep high voltage pulses

Pin-to-plane discharges in centimetre air gaps and standard conditions of pressure and temperature are generated under very high positive nanosecond scale voltage pulses. The experimental study is based on recordings of sub-nanosecond time resolved and Abel-processed light emission profiles and their complete correlation to electrical current waveforms. The effects of the voltage pulse features (amplitude between 20 and 90 kV, rise time between 2 and 5.2 ns, and time rate between 4 and 40 kV ns−1) and the electrode configuration (gap distance between 10 and 30 mm, pin radius between 10 and 200 µm, copper, molybdenum or tungsten pin material) are described. A three time period development can be found: a glow-like structure with monotonic light profiles during the first 1.5 ns whose size depends on time voltage rate, a shell-like structure with bimodal profiles whose duration and extension in space depends on rise time, and either diffuse or multi-channel regime for the connection to the cathode plane according to gap distance. The transition of the light from monotonic to bimodal patterns reveals the relative effects and dynamics of streamer space charge and external laplacian field. A classical 2D-fluid model for streamer propagation has been used and adapted for very high and steep voltage pulses. It shows the formation of a strong space charge (streamer) very close to the pin, but also a continuity of emission between the pin and the streamer, and electric fields higher than the critical ionization field (28 kV cm−1 in air) almost in the whole gap and very early in the discharge propagation.