N2-H2 capacitively coupled radio-frequency discharges at low pressure. Part I. Experimental results: Effect of the H2 amount on electrons, positive ions and ammonia formation

The mixing of N2 with H2 leads to very different plasmas from pure N2 and H2 plasma discharges. Numerous issues are therefore raised involving the processes leading to ammonia (NH3) formation. The aim of this work is to better characterize capacitively-coupled radiofrequency plasma discharges in N2 with few percents of H2 (up to 5%), at low pressure (0.3-1 mbar) and low coupled power (3-13 W). Both experimental measurements and numerical simulations are performed. For clarity, we separated the results in two complementary parts. The actual one (first part), presents the details on the experimental measurements, while the second focuses on the simulation, a hybrid model combining a 2D fluid module and a 0D kinetic module. Electron density is measured by a resonant cavity method. It varies from 0.4 to 5 109 cm-3, corresponding to ionization degrees from 2 10-8 to 4 10-7. Ammonia density is quantified by combining IR absorption and mass spectrometry. It increases linearly with the amount of H2 (up to 3 1013 cm-3 at 5% H2). On the contrary, it is constant with pressure, which suggests the dominance of surface processes on the formation of ammonia. Positive ions are measured by mass spectrometry. Nitrogen-bearing ions are hydrogenated by the injection of H2, N2H+ being the major ion as soon as the amount of H2 is >1%. The increase of pressure leads to an increase of secondary ions formed by ion/radical-neutral collisions (ex: N2H+, NH4 +, H3 +), while an increase of the coupled power favours ions formed by direct ionization (ex: N2 +, NH3 +, H2 +).N. Carrasco acknowledges the financial support of the European Research Council (ERC Starting Grant PRIMCHEM, Grant agreement no. 636829). A. Chatain acknowledges ENS Paris-Saclay Doctoral Program. A. Chatain is grateful to Gilles Cartry and Thomas Gautier for fruitful discussions on the MS calibration. L.L. Alves acknowledges the financial support of the Portuguese Foundation for Science and Technology (FCT) through the project UID/FIS/50010/2019. L. Marques and M. J. Redondo acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2019

RPC-MIP observations at comet 67P/Churyumov-Gerasimenko explained by a model including a sheath and two populations of electrons

The response of the mutual impedance probe RPC-MIP on board Rosetta orbiter electrostatically modeled considering an unmagnetized and collisionless plasma with two Maxwellian electron populations. A vacuum sheath surrounding the probe was considered in our model in order to take the ion sheath into account that is located around the probe, which is immersed in the cometary plasma. For the first time, the simulated results are consistent with the data collected around comet 67P/Churyumov-Gerasimenko (67P), but strong discrepancies were identified with the previous simulations that neglected the plasma sheath around the probe. We studied the influence of the sheath thickness and of the electron populations. This work helps to better understand the initially unexpected responses of the mutual impedance probe that were acquired during the Rosetta mission. It suggests that two electron populations exist in the cometary plasma of 67P

Space Plasma Diagnostics and Spacecraft Charging. The Impact of Plasma Inhomogeneities on Mutual Impedance Experiments

International audiencePlasma diagnostic instruments are carried into space by satellites to measure in situ the properties of space plasmas. However, due to spacecraft charging, satellites perturb the surrounding plasma, that reacts by enveloping the platform and its instruments with a short scale, strongly inhomogeneous plasma region called plasma sheath. Such plasma sheath perturbs particles and electric field measurements performed onboard the satellite. Mutual Impedance (MI) experiments are a type of in situ diagnostic technique used in several space missions for the identification of the plasma density and the electron temperature. The technique is based on the electric coupling between emitting and receiving electric sensors embedded in the plasma to diagnose. Such sensors are surrounded by the plasma sheath, which is expected to affect the plasma response to MI emissions. In this context, we quantify for the first time the impact of the plasma sheath on the diagnostic performance of MI experiments. For this purpose, we use a full kinetic Vlasov-Poisson model to simulate numerically MI experiments in an inhomogeneous medium. For the first time, we explain the locality of MI measurements. We find that MI plasma density diagnostic are not affected by the plasma sheath (dn/n < 10%). The experiment retrieves the density of the plasma unperturbed by the satellite's presence. The electron temperature diagnostic, instead, presents significant perturbations if the plasma sheath is ignored. To mitigate such electron temperature errors, the plasma sheath needs to be included in the analysis of MI measurements

In situ space plasma diagnostics with finite amplitude active electric experiments: Non‐linear plasma effects and instrumental performance of mutual impedance experiments

International audienceMutual impedance experiments are a kind of plasma diagnostic techniques for the identification of the in situ plasma density and electron temperature. These plasma parameters are retrieved from mutual impedance spectra, obtained by perturbing the plasma using a set of electric emitting antennas and, simultaneously, retrieving using a set of electric receiving antennas the electric fluctuations generated in the plasma.Typical mutual impedance experiments suppose a linear plasma response to the electric excitation of the instrument. In the case of practical space applications, this assumption is often broken: low temperature plasmas, which are usually encountered in ionized planetary environments (e.g. RPC-MIP instrument onboard the Rosetta mission, RPWI/MIME experiment onboard the JUICE mission), force towards significant perturbations of the plasma dielectric.In this context, we investigate mutual impedance experiments relaxing, for the first time, the assumption of linear plasma perturbations: we quantify the impact of large antenna emission amplitudes on the (i) plasma density and (ii) electron temperature diagnostic performance of mutual impedance instruments.We use electrostatic 1D-1V full kinetic Vlasov-Poisson numerical simulations. First, we simulate the electric oscillations generated in the plasma by mutual impedance experiments. Second, we use typical mutual impedance data analysis techniques to compute the mutual impedance diagnostic performance in function of the emission amplitude and of the emitting-receiving antennas distance.We find the plasma density and electron temperature identification processes robust (i.e. relative errors below 5% and 20%, respectively) to large amplitude emissions for antenna emission amplitudes corresponding to electric-to-thermal energy ratios up to urn:x-wiley:21699380:media:jgra57540:jgra57540-math-0001

A charging model for the Rosetta spacecraft

International audienceContext. The electrostatic potential of a spacecraft, V S , is important for the capabilities of in situ plasma measurements. Rosetta has been found to be negatively charged during most of the comet mission and even more so in denser plasmas. Aims. Our goal is to investigate how the negative V S correlates with electron density and temperature and to understand the physics of the observed correlation. Methods. We applied full mission comparative statistics of V S , electron temperature, and electron density to establish V S dependence on cold and warm plasma density and electron temperature. We also used Spacecraft-Plasma Interaction System (SPIS) simulations and an analytical vacuum model to investigate if positively biased elements covering a fraction of the solar array surface can explain the observed correlations. Results. Here, the V S was found to depend more on electron density, particularly with regard to the cold part of the electrons, and less on electron temperature than was expected for the high flux of thermal (cometary) ionospheric electrons. This behaviour was reproduced by an analytical model which is consistent with numerical simulations. Conclusions. Rosetta is negatively driven mainly by positively biased elements on the borders of the front side of the solar panels as these can efficiently collect cold plasma electrons. Biased elements distributed elsewhere on the front side of the panels are less efficient at collecting electrons apart from locally produced electrons (photoelectrons). To avoid significant charging, future spacecraft may minimise the area of exposed bias conductors or use a positive ground power system

Capacitively coupled radio-frequency discharges in nitrogen at low-pressure

Apresentação em poster; publicado em: Bulletin of the American Physical Society. 56:15 (2011).This paper studies capacitively coupled radio-frequency discharges (13.56MHz frequency) in pure nitrogen, produced within the LATMOS and the GREMI cylindrical parallel-plate reactors, surrounded by a lateral grounded grid, at 2-30W coupled powers and 0.2-1 mbar pressures. Simulations use an hybrid code [1] that couples a 2D (r,z) time-dependent fuid module for the charged particles and a 0D kinetic module for the nitrogen (atomic and molecular) neutral species. The coupling between these modules adopts the local mean energy approximation to define space-time dependent electron parameters for the fuid module and to work-out space-time average rates for the kinetic module. The model gives good predictions for the self-bias voltage and for the intensities of radiative transitions (average and spatially-resolved OES measurements)with the nitrogen SPS and FNS, and with the argon 811nm atomic line (present as an actinometer). Model results underestimate the experimental electron density (average resonant-cavity measurements) by a factor of 3-4. [1] L. Marques et al, J. Appl. Phys. 102, 063305 (2007)

Instrumentation for Ionized Space Environments: New High Time Resolution Instrumental Modes of Mutual Impedance Experiments

Mutual impedance experiments are in situ plasma diagnostic techniques for the identification of the plasma density and the electron temperature. Different versions of mutual impedance instruments were included in past and present space missions (e.g., Rosetta, BepiColombo, JUICE and Comet Interceptor). New versions are currently being devised to fit the strong mass, volume and power constraints on nanosatellite platforms for future multi-point space missions. In this study, our goal is to define and validate two new instrumental modes (i.e., chirp and multi-spectral modes) to improve the time resolution of the experiment with respect to typical mutual impedance instrumental modes (i.e., frequency sweep). Higher time resolution measurements are expected to simplify the integration of mutual impedance experiments onboard nanosatellite platforms by facilitating antenna sharing between different experiments. The investigation is performed both (a) numerically, using a 1D-1V electrostatic full kinetic Vlasov-Poisson model and, (b) experimentally, with laboratory tests using a vacuum chamber and a plasma source. From a plasma diagnostic point of view, we find that both the chirp and multi-spectral modes provide measurements identical to the (reference) frequency sweep mode. From an instrumental point of view, multi-spectral measurements are faster than frequency sweep measurements but they require larger amounts of onboard computing resources (i.e., larger power consumption). Chirp measurements, instead, outperform frequency sweep measurements both in terms of measurement duration (20 times faster) and onboard processor usage (20% less)