Jean-François Roussel

SPIS Open-Source Code: Methods, Capabilities, Achievements, and Prospects

In this paper, recent improvements in the modeling capabilities of the Spacecraft Plasma Interaction Software (SPIS) code are presented. New developments still in progress are also reported. They should in particular allow modeling of fast dynamical phenomena, including processes as challenging as the second part of ESDs, i.e., the vacuum arc and its related flashover plasma expansion. The first, electronic, part of ESDs is already modeled. The range of SPIS application domains and studies is reviewed. An interesting study case, the assessment of charging at multiple-scale levels, is presented here in more detail. Charging in geostationary-Earth-orbit conditions is simulated from the spacecraft scale down to a solar-cell-gap (hence, decameters to millimeter) scale. This self-consistent computation shows that macroscopic inverted-voltage-gradient (IVG) cases may differ at microscopic scales close to a solar-cell gap, due to the local blocking of secondary emission by the small-scale electric-field configuration. We consider this effect as the likely origin of the different ESD triggering thresholds, depending whether IVG is obtained by electrons or plasma.

Numerical modeling of plasma sheath phenomena in the presence of secondary electron emission

The steady-state, sheath between a Maxwellian plasma source and a dielectric surface was simulated numerically in the collisionless hypothesis. A method of nonphysical numerical time evolution was investigated to extend the electrostatic particle-in-cell method to a large range of ion masses at low cost in calculation time. Secondary electron emission effects were also considered and all of these results for a Maxwellian plasma were validated by comparison with previous analytical models in the entire range of ion masses and with other numerical simulations in the case of light ions. The occurrence of instabilities resulting from the interaction between the ion and the secondary electron flows was also investigated. The numerical times method was thus validated and its limitations characterized. It is now available to simulate arbitrary distribution functions of electrons, secondary electrons, and ions at low cost in calculation time.

ESDs on Solar Cells—Degradation, Modeling, and Importance of the Test Setup

Cumulative electrostatic discharges (ESDs) on spacecraft solar cells result in the degradation of their performances. In this paper, silicon solar cells are tested in inverted voltage gradient situation obtained in plasma. ESDs occurring on the cells are detected by both electrical and optical signatures. To be representative of flight ESDs, the test setup must avoid unwanted coupling with ground arcs. The degradation is then evaluated by measuring the current-voltage characteristic of the cell in darkness. The equivalent shunt resistance allows quantifying this degradation, which can be attributed to material deposition on the cell edge or to local cell carbonization due to arcing. Visual inspection of the cells allows us to correlate ESD location and local degradation of the cell. The important parameter for solar cell degradation is the amount of energy dissipated during the discharge. A model of the plasma expansion from the cathode spot is compared to measurement. This model explains the current rise during the first phase of the discharge, which is the same for normal ESDs and coupling with ground ESDs.

Modeling of Plasma Probe Interactions With a PIC Code Using an Unstructured Mesh

A new Spacecraft Plasma Interaction Software has been developed in the frame of the Spacecraft Plasma Interaction Network (SPINE). This software is designed to simulate the kinetic processes of ions and electrons, taking into account their space charge and their interaction with spacecraft surfaces. It is freely available worldwide in open source. While the development and the qualification of the software functionalities were under the responsibility of a consortium led by ONERA under contract with the European Space Agency, the test and validation of the applicability of the code for solving problems in plasma physics remained under SPINE responsibility. The validation program includes step-by-step applications of the software to sheath modeling in simple geometry, artificial plasma injection, and spacecraft charging. We report here on the progress along this program, including Langmuir probe tests with spherical and cylindrical geometry and comparison with other numerical methods.

Drifting Plasma Collection by a Positive Biased Tether Wire in LEO-Like Plasma Conditions: Current Measurement and Plasma Diagnostic

BETs is a three-year project financed by the Space Program of the European Commission, aimed at developing an efficient deorbit system that could be carried on board any future satellite launched into Low Earth Orbit (LEO). The operational system involves a conductive tape-tether left bare to establish anodic contact with the ambient plasma as a giant Langmuir probe. As a part of this project, we are carrying out both numerical and experimental approaches to estimate the collected current by the positive part of the tether. This paper deals with experimental measurements performed in the IONospheric Atmosphere Simulator (JONAS) plasma chamber of the Onera-Space Environment Department. The JONAS facility is a 9- m3 vacuum chamber equipped with a plasma source providing drifting plasma simulating LEO conditions in terms of density and temperature. A thin metallic cylinder, simulating the tether, is set inside the chamber and polarized up to 1000 V. The Earths magnetic field is neutralized inside the chamber. In a first time, tether collected current versus tether polarization is measured for different plasma source energies and densities. In complement, several types of Langmuir probes are used at the same location to allow the extraction of both ion densities and electron parameters by computer modeling (classical Langmuir probe characteristics are not accurate enough in the present situation). These two measurements permit estimation of the discrepancies between the theoretical collection laws, orbital motion limited law in particular, and the experimental data in LEO-like conditions without magnetic fields. In a second time, the spatial variations and the time evolutions of the plasma properties around the tether are investigated. Spherical and emissive Langmuir probes are also used for a more extensive characterization of the plasma in space and time dependent analysis. Results show the ion depletion because of the wake effect and the accumulation of ions upstream of the tether. In some regimes (at large positive potential), oscillations are observed on the tether collected current and on Langmuir probe collected current in specific sites.

Modeling of FEEP Plume Effects on MICROSCOPE Spacecraft

Several aspects of plume effects of field-effect electric propulsion (FEEP) were studied. We first estimated the contamination by cesium deposit due to charge exchange of fast ions and slow neutrals and to direct neutral impingement. Levels are rather low, with local maximums of a few tens or hundreds of angstroms per year, and not much more than 1-10 Aring farther from thruster nozzles. Neutralization of the ions emitted by FEEPs was also addressed, both concerning FEEP ion space-charge compensation and spacecraft net current, i.e., the floating potential issue. With the presence of a cathode-grounded neutralizer, the spacecraft was shown to float somewhat negatively with little dependence on the ambient environment.

Ground Plasma Tank Modeling and Comparison to Measurements

ONERA plasma tank JONAS is populated with drifting Ar+ argon ions representative of a low Earth orbit environment, produced by a Kaufman source, and slow ions created by charge exchange with the background pressure of argon. When testing a mock-up or space equipment in this tank, it is often important to determine the drifting and slow ion densities in various locations. The orbital and radial motion models are compared to experimental current-voltage measurements and to numerical results. In the range of parameters used in this paper, the radial motion model is more adapted than the orbital-motion-limited model to determine slow ion density. Nevertheless, the number of measurements is necessarily limited and discriminating between fast and slow ions is not easy. A complementary approach consists in performing a numerical simulation of the plasma dynamics in the tank. Provided the modeled physics is validated, and the modeling is calibrated through measurements, this approach supplies fast and slow ion densities everywhere with an acceptable factor of uncertainty. This approach has been followed by characterizing in detail a given plasma configuration in JONAS and modeling it with Spacecraft Plasma Interaction Software open source code. The mock-up was a simple plate. We measured the plasma characteristics through numerous current-voltage sweeps and current space profiles at given probe potential. The modeling was based on fast ion beam measurements close to the source and the residual pressure.

SPIS and MUSCAT Software Comparison on LEO-Like Environment

Two spacecraft charging software tools, the Spacecraft Plasma Interaction Software and the Multi-Utility Spacecraft Charging Analysis Tool, have been compared in the situation of a wake generated by an object immersed in dense drifting plasma. Each model takes account of the particle dynamics, the space charge effect on the electric field, and the currents collected by the object. The cross-comparison resulted in a good agreement between the two codes and with previous experiments conducted on the same configuration. In particular, a highly negative voltage imposed to the object rear side allows the collection of drifting ions at this location. The nontrivial shape of the current density map was correctly simulated by the two codes. Future works may allow to reach a better quantitative agreement.

Progress on the Physical Approach to Molecular Contamination Modeling

A review of the contamination physics and of the most widespread engineering approaches to contamination assessmentwas carried out. The twomain approaches are the physical and the empirical one. Themain questions still open to validate the physical approach to outgassing and deposit physics were then studied. Among others, special attention was paid to the important point of a realistic separation of chemical species, probably a prerequisite for a physicalmodeling. Several original results were obtained. Some lead to a quite clear conclusion, like the preeminence of the limitation by desorption over the limitation by diffusion for outgassing. This observed trend needs yet to be validated on other materials. Other major results are progress on the validation of the physical approach and on the ambitious species separation program.

In Situ Real-Time Quantitative and Qualitative Monitoring of Molecular Contamination

The quantitative and qualitative monitoring of contamination is an important issue for the mitigation of the risks induced by contamination. In situ and real-time measurements of contamination levels are currently performed with good accuracy by using quartz microbalances. However, they have to be completed by chemical analyses to identify the nature of the contaminants. Unfortunately, in situ techniques are limited to a very rough characterization. A transfer of the samples is then required, which prohibits real-time monitoring and may lead to the partial degradation of the samples. To tackle this challenge, an experimental technique, coupling thermogravimetric analysis and mass spectrometry, has been developed at ONERA. This method takes advantage of a preseparation of the species through the thermogravimetric analysis. A numerical tool was moreover developed to process automatically experimental thermogravimetric analysis/mass spectrometry data. It enables determination of the contribution of each spec...