Fabrice Cipriani

New SPIS Capabilities to Simulate Dust Electrostatic Charging, Transport, and Contamination of Lunar Probes

The spacecraft-plasma interaction simulator has been improved to allow for the simulation of lunar and asteroid dust emission, transport, deposition, and interaction with a spacecraft on or close to the lunar surface. The physics of dust charging and of the forces that they are subject to has been carefully implemented in the code. It is both a tool to address the risks faced by lunar probes on the surface and a tool to study the dust transport physics. We hereby present the details of the physics that has been implemented in the code as well as the interface improvements that allow for a user-friendly insertion of the lunar topology and of the lander in the simulation domain. A realistic case is presented that highlights the capabilities of the code as well as some general results about the interaction between a probe and a dusty environment.

Multi-scale simulation of electron emission from a triode-type electron source with a carbon-nanotube column array cathode

We have designed and fabricated a new type of field electron source for a novel onboard mass spectrometer. The new electron source, which is a field effect emitter in a triode configuration, consists of a CNT-column array cathode and an extraction gate with holes that are aligned concentrically with respect to the cylindrical CNT columns. In triode mode operation, cathode currents as large as ~420 μA have been emitted with an anode-to-gate current ratio of ~1.5. To account for the observed emission characteristics of the new electron source, we have carried out multi-scale simulations that combine a three-dimensional (3D) microscopic model in the vicinity of an actual emission site with a two-dimensional (2D) macroscopic model that covers the whole device structure. Because the mesh size in the microscopic 3D model is as small as 100 nm, the contributions of the extruding CNT bundle at the top edge of an electron column can be examined in detail. Unlike the macroscopic 2D simulation that shows only small field enhancement at CNT columns top edge, the multi-scale simulation successfully reproduced the local electric field strongly enough to emit the measured cathode currents and the electric field distribution which is consistent with the measured anode-to-gate current ratio.

SPIS 5: New Modeling Capabilities and Methods for Scientific Missions

Since the last version, the numerical core and the user interface of Spacecraft Plasma Interaction Software (SPIS) have been significantly improved to achieve two objectives: 1) to make SPIS more user friendly and robust for industrial use and 2) to extend the multiscale capabilities and the precision of the solvers in order to model a large range of scientific missions. The new numerical algorithm and modeling capabilities are presented in detail. This new version permits modeling of time variations of the plasma environment, spinning spacecraft, semitransparent grids, secondary emission from 1-D thin elements (e.g., wires or booms), 2-D thin elements (for example, solar arrays), the effect of v × B electric field, particle detectors, and Langmuir probes onboard spacecraft.

Initial Results From the Active Spacecraft Potential Control Onboard Magnetospheric Multiscale Mission

NASA’s magnetospheric multiscale (MMS) mission was successfully launched in March 2015. The scientific objectives of MMS are to explore and understand fundamental plasma physics processes in the earth’s magnetosphere: magnetic reconnection, particle acceleration, and turbulence. The region of scientific interest of MMS is in a tenuous plasma environment where the positive spacecraft potential may reach an equilibrium as high as several tens of volts. The active spacecraft potential control (ASPOC) instrument neutralizes the spacecraft potential by releasing the positive charge produced by indium ion emitters. While the method has successfully been applied to other spacecraft such as Cluster and Double Star, new developments in the design of the emitters and the electronics are enabling lower spacecraft potentials and higher reliability compared to previous missions. In this paper, we report the initial results from the tests of the ASPOC performance during the commissioning phase and discuss the different effects on the particle and field instruments observed at different plasma environments in the magnetosphere.

Design and Numerical Assessment of a Passive Electron Emitter for Spacecraft Charging Alleviation

In this paper, we examine the theoretical basis for using passive unheated electron field emitters to control hazardous levels of spacecraft charging. We present the experimental evidence related to the capabilities of passive and low-power active field emitters. The concept of passive field emitters is detailed looking at their characteristics and location on a model satellite typical of a commercial geostationary satellite. The assessment is performed by means of numerical simulations. The generation and extraction of electrons is simulated at micrometric scale. Their flow is modeled at spacecraft scale to assess spacecraft absolute and differential charging alleviation. The system shows good promise limiting the inverted voltage gradient situations, observed especially on solar panels where cover glasses are more positive than solar cells. This situation is known as very risky in flight, possibly leading to sustained secondary arcing powered by the photovoltaic solar arrays themselves. Finally, we review how existing design practice would be modified by the presence of such passive emitters.

Physical characterization of molybdenum microtips field emitter arrays

We used a dedicated hemispherical energy analyzer to measure energetic and angular distributions of electrons emitted from molybdenum microtips integrated in a 1 cm2 field emitter array designed by the CEA/LETI laboratory. Such cathodes typically deliver about 25 mA at an extraction voltage of 100 V, and are studied in order to replace heated wires as electron sources for space applications. We find that the energy distribution of the beam strongly depends on the extraction voltage, and is therefore expected to vary across the emission lifetime of the device, at a rate depending both on the alteration of the resistive structure with time and on the fate of adsorbed contaminants at the tip surface. A semi-empirical model of the emitters is proposed and used to determine parameters of energetic and angular distributions. The energy dispersion of the beam is found to increase from 2 eV ± 20% up to 9 eV ± 20% eV, for extraction voltages varying from 40 to 100 V. The mean angular dispersion of the beam is found to be 42° ± 20% when a null electric field is set at the grid extraction surface.

Validation Through Experiments of a 3-D Time-Dependent Model of Internal Charging

A new model of dielectric conductivities has been implemented in SPIS-IC. It allows modeling the time-dependent behavior of the charge transport and the electric potential inside payload components under realistic radiation environments in space. The simulation results have been compared to experiments on four validation cases going from simple 1-D geometry to more complex 3-D components such as a d-type connector. The comparison shows a good agreement in most of the cases, and the discrepancies are explained in certain cases. Finally, the results have been analyzed in term of discharge probability and severity. This study also shows the great importance of time-dependent modeling of the radiation-induced conductivity.

SPIS-DUST: Modeling the Interactions between Spacecraft, Plasma and Dusts

The Spacecraft Plasma Interaction Software, a.k.a. SPIS, is an open source software initially developed to model the charging of spacecraft in the GEO environments. It underwent several improvements to simulate the behavior of instruments onboard spacecraft and more recently to model the interaction of dusts with plasma and satellite. We present hereafter the models of dust and dusty soils charging and of dust interaction with the spacecraft that have been implemented as well as a realistic simulation of the dust interaction with a lunar lander and with ESA’s Philae lander that demonstrates the new SPIS capabilities.