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How do electric probes work?
Electric probes are interesting instruments as they allow to characterize the electrically charged gas, also known as “plasma”, that is ubiquitous in interplanetary space and in planetary environments. Such probes consist of electrically conducting surfaces – in this case two hollow Aluminium spheres with 8 cm diameter – where an exchange of electric charges with the plasma takes place.
Considering that the spacecraft itself, with its much larger outer surface, acts as one big electric probe, scientists measure the electric current flowing between a probe and the spacecraft when a given electric potential difference between both is imposed.
Doing so for a range of potentials, provides information on how many electric charge carriers there are in the environment (the plasma density) and on how mobile they are (the plasma energy or temperature).
Comparing such measurements from both probes allows to determine the electric field (in one direction). Because this can be done at a high cadence, it is possible to record electric waves occurring naturally in the plasma. In combination with a transmitter, one can emit a wave into the plasma and listen to the plasma response. This is an active sounding technique that provides powerful plasma diagnostics.
To avoid perturbations created by the spacecraft itself, the two probes are mounted at the tips of 1-meter-long booms.
Building test models in a national and international collaboration
After having designed the probes and benefiting from test results obtained with a prototype in the preceding years, it was our task to build the Structural and Thermal Models (STMs) and the Electric and Functional Models (EFMs).
Most of the components were ordered and manufactured by our Belgian industrial partners. A few very specific pieces were built in-house in our mechanical workshop. Also, the application of an electrically conducting black coating on the spheres was performed by Belgian industry.
Each probe assembly is built on the tip of its boom. One of the booms is deployable. It is provided by the Technical University of Braunschweig. The other boom is rigid and is built by the Swedish Institute of Space Physics in Kiruna.
Problem solving during integration: redesigning the merged probe’s inner shield
Overall, the integration of the probe assemblies on the boom tips went well. However, we did encounter one significant problem.
One of the two probe assemblies is a so-called “merged probe” that holds a magnetometer sensor inside the probe sphere. To avoid that the magnetometer would perturb the electric measurements, our probe assembly design foresees an electrically conducting screen or “inner shield” between the magnetometer and the sphere.
Initially, we had foreseen an inner shield made from a thin foil – this is a low-mass solution that worked well in the prototype instrument. However, during integration of the STM merged probe, we ran into problems with the foil – the manufacturer did not cut the foil to the right dimensions and manually modifying it did not work out well.
Moreover, and this was a more general concern, the reproducibility of the foil screen was questionable. We also had concerns about how well it would keep its shape during launch vibrations (although we had taken some measures to ensure this).
Therefore, we made a design change and switched to an inner shield made of two Aluminium pieces. All in all, this solution turned out to be much more robust, reproducible, and easier to integrate, with its mass being only a few grammes more.
In the end, the EFMs that we built are faithful copies of the flight versions that we will build in 2025. They were subsequently tested – mechanically, thermally, and electrically.
The whole STM and EFM building exercise therefore served its goal well: to identify problems with the design and solve them, to devise a complete building strategy, and to test the instruments thoroughly. We are now ready to build the flight and spare models.
