Laboratory measurements of the performances of the Sweeping Langmuir Probe instrument aboard the PICASSO CubeSat

The Sweeping Langmuir Probe (SLP) is one of theinstruments on board the triple-unit CubeSat PICASSO, an ESA in-orbitdemonstrator launched in September 2020, which is flying at about 540 kmaltitude. SLP comprises four small cylindrical probes mounted at the tip of thesolar panels. It aims to perform in situ measurements of the plasmaparameters (electron density and temperature together with ion density) andof the spacecraft potential in the ionosphere. Before the launch, theinstrument, accommodated on an electrically representative PICASSO mock-up,was tested in a plasma chamber. It is shown that the traditionalorbital-motion-limited collection theory used for cylindrical Langmuirprobes cannot be applied directly for the interpretation of the measurementsbecause of the limited dimensions of the probes with respect to the Debyelength in the ionosphere. Nevertheless, this method can be adapted to takeinto account the short length of the probes. To reduce the data downlinkwhile keeping the most important information in the current-voltagecharacteristics, SLP includes an on-board adaptive sweeping capability. Thisfunctionality has been validated in both the plasma chamber and in space, andit is demonstrated that with a reduced number of data points the electronretardation and electron saturation regions can be well resolved. Finally,the effect of the contamination of the probe surface, which can be a seriousissue in Langmuir probe data analysis, has been investigated. If notaccounted for properly, this effect could lead to substantial errors in theestimation of the electron temperature.