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Development and validation of SLP

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The Sweeping Langmuir Probe (SLP), developed at BIRA-IASB, is one of the two instruments that will fly on board the ESA CubeSat PICASSO in 2019. SLP will measure the current collected by four cylindrical probes, whose electrical potentials are swept so that the following key ionospheric parameters can be retrieved: electric charge density, electron temperature, ion temperature, and spacecraft potential with respect to the environment. The main objectives of SLP are to get insight into the magnetosphere-ionosphere coupling and aurora. These scientific investigations will be carried out by complementing the data from SLP with data from a variety of data sources: GNSS TEC tomography, whistler stations in AWDAnet, WHISPER instruments on Cluster, etc.
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SLP is a very light weight and miniaturised instrument. The electronic boards are enclosed in custom-made shielding cover to reduce electromagnetic interferences (EMI) as depicted in Figure 2. The electronics fit into a box of 104 mm x 98 mm x 25 mm and the whole instrument weighs only 152 g (including probes and booms).

Test and validation

Because the conducting area of the spacecraft (S/C) is not large enough compared to the area of the probes, the S/C will charge negatively when the probes are swept with positive bias. This charging will lead to a drift of the S/C potential during the sweep, making the data unusable. In order to avoid this problem, a specific measurement technique using two different probes simultaneously has been developed:

  1. one probe is biased and measures the current (traditional Langmuir probe technique)
  2. a second probe is used to measure the floating potential

By combining the data from these two probes, we are able to retrieve consistent current-voltage characteristics. This new measurement principle has been successfully tested and validated in the ESA plasma chamber at ESTEC, as shown in Figures 3 and 4.

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Fig. 2: Integration of SLP electronics into PICASSO.
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Fig. 3: PICASSO electrically representative mock-up in the ESA plasma chamber at ESTEC during the validation test campaign.
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Fig. 4: Left: Measured current-voltage characteristics in the plasma chamber, with respect to the spacecraft potential (red) and with respect to the floating potential (blue). Right: Floating potential measurement with respect to the spacecraft potential.
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