Nascap Analysis of the Sheath-less Planar Langmuir Probe

We have previously reported on a laboratory model of the Sheath-less Planar Langmuir Probe, SPLP. The Langmuir probe, LP, is one of the oldest plasma diagnostics, provided the plasma density and species temperature from analysis of a current-voltage curve. Of the many variations on the LP, this work combines planar geometry, PLP, with the floating double probe. This is an attractive geometry because the ram ion current is very constant over many Volts of a sweep. The PLP has been flown on the CHAMP, C/NOFS and SORTIE satellites. When a PLP is designed as a mono-element probe, it is still biased against the spacecraft, and the probe-spacecraft system is electrically floating. If the satellite is small, the system becomes a floating double probe where the physical interaction of the satellite element with the plasma is very complicated, confounding the measurement. The S-PLP combines both double probe elements in a checkerboard fashion on a scale small compared to the plasma Debye length, thus averaging the space potential to the floating potential on a scale that is independent of the plasma parameters. A simple theory of the S-PLP produces a hyperbolic tangent curve identical to some textbook treatments of the floating double probe. For future space missions however, the realities of the expected plasma must be explored, such as edge effects and light ions. For this purpose, we are using Nascap, a well-known spacecraft charging and plasma interaction model. Nascap can simulate in 3D the instrument with a highly resolved electrically floating checkerboard face, and the complete thermal ion distributions for O+, He+, and H+., and barrier effect on the electron collection. The results are very encouraging with edge effect controlled to better than 3%, and discernable light ion signatures. Approved for public release; distribution is unlimited. Public Affairs release approval #45662