The Twin-Probe Method: Improving the Accuracy of Langmuir Probes on Small Spacecraft

Abstract

A Langmuir probe (LP) is a versatile and effective in-situ space plasma instrument for measuring ion and electron densities, and electron temperatures. However, utilizing LPs on very small spacecraft presents challenges that are not experienced on larger, more traditional spacecraft. In particular, a key issue for LP operation on these very small satellites is the negative spacecraft potential induced during LP sweeps due to the limited ion current collection to the spacecraft relative to the electron current collected by the LP. This induced spacecraft charging reduces the accuracy of measurements made by the LP. To mitigate these charging effects, laboratory plasma experiments and computer modeling confirmed that the spacecraft potential can be tracked during LP sweeps using a second, identical probe configured for high impedance potential measurements. By correcting for changes to the spacecraft potential, the LP sweeps can be reconstructed as if they were referenced against a stable potential, providing more accurate measurements of the ambient plasma’s properties. This dual probe measurement is referred to here as the twin-probe method (TPM).

This dissertation focuses on the efficacy of the twin-probe method and identifies barriers that must be addressed to maximize its impact. Particle-in-cell simulations were performed using the NASA/Air Force Spacecraft Charging Analyzer Program (NASCAP-2K) to understand which physical processes and system parameters are most critical when analyzing spacecraft charging behavior. A separate MATLAB program called the Plasma-Spacecraft Interaction Codes for Low Earth Orbit (PSIC-LEO) was developed using analytic equations to model spacecraft charging effects on LP current voltage (I-V) curves. Finally, an experiment campaign, performed at NASA Marshall Space Flight Center (MSFC), studied the TPM in a laboratory plasma that approximates a high-density, low-Earth orbit environment.

Through these investigations, it was determined that induced spacecraft charging effects result in LP I-V characteristics which overestimate electron temperature and underestimate electron density. Furthermore, regions of the I-V curves have additional non-linear characteristics due to the spacecraft’s induced potential, making traditional Langmuir probe theory more difficult to apply. The TPM is shown to correct I-V curves to provide more accurate estimates of plasma properties. The magnitude of the TPM correction is dependent on the area ratio, defined as the conductive spacecraft surface area divided by the probe surface area. Greater spacecraft charging and, consequently, larger I-V curve corrections when using the TPM, are observed as the area ratio decreases. The method’s largest impact occurs for area ratios below 300. While the TPM is effective for area ratios greater than 300, overlap between measurement uncertainty and the magnitude of correction prevents definitive claims of a maximum area ratio for which twin-probe implementation is necessary. Moreover, since the TPM mitigates the effects of spacecraft charging, but does not mitigate the charging itself, a minimum area ratio of 50 is recommended for this method. Below this area ratio, the TPM can be used, but the spacecraft may charge too negatively to allow the Langmuir probe to reach the plasma potential, reducing the number of useful plasma properties obtained from the incomplete I-V curve. Finally, novel capabilities brought about using a combination of Langmuir probes and other satellite instruments are identified. These capabilities include expanding the measurable range of plasma ion distributions using charged particle energy analyzers and calibrating for environmental effects (like photoelectron current).