Comparing numerical models to laboratory experiment results using the twin-probe method: a technique to improve Langmuir probe measurement accuracy on very small satellites

Abstract:

Langmuir probe (LP) operation on CubeSats induces negative spacecraft (S/C) charging due to the limited S/C ion current collection in comparison to the LP electron current collection. This variable S/C potential reduces LP temperature and density measurement accuracy. One way to counteract the effects of induced S/C charging is to track and account for changes to the S/C potential during LP operation. This technique is best accomplished utilizing two identical LPs and is referred to here as the twin-probe method (TPM). The TPM shifts the reference of a swept LP from the variable potential of the CubeSat to the more stable reference of a high-impedance probe, which is exposed to the ambient plasma such that its potential does not change.

Numerical simulations and plasma laboratory experiments were conducted to study the TPM’s efficacy and maximize its impact. The experiments took place at NASA Marshall Space Flight Center and studied changes to the S/C potential during LP operation using the TPM in an approximated low Earth orbit (LEO) plasma environment. Results from this experiment were exhibited in the 2019 AGU Fall Meeting [1]. Additionally, the Plasma-Spacecraft Interaction Codes for Low Earth Orbit (PSIC-LEO) were developed at the University of Michigan to model the induced S/C potential due to LP operation using analytic expressions and to understand its effects on LP current-voltage characteristics. Here we present comparisons between experimental results and simulation predictions with a focus on electron temperature and density measurement correction when varying the spacecraft-to-LP area ratio. Generally, the model predictions and experimental results agree on TPM measurement corrections to within 5%. Additionally, both simulation and experiment efforts show greater corrections to LP electron density and temperature measurements as the area ratio decreases. Finally, we will use the lessons learned from these research initiatives to highlight additional applications for a twin-probe system on a CubeSat platform, such as maintaining a negative CubeSat potential to enhance ion energy analyzer performance.

[1] Leon, Omar, et al. "The twin-probe method: tracking the variable potential of small satellites to improve Langmuir probe measurement accuracy." AGUFM 2019 (2019): A41U-2688.