Testing the Validity of Using Langmuir Probe in Non-Stationary Plasma Generated from Hypervelocity Impact

Langmuir probe measurement relies on the understanding of plasma sheath theory. Plasma sheath is a thin region that forms between bulk plasma and a physical boundary such as the probe surface. The behavior of this sheath has only been studied in literature for a limited range of cases, none of which can be applied to hypervelocity impact (HVI) plasma without violating some assumptions. Theoretical studies of plasma sheath are being conducted while relaxing many of the assumptions made in current literature. Preliminary results show that the data interpretation model used for Langmuir probe in many HVI research are indeed not perfectly valid and can cause discrepancies in the final measurement. In this work, 1-D fluid simulation is performed using fluid ions and Boltzmann electrons to study the temporal and long-term behavior of the plasma sheath formed between a bulk, flowing plasma and a current absorbing boundary. Plasma sheath theory for stationary and non-stationary plasma were also derived and compared to the simulation result. This work shows that the stationary plasma sheath theory is not adequate when applied to non-stationary plasma, and further research is needed to justify using Langmuir probe in HVI plasma. This work also shows that transient formation of the plasma sheath is not an issue when it comes to making transient measurements, as sheath formation occurs on the order 10-100 times the inverse of the plasma frequency.