Effects on the collected current of the Langmuir probes located in the plasma wake

Chun-Sung Jao^1*, Sayan Adhikari², Steffen M. Brask², Lasse Clausen², Wojciech J. Miloch²

¹Department of Space Science and Engineering, National Central University, Taoyuan, Taiwan
²Department of Physics, University of Oslo, Oslo, Norway

* Presenter:Chun-Sung Jao, email:csjao899@gmail.com

The employment of Langmuir probes is widespread for plasma measurements in laboratory experiments and space missions. For the classical Langmuir probes, the current-voltage characteristics obtained from the electric voltages sweeping can provide information on physical parameters such as ion density, electron density, and electron temperature. However, space in-situ measurement missions may demand a higher sampling rate than the sweeping frequency. The so-called multi-needle Langmuir probe (m-NLP) instrument, which can provide electron density measurements with a higher sampling rate (~kHz), thus becomes a benefit. In particular, the m-NLP instrument consists of two or more small fixed-bias cylindrical Langmuir probes that are small compared to the Debye length. Based on the Orbital Motion-Limited (OML) theory, we can infer the electron density directly from the collected current by the probes with different biases. However, a traveling object can disturb the local plasma environment and lead to a plasma wake in the downstream region. It will cause measurement errors if the instrument is sitting in the plasma wake formed behind the main spacecraft/rocket body, boom, or even other probes in the instrument. Under this context, in this study, we will present the effects on the collected current of the Langmuir probes located in the plasma wake based on numerical simulations.

Keywords: Langmuir probe, Plasma simulation, Plasma wake, Plasma-spacecraft interaction, Orbital Motion-Limited (OML) theory