Analysis and Comparison of the Electron Temperatures and the Number Densities using a Single Langmuir Probe and Optical Emission Spectroscopic Diagnostic System*

In order to best characterize a radiofrequency (RF) plasma source, we identified the need for an intrusive probe able to survive high temperatures with very reactive plasma. The optimal hollow cathode materials for space propulsion systems can use argon as a source gas. The objective is to compare the intrusive Langmuir probe system and the non-intrusive optical emission spectroscopic system with each other with a focus on the electron temperatures and densities. The results can be used to relate the characteristics of a hollow cathode's plasma. The Langmuir probe analysis is based on the floating potential method which showed smaller errors compared to other available methods. The analysis of the emission spectrum (optical emission spectroscopy – branching fraction method) is based on a collisional radiative model (CRM). CRM determines possible collisional and radiative processes of atoms and ions. Literature research showed differences of ± 20% in electron temperatures and densities using RF generated plasmas between the two before mentioned diagnostic systems. The RF plasma source operated at 1000, 1250, 1500, 1750, and 2000 W with a constant pressure of 1.0 Torr. Langmuir probe and optical emission measurements have been performed at the same location inside of the quartz tube. The non-intrusive plasma measurements can reliably characterize the plasma instead of the classical intrusive measurements in hollow cathodes and other plasma-based propulsion systems.