Electron density measurements in very electronegative plasmas using different diagnostic techniques: theory and experiments

Very electronegative plasmas (known as ``ion-ion'' plasmas) are used in different applications including material processing, space propulsion and thermonuclear fusion. Diagnostics of ion-ion plasmas can be performed using different probe techniques, including Langmuir and hairpin probes, RF, microwave and optical diagnostics. However, in certain applications (for example, in the electronegative thruster PEGASES [\textit{Plasma Sources Sci. Technol.} \textbf{23} 044003 (2014)]), the electron density is too low (\textless 10$^{\mathrm{12}}$m$^{\mathrm{-3}})$ to be reliably measured by these standard techniques. This is further complicated by the presence of strong, non-homogeneous, magnetic fields in the plasma (\textasciitilde 200 G) and the relatively small plasma size (few cm). In this work we compare results achieved with a Langmuir probe, and with an independent measurement of the electron density using a matched dipole probe [\textit{Phys. Plasmas},~\textbf{22}, 073504 (2015)]. Measurements are performed in an SF6 plasma with an electronegativity in the range between a few hundred to a few thousand. We show here that though the model itself can correctly describe the plasma-probe interactions, there is a critical value of plasma electronegativity above which the electron density measured with a Langmuir probe can give only an upper limit estimation.