Experimental Studies of the difference between electron and ion densities measured by Langmuir probes in the presheath

It has recently been shown that unphysically positive measurements of the plasma potential made by Langmuir probes (LPs) in presheaths may also be related to unphysically high measurements of electron densities made my Langmuir probes in presheaths. This work attempts to quantify how far from quasineutrality Langmuir probe measurements of density are in presheaths. In our experiment, sheaths and presheaths are formed by immersing a 15cm diameter plate biased to -100V in a low pressure (P_neutral < 1 mTorr) low temperature (kT_e ~ 1 eV) plasma created in a hot-filament DC discharge device, 60 cm in diameter. It has been argued heuristically that the difference between plasma potential profiles measured by LPs and emissive probes (EPs), in the presheath, is related to ion flow caused by sheath formation. We present experimental evidence that the electron density is also overestimated by standard analysis in the presheath, and that the two anomalies are interrelated. We test the extent to which flow speed matters by preforming measurements in Ar and Helium plasma discharges.