Does plasma flow in the presheath affect sheath formation at Langmuir Probe surfaces in low temperature plasma?*

Presheath plasma flow has been observed to affect Langmuir probe measurements of the plasma potential. This work demonstrates that electron density measurements are also affected in similar and consistent manner.  In this work we compare plasma potential measurements made by Langmuir probes with that of emissive probes and compares electron and ion densities measured by Langmuir probes, focusing particularly on the presheath region. In our experiment, sheaths and presheaths are formed by immersing either a 15cm diameter plate or 10cm diameter grid (50% transparent, 25lines/cm, stainless steel), each biased to -100V,   in a Kr plasma discharge operated with neutral pressures of 0.1-0.5mTorr, 1 ≤ n_e ≤ 3 10⁹ cm^-3 and T_e ≈ 1-3 eV.   We  attempt to quantify how far from quasineutrality Langmuir probe measurements of density are in presheaths, and whether the presence of counter flowing ions in the case of the grid alters sheath formation at the probe surfaces differently.This study is part of a wider study of sheath formation and expansion surrounding surfaces entrained in flowing plasma, and attempts to provide data to benchmark with PIC codes,  pursued in collaboration with Sandia National Laboratories.