On the divergence of plasma potentials measured by cylindrical Langmuir probes and emissive probes in the presheath

It is conventional wisdom that Langmuir probes (LPs) do not work in the sheath near material boundaries, but are supposed to yield accurate measurements of plasma potentials in quasineutral plasma. Experiments that compared plasma potential measurements made by partially coated and uncoated LPs and cylindrical LPs with measurements made by emissive probes, were performed in low pressure unmagnetized argon discharges ($0.1\leq P_n \leq 1mTorr$), with electron temperatures and densities between 1 and 5 $eV$, and $ 1 \times 10^9$ and $ 1 \times 10^{10} cm^{-3}$, respectively. Presheaths were set up in the plasma using negatively biased electrodes. We used both grids and plates for the biased electrode. Results indicate that the emissive probe potential measurements (in the limit of zero emission) were more negatve than LP measurements in the presheath. In the sheath, most LP measurements did not go negative but rather became increasingly positive. Only the emissive probe measurements worked in the sheath. These divergences are thought to be caused by the inherent, diffuse, ion flow in the presheath region toward the negatively biased electrode, characteristic of sheath formation.