Sheath expansion around Langmuir Probes: is it only about the probe bias potential?*

It has recently been shown that  that Langmuir probes (LPs) measure an unphysically positive plasma potential in the presheath of low temperature plasma,  near conducting boundaries at which ion rich sheaths form [Li et al., Plasma Sources Sci. Technol. 29 (2020) 025015]. 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. More recently we have shown that in the presheath, Langmuir probes also overestimate electron density rather dramatically, especially as the probe gets close to the sheath. We present experimental evidence that the two anomalies are interrelated, and that sheaths are distorted and expanded around a Langmuir probe where ion flow is present, even when the probe bias potential passes through the local plasma potential, in weakly collisional (λ_{mean-free-path} >> λ_D), low pressure (P_neutral ≤ 1 mTorr), low temperature (kT_e ~ (1-5) eV), single ion species plasma, where the feedstock gas is Ar or He or Xe or Kr.