Does the discrepancy between Langmuir Probe and emissive probe measurements of plasma potential depend on ion flow and sheath formation? How might LIF measurements help with model formation?

 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. 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. A negatively biased plate (-100V) is immersed in a weakly collisional  (λ_mfp»λ_D), low pressure (P_n≤ 1 mTorr), low temperature (kT_e ∼1 eV), single ion species plasma formed in a hot-filament DC discharge, where the feedstock gas is Ar or He or Xe or Kr.  We are in the process of making needed upgrades to the laser-induced fluorescence (LIF) collection optics so that we can include LIF measurements with our results, which are designed to test whether the Bohm speed, v_Bohm = √(kT_e/M_i),  affects the difference between the potential profiles.