Emissive Langmuir Probes in the Strong Emission Regime for the Determination of the Plasma Properties

The determination of the plasma potential V_pl of unmagnetized plasmas by using the floating potential of emissive Langmuir probes operated in the strong emission regime is investigated. The experiments evidence that, for most cases, the electron thermionic emission is orders of magnitude larger than the plasma thermal electron current. The temperature-dependent floating potentials of negatively biased V_p <; V_pl emissive probes are in agreement with the predictions of a simple phenomenological model that considers, in addition to the plasma electrons, an additional electron group that contributes to the probe current. The latter would be constituted by a fraction of the repelled electron thermionic current, which might return back to the probe with a different energy spectrum. Its origin would be a plasma potential well formed in the plasma sheath around the probe, acting as a virtual cathode or by collisions and electron thermalization processes. These results suggest that, for probe bias voltages close to the plasma potential V_p ~ V_pl, two electron populations coexist, i.e., the electrons from the plasma with temperature T_e and a large group of returned thermionic electrons. These results question the theoretical possibility of measuring the electron temperature by using emissive probes biased to potentials V_p ≲ V_pl.