Measurements of Plasma Properties Using the Floating Potential of Emissive Langmuir Probes

The measurement of the plasma potential of unmagnetized Maxwellian plasmas by using the floating potential of emissive Langmuir probes is discussed. The temperature of the probe was monitored in order to estimate the emitted thermoionic electron current and to determine the limits of the strong electron emission regime. Under these ideal conditions, the measurements of the plasma potential of the emissive probe are cross checked against those of collecting Langmuir probes. In agreement with previous works, the current voltage curves of emissive probes show the temperature dependent electron saturation currents. An empirical expression is suggested for the dependence of this saturation current with the probe temperature which recovers the response of collecting probes when the probe is cold. The experimental data indicate that the floating potential of the emissive probe is close to the local plasma potential only when the emitted thermoionic electron current is similar to the electron saturation current. Larger electron emission currents lead to small increments the floating potential over the local plasma potential. Our results suggest that the electron saturation current of a hot emissive probe is composed by the thermal electrons from the plasma and a returned fraction of the thermoionic emitted electrons (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)