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

Departamento de F´isica y Qu ´imica Aplicada, E.U.I.T. Aeronautica. Univ. Politecnica de Madrid, 28040Madrid, SpainReceived 02 December 2011, revised 18 June 2012, accepted 22 June 2012Published online 09 January 2013Key words Langmuir probes, plasma diagnostics.The measurement of the plasma potential of unmagnetized Maxwellian plasmas by using the floating potentialof emissive Langmuir probes is discussed. The temperature of the probe was monitored in order to estimate theemitted thermoionic electron current and to determine the limits of the strong electron emission regime. Underthese ideal conditions, the measurements of the plasma potential of the emissive probe are cross checked againstthose of collecting Langmuir probes. In agreement with previous works, the current voltage curves of emissiveprobes show the temperature dependent electron saturation currents. An empirical expression is suggested forthe dependence of this saturation current with the probe temperature which recovers the response of collectingprobes when the probe is cold. The experimental data indicate that the floating potential of the emissive probeis close to the local plasma potential only when the emitted thermoionic electron current is similar to theelectron saturation current. Larger electron emission currents lead to small increments the floating potentialover the local plasma potential. Our results suggest that the electron saturation current of a hot emissive probe iscomposed by the thermal electrons from the plasma and a returned fraction of the thermoionic emitted electrons.