Measurements of Plasma Properties Using Fast Sweep Langmuir Probes in Unmagnetized Weakly Ionized Plasmas

The performances of a simple circuit for fast sweep measurements using collecting and emissive Langmuir probes are evaluated. The probes are biased by means of a time dependent ramp voltage signal with a variable pulse frequency and the current voltage curves are measured along the increasing flange of this sawtooth voltage. The response of this fast probe polarization circuit was verified under actual experimental conditions by measuring the properties of a stationary Maxwellian plasma produced by means of a glow discharge. The results are independent of the experimental conditions and essentially rely on the discharge properties for polarization pulse repetition rates below a threshold. This upper bound lies below the ion plasma frequency and is related with the faster time scale involved in the sawtooth signal probe bias voltage. The motion of ions would not follow the rapid change of the electric field around the probe associated to the short decreasing edge of the sawtooth polarization voltage and, therefore, the probe perturbs the local electric field. We conclude that these time scales should be considered for the interpretation of these measurements in addition to the electron and ion plasma frequencies in fast sweep Langmuir probe techniques. We conclude that these time scales should be considered for the interpretation of these measurements in addition to the electron and ion plasma frequencies in fast Langmuir probe techniques. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim