Langmuir probe: how to handle expanding ion collecting area when the probe is negatively biased*

Prof. Hershkowitz, being one of the greatest experimental plasma physicists, had taught me many plasma diagnostics such as Langmuir probes, double probes, emissive probes, laser induced fluorescence (LIF) and an ion-acoustic wave launcher/detector. We are all aware of importance of these techniques, and I appreciate very much for his invaluable lessons on them. However, if I am asked to say the one most important lesson from him, then I choose his skepticism before making conclusions: when I showed him data, he would often tell me that "something is wrong!" Following his skepticism, we have re-visited a well-known problem on expanding ion collecting area of a planar Langmuir probe. Ion currents collected when the probe is negatively biased tend to increase with the bias associated with sheath expansion. Typically, these effects are eliminated by fitting a linear fit or a power law fit to obtain ion saturation current which is, then, used to obtain electron information from the I-V characteristic. These fitting results are quite sensitive to the fitting range which inevitably introduces subjectivity, and quite often we observe inconsistent ion and electron densities. In this Noah Hershkowitz Memorial Session, we discuss how to properly obtain ion currents consistent with the experimental data. Note that our proposed method may not necessarily reflect the views and opinions of Prof. Hershkowitz and cannot be confirmed as he is not with us anymore.