Effect of electron temperature fluctuations on slowly swept Langmuir probe measurements

Abstract

Swept Langmuir probes are widely used to measure electron temperature (Te) in laboratory plasmas by performing an exponential fit to the measured volt–ampere (I–V) characteristic. Often the probe voltage sweep frequency is much lower than the characteristic frequencies of the plasma fluctuations and a time-averaged I–V characteristic is used for the fit. We show by numerical modeling that in the presence of Te fluctuations with frequencies well above the voltage sweep frequency this standard technique applied to a swept single probe tends to read higher than the actual time-averaged Te provided no correlated plasma potential (Vp) fluctuations are present. In the presence of coupled Te and Vp fluctuations a slowly swept single probe may read either higher or lower than the average Te, depending on the relative amplitude and phase of the temperature and potential fluctuations. In contrast, swept double probe measurements of Te are virtually unaffected by either Te or Vp fluctuations.