An appraisal of the single Langmuir probe technique in the study of afterglow plasmas

Abstract

A detailed appraisal of the collisionless sheath single cylindrical Langmuir probe technique applied to the study of electron densities and temperatures in afterglow plasmas is described. It is shown that for positive probe potentials (V) no significant disturbance (`pseudodepletion') of the plasma occurs until the electron current to the probe (i_e) approaches the natural diffusive positive ion current flowing to the reference electrodes. This is indicated by departures from linearity of i_e²-V plots obtained in the orbital-limited current régime. Departures from linearity have also been observed due to probe sheath expansion resulting in departures from cylindrical geometry above a critical value of V. It is shown that, for values of V below those critical values signifying the onset of pseudodepletion and sheath expansion, meaningful probe characteristics can be obtained in afterglows from which accurate electron densities can be obtained. Also described are the effects which arise from temporal changes of electron temperature, and the resulting changes in floating potential V_f. The latter has been measured as a function of the electron temperature (obtained from plots of the electron retarding region) and is shown to substantiate the original Langmuir theoretical expression for V_f.