A comparative experimental study of electron and positive-ion current collection by a cylindrical Langmuir probe under orbital-limited conditions

Abstract

This paper describes the results obtained from a comparative study of the electron currents (ie) and positive-ion currents (i+) which flow to a small cylindrical Langmuir probe in pure rare-gas afterglow plasmas at low pressures. It is shown that within a predictable restricted range of charge density, linear plots of ie2 and i+2 against the probe-plasma potential difference could be obtained for positively and negatively biased probes respectively; which is in qualitative agreement with the Langmuir theory of orbital-limited-current collection. However, the values of the electron density (ne) and the apparent positive-ion density (n+) deduced from the slopes of the above plots were not equal; n+ always exceeding ne and related empirically to it via the mass of the positive ion m+ (in amu) thus: n+ = (1+0·07m+½)ne.It is suggested from additional experimental evidence that the values for ne are accurate and the discrepancy arises as a result of the inadequate theoretical description of the positive-ion currents flowing to the negatively biased probe. The data points of Tonks and Langmuir (1929) and of Shaeffer (1971) obtained in arc plasmas are shown to be consistent with the present results. The significance of these results to plasma diagnostics using probes is discussed.