Experimental study of the ion current to a cylindrical Langmuir probe taking into account a finite ion temperature

Abstract

This article deals with the experimental verification of a theoretical radial model, developed by the authors, for the sheath that surrounds a cylindrical Langmuir probe immersed in a plasma in which the positive ion temperature, Ti, is not negligibly small compared to the electron temperature, Te. The theoretical model is a generalization of the classical one developed for cold ions by Allen, Boyd, and Reynolds for the case of spherical probes, and extended by Chen for cylindrical ones. According to our theory, due to the positive ion thermal motion the ion current collected by the probe is increased with respect to the case of cold ions, so its influence must be considered in plasma diagnosis. An experimental device to accurately measure the I-V characteristic of a cylindrical probe in plasma, for which Ti/Te=0, has been developed. Very good agreement has been found between the theoretical positive ion I-V probe characteristic and the experimental values by using a Sonin plot.