Improved probe-curve evaluation based upon the Self and Shih method applied to positive column in He

Abstract

The ion-current evaluation method for collision-influenced probes developed by Self and Shih (SS) and their experimental results in positive-column discharges in He are examined. SS observe a peculiar saturation in the electron (e) component of the probe current—≈ independent of all plasma and probe parameters—which they explain as a plasma effect characteristic for positive columns; attempts to reproduce this saturation in the author's laboratory were unsuccessful. To determine the space potential VS, SS resort to the traditional Langmuir scheme of extrapolating the Te line [≡ asymptote to log Je(V), with Je ≡ e current to the probe] and the (peculiar) saturation current, in spite of the error in Vs which must be expected. In the present paper a ``combination method'' is described for determining Vs from the intersection of the Te line calibrated in density, lognFF(Vf), and the function lognSS(Vf) (≡ plasma density n according to SS with Vf ≡ FP potential as independent variable). With this method the uncertainties in the location of Vs with respect to the knee of the collision-influenced probe curve are avoided, and the accuracy of n is increased (although nss is shown to be remarkably insensitive to errors in Vs). From second-derivative measurements and from the theoretical treatment of the e spectrum in the positive column in He published by a number of authors, it is estimated that the error in n caused by underpopulation of the e spectrum above ≈ 20 eV as compared to a Maxwellian is ≲ 10%. In addition, such error sources as inaccuracies in Ji and Te, e reflection, and e emission from the probe by metastables are discussed. Potential importance of the combination method for studying the location of Vs with respect to the knee of the probe curve under collisional conditions is pointed out.