The rcsults of a theoretical study of the effect of positive ions on dectron cnergy distributions derived from spherical Langmuir probe second derivative characteristics are presented. The ion contribution to the probe second derivative causes tho density of electrons in t,he high energy tail of the distribution to be overestimated. The second derivative of the ion current characteristic has been obtained by numcrical double differentiation of characteristics calculated from the theory of Allen et al. (1957) ; these are presented in a dimensionless form suitahle for application to a wide range of experimental conditions. A probe second derivative characteristic has been calculated for a helium plasma with a nlaxwellian electroi~ encrgy distribution and a ratio of probe radius to Debye lcngth ( r l , / X~) of unity. It is shown that the errors in tho electron density and moan energy obtained from the probc energy distribution are negligible : however, thcre is a large error in the ionization r a t e These errors will decrease with increasing r,,/An and ionic mass. It is concluded that a correction for the effect of the ions must be mado ill calculations of the ionization rate from probc electron energy distributions.
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