Dielectric Relaxation in Insulating Layers on Contaminated Langmuir Probes

Measurements have been made of the low-frequency admittance of contaminated Langmuir probes in a mercury-vapor plasma. It was found that the probe admittance components were frequency dependent and could be represented by an equivalent electrical circuit corresponding to a metal-insulator-plasma configuration. The frequency dispersion of the admittance components is described by the empirical equation Y=G∞−(G∞−G0)/ [1+(G0/G∞) (jωτ0)1−h], where Y is the complex admittance, G0 and G∞ are the static and infinite-frequency values of conductance, τ0 is a relaxation time constant, and h is a parameter with values between 0 and 1. The complex-plane locus of the admittance is a circular arc from which the resistance of the insulating layer on the probe surface, the plasma-sheath resistance, and the value of h are derived. The susceptance-voltage characteristic of a probe is shown to be a sensitive indicator for the presence of insulating material on the probe surface. The susceptance-voltage curve for an insulator-contaminated probe has a peak in the vicinity of plasma potential which disappears upon removal of the insulating material by sputtering.