Ion mobility measurements in a positive corona discharge

Abstract

Two methods are examined for the measurement of the positive-ion mobility in a positive cylindrical corona discharge. The first method employs the novel technique of monitoring the temporal decay of the cathode current following the cessation of the corona near threshold. A detailed analysis of the current decay is offered, and comparison with experimental results on coronas in nitrogen and dry air yield values of positive-ion mobility that agree well with those found in the literature. The second technique for the measurement of the positive-ion mobility is based upon the current-voltage relationship of the discharge near threshold. The classical interpretation of the current-voltage data is found to yield mobility values that are anomalously large. It is suggested that this classical interpretation is incomplete, as it does not consider the dependence of the anode electric field on corona current. One possible cause of such a current dependence is considered in detail: Joule heating of the gas near the anode may increase electron impact ionization, through a reduction in the gas density, in turn lowering the anode electric field. Analysis indicates that such a thermal feedback mechanism is of the correct magnitude to explain the overestimation of positive-ion mobility from the classical interpretation of the current-voltage relationships in the nitrogen and air coronas.