Observation of space charge dynamics in air under DC electric fields

Abstract

Even under corona-free conditions insulating structures (materials) may be charged in DC fields. This is due to the fact that air has a finite ionic conductivity. The finite conductivity of air results from charges generated in the air due to background radioactivity and cosmic radiation. Typical ion concentrations on ground are 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> 1/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> which leads to a conductivity ranging from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−16</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−13</sup> S/m. An experiment has been conducted to measure order of magnitude, time scales, and boundary influences of air ionic motion under DC electric fields. The experimental setup consisted of two concentric cylinders of metallic mesh. The radius of the inner cylinder was 0.5 m and that of the outer cylinder 2 m. The current was measured as a function of time after a potential was applied to the inner cylinder. The background ion concentration was monitored by an ion counter. The results were analyzed and compared with simulations using an ion-drift model considering small cluster ions of positive and negative polarity. The measurements have been found in good agreement with the model.