Digital time-resolved optical measurement of discharge currents in long air gaps

Shortcomings in the dynamic range and the frequency bandwidth of current diagnostic methods mean that measurement of the time-resolved discharge current in a long air gap discharge is still a serious problem. This work presents data from a novel discharge current sensor that takes the parameters required to determine the accuracy of the current measurement into account. The design of a coaxial current sensor with ±0.7% accuracy and a bandwidth of 75 MHz is presented. A digital optical fiber transmission system with a bandwidth that exceeds 100 MHz is used to transmit the signal. To demonstrate the efficiency of the whole system, experiments using a rod-plane discharge geometry with 2 m and 4 m air gap lengths are performed to analyze the measured current data in comparison with that taken from synchronized high-speed photographs. The results demonstrate that the investigated experimental set-up improves substantially the understanding of the fundamental mechanisms of long air gap discharges.