Electric field measurements in long gap discharge using pockels device

The optical method using a Pockels device has been used to investigate an electric field distorted by space charges due to discharge development in air. A Pockels device placed in the discharge area has the advantage of directly measuring such a field. This method is successfully applied to the measurement of the electric field in and around the leader column propagating in a rod-plane gap of 1 m subjected to positive switching impulse voltage of 0.4 MV, and 1.03 MV in a rod-plane gap of 3 m. Moreover, in the 1 m gap system, the leader propagation is effectively controlled by using a conductive thread so that the leader column passes through the Pockels device and the exact electric field in the leader column is measured. The electric field at the device increases with the approach of the leader column and reaches its peak value on the arrival of the tip of the leader column. Finally, it falls to a low constant value during the leader propagation. From the results, it is found that the peak field at the tip of the leader column is 0.8 ~ 1.4 MV/m and that, subsequently, the low field in the leader column is 0.1 ~ 0.5 MV/m, notwithstanding the gap spacing of 1 ~ 3 m. Furthermore, the electric field calculated theoretically by using a space-charge model whose shape is a cylinder of 0.5 m in radius together with a hemisphere at the head of it agrees with the measured field.