Streamer current in a three-electrode system

A study has been conducted on positive streamer discharges in air at atmosp heric conditions for a three-electrode system. The electrode system consist ed of two parallel planes (one grounded and one supplied with a negative dc voltage) and a small, insulated needle, sticking out from the center of th e grounded plane. A triggering positive square impulse voltage of 5 mus dur ation was applied to the insulated needle and the currents associated with the streamer discharge were measured simultaneously on all three electrodes . During the streamer propagation, the current measured at the needle was t he conduction current while the other two where the displacement (or capaci tive) currents generated by the movement of charge in the electrode gap. Th e objective of this study is to identify the three currents and to investig ate if simple representations of the streamer can reproduce the displacemen t currents measured at the plane electrodes. Two models for the streamer we re applied: (1) a charged sphere moving in the background field and (2) a c hannel with a constant voltage gradient extending in the gap. In both model s it was assumed that the streamer propagated with a constant velocity, whi ch was estimated from the measurements. The motion of the streamer was simu lated by a series of electrostatic calculations, using a field calculation program. Comparison of the measurements with the simulations indicates that the charge of the streamer is confined to a spherical region (i.e. streame r head) and it is increasing continuously during its advancement in the ele ctrode gap, A discussion on advantages and disadvantages with the two inves tigated models (sphere vs. channel with potential gradient) is conducted, a nd a possible hybrid model is suggested. In the proposed model, features fr om both considered streamer representations are included.