Counter-propagating opposite polarity guided streamer interactions: EFISH electric fields, propagation thresholds, and memory effects

Interactions between counter-propagating guided streamers are important in multi-jet atmospheric pressure plasma jet (APPJ) systems used to extend plasma treatment over larger areas and complex three-dimensional targets. Here, two geometrically identical helium APPJs are operated in ambient air with opposite polarity and independently controlled voltages, and their interactions are investigated using time-gated ICCD imaging and electric-field-induced second harmonic (EFISH) generation. We found that the voltage of the positive polarity jet required for streamer head merging decreases as the magnitude of the negative polarity jet voltage increases, showing that the presence of the opposing streamer strongly modifies the environment governing positive streamer propagation. Reducing the pulse repetition rate from 5 kHz to 50 Hz increases the voltage required for the positive streamer to form and reduces the positive streamer’s propagation distance, consistent with reduced memory effects. However, enhanced propagation persists at low repetition rates when the negative jet is present, consistent with an increasing relative importance of streamer interaction driven effects as memory effects weaken. Measurements of EFISH electric fields provide direct benchmarks for these opposite polarity guided streamer interactions. When the fields during dual-jet operation are compared to single-jet operation, the peak field at the positive guided streamer head is unchanged within the uncertainty of the diagnostic, whereas the field distribution ahead of the peak is elongated and approximately doubled by the presence of the negative guided streamer.