Transmission and multiple reflection mechanisms of guided streamers propagating through grounded annular electrode and interacting with grounded surface electrode

Trains of positive guided streamers are generated within an atmospheric pressure plasma jet supplied in helium and polarized by a high-voltage nanosecond pulse generator. The device is completed by a grounded annular electrode and a grounded surface electrode on which they can interact. The resulting transmitted and multiple reflected guided streamers are measured combining optical and electrical characterizations. While the electrical approach provides information on the capacitive/conductive nature of the current peaks as well as on their positive/negative value, fast ICCD imaging distinguishes whether the guided streamers are incident, reflected or transmitted. Combining these two techniques allow us to demonstrate experimentally that the reflected streamers are negative contrarily to the others. Besides, 4 types of reflections have been highlighted: a reflection (r) at the outlet of the capillary, a reflection on the grounded surface electrode (R) and two reflections (r' and r") observed when an incident guided streamer passes through the grounded annular electrode. The two techniques agree that the characteristic propagation times are always shorter for reflected negative streamers than for the positive ones propagating forward. Hence, for a grounded annular electrode placed 3 cm away from the high voltage electrode, propagation time is 80 ns for reflection versus 250 ns for transmission. These propagation times are even shorter when the annular electrode is brought closer to the surface electrode with velocities typically higher than 300 km/s. Finally, all these experimental data are utilized to build an equivalent electrical model to understand the dynamics of the guided streamers and explain their transmission and reflection modes.