In-situ optical observation of dynamic breakdown process across microgaps at atmospheric pressure

The study on breakdown characteristics at microscale is very vital to the plasma physics community and micro/nanoelectronic industries. However, the dynamic development processes including the electron avalanche and discharge channel during the breakdown event are still unclear experimentally so far due to the restriction of the in-situ diagnostic technique. In this paper, an optical diagnostic technique is put forward to achieve an in-situ observation of the dynamic breakdown process across microgaps at atmospheric pressure. By using the nanosecond temporally and micron spatially resolved optical measurement method, the light emission appearances of microgap breakdown under nanosecond pulse have been captured and imaged directly. The initial and propagation mechanisms for electrical breakdown across the very small air gap are discussed based on the light emission properties. Results show that the light emission initiates from the cathode as the initial electron avalanche, then the avalanche transits to the streamer due to the space charge effect and photoionization, and propagates towards the anode with a velocity of 4.40×10³ m/s, demonstrating a cathodedirected streamer in the pulsed breakdown. The space charge induced field enhancement and photoionization are proved to play a significant role in the pulsed breakdown across microgaps. In addition, the time to breakdown is estimated to be about 20ns while the formative time lag is about 9ns through the temporal sequence of the optical images. The in-situ optical measurement technique presented in this paper would be of a great help for better understanding the physical mechanism of microgap breakdown.