Investigation on the diffuse discharge under the double-pulse scheme in atmospheric N ₂ /O ₂ mixtures

Abstract This work experimentally investigates diffuse discharge characteristics in atmospheric-pressure N 2 /O 2 mixtures with oxygen concentrations ranging from 20% to pure nitrogen, employing both single- and double-pulse schemes. For single-pulse discharges, the diffuse morphology maintains a conical shape across oxygen concentrations (20%–0.2%), though reduced photoionization at lower O 2 levels modestly decreases propagation velocities and maximum widths. Pure nitrogen induces branching during propagation. Under double-pulse schemes, second discharges in 20% O 2 mixtures exhibit accelerated propagation but narrower widths compared to the first discharge, attributed via simulations to residual ionization simultaneously shielding electric fields and providing seed electrons. At lower oxygen concentrations, second discharges develop branching with probabilities inversely dependent on both O 2 concentration and pulse interval duration. Optical emission spectroscopy correlates branching occurrence with elevated pre-discharge gas temperatures near branching locations. While 2D simulations confirm pre-second-discharge temperature increases near the pin electrode, yet fail to resolve branching processes. Implementing 2D-derived temperature and gas density data into 3D models successfully reproduces experimental branching patterns, verifying gas-temperature-induced instabilities as the dominant branching mechanism.