Investigation of ignition of positive corona discharge in air using a time dependent fluid model

Abstract The time dynamics of positive corona ignition from an initial seed electron to the appearance of space charge effects is studied at atmospheric pressure in a spherically symmetrical geometry. The time-dependent model incorporates a recent photoionization model and boundary conditions to properly treat the corona discharge in the proximity of the anode surface. The applied voltage satisfying the self-sustainability condition of the discharge is first calculated. Then, the time dynamics of the discharge is studied for higher and lower applied voltages. The exponential growth in electron density is identified before space charge effects significantly shield the applied electric field. The related characteristic time of exponential growth in electron density is calculated as a function of applied voltage. The time scale can be tens of nanoseconds when applied voltage is close to the threshold or less than one nanosecond when increasing voltage 10% above the threshold for atmospheric pressure. Analogous to the equality of ionization timescale and dielectric relaxation time in stable streamers, a characteristic timescale is used to predict the behavior of the discharge. When the characteristic time is about one-tenth of the dielectric relaxation time, a one-tenth reduction in the applied field on the anode is observed. As the dielectric relaxation time drops below ten times the characteristic time, the discharge behavior diverges for different applied voltages, and the characteristic time is not a good predictor of further discharge dynamics.