Electrostatic instability of an AC-driven discharge in dense air

The results of fast imaging of a streamerlike discharge dynamics around a pin electrode under quasi-continuous ac excitation in air at 1-20 bar are presented. Under a relatively low pressure of 1-3 bar, the discharge filaments show a long smooth and well-branched pattern. As the pressure increases, they get smaller and become twisted. Based on the presented photographs, it was proposed that an electrostatic instability induces secondary radial filaments developing around primary channels, which change the initial straight shape of the discharge.