Observation of spiral pattern and spiral defect chaos in dielectric barrier discharge in argon/air at atmospheric pressure.

A rich variety of spiral patterns such as single-armed spiral, dipole spirals, target pattern, multiarmed spiral, and spiral defect chaos state have been observed in ac-driven atmospheric pressure gas discharge. The confined and free boundary conditions are defined by means of whether there is a sidewall in the discharge domain or not, respectively. In the free boundary condition, the spiral pattern arises when the stripe pattern undergoes core instability or notching instability. In the confined boundary condition, the spiral pattern is formed by sidewall forcing. The spiral drifts upward in the free boundary condition and meanders in the confined boundary condition. The topological charge of the spiral pattern can be changed when the spiral interacts with the dislocations. The spiral wavelength (average distance between two consecutive rolls) is a function of gas composition and decreases rapidly with increase of air concentration in discharge gas.