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.

We report the observation of a square superlattice pattern in a dielectric barrier discharge system. The correlation measurements indicate that the square superlattice pattern is an interleaving of two different transient square sublattices. The interplay between the charge pattern and the discharge filament pattern is discussed.

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The naturally persistent flow of hundreds of dust particles is experimentally achieved in a dusty plasma system with the asymmetric sawteeth of gears on the electrode. It has also demonstrated that the direction of the dust particle flow can be controlled by changing the plasma conditions of the gas pressure or the plasma power. Numerical simulations of dust particles verify the experimental observations of the flow rectification of dust particles. Both experiments and simulations suggest that the asymmetric electric potential and the collective effect are two keys in this dusty plasma ratchet.A dusty plasma (or complex plasma) consists of micron-sized dust particles immersed in plasma environments such as the ionosphere, the semiconductor manufacture and the laboratory [1,2]. These dust particles are highly charged so that they can be strongly coupled [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Due to the various heating mechanisms in plasmas, such as the plasma instability [17] and the spatial/temporal particle charge fluctuation, the kinetic energy of these dust particles can reach from several eV [10] to tens eV [18,19] or even higher.Realizing the manipulation of those energetic dust particles can lead to further scientific insights and applications such as the particle separation and the energy collection. Turning the random motion of these dust particles into the directional motion will definitely pave the way for this manipulation. The Feynman ratchet model provides a strategy to rectify the nonequilibrium fluctuations into the directional motion of particles [20,21]. Here, we demonstrate a dusty plasma ratchet to experimentally realize a rectification of dust particles in a radio-frequency (rf) plasma, i.e., the dust particles are rectified into a directional flow by the Feynman ratchet strategy. Remarkably, we can experimentally control the direction of particle flow by regulating the gas pressure or the rf power of the plasma. We further explore the rectification mechanisms of the dusty plasma ratchet by performing numerical simulations.Experiments. -A circular resin gear (named inner gear) with asymmetric sawtooth is placed concentrically with another circular resin gear (named outer gear) on a horizontal lower electrode, as shown in Fig. 1(a). The sizes of the inner and outer gears are 9 mm in height, 11.75 and 23 mm in radius, 1.5 and 4 mm in depth, respectively *

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