Square grid pattern with direction-selective surface discharges in dielectric barrier discharge

A new phenomenon that a filament discharged only once instead of twice in a cycle of the applied voltage is observed in a square grid pattern in a dielectric barrier discharge(DBD)with a larger gas gap,which is named intermittent discharge.Its spatiotemporal dynamics and the formation mechanism are studied by the multiple photomultiplier tubes and an intensified charge-coupled device.Corresponding to the positions of spots in the picture with an exposure time of 40 ms,there are some bright spots(discharge spots)and black spots(non-discharged spots)in the instantaneous image with an exposure time of 10μs(a half cycle of the applied voltage).There are at least two bright spots around one black spot and vice versa.The surface discharges(SDS)can be observed between any two adjacent spots.The intensity of the SDSbetween the bright spot and the black spot is 2.5 times greater than that between two adjacent bright spots,which indicates that the SDSare directional-selective.The intermittent discharge with positive(negative)current polarity changes to that with negative(positive)current polarity,after it sustains up to 14 voltage cycles at the longest.The spatial distribution of the electric field component is calculated through COMSOL software to solve the Poisson equation numerically.It is found that the inhomogeneous distribution of surface electric field is caused by the inhomogeneous distribution of wall charges,which leads to direction-selective SDS.The intermittent discharge is formed by the competition between the direction-selective SDSand volume discharges(VDS)in DBD.This is the reason why the intermittent discharge is generated.

A metamaterial absorber with direction-selective and polarisation-insensitive properties

This paper reports the design of a metamaterial absorber with direction-selective and polarisation-insensitive property. Both theoretical and simulated results reveal that the absorber has a distinct absorption point with direction selectivity at 7.48 GHz, which is related to the resonance of the metamaterial and is not influenced by the polarisation. The retrieved impedance indicates that the impedance of the absorber can be tuned to approximatively match the impedance of the free space on one side and not to match the impedance of the free space on the other side. This design can result in the minimal reflectance, the minimal transmission and the highest absorbance at the absorption frequency. The distribution of the power loss indicates that the absorber is an excellent electromagnetic wave collector： the wave is first trapped and reinforced in certain specific locations, and then mostly consumed. The distribution of the surface current is consistent with the design, the retrieved impedance and the distribution of the power loss. This absorber may have applications in many scientific and technological areas.